X-Planes: General Atomics X-68 LongShot

This is the sixty-ninth article in the X-Planes series, following the X-1, the X-2, the X-3, the X-4, the X-5, the X-6, the X-7, the X-8, the X-9, the X-10, the X-11, the X-12, the X-13, the X-14, the X-15, the X-16, the X-17, the X-18, the X-19, the X-20, the X-21, the X-22, the X-23, the X-24, the X-25, the X-26, the X-27, the X-28, the X-29, the X-30, the X-31, the X-32, the X-33, the X-34, the X-35, the X-36, the X-37, the X-38, the X-39, the X-40, the X-41, the X-42, the X-43, the X-44, the X-45, the X-46, the X-47, the X-48, the X-49, the X-50, the X-51, the X-52, the X-53, the X-54, the X-55, the X-56, the X-57, the X-58, the X-59, the X-60, the X-61, the X-62, the X-63, the X-64, the X-65, the X-66, and the X-67.

Every official word about this aeroplane is an item of bookkeeping, a date, a maker, an engine, a sponsor, and one name. What the aircraft is for has no official word at all.

The X-68A was allocated on 20 August 2025 to General Atomics, with an engine cell reading 1 Williams WJ38-15, a sponsor cell reading DARPA, the Defense Advanced Research Projects Agency, and a description that the register, which Andreas Parsch compiles from data the Department of Defense has released, marks as only partly official [Parsch, DOD 4120.15-L Addendum]. That row was not public on this article’s date. The register’s page as archived on 10 December 2025 and on 15 January 2026 ends its research rows at the X-66A, and the page as archived on 1 February 2026 carries the X-68A row, so the row first appeared between 15 January and 1 February 2026 [Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025] [Parsch, DOD 4120.15-L Addendum, as archived on 15 January 2026] [Parsch, DOD 4120.15-L Addendum, as archived on 1 February 2026]. It is quoted here from the copy the compiler last updated on 23 August 2026, and the Statements in This Article That Postdate Its Own Date section records it.

Longshot; Experimental air-launched UCAV for air-to-air engagements.

The name is official wording and the sentence after it is the compiler’s, whose UCAV abbreviates unmanned combat air vehicle. That distinction is visible in the register’s markup, and the second section below takes it up. The row’s other cells carry no mark and are official data by the register’s scheme, so what the Department has released is the allocation’s bookkeeping and the name, and the mission statement a reader would naturally take as the government’s description of the aircraft is not one.

The programme behind the designation is LongShot, a Defense Advanced Research Projects Agency effort to fly an unmanned air vehicle that is itself launched from an aeroplane and that carries and fires air-to-air missiles [DARPA LongShot programme]. The research question is not the range benefit, which is arithmetic. It is whether a vehicle that small can release a missile that large and still fly, and the government has said so in the same sentence in four consecutive budget justification books published before this article’s date, and in a fifth published after it [DARPA PB2023 justification] [DARPA PB2027 justification].

This article is the other side of an event the X-67 article described. That article asked why the research series skipped the number 67, and its answer turned on this allocation having been approved when both computable readings of the governing instruction gave 67 rather than 68. The X-68A is the allocation that closed the X-67, and the three readings of why the government issued 68 belong to that article.

The Research Question

The binding unknown is a mass ratio, and the government named it.

Four budget justification books running, the President’s Budget books PB2023 through PB2026, carry the same sentence in the LongShot entry, and the fifth book, which falls outside this article’s date, carries it too [DARPA PB2023 justification] [DARPA PB2024 justification] [DARPA PB2025 justification] [DARPA PB2026 justification].

This program will address the stability and control challenges of launching air-to-air missiles from a relatively small UAV in an operational environment.

That sentence is the keystone and the rest of the entry is the motivation. The motivation is that a slower air-breathing vehicle can carry a missile a long way more cheaply than the missile can carry itself, so the missile arrives at its launch point with its propellant unspent [DARPA PB2023 justification]. The arithmetic of that claim is worked below and is not in doubt. What is in doubt is whether the vehicle survives the launch.

Why the Ratio Is the Problem Rather Than the Missile

A fighter releasing an air-to-air missile sheds a small fraction of itself. An aircraft of this class releasing the same missile sheds a large one, and every consequence of a release scales with that fraction rather than with the missile’s absolute mass.

Writing $m_s$ for the mass of the store released and $m_0$ for the mass of the vehicle immediately before release, the keystone parameter is their ratio.

At a fighter’s mass fraction every consequence below is negligible and at this vehicle’s it is not. An F-15E at the midpoint between the weight and the maximum take-off weight its service fact sheet states, which is 26,875 kilograms, releasing one missile of 161.5 kilograms, gives a $\mu$ of 0.601 percent [F-15E fact sheet] [AIM-120 AMRAAM]. A vehicle of 1,800 kilograms releasing two of them gives 17.94 percent. The ratio between those two figures is 30. Subscripts $f$ and $v$ mark the fighter and this vehicle.

The fact sheet’s figures are 37,500 and 81,000 pounds, converted here at the exact definition of the pound, because the sheet’s metric conversion of its take-off weight is wrong. It prints 36,450 kilograms where 81,000 pounds is 36,741.0, a difference of 291 kilograms, and a document that converts its own headline figure incorrectly is quoted here by its pounds. The sheet is read from a public archive snapshot because the service’s site refuses every client, and that copy is current as of April 2019 [F-15E fact sheet]. At the exact definition of 0.45359237 kilograms to the pound, the conversion and the midpoint are

The missile’s mass is variant-dependent and the choice is stated. The service’s fact sheet for the missile gives a launch weight of 335 pounds [AIM-120 fact sheet], and its metric conversion is also wrong, printing 150.75 kilograms where 335 pounds is 151.95. The current heavy variant is given as 161.5 kilograms [AIM-120 AMRAAM], and this article models with the heavier figure because a programme flying in 2026 carries current missiles. Every mass fraction here scales linearly in that choice, so a reader preferring the baseline figure may scale each by the ratio of the two masses, which removes about 5.9 percent, and no conclusion moves.

That factor describes the two launchers alone, because the missile’s mass cancels out of it. Dividing the two definitions gives the identity below, from which the missile has dropped out.

Two missiles and a launcher mass ratio of about fifteen give 30, the same number, reached by cancellation where the display above reached it by substitution. Any missile carried by both aircraft produces the same factor.

That factor of 30 is the research question. It turns a problem that is negligible at a fighter’s mass fraction into the thing a demonstrator is built to retire.

The Quantity That Makes It Worse Than Linear

The consequences of a release are not proportional to $\mu$. They are proportional to an amplification of it, because the vehicle that must absorb the change is the vehicle that remains after the store has gone.

At small $\mu$ the amplification is indistinguishable from $\mu$ and at large $\mu$ it diverges, and its rate of divergence takes one line. Differentiating gives

so the amplification grows at 1.012 times the mass fraction’s rate at the fighter’s operating point and at 1.485 times at this vehicle’s. The fighter lives where the relation is effectively linear. This vehicle lives where every further kilogram of store costs half again its proportional share.

At the operating points themselves the divergence reads as follows. A fighter’s $\mu$ of 0.601 percent amplifies to 0.605 percent, a change in the third significant figure. This vehicle’s 17.94 percent amplifies to 21.87 percent, and a vehicle releasing half its mass would see its centre of gravity move by the whole store arm, since $\alpha$ reaches one there. Because $\alpha = m_s / m_1$, each value follows from the masses directly.

Nothing about this aircraft’s mass, span, wing area or internal geometry is published, so $\mu$ cannot be stated. It is swept, and so is every quantity that depends on the vehicle’s dimensions. The sweep runs from 1,000 to 3,000 kilograms, which gives $\mu$ from 32.3 percent down to 10.8 percent for a pair of missiles, and the brackets are justified under Dependent Systems from the one published anchor this aircraft has.

The Register Row, and the One Official Word in Its Description

The register marks the reliability of its entries, and this row is marked in a way no other research row is.

The addendum to the Department of Defense list of model designations, which Andreas Parsch compiles, records allocations from 19 August 1998 onward and explains in its front matter why it exists. No updated edition was published for almost six years after October 1998, the compiler requested the new assignments under the Freedom of Information Act, and the list has been maintained since [Parsch, DOD 4120.15-L Addendum]. The compiler further records that for designations assigned after October 2018 no complete official data is available, that the official description is no longer releasable to the public, and that wording which is not official Department text is therefore marked as such.

That mark sits at three levels and the three mean different things. A mark on the whole row says the allocation itself is absent from officially released data. A mark on the description cell means the allocation is official and the stated purpose is the compiler’s. A mark on a span inside the description indicates that part of the description is official wording and part is not.

So 103 of the 539 rows carry a mark of some kind. The counts are taken from the register as it stood on 18 October 2026, in the edition the compiler last updated on 23 August 2026, and every row of its tables is counted, including 7 whose designation cell is irregular, such as C-130J(CC-130J) and AGM-158B-2. The X-67 article states the same partition over the same 539 rows, and 5 of the 103 marks fall on the 7 irregular rows. The register current on this article’s date was smaller. The compiler’s update of 11 August 2025, as archived on 10 December 2025, held 526 rows and lacked the X-68A row and fourteen other later rows [Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025]. Under that version 94 rows carried a mark, being 65 unofficial descriptions, 12 partly unofficial descriptions and 17 unofficial rows against 432 official rows, and no research row carried a span-level mark. DARPA sponsored 6 rows, 3 of them research allocations and 2 of those among the 6 research rows dated in full from June 2019, Williams was named in 13 engine cells, 3 of them on research rows, and General Atomics held 14 rows, 10 of them unmanned allocations and none a research allocation. Every count below that involves the X-68A or the X-76A is a count over the later copy.

The X-68A Is the Only Research Row Marked at the Span Level

17 rows in the register carry a span-level mark and exactly 1 of them is a research allocation. The other 16 are a fighter, two light attack aircraft, two trainers, three helicopters, an airborne communications aircraft, an unmanned tanker, an assault aircraft prototype and five missiles. Each of them follows the same pattern, and once seen it explains the mark entirely. The popular name is official and the sentence explaining the aircraft is not. [Parsch, DOD 4120.15-L Addendum]

The row’s description is built as the name, then a separator, then a marked span.

<i>Longshot</i>; <span class="not-from-dod-records">Experimental air-launched UCAV
for air-to-air engagements.</span>

The Department has released the name and has not released the mission. A reader taking Longshot; Experimental air-launched UCAV for air-to-air engagements. as the government’s description of this aircraft would be taking the compiler’s sentence as the government’s, and the register is careful to prevent exactly that. The phrase air-launched UCAV for air-to-air engagements is a competent and almost certainly accurate summary. It is not an official one.

This is the sharpest available statement of how thin the official record is. The budget justification books describe the programme at length and they describe a programme rather than an aircraft. The designation register names the aircraft and describes it only through a compiler’s inference. No public Department document read for this article states what the X-68A is.

Rows Marked at the Row Level Carry No Full Date

16 rows carry the row-level mark and 0 of them carries a full date. That is not a coincidence. Once stated, it comes close to a tautology. A row whose allocation is absent from officially released data has no officially released allocation date either, so the compiler enters a year, a month and a year, or nothing.

The implication runs one way only. Every row-marked row has a partial date, and plenty of partially dated rows carry no row-level mark, so the two sets are nested and not equal.

The consequence is a trap for anyone counting the register. A count restricted to rows whose date parses, as any ordering of allocations in time must be, silently removes every row-level mark from any officiality count that uses it. The count then reports zero marks of a kind the document plainly carries, and reports it with no warning of any sort. The figures above are computed over all 539 rows for that reason.

