X-Planes: X-69 through X-75, the Leapfrogged Block
Filed under: aerospace, history, engineering
Seven consecutive numbers in the research series have no aeroplane, and the one above them was chosen to spell a year. The register of designations allocated since August 1998, which Andreas Parsch compiles from data the Department of Defense has released, records an X-68A allocated on 20 August 2025 and an X-76A allocated on 20 October 2025, 61 days later, and nothing in between [Parsch, DOD 4120.15-L Addendum]. The numbers 69, 70, 71, 72, 73, 74 and 75 were passed in a single allocation.
This is the seventieth 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, the X-67, and the X-68.
The earlier absences in this series were one number each and each had one mechanism. The X-39 was reserved and never formally requested. The X-52 was requested and refused for resembling the B-52. The X-58 and the X-67 were each consumed by an unmanned allocation that took the research series’ next number [Missing USAF and DOD Aircraft Designations]. This is a different shape of absence. No allocation in the research series carries any of the seven numbers, no single programme is named as having taken them, and the gap is seven wide [Parsch, DOD 4120.15-L Addendum].
There are three readings of a gap like this, and from the gap alone the register cannot tell them apart. The numbers might be seven skips, made at once so that a chosen number could be issued. They might be one block held in reserve for some purpose the register does not record. Or they might be seven allocations that exist in the Department of Defense’s records but had not been released when the register was compiled. Tested against the record, the first reading is the only one with a mechanism in the governing instruction and the only one the sponsoring agency’s later statement supports. The second has no instrument in that instruction, and the third fails against the series’ allocation rate.
The article also finds three things the earlier anomaly articles did not. On this article’s date the public register did not yet show the X-68A or the X-76A at all, so the anomaly was invisible in the public record when it happened [Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025] [Parsch, DOD 4120.15-L Addendum, as archived on 15 January 2026]. The X-76A is the third design number in eleven months chosen to match the last two digits of a founding year, after the YMV-75A for the Army’s 1775 and the F-47A for the Air Force’s 1947, so it is one member of a class [DARPA, 9 March 2026] [Army, 27 May 2025] [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act]. And the 2020 instruction’s rule for the next number describes only six or seven of the twenty-three allocation events since it took effect that had an earlier number to follow, so the question of which number comes after the X-76A is a question about practice rather than about text.
The Research Question
The question this article exists to settle is why seven consecutive research design numbers were passed in one allocation. The register records the X-68A on 20 August 2025 and the X-76A on 20 October 2025 and no research row between them [Parsch, DOD 4120.15-L Addendum]. The question concerns a number. No aeroplane enters it, so it names no quantity that flight could measure. It has three candidate answers, which are seven skips made at once, one block held in reserve, and seven allocations not yet released, and the record is asked to choose between them.
A second question follows from the first and the record does not yet settle it. If the numbers 69 through 75 were skipped, the governing instruction either leaves them available, as the X-49 stayed available after the X-50A, or makes them unrequestable as a skipped sequence [DAFI 16-401]. The closing subsection of the analysis, The Next Research Number Is a Test, sets out what each outcome would show.
Programme Origin
The Register Row
The X-76A’s row is short and most of it is the compiler’s. It reads 20-Oct-25, X-76A, Bell Textron, two General Electric CT7-8 engines and a Pratt and Whitney PW308C, sponsored by the Defense Advanced Research Projects Agency, hereafter DARPA, with the description that it is an unmanned demonstrator for the Speed and Runway Independent Technologies programme, hereafter SPRINT [Parsch, DOD 4120.15-L Addendum]. The description cell carries the compiler’s mark for wording that is not from Department records, so the date, the designation, the contractor, the engines and the sponsor are official and the statement of what the aeroplane is for is his. This cell-level state, in the four-state partition the X-67 article defined, is the state of seven of the thirty-one research rows in the register.
The engine cell is the strongest physical evidence in the row, and the next article in this series takes up the aeroplane. This one states only what the row is evidence of. The CT7-8 is a turboshaft and the PW308C a turbofan [EASA, CT7-series type-certificate data sheet] [EASA, PW308 series type-certificate data sheet], and a pair of turboshafts with a separate turbofan is the propulsion of an aircraft that hovers on rotors and cruises on jet thrust, which matches SPRINT’s stated aim of cruise above 400 knots with hover in austere environments [DARPA, 9 March 2026]. The programme attribution was an inference before it was a statement. Until DARPA’s announcement the compiler’s note on the row read that it was not fully confirmed that the X-76A referred to the SPRINT demonstrator, but that given the available data it was very likely [Parsch, DOD 4120.15-L Addendum, as archived on 1 February 2026].
Where the Seven Numbers Do and Do Not Appear
The block is a set of consecutive design numbers and the gap is a span of days, and both are written down here because every later test is a claim about them. Let $t_{68}$ and $t_{76}$ be the allocation dates of the X-68A and the X-76A.
\[\mathcal{B} \;=\; \{\,69, 70, \dots, 75\,\}, \qquad \lvert \mathcal{B} \rvert \;=\; 75 - 69 + 1 \;=\; 7, \qquad t_{76} - t_{68} \;=\; 61\ \text{days}\]No row in the research series carries any of the numbers 69 through 75, a result measured over every row of the register. The register, in its copy last updated 23 August 2026, holds 539 designation rows, 31 of them in the research series, and the research design numbers present run 37, 40, 44 through 51, 53 through 57, 59 through 66, 68 and 76 [Parsch, DOD 4120.15-L Addendum]. On this article’s date the register showed neither of the last two. The version then current, the compiler’s update of 11 August 2025 as archived on 10 December 2025, held 526 rows, 29 of them in the research series, and its research design numbers ran 37, 40, 44 through 51, 53 through 57 and 59 through 66, so in the public record the series ended at 66 and no block had yet been passed [Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025]. Under that version seventeen rows were marked as absent from officially released data and six of the twenty-nine research rows carried the cell-level mark. The X-68A and X-76A rows first appeared between 15 January and 1 February 2026, as the table under On This Article’s Date the Register Did Not Show It sets out, and the block is described here from the later copy.
The same seven numbers do appear elsewhere in the register, and where they appear matters for exactly one of the readings.
| Number | Rows carrying it | Basic mission | Bearing on the research series |
|---|---|---|---|
| 69 | none | none | |
| 70 | YRH-70A, ARH-70A | helicopter | none, a separate sequence |
| 71 | VH-71A, BGM-71H | helicopter, missile | none, separate sequences |
| 72 | UH-72A, UH-72B, XRQ-72A | helicopter, unmanned | the XRQ-72A carries an experimental status prefix |
| 73 | TH-73A, XRQ-73A | helicopter, unmanned | the XRQ-73A carries an experimental status prefix |
| 74 | BQM-74F | missile | none, a separate sequence |
| 75 | YMV-75A | vertical takeoff | none by rule, and everything by motive, as below |
Design numbers run within a basic mission, so a helicopter 70 and a research 70 are different numbers in the instruction’s own terms [DAFI 16-401]. Two rows still deserve a second look. The XRQ-72A of 20 March 2017 and the XRQ-73A of 12 September 2023 are unmanned designations carrying the X status prefix, and the compiler writes of the second that its number is out of sequence in both the unmanned and the research series [Parsch, DOD 4120.15-L Addendum]. The sponsor’s announcement blurs the line further. DARPA’s release of 24 June 2024 reports that the SHEPARD aircraft, a name the register expands as Series Hybrid Electric Propulsion AiRcraft Demonstration, received its official X-plane designation, XRQ-73, and that it leverages the series hybrid electric architecture of the earlier Air Force Research Laboratory and Intelligence Advanced Research Projects Activity Great Horned Owl project, which also appears on that agency’s programme pages [DARPA, 24 June 2024] [IARPA, Great Horned Owl]. So the agency that sponsors the X-76A calls an unmanned designation an X-plane. The per-mission rule exists to prevent that conflation. If the research series had treated an X-prefixed unmanned number as consumed, as the first of the X-67 article’s three readings of the X-68A proposed, then 72 and 73 would have been spent long before 2025. That reading does not reach 69, 70, 71, 74 or 75. It cannot account for the block and is set aside here.
The Pointer Moved Eight
The X-67 article introduced the measure that makes this precise, the advance of the running maximum. Let $n_k$ be the design number of the $k$-th new research allocation in date order and $h_k$ the highest research design number allocated up to and including it.
\[h_k \;=\; \max\bigl(h_{k-1},\, n_k\bigr), \qquad \Delta_k \;=\; h_k - h_{k-1}, \qquad s_k \;=\; \max\bigl(0,\, \Delta_k - 1\bigr)\]The advance $\Delta_k$ is one when an allocation takes the next number, zero when it fills a gap below the pointer, and greater than one when it passes numbers, of which there are then $s_k$. Over the register’s 23 new research allocations there are 22 steps.
| Advance | Steps | Numbers passed | Allocations |
|---|---|---|---|
| 0 | 1 | 0 | X-49A, which filled the gap the X-50A had passed |
| 1 | 16 | 0 | the ordinary case |
| 2 | 4 | 4 | X-50A, X-53A, X-59A, X-68A |
| 8 | 1 | 7 | X-76A |
Before the X-76A the largest advance in the register’s research series was two. Every earlier gap was one number wide and every one has an article in this series explaining it. The X-76A’s single step passed more numbers than the four earlier gaps together. Writing $K$ for the step that allocated the X-76A,
\[\sum_{k < K} s_k \;=\; 4 \;<\; s_K \;=\; 7 ,\]and of the eleven numbers the pointer has ever passed in this window exactly one, the 49, has come back.
On This Article’s Date the Register Did Not Show It
Every statement above uses the register as it stands in 2026, a different register from the one a reader could have consulted on 14 December 2025. The public web archive holds copies of the register page taken throughout 2025 and early 2026, and the presence of each row can be checked in each copy.
| Archived copy | Coverage the page states | X-68A row | X-76A row |
|---|---|---|---|
| 19 August 2025 | until January 2025 | absent | absent |
| 10 December 2025 | until January 2025 | absent | absent |
| 15 January 2026 | until January 2025 | absent | absent |
| 1 February 2026 | until November 2025 | present | present |
[Parsch, DOD 4120.15-L Addendum, as archived on 19 August 2025] [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]
So both rows first became public between 15 January and 1 February 2026, between 32 and 49 days after this article’s date. Writing $t_{\mathrm{pub}}$ for the date a row first appears in an archived copy and $t_{\mathrm{d}}$ for the dateline, the captures bracket the lags as
\[32 \;\le\; t_{\mathrm{pub}} - t_{\mathrm{d}} \;\le\; 49, \qquad 87 \;\le\; t_{\mathrm{pub}} - t_{76} \;\le\; 104, \qquad 148 \;\le\; t_{\mathrm{pub}} - t_{68} \;\le\; 165\]in days, so the X-68A row stayed out of public view for at least 148 days, nearly five months, and the X-76A row for at least 87, nearly three. The allocations happened inside the dateline and their publication did not. The compiler’s list of missing designations tells the same story from the other side. Its version stamped 23 March 2025, still the version served in November 2025, said that the X-67 slot would almost certainly be skipped and that the next available research number was X-68 [Missing USAF and DOD Aircraft Designations, as archived on 12 November 2025].
This article therefore describes an anomaly that had occurred and had not yet been seen. The X-39, X-52 and X-58 gaps date from 1997, 2006 and 2017, years before the dates of their articles in this series [Missing USAF and DOD Aircraft Designations] [Parsch, DOD 4120.15-L Addendum]. The X-67’s gap was not public at the date of its article either, since it became visible only with the X-68A row. Every finding here is stated from the record as it now stands, and the Epistemic State section separates what existed at the dateline from what was knowable at it.
The Compiler Named the Question and Then Withdrew the Sentence
The clearest statement of this article’s problem is not in any government document. It is a sentence the register’s compiler wrote and later removed. The version of his missing-designations page stamped 3 January 2026, as archived on 12 and 20 February 2026, gives the next available research number as X-69 and then adds a qualification.
However, the designation X-76A has already been allocated. If this was not a genuine out-of-sequence allocation, but a skipping of the range X-69 to -75 (for whatever reason), the next X-plane might also become the X-77.
[Missing USAF and DOD Aircraft Designations, as archived on 12 February 2026] [Missing USAF and DOD Aircraft Designations, as archived on 20 February 2026]
Two of this article’s three readings are named there by the register’s compiler. A genuine out-of-sequence allocation leaves the numbers 69 through 75 available, as the X-49 was available after the X-50A. A skip of the range makes them unavailable and moves the next number to 77. The two readings are two definitions of the next number, and writing them down shows that they differ only in which allocations count. Let $\mathcal{A}$ be the set of research design numbers allocated and $\mathcal{C} \subset \mathcal{A}$ the subset judged to be chosen out of sequence, which after October 2025 is the single number 76.
\[\nu^{\mathrm{hi}} \;=\; 1 + \max \mathcal{A} \;=\; 77, \qquad \nu^{\mathrm{c}} \;=\; 1 + \max\bigl(\mathcal{A} \setminus \mathcal{C}\bigr) \;=\; 69\]The first is the instruction’s last approved design number read literally. The second is the compiler’s convention, which the X-67 article found applied across three series. The block’s status is the difference between them.
By 17 May 2026 the qualification was gone and the stamp still read 3 January 2026 [Missing USAF and DOD Aircraft Designations, as archived on 17 May 2026]. The page as served on 2 October 2026 matches that later version, giving X-69 as the next number with no qualification [Missing USAF and DOD Aircraft Designations]. The removal falls after DARPA’s announcement of 9 March 2026, consistent with the compiler having read the agency’s explanation as settling the matter in favour of a genuine out-of-sequence allocation. The page does not state his reasoning, so this is an inference.
The unchanged stamp is a finding about the source rather than about the subject. A page whose last-updated date did not move when its text did cannot be dated by its stamp. Each version here is dated by the archive’s capture time instead.
Three Readings and Their Predictions
Each reading makes a different prediction about something observable, which lets the record choose between them.
Seven Allocations Not Yet Released
The register’s structure invites the third reading most, because the register does contain allocations it cannot fully show. Sixteen of its rows are marked as absent from officially released data altogether, and all sixteen carry a partial date or none [Parsch, DOD 4120.15-L Addendum]. Two of them are research rows, the X-37B and the X-56B, and both carry a series letter of B, which means each records a later model of a design number already public. Neither is a hidden new number.
The reading has a forced timing, and the timing is what makes it testable. Under the 2020 instruction’s procedure the next number is derived from the last approved one. If the seven numbers had been approved before 20 August 2025, the X-68A would have been a request below the pointer, which the instruction refuses [DAFI 16-401]. So unreleased allocations of 69 through 75 must all fall in the 61 days between the X-68A and the X-76A. With $t_n$ the allocation date of number $n$ and $h(t)$ the highest research number allocated before $t$, an allocation of any $n$ in the block before $t_{68}$ would give $h(t_{68}) \ge 69 > 68$, so
\[n \in \mathcal{B} \ \text{allocated in sequence} \;\Longrightarrow\; t_{68} \;<\; t_n \;<\; t_{76} .\]The series’ allocation rate puts a number on how likely that is. The register’s 23 new research allocations span 1999-07-07 to 2025-10-20 [Parsch, DOD 4120.15-L Addendum], a span $T$ of 26.289 years. The rate of new allocations after the first is then
\[\hat{\lambda} \;=\; \frac{K - 1}{T} \;=\; \frac{22}{26.289\ \text{yr}} \;=\; 0.8368\ \text{yr}^{-1}\]and over a window of $\tau$ equal to 61 days the expected count is
\[\hat{\lambda}\,\tau \;=\; 0.8368 \times \frac{61}{365.2425} \;=\; 0.1398 .\]Treating allocations as a Poisson process at that rate, the probability of seven or more in the window is
\[\Pr\bigl[\,N \ge 7\,\bigr] \;=\; 1 - \sum_{i=0}^{6} e^{-\hat{\lambda}\tau}\,\frac{(\hat{\lambda}\tau)^{i}}{i!} \;=\; 1.83 \times 10^{-10} .\]Because the tail is dominated by its first term, its behaviour under a larger rate is easy to state. In terms of $\mu$, equal to $\hat{\lambda}\tau$,
\[\Pr\bigl[\,N \ge 7\,\bigr] \;\approx\; e^{-\mu}\,\frac{\mu^{7}}{7!} \;=\; 1.797 \times 10^{-10}, \qquad \frac{\Pr[N \ge 7]}{e^{-\mu}\mu^{7}/7!} \;=\; 1.018 ,\]so for a small mean the tail grows as the seventh power of any factor $c$ applied to the rate.
\[\Pr\bigl[\,N \ge 7 \;\big|\; c\mu\,\bigr] \;\approx\; c^{7}\, e^{-(c-1)\mu}\, \Pr\bigl[\,N \ge 7 \;\big|\; \mu\,\bigr]\]Doubling the rate to allow for allocations that never reach any public record raises it only to $2.07 \times 10^{-8}$, against $2^{7} \times 1.83 \times 10^{-10} = 2.34 \times 10^{-8}$ from the power law alone, the difference being the exponential factor. The busiest 61-day window the series has ever had held two new allocations, and seven numbers at the series’ rate are
\[\frac{7}{\hat{\lambda}} \;=\; 8.36\ \text{years}\]of research allocation. The Poisson model is a convenience and not a claim that allocations are memoryless, since programmes cluster around budget cycles and the X-63A and X-64A were allocated on the same day [Parsch, DOD 4120.15-L Addendum]. The question is how much clustering would rescue the reading, and it has a definite answer. Solving for the rate multiplier at which the tail reaches a conventional 0.05,
\[\Pr\bigl[\,N \ge 7 \;\big|\; c^{\star}\mu\,\bigr] \;=\; 0.05 \quad\Longrightarrow\quad c^{\star} \;=\; 23.51, \qquad c^{\star}\hat{\lambda} \;=\; 19.67\ \text{yr}^{-1} .\]The research series would have to have been allocating at about twenty new numbers a year through that autumn, against a long-run rate under one and a busiest-ever window of two, and the tail is
\[\log_{10}\!\frac{0.05}{1.83 \times 10^{-10}} \;=\; 8.44\]orders of magnitude below that threshold at the observed rate. No plausible correlation between programmes supplies a factor of twenty-three. The third reading is therefore rejected on rate alone, before any statement by the sponsor is read.
One Block Held in Reserve
The second reading needs an instrument for reserving a block of design numbers, and the governing instruction contains none. The 2020 instruction’s only reservations are of administrative authority and of intermediate block numbers within a production series, which are not design numbers [DAFI 16-401]. The 2005 edition’s coordinating office would assign and reserve the next available consecutive design number, one number for one request [AFI 16-401(I), 14 April 2005].
The reservations the record does show are single numbers and they are cautionary. The compiler records that the X-39 was reserved in 1997 and lapsed because no formal request followed, and that a V-19A was reserved in 1977 and cancelled in 1980 when the responsible office could find nothing on what it had been for [Missing USAF and DOD Aircraft Designations]. No reservation of a range appears anywhere in the register, the missing-designations page or the six editions of the rule this series has read. The reading is not refuted, since an unrecorded reservation leaves no record by definition. It is unsupported, and it predicts nothing the first reading does not.
Seven Skips at the Agency’s Discretion
The first reading is the only one the instruction provides a mechanism for, and the mechanism has an awkward shape. Two clauses of the 2020 instruction govern it.
The request pertains to a new designation or modification/revision to an existing designator. Requests are not accepted for designators in reverse or skipped sequences.
AF/A8PE reserves the authority to skip design number at discretion.
Read together, the clauses mean that a requester may not ask for a skipped number and the allocating office may skip one on its own authority. The office is the Air Force Directorate of Programs, Program Integration Division, which the instruction makes the central point of contact for the designation programme and the assigner of the next-in-series designator [DAFI 16-401]. The X-76A is a skipped-sequence designator. So either the office skipped the block at its discretion, or a request for X-76 was accepted although the instruction refuses such requests.
\[\text{request for } n \text{ accepted} \;\Longrightarrow\; n \;=\; h + 1, \qquad n \;>\; h + 1 \;\Longrightarrow\; \text{a discretionary skip by the office}\]Neither alternative is recorded and the instruction requires neither to be recorded. The discretion clause carries no criterion, no duty to state a reason and no appeal, as the X-58 article found when it first cited it. The tension between a rule and a discretion to depart from it has a well-known statement in economics. Kydland and Prescott argued that discretionary policy, choosing at each moment the decision that looks best given the current situation, need not achieve the objective that a rule fixed in advance would [Kydland and Prescott 1977]. A founding-year number is a departure whose attraction peaks at one moment, the run-up to its anniversary, and the YMV-75A and X-76A were allocated in the run-up to the 250th anniversaries of 1775 and 1776 [Parsch, DOD 4120.15-L Addendum]. The F-47A fits less cleanly, since 1947 had no round anniversary in 2025 and the number’s stated meanings include the President’s own number [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act]. The first reading therefore predicts what the register shows, a single allocation passing seven numbers with no other trace, and it predicts that the sponsor will say the number was chosen.
The Sponsor’s Statement of March 2026
On 9 March 2026 DARPA announced the X-76 and said why the number was what it was.
Coinciding with the country’s 250th anniversary, the X-76 designation is a deliberate nod to the revolutionary spirit of 1776.
The contractor’s release of the same day announces that Bell is honored to receive the X-76 designation, and frames it as honoring the founding of the United States in 1776 [Bell, 9 March 2026]. A deliberate nod is a chosen number, and a chosen number is not one that arrived in sequence after seven unseen allocations. The statement does not say who chose it, and the verb receive in the contractor’s release places the choice somewhere other than with the contractor. The statement settles the reading and leaves the agent open. The X-67 article reached that outcome by a different route.
Both statements postdate this article, by
\[t_{\mathrm{DARPA}} - t_{\mathrm{d}} \;=\; 85\ \text{days} .\]Written from current knowledge, the article uses them and marks them as later. Without them, the rate argument above still rejects the third reading and the instruction still provides a mechanism only for the first.