The Register Spells the Name Differently

The register prints the name as Longshot. The sponsoring agency and the contractor both write it LongShot, with an internal capital, in their announcements [DARPA LongShot programme] [General Atomics on the X-68A]. The difference is trivial. It is recorded because the register is the only document that gives the name official standing, and the form it gives official standing to is the one nobody else uses.

Programme Origin From the Award Record

Five awards in the federal award record carry this programme [USAspending federal award record], and together they record things no press item reports. The X-65 article finds the same of the award record for its programme.

The total is 148,305,710.65 dollars. Every instrument is cost plus fixed fee. The four awarded by the sponsoring agency all cite solicitation HR001120S0037, so one broad agency announcement covers the early work and the demonstration phase alike [USAspending federal award record].

The Sponsor Holds Five Research Rows and Four Are Recent

DARPA appears in the sponsor cell of 8 rows in the register and 5 of them are research allocations, being the X-50A, the X-61A, the X-65A, the X-68A and the X-76A. 4 of those five are among the 8 research rows with a full allocation date from June 2019 onward, so an agency sponsoring five of the register’s 31 research rows overall, which is about a sixth, holds half of the recent ones. The X-56B row, dated only as 2019 with a query, is left out of that count because its place in the year is unknown, and with it counted the share is 4 of 9. The designation series’ present tempo is substantially this one sponsor’s, which is context for the register-wide statements the X-67 article made about allocation cadence [Parsch, DOD 4120.15-L Addendum].

The First Instrument Is a Navy Support Contract

N6523620F3033 begins 150 days before the earliest of the three development awards. It is awarded by a Navy information warfare office, it cites a Navy solicitation where the other four awards cite the programme’s broad agency announcement, and its product code is professional engineering support, a category outside research and development [USAspending federal award record].

So the programme was buying engineering support five months before it was buying designs. That ordering is in the record and is not in any account of the programme this article found.

The Three Early Awards Are Not Three of a Kind

One of the three is categorised as basic research and the other two as experimental development. Northrop Grumman’s instrument carries product code AC11, which the record glosses as basic research, and it runs 248 days. General Atomics’ and Lockheed Martin’s carry AC13, experimental development, and run 1,357 and 479 days respectively [USAspending federal award record].

The obligated amounts differ by much more than the categories suggest. General Atomics took 3.9 times Lockheed Martin’s amount and 10.1 times Northrop Grumman’s, for a three-award total of 41,580,384.00 dollars [USAspending federal award record]. This was not three firms doing the same work at the same scale.

The competition fields are not uniform either, which is easy to get wrong. Northrop Grumman’s action records 36 offers received under full and open competition. General Atomics’ and Lockheed Martin’s each record 5 offers under full and open competition after exclusion of sources [USAspending federal award record]. These are different procurement postures recorded on actions awarded within two months of one another, and this article states the fields as the record carries them and infers no selection mechanism from them, because the mechanism is not in the record.

General Atomics’ Early Instrument Outlives the Programme Phase It Belongs To

HR001121C0023 runs to 2024-10-31, which is 1,357 days, against Lockheed Martin’s 479 and Northrop Grumman’s 248. It therefore remains open for well over a year after the demonstration award of 2023-06-30 begins. Two instruments with the same recipient and the same programme overlap for that period, which the record shows and does not explain [USAspending federal award record].

The Demonstration Award Against the Press Figure

The record gives HR001123C0087 as 101,920,961.08 dollars with a period of performance ending 2026-12-31 and 1 offer received [USAspending federal award record]. Press accounts of this award report a figure of 94 million dollars [DefenseScoop on LongShot Phase 3]. The record’s figure is higher, and the difference is consistent with the press having reported an initial obligation while the record carries the current total, but this article has not read a document that says so and therefore does not assert it.

The period of performance ends in the quarter of the earliest flight test date the agency has given. It ends 2026-12-31, and the sponsoring agency has said the flight test campaign may begin as early as the end of 2026 [DARPA on the X-68A]. That campaign and that statement both postdate this article’s date and are treated accordingly below.

What the Award Record Does Not Account For

The award record’s 148,305,710.65 dollars is about 72 percent of the 205.740 million dollars the budget books report for fiscal years 2021 through 2025. That total is the largest figure the funding table below gives for each of those five years, and the share follows from it.

The remainder is not accounted for by any award in the record that carries this programme. It may include government labour, test facilities and work bought through instruments that do not name the programme, and which of these applies is not recorded. A programme’s cost is not its contracts and the two records are not substitutes for one another.

The Solicitation Was Not Read

HR001120S0037 is named by four awards and this article has not read it. The federal opportunity portal serves a single-page application whose content is not in the delivered document, the public web archive holds no snapshot of the announcement, and the sponsoring agency’s site returns a not-found response for it. Its address in the portal is nonetheless real and is cited as an address [Solicitation HR001120S0037], so that a reader with a session can reach what this article could not, and no claim here depends on the announcement’s content.

Two Names Appear as Programme Manager

The sponsoring agency’s programme page names one person and its later announcement names another. The programme page gives Shawn Naigle of the Tactical Technology Office [DARPA LongShot programme]. The announcement of February 2026 quotes Colonel John Casey as LongShot programme manager [DARPA on the X-68A]. This article reports both with their sources and does not resolve which is current, since a programme running six years may simply have had two.

What Seven Budget Books Said About the Concept

The programme’s description in the budget justification books is boilerplate that is rewritten when the concept changes, so diffing it across books dates the change without any press item at all. Seven books carry a LongShot entry, from the book of February 2020 to the book of April 2026, and they carry six distinct descriptions between them [DARPA PB2021 justification] [DARPA PB2027 justification].

The Programme Began as a Weapon System

The first two books describe a munition. PB2021 and PB2022 both open the entry the same way [DARPA PB2021 justification] [DARPA PB2022 justification].

The goal of the LongShot program is to develop and flight demonstrate a weapon system using multi-mode propulsion that significantly increases engagement range and weapon effectiveness against adversary air threats. LongShot will explore new engagement concepts for multi-modal, multi-kill systems that can engage more than one target.

Three phrases in that passage do not survive into the later books. It is a weapon system rather than an air vehicle, its propulsion is multi-mode, and it is multi-kill. Both books also spell out the benefit in two numbered parts, and those two parts are the quantitative claims this article works through below [DARPA PB2021 justification] [DARPA PB2022 justification].

First, the weapon system will have a much-increased range over their legacy counterparts for transit to an engagement zone. Second, launching air-to-air missiles closer to the adversary increases energy in terminal flight, reduces reaction time, and increases probability of kill.

The Concept Changed Once, in a Window of 11 Months

PB2023 and PB2024 open the entry differently [DARPA PB2023 justification] [DARPA PB2024 justification].

The LongShot program is developing and flight demonstrating an air-launched Unmanned Aerial Vehicle (UAV) capable of engaging multiple adversary targets from standoff ranges using existing air-to-air missiles.

Four things changed at once. The weapon system became an unmanned air vehicle. Multi-mode propulsion disappeared and has not returned. The missiles became existing missiles, and that word marks what the programme will not develop. And the keystone sentence appeared. [DARPA PB2022 justification] [DARPA PB2023 justification]

The change is bounded by the two book dates and the window is 11 months, from the book of May 2021 to the book of April 2022. That is as tight a dating as the budget record can give, and it required no press item to reach.

The Award Record Corroborates the Change Independently

One contract description preserves the original concept in the government’s words and is dated by its award. Northrop Grumman’s instrument of 2021-01-04 carries this as its description of the programme [USAspending federal award record].

LONGSHOT, THE LONGSHOT PROGRAM WILL DEMONSTRATE THE USE OF MULTIPLE MODES OF PROPULSION TO ACHIEVE OPTIMAL LONG-RANGE AIR-TO-AIR WEAPONS EMPLOYMENT.

Multiple modes of propulsion, in a contract file, from January 2021. The budget books and the award record are separate systems maintained by separate offices for separate purposes, and they agree on what this programme was at the start. Agreement between two independent systems is corroboration.

And the engine is the physical trace of the change. A vehicle whose concept is multi-mode propulsion does not fly on one cruise-missile engine. The engine in the register’s cell is single mode, and this article takes that as the strongest available evidence that the change in the books was a change in the aircraft and not in the prose describing it.

The Later Changes Are Matters of Vocabulary

Three further versions follow and none of them changes the aircraft. The book after PB2023 and PB2024 drops the expansion of unmanned air vehicle from the opening sentence and says an air-launched system. The next replaces a relatively small UAV with a relatively small uninhabited system in the keystone sentence. The last replaces which provides with providing [DARPA PB2025 justification] [DARPA PB2026 justification] [DARPA PB2027 justification].

The first two remove first the expansion of the word unmanned and then the word itself, and the third is copy editing. A reader diffing these books without separating concept from vocabulary would count five changes where there was one, and this article identifies which is which.

The Funding Line Reports One Year Three Ways

A budget book states a request for its budget year and figures for the two years before it, so a fiscal year appears in up to three books and can appear with three different figures. Reading only the latest book hides every restatement.

4 fiscal years are restated and 2 are not. The stable pair, FY2022 and FY2023 in the books’ abbreviation for fiscal year, each hold 36.000 million dollars across three books without moving. Their stability establishes that the restatements elsewhere are not an artefact of how the books are compiled [DARPA PB2023 justification] [DARPA PB2024 justification] [DARPA PB2025 justification].

The largest restatement is fiscal year 2025, which grew 74.8 percent above its own request, from 36.742 million dollars to 64.213 million [DARPA PB2025 justification] [DARPA PB2026 justification] [DARPA PB2027 justification], and every share in the table’s last column is the spread over the lowest figure in the same way.

A request that grew by three quarters in execution is consistent with a programme being accelerated, and the books do not say why or by whom.

The Programme Moved Programme Elements

The line did not stay where it started. Through the book of February 2020 and five books after it the entry sits in programme element 0603286E, Advanced Aerospace Systems, project AIR-01. From fiscal year 2026 it sits in programme element 0603468E, Advanced Complex Systems, project ACX-01, and each book from that point carries the migration in a dedicated sentence [DARPA PB2021 justification] [DARPA PB2026 justification].

Beginning in FY 2026, this program will be funded in PE 0603468E, Project ACX-01.

The migration matters to anyone reconstructing the funding history, because a search of the old programme element after fiscal year 2026 finds the programme named and funded at nothing [DARPA PB2027 justification]. The X-65 article met the same structure in the same programme element and recorded it for the same reason.

The project identifier must be read from that sentence and not from the page. The books print a shared footer, and a reader taking the nearest project identifier on the page takes a neighbouring programme’s.

Sizing From First Principles

The Symbols This Article Uses

What Leaves and What the Remainder Must Absorb

The release is an instantaneous change in the vehicle’s mass, its centre of gravity, its inertia and its trim, and all four follow from the one ratio. The mass after release is immediate.

The centre of gravity is where the problem starts. Taking moments about any datum before and after, and writing the store’s position relative to the original centre of gravity as $d_s$, the remaining vehicle’s centre of gravity sits at

which, substituting $x_s = x_{cg,0} + d_s$ and dividing through by $m_0$, reduces to a shift that depends only on $\mu$ and the arm.

The sign is the physically obvious one. Releasing a store that sits aft of the centre of gravity moves the centre of gravity forward. The magnitude is the part that is not obvious, because the arm is scaled by the amplified fraction where intuition expects the plain one, and the amplification distinguishes this vehicle from the aeroplane that carries it.

The First Consequence Is Vertical and Precedes Any Rotation

The vehicle trimmed its lift against a weight that is suddenly smaller, so the first consequence of a release is translational. At the instant the store leaves, the lift is unchanged and the weight has stepped down, and Newton’s second law for the remaining vehicle gives the net upward acceleration directly.

The amplification factor appears a third time, now as a load factor step. The fighter of the comparison rises at 0.0060 of a gravity, which its occupants would not notice. This vehicle rises at 0.219 of a gravity, or 2.14 metres per second squared, an uncommanded manoeuvre at the moment the centre of gravity is also moving.

Restoring level flight means shedding lift in proportion to the mass that left. At constant speed and dynamic pressure the required lift coefficient steps down by the mass ratio,

a 17.94 percent reduction, commanded through the same control surfaces that are simultaneously retrimming the pitching moment. The two demands arrive together and are coupled through the same actuators, and that coupling adds one more sentence of content to the government’s phrase stability and control challenges.

The Static Margin Can Be Spent in One Event

The neutral point does not move when a store is released from inside the vehicle, since the store contributes no lift of its own and the external shape is unchanged once the bay is closed. The entire change in static margin is therefore the centre-of-gravity shift expressed in chords.

This result turns the government’s sentence from an engineering detail into a research question. For a vehicle of 1,800 kilograms releasing two missiles, with a mean aerodynamic chord of 1.2 metres, the static margin moves by 5.5 percentage points for a store arm of 0.3 metres and by 21.9 percentage points for an arm of 1.2 metres.

The band a design accepts is taken here as a few percent of chord, an assumption that no source supplies. A release that moves the margin by tens of percentage points then does not perturb the vehicle’s stability. It redefines it. The vehicle must be stable both before and after, which is a constraint on where the store may sit rather than a constraint on the control system.

The constraint can be written directly. Requiring the static margin to stay within a band $[K_{\min}, K_{\max}]$ both before and after release bounds the arm.

So the admissible store arm is inversely proportional to the amplification factor. At a fighter’s $\mu$ the bound is metres and does not bind. At this vehicle’s it is a fraction of a chord, and the store must sit very close to the centre of gravity. A stability requirement has become a layout requirement, the kind of coupling a demonstrator exists to discover.

Retrimming Costs Control Authority That Was Sized for Something Else

The shift turns the lift the vehicle is already carrying into an unbalanced pitching moment of magnitude $C_L \Delta x/\bar c$, which the longitudinal control must cancel.

The deflection is proportional to the lift coefficient, so the release is hardest to trim out at the low speed and high lift coefficient where a small vehicle has least control power to spare, and easiest in fast level flight. A release envelope therefore exists, narrower at the slow end.

The Inertia Changes Twice and Taking One Correction Is the Common Error

Two parallel-axis corrections apply to a release and only one of them is intuitive. The store’s own inertia and its $m_s d_s^2$ term leave with the store. The remaining vehicle must then be re-referenced to a centre of gravity that has moved, a second correction of opposite effect.

The second term is small when $\mu$ is small and is not negligible here. An analysis that stops after the first term overstates the remaining inertia, and overstating inertia understates every angular rate the release produces, an error in the optimistic direction.

Releasing One of a Pair Is a Different Problem From Releasing Both

The symmetric arithmetic above misses the case that is harder. A vehicle carrying two stores and releasing one is left with a lateral mass asymmetry whose rolling moment the lateral control must hold against.

Setting that against the roll control’s authority at the current dynamic pressure gives the deflection required to hold the wings level.

This scales with $\mu$ directly rather than with $\alpha$, because the asymmetry is a weight and not a redistribution, and it scales inversely with dynamic pressure. A single release at low speed under load factor is therefore the binding case for lateral control, and a vehicle firing one missile and keeping another would fly it.

Before the control responds at all, the moment produces a roll transient whose arithmetic is rigid-body rotation and nothing else. The angular acceleration and the open-loop bank growth are

and no roll inertia for this vehicle is published, so $I_x$ is estimated as $m_1 k_x^2$ with the radius of gyration swept and the estimate labelled as one. At a load factor $n_z$ of one, which is level flight, at $k_x$ of 0.4 metres and a lateral arm of 0.3 metres the kept missile’s moment is 475 newton metres, the open loop acceleration is 1.8 radians per second squared, and the bank angle half a second after release is about 13 degrees, for a vehicle of 1,800 kilograms whose remaining mass is 1,638.5 kilograms.

The estimate brackets $k_x$ between 0.3 and 0.5 metres and $y_s$ between 0.2 and 0.4 metres, and its two corners bound the bank angle from 6 to 31 degrees, since $\phi$ grows with $y_s / k_x^2$.

Half a second is a generous allowance for a control system to notice, and a vehicle that banks tens of degrees in it has a requirement on control bandwidth that came from carrying its stores side by side.

The Ejector Pushes Back

A store is not dropped, it is pushed, and momentum is conserved. An ejector that imparts velocity $v_e$ to the store gives the vehicle a recoil in the opposite direction.

At an assumed ejection velocity of 7 metres a second the vehicle takes 1.53 metres a second if both stores leave together, or 0.69 for one of the pair at the single-missile fraction of 8.97 percent. The same ejector on the F-15E of the comparison above gives 0.042 metres a second. The amplification factor appears again, and this time it multiplies a velocity the cartridge was sized to produce for the store’s benefit alone.

Treated properly the ejection is a two-body problem, governed by a reduced mass. The cartridge’s impulse $J$ acts equally and oppositely on store and launcher, so the relative separation velocity is the impulse over the reduced mass,

which for one missile here is 147 kilograms against the missile’s 161.5. The energy the cartridge must deliver is $J^2 / 2 m_r$, about 4.3 kilojoules for an impulse of 1,130 newton seconds, and the launcher’s share of that energy follows from momentum conservation alone.

The launcher absorbs exactly its store’s mass fraction of every cartridge it fires. A fighter takes 0.60 percent of each shot and this vehicle takes 9.0 percent, about 390 joules per release delivered into its structure, a repeating structural load whose size the keystone parameter set, whatever the ejector’s designer chose.

The ejector acts off the centre of gravity, so it also produces rotation. A force $F_e$ at an arm $d_s$ for a stroke time $\tau$ delivers an angular impulse.

The inertia in that denominator is the inertia after release, computed with both corrections above, and the second correction matters for that reason. An ejector sized to clear the store cleanly is not sized to leave the launcher undisturbed, and for a vehicle of this mass fraction those are different requirements.

A worked example, with every assumption stated, shows the size of the effect. The pitch inertia of a slender body of uniform density is estimated as

which at 6 metres of length gives 4,916 kilogram square metres, and the estimate is an assumption with no datum behind it. The cartridge impulse above delivered over 50 milliseconds is a force of 22.6 kilonewtons, and acting at the admissible arm of 0.3 metres it leaves a residual pitch rate of about 4.0 degrees per second, or 7.9 at twice the arm. A tip-off rate of a few degrees per second is modest but not zero, and it arrives at the same instant as the heave, the trim change and the static margin step above. The release is all four at once, and the word transient carries all four in the budget books’ sentence.

The Premise as an Exchange Rate

The programme’s motivation is usually stated as extra reach and is better stated as an exchange rate. The budget books of February 2020 and May 2021 give two benefits and both of them reduce to one asymmetry between how an air-breathing engine and a solid rocket motor convert propellant into distance [DARPA PB2021 justification] [DARPA PB2022 justification].

An Engine Does Not Carry Its Oxidiser

The first and largest term is specific impulse. A rocket motor carries its oxidiser and an engine takes oxygen from the air it flies through, so the propellant each is charged for differs by roughly the mass of the oxidiser. Writing an engine’s thrust specific fuel consumption as $c_T$ gives its effective specific impulse directly.

The ratio of the two is the premise.

At 3,500 seconds for a small turbofan and 235 seconds for a solid motor, $\Lambda$ is about 14.9. Neither figure is published for this aircraft or for the missile it carries, so both are assumed and swept, the engine’s from 3,000 to 4,500 seconds and the motor’s from 220 to 250, which moves $\Lambda$ between 12.0 and 20.5 without changing its order of magnitude. A kilogram of fuel burned in the engine delivers something near fifteen times the impulse of a kilogram of solid propellant.

The Atmosphere Every Number Below Is Computed In

Two constants run through everything that follows, and both are derived here. Every coast, cruise and climb figure in this article is computed at 10,668 metres, which is 35,000 feet, and the air there is described by three classical relations and a gas property. The temperature falls linearly through the troposphere, the pressure follows from hydrostatic balance combined with the ideal gas law, and the density is the state equation applied to the two.

With a lapse rate of 6.5 kelvin per kilometre and a sea-level temperature of 288.15 kelvin, the exponent evaluates to 5.256, and the altitude gives a temperature of 218.8 kelvin, a pressure of 23,842 pascals, and a density of 0.3796 kilograms per cubic metre. The sound speed follows from the gas alone,

which evaluates to 296.5 metres per second. Those are the two numbers every coast, cruise and climb figure in this article uses, and both follow from four constants of the standard atmosphere.

The constants are read from the defining document. The 1976 standard adopts $g_0$ of 9.80665 metres per second squared exactly, a sea-level pressure of 101,325 pascals, a sea-level temperature of 288.15 kelvin and a heat-capacity ratio of 1.4 in its table of adopted constants, and its table of defined gradients gives the first atmospheric layer a gradient of minus 6.5 kelvin per kilometre [U.S. Standard Atmosphere, 1976]. The scan is image-only and the two defining pages were read as page images. One nuance follows from those pages. The defining relations take geopotential height as their argument, and the figure of 10,668 metres is treated here as geopotential. The same document relates geometric height $Z$ to geopotential height through an effective Earth radius $r_0$ of 6,356,766 metres [U.S. Standard Atmosphere, 1976],

so the geometric difference is 17.9 metres, or 1.7 parts in a thousand. Read the other way, as a geometric 10,668 metres, the altitude would raise the density by about 0.2 percent to 0.3805 kilograms per cubic metre and the sound speed to 296.6 metres per second. Each change is in the fourth significant figure.

A Missile’s Reach Is Logarithmic

A missile does not cruise, it coasts. After burnout it decelerates against drag, and writing the deceleration against distance rather than time gives an exponential decay with a characteristic length.

For a missile of the mass and diameter the programme expects to carry, at cruise altitude, $\ell$ is about 114 kilometres. A missile needs residual speed at arrival in order to manoeuvre against a manoeuvring target, so its reach is set by a terminal-speed requirement rather than by coming to rest.

At a burnout speed of 1,186 metres a second and a terminal requirement of Mach 1.5, that gives 112 kilometres. Across a terminal requirement swept from Mach 2.0 down to Mach 1.2 the reach runs from 79 to 137 kilometres, which spans the 90 kilometres the encyclopedia gives for one variant of the missile family the programme names and reaches the lower end of the 130 to 160 kilometres it gives for a later one [AIM-120 AMRAAM]. The model is therefore calibrated against the one thing about this system that is published, and that calibration supports the comparison that follows more than the absolute values in it.

Reach being logarithmic in the speed ratio creates the asymmetry. Inverting the relation gives the burnout speed a motor must deliver for a required reach, and the exponential is now working against the designer.

What It Costs Each Vehicle to Buy the Same Distance

Doubling the missile’s reach with its own motor means squaring its speed ratio. From 112 kilometres to twice that requires a burnout speed of 3,163 metres a second, which is 2.67 times what the motor delivers now. The rocket equation turns that into propellant.

The propellant fraction rises from 0.402 to 0.746. A missile that is already two-fifths propellant must become three-quarters propellant, which leaves no mass for a warhead, a seeker or a structure. The missile cannot be made to fly twice as far. It is not a matter of expense.

The carrier buys the same distance on the Breguet relation, and a one-line derivation shows where the asymmetry comes from. In steady level flight the thrust equals the drag, the drag is the weight over the lift-to-drag ratio, and the fuel flow is the thrust over the specific impulse, so each increment of distance costs fuel in proportion to the mass it is carrying,

and the logarithm appears on integration rather than by assumption. The missile’s logarithm sits in the exponent of its reach and the carrier’s sits inside its range, and that placement is the difference between the two vehicles stated grammatically.