Why, So Far As the Record Says
A Number Chosen to Spell a Year
The X-76’s number is the last two digits of 1776. The register’s design numbers run from 1 to 999, so a two-digit number can stand for a year only by dropping the century [DAFI 16-401].
\[n \;=\; y \bmod 100\]The map is many-to-one and the number alone cannot say which century it means. The 76 is equally 1876, the year of the Centennial Exhibition in Philadelphia [Centennial Exposition], and 1976, the bicentennial, which the National Aeronautics and Space Administration, hereafter NASA, marked with a bicentennial survey of two hundred years of flight in America [Emme 1977] [Lundin 1975]. The sponsor’s statement fixes 1776, and the occasion it names is the 250th anniversary of the Declaration of Independence, dated in Congress on 4 July 1776 [Declaration of Independence, National Archives transcript]. Congress fixed the occasion in law a decade earlier. The United States Semiquincentennial Commission Act of 2016 finds that 4 July 2026 is the 250th anniversary of the founding of the United States, as marked by the Declaration of Independence in 1776, and establishes a commission to plan its observance [Public Law 114-196]. The behavioural literature finds that particular numbers act as reference points, people exerting effort to finish just above rather than just below a round number [Pope and Simonsohn 2010], and an anniversary is a round number on the calendar.
It Is the Third Such Number in Eleven Months
The register holds two earlier design numbers chosen the same way, and all three fall within 329 days. [Parsch, DOD 4120.15-L Addendum]
| Designation | Allocated | First public | Lag in days | Year encoded | Stated by |
|---|---|---|---|---|---|
| YMV-75A | 2024-11-25 | 2025-05-14 | 170 | 1775, the Army’s founding | Army |
| F-47A | 2025-04-23 | 2025-03-21 | minus 33 | 1947, the Air Force’s founding, among other meanings | Air Force |
| X-76A | 2025-10-20 | 2026-03-09 | 140 | 1776, the nation’s founding | DARPA |
The Army’s statement for the tiltrotor that won its long-range assault competition is the plainest of the three. The release of 27 May 2025 by the Army’s Program Executive Office for Aviation writes that the 75 commemorates the Army’s founding year, 1775, and names the prototype as the YMV-75A [Army, 27 May 2025]. An Army public affairs officer had told The War Zone on 15 May 2025 that the number 75 is a homage to 1775, the birth year of the United States Army [The War Zone, 14 May 2025, updated 15 May 2025]. The year it encodes follows the Army’s account of its founding. The Army’s history of the date quotes the Continental Congress’s resolution of 14 June 1775 that six companies of expert riflemen be immediately raised in Pennsylvania, two in Maryland and two in Virginia [Army, 14 June 1775, 250th Anniversary of the Infantry and the U.S. Army]. The vertical-takeoff series’ highest number before it was 25, from the XV-25A of March 2024 [Parsch, DOD 4120.15-L Addendum], so the 75 passed
\[s \;=\; 75 - 25 - 1 \;=\; 49\]numbers, seven times as many as the X-76A passed.
The Air Force’s statement for the F-47 gives three meanings and the record shows them assembled over two days. It reads that the number honors the legacy of the P-47, pays tribute to the founding year of the Air Force, and recognizes the 47th President’s pivotal support for the programme [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act]. The founding year it names is statutory. The National Security Act of 1947 provided for a Department of the Air Force alongside the Departments of the Army and the Navy [National Security Act of 1947, Public Law 80-253]. The F-47 is the only one of the three whose public announcement preceded its allocation, by 33 days, so the number was spoken by the President before the register recorded it [Air Force Reserve Command, 21 March 2025] [Parsch, DOD 4120.15-L Addendum].
The X-76A is the only one of the three in the research series, and the only one whose sponsor is not a military service. [Parsch, DOD 4120.15-L Addendum] Its number also spells the founding of the country, where the other two spell a service’s, and its occasion, 4 July 2026, falls latest of the three.
The One Case Where the Record Names Who Decided
For the X-76 and the YMV-75A the record gives the reason and not the chooser. For the F-47 a released set of Air Force public affairs emails gives both. The emails were released under the Freedom of Information Act and are hosted by Bloomberg, and their content is quoted here directly from the released pages. The released file’s metadata shows a creation date of 17 June 2025, inside this article’s dateline, though the date it first became public is not established here [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act].
On 20 March 2025, the day before the announcement, an urgent request from the Chief of Staff’s office asked the Air Force’s history office for background on the P-47’s naming and its record in the Second World War, and a two-page talking paper was returned that afternoon. On 21 March a proposed statement was sent for coordination with the Secretary of Defense’s office, reading that the number 47 was chosen in consultation with the Secretary of Defense and carries multiple significant meanings. On 25 March, asked by a reporter why the nomenclature system had been abandoned in this case and who decided, the public affairs office replied that General Allvin made the decision [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act].
The same exchange records the reporter’s statement of the system’s purpose, that the nomenclature system exists to maintain an orderly, non-chaotic process of naming systems, and the observation that the Air Force has usually honored earlier aircraft with a popular name rather than with the designation [Air Force public affairs correspondence on the F-47, released under the Freedom of Information Act]. The office answered the question of who and added nothing to the question of why.
This document is the only one this article found that names a design number’s chooser, and the chooser it names is not the allocating office. The instruction gives the discretion to skip to the Program Integration Division [DAFI 16-401]. The person named as deciding is General Allvin, whom the Air Force’s biography gives as Chief of Staff of the Air Force from November 2023 [Air Force, General David W. Allvin, as archived on 27 November 2024], and the proposed statement places the choice in consultation with the Secretary of Defense. Whether the X-76 followed the same path is not recorded, and the article does not infer it from the F-47.
DARPA Has Done This Once Before, Under the Other Rule
The research series has carried a chosen number before, under DARPA’s sponsorship. The compiler’s note on the X-50A records that the X-49 was skipped because DARPA explicitly requested the X-50A for its Canard Rotor/Wing demonstrator, wanting the number 50 because the aircraft would be the first true 50/50 marriage of helicopters and high speed fixed wing aircraft [Parsch, DOD 4120.15-L Addendum]. The same note records that the X-49 was not permanently skipped and was assigned when the next research designator was requested, which the X-49 article told from the aeroplane’s side.
A second primary carries the same story with two details the compiler’s note lacks. NASA’s inventory of the X-vehicles through the X-50, published for the centennial of flight, says that as of early 2003 the X-49 designator had apparently been skipped because DARPA requested the X-50 designation, that DARPA reportedly wanted the number 50 because the aircraft would be the first true 50/50 marriage of helicopters and high-speed fixed wing aircraft, and that it is likely that the X-49 designation will be assigned when the next X designator is requested [Jenkins et al 2003]. The same entry quotes Boeing’s X-50 programme manager, Steve Bass, confirming that Boeing got the number out of sequence by special request, the designation being fitting for a concept that is 50 percent helicopter and 50 percent airplane. The aircraft it names is the Dragonfly Canard Rotor/Wing demonstrator that began flight testing at Yuma in December 2003 [Boeing, 4 December 2003].
The three accounts agree that the number was asked for and disagree about who asked. The compiler and NASA name DARPA and the contractor’s programme manager names Boeing. The X-76 raises the same open question of agency, and it was never settled for the X-50 either. And NASA’s monograph made in 2003 the kind of prediction made at the end of this one, that the passed number would be issued with the next request. It was, on 23 May 2003 [Parsch, DOD 4120.15-L Addendum].
So DARPA has sponsored a research number chosen for its meaning twice, 23 years apart, and the two choices fall on either side of the change in the rule the X-67 article dated.
| X-50A | X-76A | |
|---|---|---|
| Allocated | 2002-02-13 | 2025-10-20 |
| Meaning | a pun on half rotorcraft and half fixed wing | a year |
| Numbers passed | 1 | 7 |
| Rule then in force | the next available consecutive number | the last approved number, skipped sequences refused |
| What happened to the passed numbers | the 49 was issued 464 days later | not yet known |
The next-available rule’s sentence is short, as the 1994 edition gives it.
HQ ASC/ENOS will assign the next available consecutive design number within each basic mission for new vehicles.
[AFJI 16-401, 9 September 1994]
Under the next-available rule the passed number stayed available and was used. Under the 2020 rule a passed number is in a skipped sequence and requests for it are not accepted. [DAFI 16-401] Read literally, the X-76A has made 69 through 75 unrequestable. Read by the X-50A’s precedent, they are waiting.
Sizing From First Principles
There is no vehicle to size. The register allocates none of the numbers 69 through 75 in the research series and names no programme that took them [Parsch, DOD 4120.15-L Addendum]. The X-76A, whose number passed them, is left to the next article in the series. The only physical evidence read here is the X-76A row’s engine cell, two General Electric CT7-8 engines and a Pratt and Whitney PW308C, and The Register Row, above, reads it only as evidence of an aircraft that hovers on rotors and cruises on jet thrust.
The quantities this article derives from first principles describe the numbering, since there is no airframe to describe. They are the allocation rate and its Poisson tail, the ordinal bound for the X-76A and the count of two-digit numbers left above the research pointer. Epistemic State lists them as engineering analysis derived here and lists the pointer walk beside them as measured from the documents.
Dependent Systems
The block has no systems because it has no aircraft. The systems this article depends on are administrative. They are the register that records allocations, the instruction that governs them, and the allocating office that holds the authority to skip a design number at its discretion [DAFI 16-401]. How those three behaved is the subject of Three Readings and Their Predictions and of What the Data Changed. The X-76A’s propulsion and airframe are excluded and listed under Out of Scope.
The Flight Test Record
Nothing numbered 69 through 75 has flown, because nothing numbered 69 through 75 exists in the research series. The register holds no research row for any of the seven, and no public record as of 2 October 2026 shows an X-69 [Parsch, DOD 4120.15-L Addendum]. Whatever flight record the X-76A accumulates is a matter for the X-76 article that follows.
The record that does exist is one of publication. The X-68A and X-76A rows first appear in an archived copy of the register between 15 January and 1 February 2026, and the sponsor’s statement of the reason followed on 9 March 2026 [Parsch, DOD 4120.15-L Addendum, as archived on 1 February 2026] [DARPA, 9 March 2026]. That sequence stands in for a flight log.
Comparison With Ground Prediction
No vehicle flew, and there is no flight result to compare with a ground prediction. What the record does hold are predictions made on paper about the numbering, and each can be set beside what followed.
NASA’s 2003 inventory, describing the position as of early 2003, predicted that the passed X-49 would be assigned when the next research designator was requested [Jenkins et al 2003], and it was, on 23 May 2003 [Parsch, DOD 4120.15-L Addendum]. The compiler’s page stamped 23 March 2025 gave the next available research number as X-68, and the X-68A was allocated on 20 August 2025 [Missing USAF and DOD Aircraft Designations, as archived on 12 November 2025]. Neither prediction anticipated a single allocation passing seven numbers.
The 2020 instruction’s rule predicts that each new design number is one above the last approved number, and six of the 23 allocation events with a predecessor since November 2020 follow it, or seven if one unmanned row whose official form is uncertain is set aside [DAFI 16-401]. The count is argued below, under The 2020 Rule Describes Less Than a Third of the Allocations Made Under It. The compiler’s page stamped 3 January 2026 gave X-69 as the next research number with a qualification that it might be X-77, and no public record as of 2 October 2026 shows either [Missing USAF and DOD Aircraft Designations, as archived on 12 February 2026].
This article makes a single prediction, set out in The Next Research Number Is a Test, and no outcome exists to compare it with until the next research designation is issued.