Flying 112 kilometres costs the carrier a fuel fraction of 0.013. Against the missile’s 0.746 that is a factor of about 59.

The same comparison can be made at the margin, where it needs no fuel fraction at all. Differentiating each range with respect to the propellant that buys it gives the exchange rate of distance for mass on each side,

and the next kilogram of fuel buys the carrier 4,901 metres while the next kilogram of solid propellant buys the missile 1,371, a ratio of 3.6 at the midpoint of every sweep. The marginal ratio is smaller than the integral comparison suggests because the missile’s first kilograms of propellant are cheap and its last are ruinous, as a logarithm in the exponent requires.

That ratio, and not any statement about extra reach, is the programme. The two vehicles are not competing to fly the same distance at different efficiencies. One of them can fly the distance and the other cannot fly it at any propellant fraction below one.

The Reach Gain, Stated Conservatively

With the carrier flying $R_c$ and the missile flying $R_m$ from wherever the carrier releases it, the total is additive and the gain is a ratio.

In this honest form the benefit is linear rather than exponential. A carrier flying as far as the missile doubles the reach. This article does not state a figure for $G$, because $R_c$ depends on the vehicle’s fuel fraction and lift to drag ratio, neither of which is published, and a gain computed from two swept quantities would be a sweep presented as a result.

Why the Terminal-Energy Claim Must Be Stated Carefully

The budget book’s second benefit is that launching closer increases energy in terminal flight, and the exponential makes that claim easy to overstate. [DARPA PB2022 justification] Arrival speed decays as $e^{-s/\ell}$ and arrival kinetic energy therefore as $e^{-2s/\ell}$, so removing carrier transit from the missile’s fly-out multiplies its arrival energy by the reciprocal.

Evaluated at a carrier range of several hundred kilometres this expression returns factors in the thousands, and those factors are meaningless. The missile never flies that far, so the denominator describes an engagement that cannot occur. The expression is a reach ceiling wearing the clothes of a benefit curve, valid only for $R_c$ small against $\ell$.

Stated within its domain the claim is still strong, and only the bounded version should be made. Because reach is logarithmic, every kilometre the carrier flies is a kilometre the missile does not have to spend speed on, and the speed it does not spend is available at arrival. The benefit is real, it is bounded by the missile’s reach, and the unbounded version of it is an artefact.

Reaction Time Is the Same Exponential Read Differently

The reduced reaction time, which the second stated benefit also claims, follows from integrating the decaying speed over the fly-out.

Flight time is convex in distance, so the last kilometres of a long shot cost disproportionately more time than the first. Moving the launch point forward removes time from the expensive end of that curve, the sense in which the book claims reduced reaction time, and a stronger statement than a proportional saving would be.

Dependent Systems

The Engine and Its Conflicting Designations

The register’s engine cell is the only independent published statement of this aircraft’s propulsion, and its designation conflicts with the engine’s published type. The cell reads 1 Williams WJ38-15 [Parsch, DOD 4120.15-L Addendum]. In the military aero-engine designation system, which the same compiler documents, the letter J denotes a turbojet and the letter F a turbofan [Designations of U.S. Military Aero Engines].

The same engine is identified elsewhere as the powerplant of the Taurus KEPD 350 cruise missile [The War Zone on the X-68A], whose manufacturer designation is P8300-15, described as a turbofan of 6,672 newtons, or 1,500 pound force [Taurus KEPD 350]. The missile’s manufacturer uses the same word, its datasheet describing a high-thrust turbofan engine and stating the 1,400 kilogram mass on which this article’s thrust-to-weight anchor depends [Taurus KEPD 350 datasheet]. The thrust figure appears in neither manufacturer document and rests on the encyclopedia alone, which the epistemic state records.

So the register’s engine string reads as a turbojet in that system, and the engine is described as a turbofan under its manufacturer designation. The X-65 article met the mirror image of this problem, where the register gave a turbofan and the secondary index gave a turbojet, and recorded it on the same grounds. This article does not resolve the conflict. Whether the string WJ38-15 parses as a standard military engine designation at all is not established by any document read here, and a guess would be worse than the open question.

One further caution about the engine concerns sourcing rather than hardware. Press accounts of this aircraft’s propulsion attribute the engine to the designation register itself [The War Zone on the X-68A]. They are therefore not independent corroboration of it, and the engine fact has exactly one source however many places it appears.

The Same Engine Maker Appears on Three of Four Consecutive Subjects of This Series

15 rows in the register name Williams in their engine cell and 4 of those are research allocations, being the X-48A, the X-50A, the X-65A and the X-68A. So this aircraft, the X-65, and the unmanned allocation that took the number the X-67 article is about, all carry an engine from one maker [Parsch, DOD 4120.15-L Addendum].

The match is on the word and the word undercounts. The X-61A’s cell reads 1 F107-WR-105, which is a Williams engine the cell does not name as one, and 2 further rows carry an F107 without the maker’s name. [Parsch, DOD 4120.15-L Addendum] So this is a count of what the register prints rather than a census of engines, and the article states the match for that reason without implying the stronger claim.

Read as a fact about the register it is unremarkable, though still recorded here. One plausible reading, which this article does not test, is that few firms build engines in the thrust class a small demonstrator or a cruise missile needs, so that a series of small unmanned research aircraft keeps meeting the same names.

The Contractor Holds Sixteen Rows and Exactly One Is a Research Allocation

General Atomics holds 16 rows in the register, of which 11 are unmanned allocations, 4 are unmanned-control allocations, and 1 is a research allocation [Parsch, DOD 4120.15-L Addendum]. This one.

That distribution shows what kind of firm this is. A contractor with eleven unmanned aircraft and four control stations in the designation register is an established builder of unmanned systems rather than a research house, and the X-68A is its only row among the register’s research allocations, although the contractor counts the XQ-67A as its first X-plane and this as its second [General Atomics on the X-68A]. The X-67 article recorded that the same firm holds both the allocation that took the number 67 and the allocation that closed it, and claimed no intent in that coincidence. This article claims none either.

The Engine as the Only Anchor for the Airframe

No mass, span, area or dimension of this aircraft is published, so the engine is the one anchor available, and this article uses it as far as it reaches and no further.

The engine was built for an airframe whose manufacturer states 1,400 kilograms [Taurus KEPD 350 datasheet], which with the encyclopedia’s thrust figure gives that airframe a thrust to weight ratio of 0.486.

Across the swept mass range the ratio runs from 0.680 at 1,000 kilograms to 0.227 at 3,000 kilograms.

A thrust-to-weight ratio sets a climb gradient, and the point-mass relation turns the table above into performance. In a steady climb the thrust carries the drag plus the weight component along the flight path,

where the second expression is the specific excess power at constant speed. At sea level the sweep gives gradients from 22.7 degrees at 1,400 kilograms through 16.1 at 1,800 to 7.3 at 3,000, at a lift-to-drag ratio of ten.

But the vehicle does not fly at sea level, and a turbojet’s thrust falls roughly with the density. Taking the simplest lapse,

the density ratio at cruise altitude is 0.31, and the climb table collapses. At 1,400 kilograms the gradient falls to 2.9 degrees, at 1,800 to 1.0 degrees with 4.3 metres per second of specific excess power, and at 3,000 kilograms the gradient is negative, the thrust at altitude falling short of the drag, so a vehicle of that mass could not hold this altitude at this lift-to-drag ratio at all.

That negative sign is the most useful number the engine anchor produces, because it closes the mass sweep from above by classical mechanics alone. The sweep’s upper bracket is not a judgement about what seems reasonable. It is the mass beyond which the one published engine cannot sustain the cruise condition this article assumes. The brackets remain an inference and are labelled as one in the epistemic state, and the upper one stands on a mechanism rather than on a plausibility argument.

The engine’s provenance also constrains the speed. A cruise missile powerplant sized for high subsonic flight does not make a supersonic aeroplane, which is consistent with the budget books’ repeated description of a slower, fuel-efficient vehicle for ingress [DARPA PB2023 justification] [DARPA PB2026 justification]. The vehicle is slow by design and the design reason is the premise above.

Structure Around the Bay

A vehicle carrying a store at a large mass fraction is a structure wrapped around a cavity. The missile the programme names is 3.655 metres long and 177.8 millimetres in diameter, which are the service fact sheet’s 143.9 inches and 7 inches [AIM-120 fact sheet] converted at the exact definition of the inch.

A vehicle that carries it internally cannot be shorter than that plus a nose, a tail and an ejector stroke, which sets a minimum length independently of any aerodynamic consideration.

The stability result above then sets where along that length the store may sit. The admissible arm was bounded by $\bar c (K_{\max}-K_{\min})/\alpha$, and the bound tightens as the vehicle gets smaller because $\alpha$ grows. A smaller vehicle is harder to balance around the same missile, and the difficulty grows faster than the size shrinks. That coupling between the keystone and the layout makes the configuration question and the stability question the same question.

Control Across Two Configurations and the Transition

The vehicle must be controllable loaded, controllable empty, and controllable during the event between them. The first two are static problems and the third is not.

Published descriptions of the configuration are consistent on one point. The vehicle carries main wings and canards that are stowed during carriage and deploy after release from the launch aircraft [The War Zone on the X-68A]. A folding surface is a requirement imposed by the launch aircraft rather than by flight, since a vehicle carried externally on a fighter station or internally in a bay must fit a volume sized for a store.

That creates a third configuration nobody flies in but everybody must survive. Between release from the launch aircraft and deployment of the surfaces the vehicle has neither the launch aircraft’s support nor its own aerodynamic controls. The deployment is therefore a timed event in a transient, and its reliability is a flight-safety matter and not a performance one.

The deployment is a hinge problem and its governing relations are two lines. A panel of hinge-axis inertia $I_h$ driven through a sweep angle $\Delta\theta_h$ by a torque $M_h$ obeys the rigid-body rotation equation, and from rest under constant torque the deployment time follows,

where $M_a$ is the aerodynamic moment resisting or assisting the panel, which depends on the attitude the vehicle happens to hold during the fall. No inertia, torque or sweep angle for this vehicle is published, so the relations are shown without a worked number, and one consequence needs no numbers at all. The torque that drives each panel reacts on the fuselage, two panels deploying symmetrically cancel in roll by Newton’s third law, and a single panel that lags or jams delivers its whole hinge torque as an uncancelled rolling moment to a vehicle that has, at that moment, no aerodynamic control to oppose it. The reliability requirement on the deployment mechanism is therefore a symmetry requirement and not only a completion one.

Recovery Serves the Test Programme

The vehicle has a parachute recovery system, and combat is not its purpose. A contractor spokesperson has been quoted directly on the point, saying that recovery is not practical in combat scenarios and that the price point does not make it necessary, while for test and training the vehicle is recoverable [The War Zone on the X-68A].

That is an unusually clean statement of an attritable vehicle’s economics and it resolves what would otherwise be a puzzle, since a recovery system is mass that a one-way vehicle should not carry. It is carried for test and training, not because the operational vehicle would use it. The milestone confirming the system postdates this article’s date, and the spokesperson’s statement is reported only as made during 2025.

The Flight Test Record

Nothing had flown. At 13 December 2025 the designation was 115 days old, no public document read for this article records a flight of this vehicle, and the budget books’ forward plans place flight testing ahead and not behind [DARPA PB2026 justification].

The absence of a flight is a finding, and this section reports it as one. A flight test record sets the envelope reached against the envelope designed for, and here the former is empty. An article that inferred a flight history would be worse than one that states there is none.

Reading the Plans in Sequence

A budget book plans work for its own budget year, so a milestone that slips appears in book after book, each time attached to the year of whichever book is current. One book shows a plan. The sequence shows whether the plan moved.