What the Data Changed, What the Anomaly Reveals About the Designation System
Recorded Reasons for Out-of-Sequence Numbers
The register’s notes give or disclaim a reason for 24 designations that are out of sequence or that caused a skip, and the reasons fall into a small number of kinds. [Parsch, DOD 4120.15-L Addendum] The classification below is made here from the compiler’s notes, and a note giving two reasons is counted under both.
| Kind of reason | Count | Designations |
|---|---|---|
| none known, or none given in the note | 8 | F-35, LGM-35, AIM-260A, AIM-424, QUH-101A, YFQ-42A, YFQ-44A, XRQ-72A |
| a founding year | 3 | YMV-75A, F-47A, X-76A |
| avoiding confusion with another number | 3 | X-53A, VH-71A, ES-14A |
| borrowed from another series’ queue | 2 | XQ-58A, XQ-67A |
| a manufacturer’s model number | 2 | KC-767A, RQ-170 |
| continuing or reusing an older number | 2 | C-143A, EA-37B |
| following on from a predecessor’s number | 1 | XRQ-73A |
| a meaning requested by the sponsor | 1 | X-50A |
| superstition | 1 | ES-14A |
| a refused request, later filled | 1 | RQ-12A |
| skipped with no reason given | 1 | RQ-14A |
The column sums to one more than the number of notes, the extra assignment being the ES-14A, whose note gives both a confusion and a superstition as reasons.
Two features of the table matter. The largest class is the one with no reason, so
\[\frac{8}{24} \;=\; \frac{1}{3}\]of the register’s recorded departures from sequence carry no reason in the note that records them. And the founding-year class has three members, all allocated between 25 November 2024 and 20 October 2025, with none before [Parsch, DOD 4120.15-L Addendum]. No member of the class predates November 2024, and all three fall within an eleven-month window that closes about eight and a half months before the semiquincentennial, the 250th anniversary of 1776. The class is new, and it arrived as a cluster. The tightness of the cluster can be given a scale. If $J$ members were placed independently and uniformly over a window of $W$ days, the probability that all of them fall within some span of $w$ days is the distribution of their range $R$,
\[\Pr\bigl[\,R \le w\,\bigr] \;=\; J\,x^{J-1} \;-\; (J-1)\,x^{J}, \qquad x \;=\; \frac{w}{W} .\]With the three members spanning $w$ equal to 329 days, and the register’s window from 19 August 1998 to the end of November 2025 being $W$ equal to 9,965 days, $x$ is 0.03302 and
\[\Pr\bigl[\,R \le 329\ \text{days}\,\bigr] \;=\; 3\,(0.03302)^{2} - 2\,(0.03302)^{3} \;=\; 0.00320 .\]The figure gives a scale. It is not a test. The uniform null is the hypothesis that founding-year numbers arrive with no occasion, which nobody holds, and the window was drawn after the cluster was seen. It shows that three such numbers falling together by accident would be about a one-in-three-hundred event, so the clustering is better read as the occasion’s signature than as chance.
The 2020 Rule Describes Less Than a Third of the Allocations Made Under It
The instruction directs a requester to research the last approved design number and request the next one, which gives its rule a testable consequence for every allocation made since 3 November 2020. [DAFI 16-401] Grouping allocations made on the same day in the same basic mission into one event, the register holds 24 events of new design numbers on or after that date [Parsch, DOD 4120.15-L Addendum].
| Outcome against the highest approved number $h$ | Events | Examples |
|---|---|---|
| exactly $h + 1$, the rule as written | 6 | X-62A, X-63A with X-64A, T-54A, X-65A, X-66A, XV-25A |
| below $h$, a reverse sequence in the instruction’s words | 13 | MQ-35A, XQ-67A, F-47A, AGM-188A with AGM-189A |
| above $h + 1$, a skipped sequence | 4 | ES-19A with ES-21A and ES-23A, YMV-75A, X-68A, X-76A |
| first in its basic mission | 1 | ML-1A |
Of the 23 events that had a predecessor, six follow the rule as written, a fraction of about a quarter. The count depends on one row, so both values are stated. The register gives the RQ-170 without a series letter, and the compiler notes that its official form may be RQ-170A [Parsch, DOD 4120.15-L Addendum]. If that row is set aside, the unmanned series’ highest number before September 2023 is 72, the XRQ-73A becomes one above it, and seven of the 23 events follow the rule.
\[f' \;=\; \frac{7}{23} \;=\; 0.304\]This matches the compiler’s follow-on reading of the 73, seen from the instruction’s side, and either way the rule as written describes less than a third of the allocations made under it. The thirteen below the highest number are mostly in series whose maximum is an outlier, such as the unmanned series with its 170 and the missile series with its 260, and they are what any working allocator must produce if it ignores outliers [Parsch, DOD 4120.15-L Addendum]. The convention is the compiler’s, and the instruction’s text does not contain it, as the X-67 article established.
The research series was the exception until 2025. Its first four events under the rule, from the X-62A through the X-66A, all took the next number [Parsch, DOD 4120.15-L Addendum]. Its next two, the X-68A and the X-76A, both skipped. For the research series alone, with $f_X$ its fraction,
\[f_X^{\,\text{2021 to 2023}} \;=\; \frac{4}{4} \;=\; 1, \qquad f_X^{\,\text{2025}} \;=\; \frac{0}{2} \;=\; 0 .\]So the series that best followed the rule stopped following it in the same year that the founding-year class appeared.
The Design Number Is No Longer an Ordinal
The instruction illustrates the design number by saying that the 16 in F-16A represents the sixteenth approved designator for an aircraft with a fighter mission [DAFI 16-401]. That makes a design number an ordinal, and a skipped number makes the ordinal wrong by the count of numbers skipped below it. Let $G$ be the set of research numbers never allocated.
\[o(n) \;=\; n \;-\; \bigl|\, G \cap [1, n) \,\bigr|\]The compiler records the research numbers 23, 39, 52, 58 and 67 as never allocated, and the seven numbers of the block join them [Missing USAF and DOD Aircraft Designations]. With twelve numbers missing below 76,
\[o(76) \;=\; 76 - 12 \;=\; 64, \qquad \frac{76}{64} - 1 \;=\; 0.1875 .\]The X-76A is at most the sixty-fourth research design number allocated, and its number overstates its place by 18.75 percent. The bound is an upper one because it assumes that every number below 76 not on the compiler’s list was allocated. The X-23 is on the list because the compiler finds no allocation for it, and this series treated that aircraft at full length while recording the disagreement. Before the X-76A the overstatement was five numbers in 68. The X-76A added seven in a single allocation. The share of printed numbers below the pointer that stand for no aircraft more than doubled.
\[o(68) \;=\; 68 - 5 \;=\; 63, \qquad \frac{5}{68} \;=\; 0.0735 \;\;\longrightarrow\;\; \frac{12}{76} \;=\; 0.158\]A Year Can Be Spent Only Once, and Only Upward
A founding-year number is also constrained by the pointer. Under the 2020 rule a number at or below the highest approved number is a reverse sequence [DAFI 16-401]. So a year can be encoded in two digits only if
\[y \bmod 100 \;>\; h_t ,\]and every allocation that raises the pointer closes off every year whose last two digits it passes. With the research pointer now at 76, the two-digit numbers still above it are 77 through 99, which is
\[99 - 76 \;=\; 23\]values, a fraction
\[\frac{99 - 76}{100} \;=\; 0.23\]of the years in any century. The X-76A therefore used the last two-digit number in the research series that can stand for 1776, and it also passed 69 through 75, which closed the years ending in those digits for as long as the 2020 rule is read literally. The rule that refuses skipped sequences turns each chosen number into a one-way door. Under the next-available rule the X-50A’s choice cost nothing permanent, because the 49 came back.
What an Observer Counting From the Top Would Infer
Inferring how many items exist from the highest serial number seen is a classical estimation problem, known from its wartime use as the German tank problem, in which serial numbers on captured equipment were used to estimate production [Ruggles and Brodie 1947], and set out in general form as serial-number analysis [Goodman 1952] [Goodman 1954]. The neighbouring problem of estimating how many kinds exist from the ones observed has a separate classic literature [Good 1953] [Efron and Thisted 1976]. Their method assumes that numbers are issued in sequence without choice. A chosen number breaks that assumption at its root. An observer who read the X-76A as the seventy-sixth research aircraft, as the instruction’s ordinal language invites, would overcount by
\[76 - o(76) \;=\; 12 .\]The classical estimator makes the same error in a sharper form. For $\kappa$ distinct serial numbers seen with largest $m$, the minimum-variance unbiased estimate of the total issued, which the literature gives as the best estimate from the largest serial number seen [Anthony Lee and Steven Miller 2023], is
\[\hat{M} \;=\; m\Bigl(1 + \frac{1}{\kappa}\Bigr) - 1 .\]Even an observer who had seen every research number allocated, at most 64, which is the most favourable case, would estimate 76.19 with $m$ equal to 76, overcounting by more than twelve. The estimator is right about a sequence issued without choice and wrong about this one by the twelve numbers never allocated and a fraction, seven of them the numbers the choice passed. The register’s design numbers have become, in part, a channel for messages, and a channel for messages is a poor counter. The X-67 article measured the same drift in the unmanned series in bits, and the founding-year class supplies a mechanism that explains why it is spreading.
The Next Research Number Is a Test
The record does not say whether 69 through 75 are spent, and the next research allocation will.
- If the next research designation is the X-69A, the X-49 precedent governs in practice, the compiler’s convention is the allocator’s, and the instruction’s refusal of skipped sequences is not applied to numbers passed by a chosen allocation.
- If it is the X-77A, the 2020 text governs as written, and the seven numbers are permanently lost.
- If it is neither, the series has another chosen number, which would make the founding-year class a practice rather than an episode.
The evidence points in two directions and the article reports both. The unmanned series since 2020 has continued its contiguous run, with the RQ-29A and YRQ-30A taking the numbers after 28 although the series’ maximum is 170 [Parsch, DOD 4120.15-L Addendum], which favours the compiler’s convention. In the two definitions written down earlier, applied to the unmanned series,
\[n_{\text{RQ-29A}} \;=\; 29 \;=\; \nu^{\mathrm{c}}, \qquad n_{\text{YRQ-30A}} \;=\; 30 \;=\; \nu^{\mathrm{c}}, \qquad \nu^{\mathrm{hi}} \;=\; 171 ,\]so the unmanned series has followed $\nu^{\mathrm{c}}$ and not $\nu^{\mathrm{hi}}$ twice since the rule took effect. But the research series’ last test went the other way. On a reading that keeps passed numbers available within a basic mission, the X-68A should have been the X-67A, and it was not, so the research series has already once treated a number as spent without allocating it in the series. No public record as of 2 October 2026 shows either an X-69 or an X-77. The absence covers about a year of allocations and is not evidence that none has been made [Parsch, DOD 4120.15-L Addendum] [Missing USAF and DOD Aircraft Designations].
Symbols
| Symbol | Meaning | Unit |
|---|---|---|
| $\mathcal{B}$ | the block of research numbers 69 through 75 | none |
| $t_{68}$, $t_{76}$, $t_n$, $t_{49}$, $t_{50}$, $t_{\mathrm{pub}}$, $t_{\mathrm{d}}$, $t_{\mathrm{DARPA}}$ | a date, being respectively the allocation of the X-68A, of the X-76A, of design number $n$, of the X-49A and of the X-50A, the first appearance of a row in an archived copy of the register, this article’s editorial date of 14 December 2025, and DARPA’s announcement of 9 March 2026 | date |
| $n_k$ | design number of the $k$-th new research allocation in date order | none |
| $h_k$, $h_t$, $h(t)$ | highest research design number allocated up to allocation $k$, or before time $t$ | none |
| $\Delta_k$ | advance of that highest number at allocation $k$ | none |
| $s_k$, $s$, $s_{50}$, $s_{76}$ | numbers passed at allocation $k$, by a single allocation, or by the X-50A and the X-76A | none |
| $K$ | number of new research allocations in the register, and the step that allocated the X-76A | none |
| $\mathcal{A}$ | set of research design numbers allocated | none |
| $\mathcal{C}$ | subset of $\mathcal{A}$ judged chosen out of sequence | none |
| $\nu^{\mathrm{hi}}$, $\nu^{\mathrm{c}}$ | the next number, as one above the highest allocated, and as one above the highest allocated in sequence | none |
| $T$ | span of the research allocations | years |
| $\hat{\lambda}$ | estimated rate of new research allocations | per year |
| $\tau$ | the window between the X-68A and the X-76A | days |
| $\mu$ | expected allocations in that window, $\hat{\lambda}\tau$ | none |
| $N$ | number of allocations in that window under the Poisson model | none |
| $i$ | summation index | none |
| $c$, $c^{\star}$ | a multiplier on the rate, and the multiplier at which the tail reaches 0.05 | none |
| $n$, $n_{\text{RQ-29A}}$ | a design number, generally or of a named allocation | none |
| $h$ | highest approved design number in a basic mission at the time of a request | none |
| $y$ | a calendar year | year |
| $\delta t$ | public announcement date minus allocation date | days |
| $J$ | number of founding-year allocations | none |
| $R$ | range of their allocation dates | days |
| $w$, $W$ | observed span of the founding-year class, and the register’s window | days |
| $x$ | ratio $w / W$ | none |
| $f$, $f’$ | fraction of allocation events since November 2020 that follow the rule as written, with and without the RQ-170 row | none |
| $f_X$ | the same fraction within the research series alone | none |
| $G$ | set of research design numbers never allocated | none |
| $o(n)$ | ordinal position of design number $n$ among those actually allocated | none |
| $\kappa$ | number of distinct serial numbers an observer has seen | none |
| $m$ | largest serial number an observer has seen | none |
| $\hat{M}$ | estimate of the total number issued | none |
The Contemporary Literature
| Cluster | Records |
|---|---|
| Designation, nomenclature and configuration identification | 42 |
| Identifier allocation, registries and numbering plans | 187 |
| Version numbering | 14 |
| Classification and numerical taxonomy | 16 |
| Why particular numbers are chosen | 177 |
| Inferring unseen counts from numbered items | 14 |
| Anniversaries, centennials and bicentennials | 129 |
| Commemoration and collective memory | 252 |
| Commemorative naming and critical toponymy | 186 |
| Military heritage, tradition and naming | 9 |
| Symbolic politics and the politics of naming | 7 |
| Discretion within rules | 199 |
| Total | 1,232 |
There is no literature on the X-69 through X-75, and the survey begins by stating that. A block of numbers that was never allocated produces no technical papers, and no research record cited here concerns them. What the survey maps is the set of fields this question sits in. They are the engineering literature of designation and identifier allocation, the behavioural literature of why particular numbers are chosen, the statistical literature of inferring unseen counts from numbered items, the humanities and social science literature of commemoration and commemorative naming, and the public administration literature on discretion within rules.
The table counts 1,232 research records in twelve clusters. A record that belongs to several clusters appears once, under the first of them in the order above, and the rule that admits a record is set out under The Source Base, in the subsection headed What the Survey Admits.
The shape of that table is the argument restated. The commemoration clusters together, which are anniversaries, memory, commemorative naming and military heritage, are the largest group, holding 576 of the 1,232 records, because the X-76’s number is an act of commemoration and those fields study such acts. The literature closest to the X-76’s motive is not aeronautical. One of its founding papers observes that street names are ostensibly visible, quintessentially mundane and seemingly obvious, which it gives as the reason social scientists had hardly studied them as instruments of authority, and it treats the use of street names for commemoration as a fundamental feature of modern political culture [Azaryahu 1996]. The same author’s titles follow renaming through changes of regime, in Berlin from 1945 to 1948 and in East Berlin after reunification [Azaryahu 2011] [Azaryahu 1997]. A later review describes the field’s critical turn away from etymology and taxonomy toward the politics of place-naming practices [Rose-Redwood et al 2009] [Rose-Redwood et al 2008], and the commemoration of Martin Luther King on American streets is among its cases [Alderman 2003] [Alderman and Inwood 2013]. Behind it sits the literature of collective memory and commemoration [Nora 1989] [Schwartz 1982], including the shaping of the national holiday calendar [Schwartz 2008], the invention of traditions [Hobsbawm 2012], and the American bicentennial of 1976 itself [Rymsza-Pawlowska 2017] [Hattem 2024]. A design number is visible, mundane and seemingly obvious in the sense that abstract uses. That gives a reason to read the founding-year class through this literature rather than through aeronautics. The literature of commemorative naming, much of which calls itself critical toponymy, studies who renames streets and squares, when they do it and what the names are made to say. Of the 186 records the cluster holds, 51 have titles that name an anniversary, commemoration or memory, 32 name renaming, and 24 name a regime, a revolution, independence, colonialism, apartheid or nationalism. As shares of the cluster,
\[\frac{51}{186} \;=\; 0.274, \qquad \frac{32}{186} \;=\; 0.172, \qquad \frac{24}{186} \;=\; 0.129 .\]The shares measure titles and do not interpret findings. They show that the field’s vocabulary is the vocabulary of the founding-year class. The aerospace community marked the last such anniversary as well, with a bicentennial survey of two hundred years of flight in America among the records this survey cites from NASA’s Technical Reports Server [Emme 1977].
Designation, nomenclature and configuration identification
This is the system’s own literature. It covers the designating instruction itself, designation and nomenclature systems for aircraft and for manufactured items such as steels, lamps, bearings and railway vehicles, the configuration identification of manufactured items, and NASA’s records of the X-plane programme as a programme, being its inventories, history, database, cost and guidelines.
42 records. [Hirshorn et al 2024] [Van Dalsem 2019] [Sterk et al 2018] [Cumming et al 2017] [Lechniak and Melton 2017] [Sterk et al 2016] [Jenkins et al 2003] [Miller and Jenkins 2000] [Hicks and Trippensee 1997] [Louis M Edelman] [Gorn and De Chiara 2021] [Appendix 1 Standard nomenclature 2019] [Bryan 2009] [Configuration Management Identification 2007] [System Configuration Identification 2005] [Configuration Identification 2004] [How Configuration Identification Works 2004] [Appendix A Aircraft Nomenclature 1997] [Configuration Identification 1995] [Buckle 1982] [Martin 1956] [Aircraft Propulsion System Performance] [Aircraft Propulsion System Performance] [Carbon fibre. Designation system] [Configuration Identification] [Configuration Identification for Digital] [Construction drawings. Designation systems] [Dentistry. Designation system for] [Designation system for steels] [Designation systems for steels] [Founding. Designation system for] [Glass bulb designation system] [New designation system for] [Nomenclature for glass bulb] [Railway applications. Designation system] [Recycled carbon fibres �] [Refrigerants. Designation system] [Rolling bearings. Tapered roller] [Technical product documentation. Reference] [Titanium and titanium alloys] [Welding and allied processes] [Department Of The Air Force Washington Dc 1997]
Identifier allocation, registries and numbering plans
The questions a designation office faces are better documented for other registries. Telephone numbering plans, address allocation, registration authorities and persistent identifiers all face the choice between issuing the next number and issuing a meaningful one, and between reclaiming a passed number and retiring it.
187 records. [Pérez Vera 2026] [Assessing the Impact of 2026] [Chigwada et al 2026] [Martin et al 2026] [Golovachev 2026] [Czerniak et al 2026] [Hsu and Edmunds 2026] [Crossref 2026] [Cebeci 2026] [Bronselaer 2025] [Webster 2025] [Carpenter et al 2025] [Hernández Gutierrez et al 2025] [Reyes-Lillo et al 2025] [Yao 2025] [Nasrullah et al 2025] [Abedrapo Rosen and Hartley Belmar 2025] [Panter et al 2025] [Kandler 2025] [Fenner 2025] [Stoll and de Lamotte 2025] [Bhirawa 2025] [Ajibode et al 2025] [Wang et al 2025] [Iman 2025] [Chala et al 2024] [Chodacki et al 2024] [Park et al 2024] [Nizami 2024] [Chapman 2024] [Mandal and Mandal 2024] [Riley 2024] [Owango et al 2024] [Pinckney et al 2023] [Li et al 2023] [Tournier 2023] [van Veenendaal 2023] [Fenner 2023] [Islam 2023] [Minihan et al 2023] [Brower and Narlock 2023] [Murillo-Gonzalez and López 2023] [Burton et al 2023] [Carvalho 2023] [Page 2023] [Aghassibake et al 2023] [Johaadien et al 2022] [Ochoa et al 2022] [Ziegler and Crettaz 2022] [Zhang et al 2022] [Gould 2022] [Winston 2022] [Winston 2022] [Herb 2022] [Hisseine et al 2022] [Islam et al 2022] [Farach 2022] [Hardisty et al 2021] [Klein and Balakireva 2021] [Opdebeeck et al 2021] [Gould and Praetzellis 2021] [Mangahis 2021] [Retteen and Hall 2021] [Goddard 2021] [Mejias 2021] [Herb 2021] [Housley 2021] [Grenz et al 2021] [Decan and Mens 2021] [Ziedorn 2020] [Ananthakrishnan et al 2020] [Schirrwagen et al 2020] [Rzepa 2020] [Plomp 2020] [Page 2020] [Lam et al 2020] [Paglialonga and Schirnick 2020] [Madinah 2020] [Myrda and Panecki 2020] [Sicilia et al 2019] [Yun 2019] [Wittenburg 2019] [Sharan 2019] [Meadows et al 2019] [Lehnert et al 2019] [Overton et al 2019] [Mainz et al 2019] [Milne and Chan 2019] [Chen et al 2019] [Thompson and Tong 2018] [Meadows 2018] [Rzepa 2018] [Meadows and Haak 2018] [Early 2018] [Parsons and Fox 2018] [Klump and Huber 2017] [Berber and Yahyapour 2017] [Mayernik and Maull 2017] [Dappert et al 2017] [Berber and Yahyapour 2017] [Rueda et al 2017] [Digital Object Identifier DOI® 2017] [Golodoniuc et al 2017] [Hendricks and Bilder 2017] [Hu et al 2017] [Bangert and Frances 2017] [Wang et al 2017] [Hewlett 2017] [Aryani 2017] [Raemaekers et al 2017] [Klensin and Hakala 2017] [Lemieux 2016] [Kennickell 2016] [Cook et al 2016] [Wannenwetsch and Majchrzak 2016] [Bolikowski et al 2015] [Schmitt et al 2015] [Fenner 2015] [PID Service an 2015] [Fenner 2015] [Fenner 2015] [Bunakov 2015] [Weigel et al 2014] [Lemieux 2014] [Bellini et al 2014] [Housley 2014] [Housley 2014] [Bhamare et al 2014] [Van de Sompel et al 2014] [Haak 2014] [Ying et al 2014] [Sugang Xu et al 2013] [Bazzanella et al 2013] [IMF Participation in the 2013] [Housley 2013] [Simons and Richardson 2013] [RP 2079 2013 Digital 2013] [Chan and Milne 2013] [Bellini et al 2012] [Laibe 2011] [Hoffman 2010] [Ceravolo and Bellini 2010] [IP Address Allocation 2010] [Sommer 2010] [Askitas 2010] [Cox 2008] [Hsia-Ling Chiang et al 2008] [Abdallah et al 2008] [Jennings and Gurbani 2008] [Wang et al 2007] [Chandrakar 2006] [Camarillo 2006] [Yuan-Ying Hsu and Chien-Chao Tseng 2005] [Schroeder 2005] [Rees and Murray 2004] [Camarillo 2004] [Haucap 2003] [Dobrovolskis 2003] [Hamilton 2002] [Ricketts 2002] [McPherson and Dykes 2001] [Jan and Wu 2000] [Paskin 1999] [George 1998] [IPv6 Address Allocation Management 1995] [Butler 1995] [Rekhter and Li 1993] [Cerf 1990] [Donald 1986] [Mellichamp 1979] [Keevers 1966] [Bergmann and Haferd 1964] [Myers 1961] [Nunn 1952] [Nunn 1952] [DIN 31646 2013-01, Information und] [Digital Object Identifier DOI] [Digital Object Identifier system] [Digital token identifier DTI] [Embossed credit cards. Specifications] [Health informatics. Health cards] [Information and documentation. Digital] [Information technology – Procedures] [Information technology. Open systems] [Wannenwetsch] [Procedures for the operation] [de Castro et al]
Version numbering
Software version numbers are a familiar case of a number that is supposed to be an ordinal and is sometimes chosen for what it says. The literature on semantic versioning and release numbering studies what a number promises and what breaks when the promise is not kept.