The design reviews moved by a year each and then settled. The preliminary design review was planned FY2021 to FY2022 and the critical design review FY2022 to FY2023, in each case by two books that each claimed the review for themselves [DARPA PB2021 justification] [DARPA PB2022 justification] [DARPA PB2023 justification]. That is ordinary.

Captive carry is not ordinary. It appears in three consecutive books, and every one of them plans it for the year that book requests, moving it from FY2024 to FY2026. A slip of two fiscal years, from PB2024 to PB2026, with the milestone always one year away [DARPA PB2024 justification] [DARPA PB2025 justification] [DARPA PB2026 justification]. The first book after this article’s date plans flight testing of the vehicle on a host aircraft for FY2026 as well, which describes captive carry without the term, so the slip had not grown by April 2026 [DARPA PB2027 justification]. The X-65 article measured a slip of the same size in the same programme element over the same period, which suggests a cause not specific to either programme.

The Missile Release Demonstration Slipped Two Years and Then Went Unscheduled

This is the finding, and it bears on which test is in the plan as well as when.

The first promise was a ground test. PB2022 and PB2023 both plan a missile separation test for FY2022. After PB2023 the phrase does not appear in any book read for this article [DARPA PB2022 justification] [DARPA PB2023 justification].

The second promise was a flight test, and in this wording it appears in exactly one book. PB2024, of March 2023, plans this for FY2024 [DARPA PB2024 justification].

Conduct initial series of flight demonstrations validating separation of air-to-air missile from host flight test vehicle.

That is the keystone as a flight demonstration. A missile of substantial mass fraction leaving the vehicle in flight is precisely the event the programme’s recurring sentence describes as its challenge.

The next book rewords it. PB2025 plans this for FY2025 [DARPA PB2025 justification].

Conduct a series of flight demonstrations validating air vehicle stability and controls upon separation from host-aircraft and prior to, during, and after separation of an air-to-air missile payload.

Read the two carefully, because the second adds a separation. The first validates separation of the missile from the vehicle. The second validates the vehicle’s stability and control upon separation from the host aircraft, which is carriage release, the subject of the established store separation discipline gathered in the cluster below, and also before, during and after separation of the missile, the research question in the programme’s terms.

The book after that adds a ground test of a substitute mass. PB2026 plans this for FY2025 [DARPA PB2026 justification].

Conduct pit drop testing of missile mass simulant from LongShot ground test vehicle.

The same book moves the flight demonstration a year. Its entry under the new programme element plans, for FY2026, flight demonstrations validating the vehicle’s stability and control prior to, during and after separation of an air-to-air missile payload, and live fire flight test demonstrations [DARPA PB2026 justification].

The latest book plans the release of the vehicle and no longer names the missile. PB2027, of April 2026 and outside this article’s date, plans for FY2026 weapon integration and ground testing, a line PB2025 had planned for FY2024, then flight testing on a host aircraft and flight demonstrations validating vehicle release from a host aircraft and the vehicle’s performance, stability and control through all phases of flight, and for FY2027 it continues both demonstrations [DARPA PB2025 justification] [DARPA PB2027 justification]. None of its FY2026 or FY2027 plans names a missile. The announcement of February 2026 does name a store. It states that the coming flight tests will demonstrate the vehicle’s ability to safely eject a captive sub-munition, and it places the campaign as early as the end of 2026 [DARPA on the X-68A]. The press account of the same day records that the first flight had been hoped for before the end of 2023 [The War Zone on the X-68A]. So the latest official schedule gives a year to the vehicle’s release and to no missile’s release, and the one later official statement that a store will leave the vehicle in flight carries no fiscal year.

A pit drop of a mass simulant is the right first test and it does not answer the question. It measures the separation trajectory of a representative mass from a representative vehicle while the vehicle is held still. The programme’s keystone sentence specifies an operational environment, and a pit is not one.

What the pit measures is set out here, because it is clean classical mechanics and because the arithmetic contains a bias specific to this vehicle. With the launcher clamped, the store leaves at the cartridge impulse over its own mass and then falls, so the separation distance is

and the pit therefore measures the impulse, the tip-off rate and the mechanism’s repeatability, each of them exactly. Those are the quantities a release clearance needs first and the pit is the correct instrument for all three.

The test belongs to a prescribed catalogue and the catalogue’s governing document is named here even though it could not be read. The Department’s handbook for aircraft and stores compatibility, MIL-HDBK-1763 of 15 June 1998, which supersedes MIL-STD-1763A and whose title names systems engineering data requirements and test procedures, governs the ground and flight tests of this discipline [EverySpec record for MIL-HDBK-1763]. It is distributed through the Department’s standards repository, whose public search page is cited as the address [ASSIST Quick Search], and whose document could not be read without a session, so no claim here draws on the handbook’s content beyond what its index record states. That the pit drop is standard practice is established independently, by the discipline’s literature in the clusters below, which includes the modelling of ejection accelerations from such tests.

But the clamp changes the physics, and it changes it by the keystone parameter. In flight the launcher recoils, the same impulse divides by the reduced mass rather than the store’s mass, and the ratio of the two separation speeds is the denominator of the amplification,

A cartridge that gives the simulant 7 metres a second in the pit separates the same store at 7.69 in flight. For a fighter the correction is 0.6 percent and the pit is a faithful simulator, which is why the technique is standard. For this vehicle it is 9.9 percent for one missile of the pair and 21.9 percent for both, so the ground test underreads the flight separation rate by a tenth to a fifth for the same cartridge, as a matter of momentum conservation rather than of aerodynamics. The standard ground test’s fidelity degrades with the very parameter the programme exists to study, and nothing in the open record read for this article states whether the test’s analysis carries the correction, because no open record describes the analysis at all.

What the pit cannot measure even in principle is the aerodynamic increment. In flight the store leaves into the vehicle’s flow field, and the aerodynamic force $F_a$ on it integrates into a trajectory deviation that grows exactly as the gravity term does,

so a store trajectory cannot be separated into a mechanism part and an aerodynamic part by timing alone. That is why the discipline’s wind tunnel and captive-trajectory literature exists, and the cluster below carrying it is this article’s largest.

What This Does and Does Not Establish

A plans list is a summary and not a schedule, so a milestone’s absence from a later book is not proof that the test was cancelled. The work may continue under a heading the book does not itemise, or inside a classified annex, or simply below the threshold of detail the exhibit carries.

What the record does establish is narrower. Across six books inside this article’s date, three plan an in-flight demonstration that includes the release of a missile from the vehicle, PB2024, PB2025 and PB2026, each for its own budget year, and only PB2024 names the missile’s separation as the thing validated. The demonstration the programme says it exists to perform slipped two fiscal years as captive carry did, and the first book after this article’s date plans vehicle release from a host aircraft and stability through all phases of flight without naming a missile at all [DARPA PB2024 justification] [DARPA PB2025 justification] [DARPA PB2026 justification] [DARPA PB2027 justification]. The announcement of February 2026 still promises the safe ejection of a captive sub-munition in flight test, and gives it no fiscal year [DARPA on the X-68A]. The missile release was postponed and then left without a date, and no document read for this article states that it was removed.

The observation is about documents and makes no accusation about engineering. The most likely explanation is the ordinary one, that a demonstration retires risk in the order the risks can be retired, and that firing a missile off a vehicle requires first proving the vehicle flies. The observation is recorded because a reader of any single book would not see it.

Comparison With Ground Prediction

There is no flight result to set beside any ground prediction, because at this article’s date nothing had flown. A comparison of prediction with flight needs both halves, and here only the first half exists. What the record does hold is a set of ground predictions of three kinds, and each is gathered here with the place where it is argued.

The first kind is the planned ground test of the keystone. PB2022 and PB2023 both planned a missile separation test for FY2022 [DARPA PB2022 justification] [DARPA PB2023 justification], and PB2026 planned pit drop testing of a missile mass simulant from a ground test vehicle for FY2025 [DARPA PB2026 justification]. No document this article read states a result of either test. Under The Flight Test Record above, this article sets out what each would measure and what a pit cannot measure even in principle.

The second kind is a prediction about the ground test, and this article makes it. Momentum conservation implies that a pit, which holds the launcher still, separates the store more slowly than flight will for the same cartridge. A cartridge that gives the simulant 7 metres a second in the pit separates the same store at 7.69 in flight, and for this vehicle the correction is 9.9 percent for one missile of the pair and 21.9 percent for both, from the ratio $1/(1-\mu)$ displayed with the pit drop analysis above.

That is a prediction of how a ground measurement will compare with a flight measurement, made before either exists, and the flight half of it is the half the record cannot yet supply.

The third kind is the closed-form sizing. The centre-of-gravity shift, the static margin step, the heave, the trim demand, the two inertia corrections, the roll transient of an asymmetric release and the ejector’s recoil are each derived under Sizing From First Principles, and each is computable before the vehicle flies. None of them is yet a prediction about this aircraft in particular, because no mass, dimension or aerodynamic coefficient of it is published, so a flight result could test the relations only once those inputs were known as well.

One ground milestone is reported only after this article’s date. The announcement of 17 February 2026 reports a full-scale wind tunnel milestone among others [DARPA on the X-68A], and this article reports no result from it. The comparison this section exists to make is therefore the first thing a flight test campaign would supply, and at this article’s date it supplies nothing.

What the Data Changed

No flight data exists, so this section is about what the programme has already settled without flying and what remains open.

What Is Already Settled Concerns Procurement

The concept change the budget books date counts as a result. A programme that began as a multi-mode-propulsion weapon and became an air-breathing vehicle carrying existing missiles has answered a question, concluding that the mission is better served by separating the carrier from the warhead than by building one object that does both.

That answer is visible in three places and they are independent. It is in the budget books’ rewritten description, dated to a window of 11 months. It is in the award record, where one contract description preserves the earlier concept verbatim. And it is in the engine, a single-mode cruise missile powerplant that could not have been chosen by a programme still pursuing multi-mode propulsion.

The reach arithmetic above explains why that answer is the right one. A missile cannot be made to fly twice as far, at any propellant fraction below one, because its reach is logarithmic in its speed ratio. Separating the transit from the endgame is not an optimisation. It is the only available move.

What the Flight Data Would Settle

One number. Whether a vehicle at a store mass fraction near 17.94 percent, the value of $\mu_v$ displayed under The Research Question, can release that store in flight and remain controllable through the transient. Everything derived above is a closed-form consequence of geometry and mass, and every one of those consequences is computable before the vehicle flies. What is not computable is whether the transient is benign, because the transient involves the store’s wake, the bay’s flow, the plume if the motor lights near the vehicle, and the control system’s response to a plant that changed discontinuously.

A demonstration would convert the stability result from a constraint into a measurement. The admissible store arm derived above is a bound from a static-margin band, and the band is a judgement. Flight data would replace the judgement with a number, and that number would transfer to every later vehicle of this class whatever its size.

What It Would Feed

The programme names its transition partners in every book, as the Navy and the Air Force, and names them in the same sentence in all seven [DARPA PB2021 justification] [DARPA PB2027 justification]. The later plans widen the scope considerably, reaching toward alternate payloads, bomber weapons station integration, palletised and ground launch options, and alternate engines. Those plans lie outside this article’s date and are treated as such.

What It May Fail to Influence

A demonstrator that proves a vehicle can fire a missile has not proved anyone will buy it. A demonstration can answer its question and change nothing, and at this date the programme’s influence is unmeasurable because its demonstration has not happened.

One structural observation is available, concerning the designation rather than the aircraft. The programme was given a research designation in the experimental series rather than a prototype designation in a mission series, which places it among demonstrators and not among candidate weapons. The designation carries no information about which outcome is intended.

The Contemporary Literature

This article’s subject sits across 11 bodies of work and each is cited as a body and not through an exemplar. Every research record appears in exactly one cluster below, and the row under each heading counts its records. Three of the clusters are small enough that the cluster’s opening sentence says so.

Store separation, which is the keystone’s own discipline

This is the keystone’s own discipline and it is old, large and quantitative. The question of what a store does when it leaves an aeroplane has its own wind tunnel techniques, its own captive trajectory rigs, its own validation literature and its own flight clearance practice, and it is the body of work a programme releasing a missile from a small vehicle inherits whole.

178 records. [Store Separation Testing Techniques] [Mathematical Fluid Dynamics of] [A Method for Predicting] [Prediction of Six-Degree-of-Freedom Store] [Transonic Wind Tunnel Tests] [CFD Wing/Pylon/Finned Store Mutual] [Aeroacoustic Environment of a] [Aircraft Store Trajectories Predicted] [Transonic Wing/Store Flow-Field Measurement] [Wing/Store Flow-Field Measurement at] [Dynamic Aeroelastic Instabilities of] [Comparison of Wind Tunnel] [The Rapid Prediction of] [Captive Trajectory System Test] [User Requirements and Information] [Comparison of Store Trajectory] [Insensitive Munitions Technology Transition] [Viscous flow simulations of] [Tangential, semisubmerged, and internal] [Tangential, semisubmerged, and internal] [New test techniques to] [Navier-Stokes Solutions for Flows] [Zhang et al 2026] [Das et al 2026] [Türk and Güleren 2025] [Vishwajeet et al 2025] [Cenko 2025] [Coley 2024] [Ahn et al 2024] [Smith et al 2024] [Savery and Coder 2024] [Campbell 2024] [Pandey et al 2023] [Ailneni et al 2023] [Interface Standard, Miniature Mission 2023] [Shin et al 2023] [Gothard and Granlund 2022] [Song 2021] [Tamer 2021] [Fontanella et al 2021] [Song and Ai 2021] [Mizrahi and Raveh 2019] [Gong and Wang 2019] [Dansie 2019] [Schoppert 2019] [de Vasconcelos et al 2019] [Sinha and Garg 2018] [Guigue 2018] [Unal and Baran 2018] [Demir 2018] [Mizrahi and Raveh 2018] [Khaware et al 2018] [Generic Aircraft-Store Interface Framework 2017] [Aircraft/Store Common Interface Control 2017] [Hiwale et al 2017] [Masud et al 2017] [Demir et al 2017] [Interface Standard, Miniature Mission 2017] [Özgür et al 2016] [Lei and Zheng-yin 2016] [Osman et al 2016] [Padmanabhan et al 2016] [Osman et al 2015] [Kim et al 2015] [Simulation of Store Separation 2014] [Barone and Arunajatesan 2014] [Flora et al 2014] [Ruchała and Placek 2013] [Di et al 2013] [Carter and Lind 2012] [Binkley and Vanderwyst 2012] [Carter and Lind 2012] [Carter 2012] [Hallberg et al 2011] [Berglind and Tysell 2011] [Rusong et al 2011] [Kraft and Lofthouse 2011] [Berglind and Tysell 2010] [Smith et al 2010] [Finney 2010] [Cureton et al 2010] [Morgret et al 2009] [Keen et al 2009] [Oktay et al 2009] [Roughen et al 2009] [Nelson and Cain 2009] [Perillo et al 2009] [zhang et al 2008] [Johnson et al 2008] [Hallberg et al 2008] [Spinetti and Jolly 2008] [Charlton and Davis 2008] [Vaughn and Milton E. 2008] [Harding and Barton 2007] [Prananta et al 2007] [Sickles et al 2007] [Babcock 2007] [Hallberg and Godiksen 2007] [Cenko et al 2007] [Harish et al 2006] [Hallberg et al 2006] [Guigue et al 2006] [Mani et al 2006] [Freeman 2006] [Freeman 2006] [Jiang et al 2005] [Gleissl and Deslandes 2005] [Malmuth et al 2005] [Dudley and Westmoreland 2004] [Martel 2004] [Freeman and Jolly 2004] [Noel et al 2003] [Liu 2003] [Lesieutre et al 2002] [Chen et al 2002] [Davids and Cenko 2001] [Marcum 2001] [Ching 2000] [Deslandes and Seifert 2000] [Pan et al 2000] [Kim and Lee 2000] [Nichols and Denny 1999] [Chan and Zhu 1999] [Cavallo et al 1999] [Hoffren and Salminen 1999] [Garrell et al 1999] [Stokes et al 1999] [Tomaro et al 1999] [Lijewski and Lijewski 1997] [Cline et al 1996] [Welterlen and Leone 1996] [Coleman et al 1996] [Kaykayoglu 1996] [Carlson et al 1995] [Atwood 1995] [Moyer et al 1995] [New applications of engineering 1994] [Cenko et al 1994] [Lijewski and Suhs 1994] [Lan et al 1994] [Ferguson and Getson 1994] [Kraft 1994] [Ahmad et al 1993] [Cenko and Madson 1993] [Massengill 1993] [Newman et al 1992] [Lijewski and Suhs 1992] [Chakravarthy and Szema 1991] [Szymkowiak and Silver 1990] [Formaggia et al 1988] [Cenko et al 1986] [Sorrells et al 1984] [Triplett 1984] [Turner 1982] [Stallings 1982] [A new approach to 1981] [Aircraft store separation motion 1980] [Maddox 1980] [Carman et al 1980] [Billingsley et al 1979] [Maddox et al 1979] [Kiber 1977] [Spahr 1975] [Dillenius et al 1975] [Spahr 1974] [Daniels and Clare 1973] [Burkhard 1973] [Brooks 1971] [Markarian 1971] [Black and Picklesimer 1971] [Black 1968] [Bacci and Saddington 2026] [Ben-Gida 2023] [Loupy et al 2018] [Bower 2008] [Bower et al 2004] [Cenko et al 1995] [Cenko 1994]

Carriage loads and the weapons bay

Before a store separates it is carried, and carriage is a separate problem. A store in a bay or on a pylon changes the flow around both itself and the aircraft, and an internal bay adds an unsteady cavity flow that a pylon does not have. The aerodynamic prediction of the store itself, without which neither its carriage loads nor its separation can be computed, is gathered here as well, and so is the weapon bay of the unmanned combat aircraft.

54 records. [Static Stability and Drag] [Computational Fluid Dynamics Modeling] [Aerodynamic Stability Technology for] [Method to predict external] [Predicted Performance of a] [Lee and Lua 2026] [Chang 2025] [du Rand et al 2024] [Prakash et al 2024] [Gonzalez-Martino et al 2023] [Waghmare et al 2022] [Tembhurnikar et al 2021] [Chin et al 2020] [Lesieutre 2017] [Kholodar 2016] [Feng et al 2010] [Heinze et al 2009] [Cenko et al 2008] [Mifsud et al 2006] [Vaughn 2006] [Moore 2000] [Welterlen 2000] [Moore et al 2000] [Packard and Miller 2000] [Tang and Dowell 1998] [Nichols et al 1997] [Ozcan et al 1995] [Ozcan et al 1994] [Ozcan et al 1993] [Cenko 1992] [Nichols et al 1992] [Eastman 1992] [Blair and Stallings 1989] [Rosen 1988] [Blair and Stallings 1988] [Cenko et al 1983] [Hemsch and Nielsen 1983] [Williams 1982] [Anderson 1981] [Krieger and Williams 1981] [Vukelich and Jenkins 1981] [Shadow 1978] [Quam 1978] [Kain et al 1978] [Martin and Walkley 1975] [Martin et al 1975] [Chadwick 1974] [Liu et al 2023] [Hallissy and Hariharan 2014] [Lawson and Barakos 2012] [Lawson and Barakos 2010] [Maxwell and Dawson 2004] [Dawson and Maxwell 2003] [Dawson and Maxwell 2006]

The release mechanism, the ejector and the pit drop

The mechanism that pushes the store is a discipline in itself and it is a small literature beside the separation work it serves. Its records are rack and cartridge test reports, rack load and stress analyses, and a recent group of ejector force models. The ground test the budget books plan appears in this literature under its own name, as the modelling of ejection accelerations from store pit drop tests and as static ejection testing, which is how this article knows the test is standard practice and not improvised.

16 records. [Ailneni et al 2023] [Kariv and Raveh 2020] [Lucey 2019] [Stoykov and Atanasov 2016] [Krejci 2013] [Chen and Zhang 2013] [O’Bannon 1964] [Nguyen and Gebert 2023] [Stenftenagel and Gebert 2022] [Tian et al 2022] [Bilal and Shah 2016] [Kenkelen and Wieseman 1972] [Tactical Air Command Langley Afb Va 1963] [Erickson 1948] [Schoppert and Means] [Nguyen et al 2026]

Mass, inertia and what a release does to stability

A release is a discontinuous change in the plant, and the control literature treats that as its own problem. Mass properties measurement, a centre of gravity that moves in flight, the response and control of an aircraft releasing or carrying asymmetric stores, and the stability analysis of time-varying systems are its four parts.

26 records. [Flight Article MASS Properties] [Flight Article MASS Properties] [Flight Article MASS Properties] [AIR Force Plant 77] [Control and Identification of] [Change-Point Detection and Adaptive] [Applied Time Domain Stability] [Medwetz 2024] [Li et al 2017] [Vengate et al 2016] [Liu and Lu 2011] [Sadraey 2009] [Mukherjee and Balaram 2008] [A Study on Improvement 2006] [Shamma 1995] [Li et al 2025] [Veeresha et al 2025] [Antonić et al 2024] [Park et al 2024] [Ji et al 2021] [Klappa and Fields 2021] [Manshadi and Saghafi 2018] [Safi’i et al 2018] [Jing Zhang et al 2009] [Cadwell] [Gade and Inman 1999]

Air launch and the carrier aircraft

Launching one air vehicle from another is a recurring idea with a long record, and the separation problem it poses is the same problem as a store’s with the sign of the interest reversed, because here the released object is the one that must survive.

17 records. [Air-Launched Windsonde] [Air-Launched Balloon System] [AIR Launched Rocketsounding Study] [Accuracy and Dispersion of] [Navy Air-Launched Missile Operating] [Flight Tests of the] [Thermal Stability of Six] [Report on Human Factors] [Stewart et al 2026] [Cai et al 2023] [Aleksandrovich and Nikolayevich 2020] [Haglind et al 2007] [Graves and Robins 1979] [Brunner et al 1976] [Goerttler and Schnepf 2024] [Goerttler and Schnepf 2022] [Anderson and Lopata 1996]

Folding surfaces and their deployment transient

A vehicle carried in a volume sized for a store must unfold, and the deployment is a transient with its own dynamics, its own structural loads and its own failure modes.

40 records. [Li et al 2026] [Liu et al 2026] [Zhang and Qin 2026] [Molz and Breitsamter 2026] [Karatzas et al 2026] [Wei et al 2026] [Wang 2026] [Zhang et al 2026] [Huang et al 2025] [Hua et al 2025] [Wang and Guo 2024] [Qi et al 2024] [Wang and Guo 2024] [Han et al 2024] [Watson and Gonzalez 2023] [Huang et al 2023] [Zhao et al 2023] [Zhang et al 2023] [Zhang et al 2023] [Wang et al 2022] [Gao et al 2021] [Yue and Zhao 2021] [Xu et al 2019] [Geva et al 2019] [Otsuka et al 2019] [Zhang and Yang 2018] [Ya 2017] [Mills and Ajaj 2017] [Hayes et al 2017] [Lee and Han 2016] [Fujita et al 2014] [Yue et al 2013] [Zhao and Hu 2013] [Yue et al 2013] [Coffin et al 2011] [Wang and Dowell 2011] [Obradovic and Subbarao 2011] [Ben and Jacob 2010] [Grant and Lind 2007] [Henry et al 2005]

The premise, air-breathing against rocket

The programme’s motivation is a propulsion comparison and the comparison has a literature. Specific impulse methodology, the range equation and its extension to a payload released in flight, and the propulsion of cruise and tactical missiles are where the premise above is worked out properly.