14 records. [Wang et al 2026] [Noguchi 2026] [Shima 2025] [Serbout and Pautasso 2024] [Abdalkareem et al 2023] [Carvalho and Seco 2021] [Broyles 2020] [Opdebeeck et al 2020] [Struzik et al 2020] [Harrison and Živanović 2017] [Vion-Dury and Lagos 2016] [Asif and Karim 2014] [Raemaekers et al 2014] [Keller and Ullman]
Classification and numerical taxonomy
A designation is a classification as well as a count, and the X-67 article draws on this literature for the question of what counts as a new design. The cluster holds the method of numerical classification, its principles, its character selection and its consensus methods, and not its applications to particular organisms or products.
16 records. [Numerical Taxonomy 2017] [D’Agostino and Russell 2014] [Principles of Numerical Taxonomy 2013] [Barthélemy and Brucker 2000] [Gyllenberg and Koski 1996] [Numerical Taxonomy and Cluster 1994] [O’brien and Colwell 1988] [Sneath 1985] [McNeill 1984] [Fiala 1983] [Hubac 1983] [Numerical Taxonomy 1983] [Numerical Taxonomic Methods 1982] [Leuschner 1981] [Quesne 1969] [Maze 1966]
Why particular numbers are chosen
The behavioural literature on number choice is large and quantitative. It covers digit preference and heaping in reported figures, the pull of round numbers, lucky and unlucky numbers and the prices paid for them, number symbolism, and the effect of numerals in brand and product names. A founding-year design number is a case of a number chosen for its symbolism rather than its place, which is this literature’s subject.
177 records. [Kudtarkar 2026] [Pandit et al 2026] [Lee 2026] [Han and Lee 2026] [Roychoudhury 2026] [Choi 2026] [Lau et al 2025] [Ferber and Baten 2025] [Mangombe et al 2025] [Piotrowicz et al 2025] [Gómez-i-Aznar 2025] [Dube et al 2025] [Huntington 2025] [Lengerich et al 2025] [Colvin et al 2024] [Tollnek and Baten 2024] [Rönnbäck et al 2024] [Breunig et al 2024] [Breunig 2024] [Rosen and Spaenjers 2024] [Gu and Feng 2024] [Chandel et al 2024] [Putri et al 2023] [Helleringer et al 2023] [Fitzgerald 2023] [Lucky numbers 2023] [O’Grady 2023] [O’Halloran et al 2022] [Baten and Nalle 2022] [A’Hearn et al 2022] [Luo et al 2022] [Lengerich et al 2022] [Dahiru and Dikko 2022] [Basannar et al 2022] [Baten et al 2022] [Nguyen et al 2022] [O’Halloran et al 2022] [Barrett 2021] [Singh et al 2021] [Malik 2021] [Lapostolle et al 2021] [Romero Prieto et al 2021] [A’Hearn et al 2021] [Billig 2021] [Beltrán Tapia et al 2021] [Foti et al 2020] [Sakaguchi et al 2020] [Fayehun et al 2019] [Lee 2019] [Chow 2019] [Suhartini 2019] [The Law of Round 2019] [Gunasti and Ozcan 2019] [den Bakker and Damhuis 2018] [Samuel O 2018] [Blum and Krauss 2017] [Szołtysek et al 2017] [Westjohn et al 2017] [Calzolari 2017] [Gideon et al 2017] [Wu and Sun 2017] [Cappuccini 2017] [Baten and Sohn 2017] [Hom 2017] [Lyons-Amos and Stones 2017] [Camarda et al 2016] [Barootes 2016] [Chooi and Chia 2016] [Gao 2016] [Plumb et al 2016] [Backus et al 2015] [Kara et al 2015] [Pena-Marin and Bhargave 2015] [Gunasti et al 2014] [Gunasti and Ozcan 2014] [Wagner and Jamsawang 2014] [Mission Number Thirteen 2014] [Number Preference Model 2014] [Kettle et al 2014] [Kara et al 2014] [Stratford et al 2013] [Yazdanparast et al 2012] [Bhattacharya et al 2012] [The Likely Number Thirteen 2012] [Manzel et al 2011] [Földvári et al 2011] [Alsanjari et al 2011] [Pardeshi 2010] [Baten et al 2010] [Gunasti and Ross 2010] [Gunasti and Ross 2010] [Clucas 2010] [Pope and Simonsohn 2010] [Pope and Simonsohn 2010] [Elders 2009] [Number Symbolism 2009] [A’Hearn et al 2009] [Hayes 2008] [Burnier and Gasser 2008] [Camarda et al 2008] [Johnson et al 2008] [Woo et al 2008] [Lucky numbers 2007] [Renedo and Sobrino 2007] [The Carrez Law a 2007] [Price and Seaman 2006] [Locker and Mason 2006] [Nietert et al 2006] [Lucky numbers 2006] [Li et al 2005] [Thirteen is a Lucky 2005] [Doucouliagos 2004] [Crawford 2002] [Beaman 2002] [Miller and Anderson 2002] [Wingfield et al 2002] [Lucky numbers 2001] [Jansen and Pollmann 2001] [Crockett et al 2001] [Crawford et al 2000] [Bieniaszewski et al 2000] [Spriet and Dupin-Spriet 1997] [Salsburg 1997] [Beaman et al 1997] [Edouard and Senthilselvan 1997] [andNa 1995] [Kaufmann 1995] [Ormerod 1994] [Tarrant and Manfredo 1993] [Duncan 1993] [Baker 1992] [Pickering 1992] [CHAPTER IV. Donne and 1991] [Budd and Guinnane 1991] [Ridout and Morgan 1991] [Thirteen is Not an 1991] [Heitjan and Rubin 1990] [Language Symbolism and Number 1989] [Halpern and Devereaux 1989] [Sigurd 1988] [Hessel 1986] [Russell 1983] [Nuessel 1982] [McCutcheon 1981] [Wells 1981] [Brooks-Davies 1980] [Williams 1979] [Wicks 1975] [Wicks and Stockwell 1975] [Stockwell and Wicks 1974] [Nagi et al 1973] [Brown 1972] [Heywood 1972] [Baker 1971] [Prescott 1969] [Owen 1968] [Stockwell 1966] [Chapter 7. Adjusting for 1963] [Batts 1963] [Zelnik 1961] [Ekvall 1959] [Clar 1958] [Baily 1952] [The Holy Number Thirteen 1947] [London 1947] [C. T. S. 1947] [I. Elementary Number Symbolism 1938] [Rhedecynian 1938] [Hopper 1938] [Mendizabal 1923] [Murray 1920] [Keith 1916] [Jordan 1911] [Winslow 1900] [Penny 1897] [Jones 1896] [Walcott 1855]
Inferring unseen counts from numbered items
This is among the smallest clusters and it is the most directly applicable. It covers the German tank problem, estimation from serial numbers, and the estimation of unseen classes, all of which assume that numbers are issued without choice. A chosen number violates that assumption, which is why the article’s ordinal section reaches this literature.
14 records. [Ruggles and Brodie 1947] [Good 1953] [Efron and Thisted 1976] [Miller et al 2024] [Simon 2023] [Anthony Lee and Steven Miller 2023] [Clark et al 2021] [Roy et al 2018] [Thomopoulos 2017] [Sharma and Sharma 2012] [Çalik and Güngör 2004] [Discrete Uniform Distribution 2003] [Goodman 1954] [Goodman 1952]
Anniversaries, centennials and bicentennials
The X-76’s occasion is an anniversary, and anniversaries have their own literature. It covers national centennials and bicentennials, the Centennial Exhibition of 1876, the American Revolution Bicentennial of 1976, and the uses states make of round-number years. The 76 in the X-76 is the last two digits of all three American years.