33 records. [A General Method for] [Palaia 2026] [Batra et al 2026] [Nguyen et al 2025] [Frederick and Petrof 2024] [Luca et al 2024] [Batra et al 2023] [Cestino et al 2023] [Harish et al 2022] [Staack et al 2021] [Chen 2021] [Wroblewski and Ansell 2020] [de Vries et al 2020] [Ravishankar and Chakravarthy 2018] [Randle et al 2011] [Fleeman 2008] [Cavcar 2006] [Bardell 2000] [Schindel 1999] [Chase et al 1999] [Hoy 1989] [Stricker 1989] [High specific impulse propulsion 1987] [Turk and Zeiner 1986] [Davis 1985] [Davis 1984] [Cruzen 1983] [Locke 1981] [Kramer and Buehler 1979] [Allen and Byrne 1965] [Powers 1960] [Deuschle 2025] [Tsukanov and Riabkov 2021]

A missile’s energy, its coast and its reach

This cluster is small, and what it holds is method rather than measurement. Its records are missile trajectory models and six-degree-of-freedom equations, launch range and launch acceptability region documents, missile drag against flight test, and ballistic coefficient estimation for other flying bodies. None of its titles names a particular missile’s reach, which is unsurprising for weapons effectiveness and is stated here because a reader should not take a thin record of the numbers for a thin subject.

20 records. [Zaikang and Defu 2019] [Warchulski and Warchulski 2016] [Herrmann 1996] [Holley et al 1996] [Cliff and Kelley 1989] [Henton 1971] [Anderson 1970] [Barnett 1966] [Barnett 1962] [Eastman 1993] [Krieger 1983] [Krieger 1982] [Algorithms for Estimating a] [Influence of projectile ballistic 2018] [Murthy 2015] [Li et al 2022] [Common Launch Acceptability Region] [Common Launch Acceptability Region] [Common Launch Acceptability Region] [Common Launch Acceptability Region]

Attritable and expendable air vehicles as a class

An attritable vehicle is a cost argument before it is an engineering one, and the literature accordingly weighs reliability, life-cycle cost and expendability alongside the guidance and identification of small, low-cost vehicles, and says little about their aerodynamics.

10 records. [Ma et al 2021] [Comparing the Cost-Effectiveness of 2021] [Colombi et al 2017] [Krings et al 2013] [Erdman and Mitchum 2013] [Dorobantu et al 2013] [Kahn and Edwards 2012] [Bateman et al 2007] [Case Study Micro Tactical 2007] [Hawkins 1989]

Recovery of a test article

A recovery system on a vehicle meant to be expended is a test-programme artefact, and the parachute and decelerator literature is where its sizing lives.

21 records. [Drag and Performance Characteristics] [To 2024] [Dinh 2024] [Drynan 2024] [Pepermans et al 2018] [Clark et al 2015] [Gallon and Witkowski 2015] [Cook 2013] [Guidotti et al 2012] [Wolf and Fallon 2011] [Morris and Olson 2011] [Witkowski 1999] [Behr 1989] [Pepper 1984] [Pepper 1980] [Pepper 1979] [Pepper 1977] [Whalley 1973] [Flatau and Miller 1970] [Babish III 1966] [Ericksen et al 1966]

Demonstrator practice

This cluster is nearly empty, and for a reason of its own. Writing about how to run a demonstrator programme is mostly institutional rather than indexed, so the indexed literature holds almost none of it. The practice is real and the literature about the practice is thin.

2 records. [Artificial Intelligence Techniques for] [Flight Test Programme 1970]

Where the Framing Breaks Down

Treating this aircraft through a store mass fraction is the right frame, yet it misleads in at least seven ways, each named here so that a reader need not find it.

The Keystone Parameter Cannot Be Evaluated

$\mu$ is unpublished because the vehicle’s mass is unpublished, so every figure in this article that depends on it is a sweep rather than a measurement. The sweep’s brackets are argued from the engine’s provenance and that argument is an inference. A reader who takes the 17.94 percent of the $\mu_v$ display under The Research Question as this aircraft’s store mass fraction is taking the middle of a sweep as a fact, and this article’s only defence against that is to keep saying so.

A Static Margin Band Is a Judgement and the Bound Inherits It

The admissible store arm was derived from a requirement that the static margin stay inside a band. The band is not derived from anything here. A vehicle with a full-authority digital control system and a deliberately relaxed static margin may tolerate a far larger excursion than a conventionally stable aeroplane, and some combat aircraft are flown statically unstable on purpose. The bound is therefore conditional on a stability philosophy this article does not know.

The Release Is Treated as Instantaneous

Every relation above compares a before state with an after state. A real release has a duration, during which the store is partly supported, partly free, and aerodynamically interfering with the vehicle it is leaving. The difficulty lives in the transient, the part that closed-form mass arithmetic cannot reach. This article’s mathematics describes the endpoints of the event the programme exists to study.

The Plume Is Not Treated At All

If the missile’s motor ignites within a few body lengths of the vehicle, the vehicle is in a rocket exhaust. Pressure, temperature and particulates on a structure sized for cruise are a design driver, and nothing in this article addresses them. Whether the missile is ejected and coasts clear before ignition, or fires while attached, is not established by any document read here, and the two cases are not close to each other in difficulty.

The Drag Model Behind the Reach Numbers Is One Coefficient

The coast derivation assumes a constant drag coefficient and a constant density. A real missile flies through a changing atmosphere at a changing Mach number with a drag coefficient that varies considerably across the transonic region, and it manoeuvres, which costs further energy. The calibration against published reach figures is the reason to trust the comparison between the carrier and the missile, but gives no reason to trust any single reach figure as a prediction.

The Comparison Flatters the Carrier by Ignoring What It Costs to Carry

The carrier must itself be carried. A launch aircraft giving up a station or a bay volume to a vehicle of this size gives up the stores it would otherwise have carried there, and the reach gain should properly be computed per unit of launch-aircraft capacity rather than per missile. This article does not compute that, because the vehicle’s volume and the exchange ratio against conventional stores are both unpublished. The omission is in the optimistic direction and is the most important of the seven.

The Mission Statement Under the Frame Is Not Official

The register’s mission sentence is the compiler’s, as the second section established. The budget books describe a programme and not an aeroplane. So the frame is built on an official programme objective and an unofficial aircraft description, and a reader should hold the aircraft-level claims more loosely than the programme-level ones.

The Source Base

The research survey holds 417 records across 11 clusters, cited alongside 26 primary sources read directly. Its records come from NASA’s Technical Reports Server, the Defense Technical Information Center and the journal, conference and standards registries that Crossref indexes. 365 of the 417 carry a year and 349 of those are dated 2025 or earlier, with a median year of 2009 and a range from 1948 to 2026, and the count is reported beside the fraction because adding contemporary work lowers the fraction while leaving the count unchanged.

The Reports Server Is the Wrong Server for This Subject

Only 8 of the 417 cited records carry an address on NASA’s Technical Reports Server, against 58 that carry a Defense Technical Information Center DOI.

That is not a thin literature, it is a subject living somewhere else. Store separation is a weapons integration discipline, and weapons integration is a defence activity and not a civil aeronautics one. An account of the literature drawn from the civil server alone would describe it as thin when it is large.

What This Article Read in Full

26 sources were read directly rather than surveyed by title. The designation register and its front matter. All seven budget justification books, for the programme entry, the funding columns, the plans lists and the programme element migration sentences. The sponsoring agency’s programme page and its announcement. The contractor’s announcement. The federal award record, for five awards and their contract data. Three defence journalism accounts. The compiler’s page on the designations of military aero-engines. An index record of the compatibility handbook, for its title, its date and the standard it supersedes. And the reference pages for the cruise missile that shares this aircraft’s engine, the missile the programme says it will carry, and the aircraft that will launch it. And the register’s page as archived on three dates, for the note under Statements in This Article That Postdate Its Own Date.

Every load-bearing anchor but the engine’s thrust rests on a primary document. The launch aircraft’s weights come from the service’s own fact sheet, read from a public archive snapshot because the service’s site refuses every client, and quoted by its pound figures because the sheet’s own metric conversion of its take-off weight is 291 kilograms low. The cruise missile’s mass and the word turbofan come from the manufacturer’s own datasheet. The atmosphere subsection’s constants carry the 1976 standard’s own tables, read as page images from an image-only scan. The missile’s dimensions and its baseline launch weight carry the service’s fact sheet, read from a public archive snapshot. The engine’s thrust figure is encyclopedia-sourced because neither manufacturer document states it, and the modelling mass for the missile is the encyclopedia’s heavy-variant figure by a stated choice, the service sheet giving the baseline variant. The thrust is the weakest link in the chain, anchoring as it does the thrust-to-weight table, the climb gradients and the mass sweep’s brackets.

And a small finding about the fact sheets themselves. Both of the service’s sheets convert their own pound figures to kilograms incorrectly, the aircraft’s take-off weight printed 291 kilograms low and the missile’s launch weight 1.2 kilograms low. This article therefore quotes both documents by their pound figures and converts at the definition of the pound, so that neither discrepancy becomes this article’s own.

And two documents are cited as addresses rather than as readings, which are different things and are kept apart. The broad agency announcement HR001120S0037, named by four awards, could not be retrieved from the federal opportunity portal, the public web archive or the sponsoring agency’s own site, and the discipline’s governing compatibility handbook sits behind the standards repository’s login, so only its index record was read. Each is cited by its nominal address, the portal page and the repository’s public search page, neither is counted among the 26 sources read directly, and no claim in this article rests on either document’s content.

The Definition of Primary

Primacy is a property of the document, and this survey derives it from the address. A record is a report primary when its address is on NASA’s Technical Reports Server, the Defense Technical Information Center or the Department of Energy’s Office of Scientific and Technical Information, or when its digital object identifier, or DOI, is one of their report DOIs. 67 of the 417, or 16.1 percent, are report primaries by that rule. Store separation is an aerospace engineering discipline published mainly through one professional society’s conferences and journals, and 222 of the 417, or 53.2 percent, carry a DOI under the prefix 10.2514 of the American Institute of Aeronautics and Astronautics, so most of the discipline’s first-hand work sits in the article registries and not on the government report servers. The rule therefore understates how much of the survey is first-hand, since a conference paper reporting a wind tunnel or flight test is a primary account of that test whatever its address.

Thin Subjects

Six subjects that this article’s derivations depend on have thin literatures. Each count below is the number of research records in this survey whose title carries at least one of the listed terms, with a hyphen read as a space. Where a row joins two groups of terms with the word together, a title must carry a term from each group.

The literature on unmanned carriers studies their bays more than their releases. Of the 9 titles, 4 name a weapon bay, and 3 concern something released from the unmanned vehicle itself, of which 1 names a weapon. The keystone, a small unmanned vehicle releasing a store of large mass fraction, therefore rests on this article’s own arithmetic rather than on a published analysis.

The ejector’s force has become a modelling subject only recently. Of the 15 titles, 4 model the ejector’s force or the acceleration it imparts, and all 4 date from 2022 or later. The recoil and reduced-mass relations under The Ejector Pushes Back are classical mechanics and do not depend on those models, but the cartridge impulse they take as an input does.

The plume appears once, in a simulation of missile plumes for aircraft and store compatibility assessments, which is consistent with this article’s own decision to leave the plume out of scope.

The reach literature is method without a missile. None of the 6 titles names a particular missile, so the reach figures under The Premise rest on the coast model calibrated against the encyclopedia’s published range figures.

Inertia and trim are named almost nowhere. One title names inertia, and it concerns the time-varying inertias of a variable-sweep morphing aircraft rather than a release. No title names trim, so the two inertia corrections and the retrim demand derived above stand on the relations shown and not on a measured case.