129 records. [Hobsbawm 2012] [Hattem 2024] [National Aeronautics and Space 1983] [Schneider 1977] [Emme 1977] [Lundin 1975] [Rymsza-Pawlowska 2026] [Gabilliet 2026] [Coleman 2026] [Enzerink and King-Savic 2026] [Reyes 2026] [Hart 2026] [Young and Cordova 2026] [xheritye 2026] [Johnson 2025] [Johnson 2025] [Spitzer 2025] [Colman 2024] [Drzewiecka 2024] [Hendershot and Marsh 2024] [Kennedy 2023] [Byrd 2023] [Kinslow 2023] [Kryńska 2023] [Appendix 2 Ludlow Centennial 2022] [Souvenir from the Centennial 2022] [Gordon 2021] [Kis 2021] [Tsoukalas 2021] [The Philadelphia Centennial The 2020] [Budrina 2020] [Inventing America at the 2020] [The Bicentennial Commemoration Imagining 2020] [An 2020] [National Symbols and Commemorations 2018] [“Fit for Citizenship” Scouting 2018] [A Continental Centennial Situating 2018] [New Nationalism in the 2018] [Children of a Common 2018] [Rouleau 2018] [King 2018] [Introduction. Celebrating Canada Commemorations 2018] [Brune 2018] [Aguirre 2017] [Conclusion Canada’s 1967 Centennial 2017] [Liu 2017] [Introduction Canada’s 1967 Centennial 2017] [Memorial Hall, Centennial Exhibition 2017] [National Identity and Its 2017] [Uy 2017] [Ulysses S. Grant’s Speech 2017] [Uslenghi 2016] [Noe 2016] [Mead 2015] [Kremer 2014] [Watson 2014] [Riccucci and Thompson 2013] [Visser 2013] [Guidotti-Hernández 2011] [Bause 2011] [Schleifer 2010] [Fergus 2010] [Woods 2009] [Shank 2009] [Judging the Progress of 2009] [Sygkelos 2009] [Gold 2008] [Franklin 2008] [Finley 2006] [Ihalainen 2005] [Pitman 2002] [Cole 1999] [Giberti 1999] [To remind ourselves that 1997] [Redfield 1992] [Bickford 1990] [Maniquis 1989] [Marshall 1987] [Arndt 1985] [Appendix B Resolutions in 1984] [Yoshikawa 1983] [United States Bicentennial exhibition 1978] [Morris 1977] [Mason 1976] [American History for Young 1976] [Belcher 1976] [Schroeder 1976] [Calkin 1976] [MENC Bicentennial Commission Selective 1976] [Lemisch 1976] [Baker 1976] [Stool et al 1975] [American Pharmacy and the 1975] [Llewellyn 1975] [Greening 1974] [America’s Bicentennial 1974] [Peter 1967] [Norton 1877] [American Centennial Celebration 1876] [C. 1876] [Carriage Wheels at the 1876] [Centennial Exhibition 1876] [Centennial Exhibition U 1876] [Centennial Exhibition. United States Department 1876] [Westcott 1876] [Ericsson 1876] [Curious Boats at the 1876] [Dimmock 1876] [Glass Making at the 1876] [C. 1876] [C. 1876] [Photographs at the Centennial 1876] [Progress of the Centennial 1876] [The American Society of 1876] [Morford 1876] [The Centennial Commissioners 1876] [The Centennial Exhibition 1876] [The Centennial Exhibition 1876] [The Centennial Exhibition-The State 1876] [The Centennial Exposition 1876] [The Centennial Exposition guide 1876] [The Centennial International Exhibition 1876] [The Exhibition of Fish 1876] [The Horticultural Hall at 1876] [Shinn et al 1876] [Trial of Steak Fire 1876] [Webster 1875] [The Proposed Centennial Exhibition 1871] [Morse]
Commemoration and collective memory
Commemoration is studied as something institutions do on purpose and for reasons. This cluster covers collective and public memory, the politics of memory, and national and military commemoration.
252 records. [Nora 1989] [Zerubavel 2014] [Bineham 2013] [Cook and Willis 2009] [Renstrom et al 2002] [Chapter 5 The Making 2026] [Scates 2026] [Commemorative Memorialization 2026] [Park and Roy 2026] [Bellisari 2026] [Füssel 2026] [An 2026] [Xu et al 2026] [Fourneir-Peneff 2026] [Elbourne 2025] [Obelisk The Worth Monument 2025] [Harrison 2025] [Tixell 2025] [Georgakis 2025] [Tyrała 2025] [Commemoration and National Identity 2025] [Marutyan 2025] [Salchinkina 2025] [Wien 2025] [Muzaini 2025] [National and Transnational Commemoration 2025] [Soyer and Tunca 2025] [Kenar 2025] [The Settler Politics of 2024] [Commemoration and Creating Identity 2024] [Bekliamishev 2024] [Brunssen 2024] [Maasing 2024] [Johansen and Bauer 2024] [Krzyżanowska 2024] [Al Nashmi 2024] [Gyollai 2024] [Walden and Makhortykh 2023] [Lincoln 2023] [Sonvilla-Weiss 2023] [Duthille 2023] [Fihurnyi 2023] [Ebury 2023] [Oktaviana and Junawaroh 2023] [Guthrie 2023] [Deputies of the State 2023] [Kraev 2023] [Rhyder 2023] [Ngoaketsi 2023] [Fridman and Gensburger 2023] [Palinhos and Vanhaesebrouck 2023] [Moliner and Bovina 2023] [Philips and Savelsberg 2023] [Sorokin 2023] [Martin 2023] [McNeilly 2023] [THE AESTHETICS OF RELATEDNESS 2022] [“We Died There, Too” 2022] [Lee 2022] [Langenbacher 2022] [Chapter 7 Memorializing Immortality 2022] [McAuliffe et al 2022] [Jaśniewicz-Downes 2022] [Commemoration of an Epoch 2022] [Discussion Nationalism, Friendship, and 2022] [Malone 2022] [Lipiński and Szabo 2022] [Politics in the Pantheon 2022] [Stibbe et al 2022] [Mačiulis 2022] [Jørgensen 2021] [Moulton 2021] [Fekete-Nagy 2021] [Silletti 2021] [Forster 2021] [Haji Ingiriis 2021] [Acosta and Braun 2021] [Fitzpatrick 2021] [Maurantonio 2021] [Jethro 2020] [Hill 2020] [Chupriy and Den Sik 2020] [Holocaust, Independence, and Remembrance 2020] [In Commemoration of the 2020] [In Commemoration of the 2020] [Riotto 2020] [Williams 2020] [Kabakchieva 2020] [Sharma 2020] [Izumi 2020] [Nayar 2020] [Klymenko 2020] [Women and War Commemoration 2020] [Molloy 2019] [Rozhdestvenskaya 2019] [Anagnostopoulos 2019] [Heath-Kelly 2019] [Baggiarini 2019] [Korhonen 2019] [Mountford 2019] [Bergman 2019] [Simov 2019] [Árvay and Foote 2019] [Conclusion The Importance of 2018] [Chielens 2018] [Competing Pasts, Multiple Identities 2018] [Kaplan 2018] [Valentina Rozas-Krause 2018] [Shub 2018] [Kenaga 2018] [Schaff 2018] [Hakim and Adams 2018] [Brown and Arriaza Ibarra 2018] [Barrett 2018] [Kennell et al 2018] [Brown 2018] [Moazzen 2018] [Arnason 2018] [Mangena 2018] [Mamvura et al 2018] [Peleggi 2017] [Chapter 6 Monuments, Commemoration 2017] [Collective Memory 2017] [Beiner 2017] [Courtly Traditions and National 2017] [Qasmi 2017] [Rendle and Lively 2017] [Museums, Monuments, and Memory 2017] [Lapp 2017] [Statement by the Prime 2017] [The Commemoration Industry and 2017] [Rymsza-Pawlowska 2017] [Jisun Yee 2017] [Duncan 2017] [Merrill 2017] [Sériot 2017] [Zhang 2016] [Stubbs 2016] [Celebrating the Medieval Past 2016] [McHugh 2016] [Lee and Chan 2016] [Chan-Seung Park 2016] [Gough 2016] [D’ascenzo 2016] [Mascall-Dare 2016] [Chaniotis 2016] [Ramsey 2016] [Smith 2016] [Moazzen 2016] [Troublesome Anniversary The Rise 2016] [West 2016] [West 2016] [Reeves et al 2015] [Logan 2015] [Lorenzo-Dus 2015] [The Politics and Rhetoric 2015] [‘If They Were Rebels 2014] [Tomsky 2014] [Amarilyo 2014] [Schudson 2014] [Üngör 2014] [Coombes 2014] [Coleman 2014] [Vlossak 2014] [Revolution and Commemoration －the 2014] [Wolffe 2014] [Vassallo 2014] [A British Dimension to 2013] [Casquete 2013] [Stephens 2013] [Dwyer et al 2013] [Commemorative and ‘historical event’ 2013] [Commemorative landscapes the politics 2013] [Involuntary commemorations post-traumatic stress 2013] [Monuments, commemorative space and 2013] [National narratives, war commemoration 2013] [Alderman and Inwood 2013] [Prescott 2013] [Marschall 2013] [The politics of war 2013] [War commemoration in Western 2013] [Jeff Fortney 2012] [Carr 2012] [Azaryahu 2012] [Oppelland 2012] [Dupuy 2012] [Chaniotis 2012] [Historical Linkage and Political 2011] [Cash 2011] [Commemoration, monument and identity 2011] [Azaryahu 2011] [From collective memory to 2010] [Conway 2010] [The Two World Wars 2009] [Collective Memory 2008] [Schwartz 2008] [Hyder 2008] [Jones 2008] [Palonen 2008] [Tileagă 2008] [Collective memory 2007] [History, Commemoration and National 2007] [Jui-te 2007] [White 2007] [Seraphim 2006] [Beattie 2006] [Social Memory and Commemoration 2005] [Ben-Amos and Bet-El 2004] [Blair 2004] [Alderman and Dwyer 2004] [Social memory, commemoration and 2004] [Freestone and Veale 2004] [Vandkilde 2003] [Brog 2003] [“Blood-Bought Fame” National Identity 2002] [Lloyd 2002] [Dolan 2002] [Peifer 2001] [International Space Forum in 2001] [Meigs 2001] [Finch 2000] [Commemorative naming in the 1999] [Commemorative Naming in the 1998] [Weiss 1997] [Schudson 1997] [Spillman 1997] [Berg 1995] [Bodnar 1994] [Piehler 1994] [Schama and Hoffmann 1990] [French Revolution Commemoration 1789-1989 1988] [Stump 1988] [Thomas 1987] [Schwartz 1982] [Adam 1980] [150th Anniversary Commemoration April 1975] [In commemoration of the 1973] [Commemoration of Copernicus in 1943] [Commemoration of the 450th 1943] [Wood] [Chapter 13 Commemoration and] [Chapter Four National commemoration] [Contested Sites of Memory] [Murray] [Hare] [Todd] [Bolinger] [National Commemoration after the] [Bodnar] [Browness] [The Entangled Politics of] [Wars Afterwards The Repression]
Commemorative naming and critical toponymy
This is the literature closest to the founding-year class. It studies the naming and renaming of places, streets, buildings and institutions as political and commemorative acts, and naming as a political act more widely, including the naming of parties, groups, chemical elements and people.