What the Survey Admits

A record is admitted only when a person reading its title finds it on this article’s subject, and twin deposits of one document under one title are merged so that each document is cited once. The subject is read as the eleven literatures the clusters name, each of which a section of this article uses, from the release of a carried store and its carriage, mechanism and effect on the carrier, through air launch, folding surfaces and recovery, to the propulsion premise, a missile’s reach, attritable vehicles and demonstrator practice.

Several words this subject depends on also name something else, and a record that shares only such a word is excluded. Breguet names a watchmaker and an aircraft maker as well as the range equation. Store also names a shop, a memory in cognitive psychology and the storey of a building, and centre of gravity also names a concept in economic geography. Trajectory also describes strategic and ballistic missile flight from burnout to impact and boost-glide flight, which are missile flight rather than store release, and it describes missile guidance, autopilot design, evasion and terminal trajectory optimisation, which are excluded for the same reason. Specific impulse also belongs to spacecraft electric and laser propulsion, and air-breathing propulsion also names combustion, thermal management and teaching, which are excluded because the premise is a comparison and not a survey of engines. Air-launched also describes oceanographic instruments dropped from aircraft, loading also describes the ground equipment that loads an aircraft gun, ejection also names a pilot’s seat and a hydrocyclone, and time-varying also names classes of systems with delay, periodicity or special structure, which are excluded because a release is a step change. Correction notices, withdrawal notices and withdrawn papers are excluded as well, because a survey counts research records and those are editorial events rather than works.

Separation is the hardest of these words, because its largest other use is aeronautical. Boundary-layer separation, flow separation, leading-edge separation and turbulent separation control are the largest aeronautical users of the word separation. They are aerodynamics, so no test of whether a title is aeronautical tells them apart from store separation. A title is therefore read as on this subject only when it names a thing that is carried and released, and the word separation standing alone admits nothing.

Some records of doubtful relevance are kept. Air-launched meteorological sondes and balloon systems, ballistic coefficient estimation for re-entry bodies and non-lethal projectiles, mass properties reports for an intercontinental ballistic missile, air-breathing and rocket propulsion compared for trans-atmospheric and space transport vehicles, propellant chemistry for an air-launched missile, cooperative air combat tactics read through weapon engagement zones, and a fixed-wing unmanned aircraft delivering a payload each share either the carried and released object or the physics of a release or a coast with this subject, so each is kept and a reader may weigh it accordingly.

Every research title has been read for relevance, so the off-topic share that remains is a matter of reading judgement. A reading of titles cannot show that a work is relevant in substance, so this is a statement about titles only.

Epistemic State

Historical fact, from documents read for this article.

The X-68A designation was allocated on 20 August 2025 to General Atomics, with the engine, sponsor and description cells as quoted, and the description carries a span-level officiality mark whose effect is that the name is official Department wording and the mission sentence is not. The programme is LongShot, sponsored by the Defense Advanced Research Projects Agency. Five awards in the federal award record carry it, totalling 148,305,710.65 dollars, with the parties, dates, amounts, offer counts and product codes as tabulated. Seven budget justification books carry a programme entry, the funding figures and restatements are as tabulated, and the programme moved from programme element 0603286E project AIR-01 to programme element 0603468E project ACX-01 beginning in fiscal year 2026 [Parsch, DOD 4120.15-L Addendum] [USAspending federal award record] [DARPA PB2021 justification] [DARPA PB2027 justification].

Engineering analysis, derived here from published anchors and stated assumptions.

Every relation in the sizing and premise sections is derived here and not cited, and each is a closed-form consequence of mass, geometry and the two propulsion relations. None of it is measured from this aircraft, because no mass, dimension or aerodynamic coefficient of this aircraft is published. The ballistic decay model is calibrated against the published reach of the missile family the programme names, which is the only external check available to it.

Where each load-bearing anchor rests. The launch aircraft’s weights are the service fact sheet’s pound figures exactly converted. The cruise missile’s mass and its engine’s type are the manufacturer’s datasheet. The atmosphere constants are the 1976 standard’s adopted tables. The carried missile’s dimensions and baseline weight are the service’s fact sheet, with the modelling mass the encyclopedia’s heavy-variant figure by the stated choice. The engine’s thrust is an encyclopedia’s figure and nothing stronger, and every result standing on it, the thrust-to-weight table, the climb gradients and the mass sweep’s brackets, inherits that weakness. The ratios and closed forms do not, since they are arithmetic whatever the inputs. Two reference definitions are addresses and not readings, the solicitation’s portal page and the standards repository’s search page for the compatibility handbook, each marked as such where cited, and neither document’s content informs any claim here.

Inference, labelled as such.

That the vehicle’s mass lies in the swept range of 1,000 to 3,000 kilograms is an inference from the engine’s provenance and is not stated anywhere. That the concept change the budget books record was a change in the aircraft rather than in the prose is an inference, supported by the award record’s preserved wording and by the engine being single mode. That the press figure for the demonstration award differs from the record’s because one is an initial obligation and the other a current total is a plausible explanation this article has not confirmed in any document.

What the record does not settle.

Whether WJ38-15 parses as a standard military engine designation, and therefore whether the register’s turbojet or the manufacturer designation’s turbofan is correct. Which of the two named individuals is currently the programme manager. Whether the missile fires while attached or coasts clear before ignition, which is the difference between two very different structural problems. Whether the in-flight missile separation demonstration that PB2024, PB2025 and PB2026 plan, and PB2027 does not name, was deferred, reorganised or removed, and in which year the ejection of a captive sub-munition that the announcement of February 2026 promises is to be flown. And the keystone itself, since no vehicle of this class has released a store of this mass fraction in flight in any record read here.

Statements in This Article That Postdate Its Own Date

This article is dated 13 December 2025 and a number of its statements are later. The survey is written from current knowledge and this section marks where, and this article has more such statements than most because its subject became public after its date.

The designation was allocated 115 days before this article’s date and announced 66 days after it, on 17 February 2026. So at the article’s date the designation had been allocated and had not been publicly announced.

A note on the register. The register row this article reads was not public at this article’s date either. The register’s page as archived on 10 December 2025 [Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025] and on 15 January 2026 [Parsch, DOD 4120.15-L Addendum, as archived on 15 January 2026] ends its research rows at the X-66A, and the page as archived on 1 February 2026 [Parsch, DOD 4120.15-L Addendum, as archived on 1 February 2026] carries the X-68A row, so the row first appeared between 15 January and 1 February 2026, 148 to 165 days after the allocation it records. Every reading of the row in this article is therefore a reading of a page later than the article’s date, like the announcement.

The following are outside the date and are used anyway, each marked where it appears.

• The announcement of 17 February 2026 by the sponsoring agency and the contractor’s response.
• The full-scale wind tunnel, parachute recovery and weapons-release milestones those announcements report.
• The F-15 as the launch aircraft for the first flight tests, the statement that flight testing could begin as early as the end of 2026, and the statement that those tests will demonstrate the safe ejection of a captive sub-munition.
• The name of the programme manager quoted in that announcement.
• April 2026’s budget justification book, which is the seventh and the only one outside the date [DARPA PB2027 justification]. Its funding figures for fiscal years 2025 through 2027 and its plans lists are used, and every table marks which column came from it.
• The statement that parachute recovery is intended for test and training and not combat.
• The two defence journalism accounts of 2026 [The War Zone on the X-68A] [Air and Space Forces on the X-68A].
• The federal award record, which was read in October 2026. The amounts stated are the record’s current totals and reflect contract modifications through May 2026, so the Phase 3 figure in particular may be larger than the figure that stood at this article’s date [USAspending federal award record].
• The register row itself, as the note above explains.
• The research literature, which is drawn from current indexes and includes records dated to 2026.

And two sources sit inside the date by their content and outside it by their reading. The two service fact sheets were read from archive snapshots taken on 1 and 4 December 2025, both inside this article’s date, so the pages cited are the pages as they stood at the time. The readings themselves, like every reading here, happened later.

Out of Scope

The engagement, the subject most writing about this programme chooses. Whether a forward-launched missile carrier changes air combat, what an adversary does about it, and how it interacts with the collaborative combat aircraft programmes are questions about operations rather than about an aeroplane, and this article is about an aeroplane.

The guidance and the datalink. A vehicle launching a missile at a target it did not find needs targeting data from somewhere, and the architecture that supplies it is a separate subject with its own literature. The PB2025 book plans weapon integration and ground testing as a separate line [DARPA PB2025 justification], and that integration is not treated here.

The missile itself. The programme plans to use existing missiles and this article takes one of them as a mass, a length and a published reach. Its seeker, its fuze, its warhead and its control system are out of scope.

The plume and the structural response to it, named in the framing section as a gap rather than dismissed. It would need a thermal and pressure analysis this article does not attempt.

Aeroelasticity. Store-induced flutter appears in the literature cluster because it is part of the discipline, and this article’s mathematics is rigid-body throughout. A folding surface on a slender vehicle releasing a heavy store is an aeroelastic problem and is not treated as one here.

The cost of the vehicle. The attritable literature is cited as a body and the vehicle’s unit cost is not public, so the economics that would decide whether this is bought are absent.

The other designations in this article’s neighbourhood, which have their own articles in this series.

Conclusion

The X-68A is an aeroplane whose every official word is bookkeeping, and whose purpose has no official word but its name. The register that names it marks its mission sentence as the compiler’s, and no other research allocation in that register is marked that way. The budget justification books describe the programme at length, though not the aircraft. No public Department document read for this article says what the X-68A is.

What those books do say, four times running, is what the programme is for. It exists to address the stability and control challenges of launching air-to-air missiles from a relatively small unmanned vehicle, and that sentence is the research question. The reach benefit that every account of the programme leads with is the motivation, and the motivation is arithmetic. A missile’s reach is logarithmic in its speed ratio, so doubling it with a bigger motor would take the propellant fraction from about 0.402 to about 0.746 and leave nothing for a warhead. The same distance costs an air-breathing carrier a fuel fraction near 0.013. Separating the transit from the endgame is not an optimisation but the only available move, and that is why the programme exists.

The research question is a mass ratio. A fighter releasing one of these missiles sheds about 0.601 percent of itself. A vehicle of this class releasing two sheds something near 17.94 percent, a factor of about 30. Everything follows from that one number, amplified by the factor that grows faster than it does. The centre of gravity moves, the static margin can be spent in a single event, the trim demand lands hardest where control power is scarcest, the inertia changes by two corrections rather than one, an asymmetric release loads the lateral control, and the ejector pushes the launcher as hard as it pushes the store.

The budget books also show the programme becoming this aeroplane. It began as a weapon system with multi-mode propulsion, and in a window of 11 months between two books it became an air-breathing vehicle carrying existing missiles. The award record corroborates that independently, holding the original concept verbatim in a contract description dated January 2021, and the single-mode cruise missile engine in the register’s cell is the physical trace of the same change.

And the plans tell one more thing that no single book reveals. The in-flight release of a missile from the vehicle, which is the keystone as a demonstration, is planned by three consecutive books for three different years, is named as the missile’s separation only in the first of them, is joined by a ground drop of a substitute mass in the third, and is not named at all in the first book after this article’s date, which plans the vehicle’s release for FY2026 and FY2027 while the announcement of two months earlier promises the ejection of a captive sub-munition without a year. A plans list is a summary and not a schedule, so this is an observation about documents rather than a claim about engineering. It is recorded because the thing the programme states it exists to retire slipped two fiscal years like any other milestone and then lost its year in the published plans, and a reader of any one book would not see it.

At this article’s date nothing had flown. The designation was 115 days old and unannounced, the flight test campaign had not begun, and captive carry had been promised by three consecutive books for three different years. The question remains open, and a good one, because the answer transfers to every vehicle that tries to carry a weapon it is barely larger than.

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