186 records. [Azaryahu 1996] [Azaryahu 1997] [Rose-Redwood et al 2009] [Rose-Redwood et al 2008] [Alderman 2003] [Kim 2026] [Strenga 2026] [Peteshova 2026] [Shorer and Mahat-Shamir 2026] [Firdaus et al 2026] [Gnatiuk and Basik 2026] [Theocharous et al 2026] [Kárníková 2026] [Nkama 2026] [Peteshova 2026] [Ndlovu and Erasmus 2026] [Tian 2026] [Turai 2026] [Tahereen 2026] [Bouviet 2026] [Aso et al 2026] [Kim 2025] [Zymovets 2025] [Ramutsindela 2025] [Chidora and Tivenga 2025] [Caballero-Cordero et al 2025] [Kim 2025] [Dickinson 2025] [Jordan 2025] [Burdin et al 2025] [Petković 2025] [Palonen 2025] [West 2025] [Rodionov 2025] [Madlome 2025] [Evstifeev and Evstifeeva 2025] [Felecan 2024] [Domby 2024] [The Colonial Politics of 2024] [Nkansah and Bonsu 2024] [Lytvynovska and Olitskyi 2024] [Fabiszak et al 2024] [Carter 2024] [Renaming Cities in the 2024] [Theocharous 2024] [Theocharous 2024] [Ramón 2024] [Theocharous 2024] [Theocharous 2024] [Infante Batiste 2024] [Hanafi 2024] [Family Politics Revealed Through 2023] [Doss 2023] [Girma 2023] [Choo 2023] [Dalgıç and Yildirim Okta 2023] [Jethro and Merrill 2023] [Annabell 2023] [Paul 2023] [Scates and Yu 2023] [Soloviova and Koloiz 2023] [Munnik 2023] [Nick 2023] [Chandrashekar 2023] [Tkachenko 2023] [Bulgarova 2023] [Schenk 2023] [Schembri and Gauci 2023] [Yankson 2023] [Wijeyesinghe 2023] [Perera 2023] [Mamvura and Marowa 2023] [Brunila et al 2023] [Szuba 2023] [Manning et al 2022] [Alderman 2022] [Shivan 2022] [Basik 2022] [Masters 2022] [Masters 2022] [Giraut and Houssay-Holzschuch 2022] [Bitušíková 2022] [Ben Arrous and Bigon 2022] [Sysiö et al 2021] [Conclusions The Vedii A 2021] [Nick 2021] [Larionova/Ларионова and Tsareva/Царёва 2021] [Paschalidou 2021] [Mann 2021] [Seits 2021] [Vishal Surbun 2021] [The Politics of Naming 2021] [Düzgün 2021] [Rose-Redwood 2021] [Viala-Gaudefroy and Lindaman 2020] [Vivant 2020] [Rose-Redwood and Kim 2020] [Ćwiek-Rogalska 2020] [Mamvura 2020] [Mamvura 2020] [Havryliuk 2019] [Webster 2019] [Balogun and Fasanu 2019] [Brocket 2019] [Kapur 2019] [Stolberg 2019] [Obasi et al 2019] [Riazi 2019] [Gjesdal 2019] [Kachalkova and Ruth 2019] [Commemorative Groups and Commemorative 2018] [Çetin 2018] [Atencio 2018] [Dube 2018] [Bass and Houghton 2018] [Gnatiuk 2018] [McAuley 2018] [Bartolini 2017] [Choi 2017] [Wideman and Masuda 2017] [Smith 2017] [Yeoh 2017] [Vuolteenaho 2017] [Dietze-Schirdewahn 2017] [Hui 2017] [Golomidova 2017] [Palonen 2017] [Różycki 2017] [Alderman and Inwood 2017] [Duminy 2017] [Light and Young 2017] [Ndlovu 2017] [Kank 2016] [Bigon 2016] [Power, politics, and naming 2016] [Bigon 2016] [Bigon 2016] [Lucas 2015] [Creţan and Matthews 2015] [Hanna and Hodder 2015] [Myong and Chun 2015] [Mitchell and Alderman 2014] [Anger and the Politics 2014] [Machar 2014] [Orel 2014] [Naming China-political art as 2014] [Arrington 2014] [Jelaska Marijan 2014] [The Politics of Naming 2014] [Warman 2013] [Boonprong 2013] [Gabor 2013] [Waller 2013] [Oliver 2012] [Azaryahu 2012] [Rom and Benjamin 2011] [Blankenship 2011] [Fearn 2010] [Hart 2009] [Kirk 2007] [Howard-Hassmann 2007] [Alia 2007] [Tarpley 2006] [Miller and Goldblum 2006] [The Politics of Naming 2006] [Aboriginal place names charting 2004] [CHAPTER 3 Naming, Identity 2004] [Castillo 2003] [Wilks and Brick 2001] [Galasiński 2001] [Alderman 2000] [Alderman 2000] [Kelly 1999] [Herbert 1997] [Andrews 1996] [Nash 1993] [O’Brien 1993] [Azaryahu 1992] [Diament 1991] [Krumpelmann 1951] [Jones 1914] [Silverman] [Chapter 5. The Politics] [Radovic] [Yang] [Laqueur]
Military heritage, tradition and naming
The armed services name ships, weapons and mascots and keep traditions and a heritage of their own, and that practice has a literature. Its titles concern the naming of warships, tanks and mascots, army tradition and honour, and military heritage, and none of them concerns design numbers.
9 records. [Department Of The Navy Washington Dc 2012] [Ihrke 1992] [Rijnoveanu 2026] [Starchenko 2023] [Honour and Dishonour in 2022] [Mamet 2020] [Koopman 2017] [Malcomson 2009] [Keeping in Step The]
Symbolic politics and the politics of naming
This is a small cluster on naming as a political act and on symbolic action by governments.
7 records. [Assanbayeva 2024] [Drescher et al 2020] [Happaerts 2012] [The Political Symbolism of 2011] [Battaglio and Legge 2009] [Fox 1996] [Richmond 1978]
Discretion within rules
The instruction pairs a rule with an unconditioned discretion to depart from it, and public administration has studied that pairing for decades. This cluster covers administrative and bureaucratic discretion, street-level bureaucracy, and the political control of discretion.
199 records. [Kydland and Prescott 1977] [Nalubolu 2026] [Street-Level Bureaucracy 2026] [Administrative Discretion 2026] [Luparev 2026] [Salah and Alnoor 2026] [Hossain 2026] [Anastasopoulos 2026] [Hashimoto 2026] [Purohit 2026] [Nasir 2026] [PhD 2026] [Lagrotta 2026] [Freeman 2026] [Julianis et al 2026] [Middleton and Wigginton 2026] [Verma 2026] [Wang et al 2026] [Poelen and Van Oers 2026] [Lutsukh 2026] [Lotta 2026] [Webeck and Banerjee 2025] [Zyuzin 2025] [Gong et al 2025] [Renan Barzilay et al 2025] [Kühn et al 2025] [Sanabria-Pulido and Langbein 2025] [Gegenava 2025] [Ahuja 2025] [Shah 2025] [Jermann and Xiang 2025] [Ferrey 2025] [Busch and Henriksen 2024] [Shah 2024] [Administrative discretion abuse of 2024] [Munir 2024] [Rowe 2024] [Leijon and Moberg 2024] [Chaudhry 2024] [Jacquin 2024] [Rowe 2024] [Street-Level Bureaucracy in Weak 2024] [Akella 2024] [Lotta et al 2024] [Meza et al 2024] [Rowe 2024] [Rowe 2024] [Vredenburgh 2023] [Duvanova 2023] [Duvanova 2023] [Ryan 2023] [Kessy 2023] [Ryan 2023] [Durose and Lowndes 2023] [Shveda 2023] [Slee 2023] [O’Brien 2023] [Heymann et al 2023] [Dippel et al 2022] [Young and Tanner 2022] [Lin and Chen 2022] [Hupe 2022] [Bureaucratic Discretion in the 2021] [Bylik and Osadcha 2021] [Dvorak et al 2021] [Iskandar and Alwi 2021] [Boland and Godsell 2021] [Hill and Varone 2021] [Administrative Discretion 2020] [Ginther and Heggeness 2020] [Bureaucratic Discretion Political Control 2020] [Spanou 2020] [Khair and Shahan 2020] [Cordelli 2020] [Hong 2020] [Rice 2019] [Hupe 2019] [Kuhlmann 2019] [Klenk and Cohen 2019] [Raaphorst and Groeneveld 2019] [Szot 2019] [Judicial Review of Administrative 2019] [Hupe 2019] [Gofen et al 2019] [Methodological issues in street-level 2019] [Schott and van Kleef 2019] [Loyens 2019] [Hupe and Hill 2019] [Thomann 2019] [Kekez 2019] [van Engen 2019] [Estrada 2019] [Van Parys 2019] [Zacka 2019] [Street-level bureaucracy as a 2019] [Byers 2019] [Lieberherr and Thomann 2019] [Hupe and Keiser 2019] [Harrits 2019] [Lotta and Pires 2019] [Stensöta 2019] [Høybye-Mortensen 2019] [Møller 2019] [Koven 2019] [Nenko 2019] [Hupe 2019] [Theoretical issues in street-level 2019] [Harrits 2019] [Corduneanu-Huci 2018] [Langbein 2018] [Thompson et al 2018] [Kjaerulff 2018] [Choi 2018] [MacKenzie 2018] [Szot 2018] [Song 2017] [McHarg 2017] [Mendes 2017] [Chapter 1 Street-Level Discretion 2017] [Ehrhardt 2017] [Montagnes and Wolton 2017] [Park 2017] [Bureaucratic Discretion and Hierarchical 2016] [Social Services-Street-Level Bureaucracies? 2016] [The Street-Level Bureaucracy Perspective 2016] [Presidential Portrait Prosecutorial Discretion 2015] [Williams 2015] [Velázquez-López Velarde 2015] [Hupe et al 2015] [Hupe et al 2015] [Evans 2015] [Schuppan 2015] [Harrison 2015] [Wilkins and Wenger 2015] [Winter and May 2015] [Baker Collins 2015] [Osiander and Steinke 2015] [Understanding Street-Level Bureaucracy 2015] [Cai and Whalley 2014] [Antonelli 2014] [Balla 2014] [Kondo 2014] [Administrative Discretion in Action 2013] [Flanigan 2013] [Parinandi 2013] [Hupe 2013] [Liu et al 2013] [Scourfield 2013] [Tummers and Bekkers 2013] [Rabin 2013] [Mahaney-Walter 2012] [Beazer 2012] [Brown 2011] [Huang 2010] [Workman et al 2010] [Evans 2010] [Khare 2009] [Kochtcheeva 2009] [Hunold and Peter 2008] [Alexander and Richmond 2007] [MacDonald and Franko 2007] [Furlong 2007] [Heinen and Scribner 2007] [Erler 2007] [Daintith 2005] [Schneider and Baltz 2005] [Evans 2004] [Sossin 2004] [Levin 2003] [Gailmard 2002] [Grady and Simon 2002] [Majumdar and Marcus 2001] [Administrative Discretion, the Rule 1999] [Seidenfeld 1998] [Loveland 1995] [LaFave 1990] [Shull 1989] [O’Neill 1988] [The Dilemma of Administrative 1985] [Rulemaking and Bureaucratic Discretion 1985] [Thomas 1984] [Thompson 1982] [Lungu 1982] [Shnit 1979] [Administrative Discretion in Zoning 1969] [Government Liability, Administrative Discretion 1959] [Garner 1949] [Leys 1943] [Laski 1923] [Administrative Law. Review of 1921] [The educational selectivity effects] [Lai] [Mak] [Langbein] [Chapter VI. the Exercise] [Chapter VII. the Exercise] [McManus] [The Rule of Policy] [Ho]
Where the Framing Breaks Down
The series frames each article around an aircraft, and this article has none. The sections on sizing, dependent systems and the flight test record therefore say little, and what carries the article is an argument about a register, an instruction and a handful of statements.
The article’s models break down in places it names. The Poisson model of the allocation rate is a convenience that does not claim allocations are memoryless, and the article answers that limit by stating how much clustering would rescue the third reading, without assuming that there is none. The uniform null for the founding-year cluster gives a scale and cannot serve as a test, because the window was drawn after the cluster was seen. The ordinal bound for the X-76A is an upper bound only, since the X-23 is on the compiler’s list of numbers never allocated [Missing USAF and DOD Aircraft Designations] while this series treated that aircraft at full length. The classification of out-of-sequence reasons was made by this article from the compiler’s notes, and the compiler did not make it.
Two limits concern the readings themselves. The reserve reading is unsupported rather than refuted, because an unrecorded reservation leaves no record. And the evidence that decides between the readings postdates the article’s date of 14 December 2025, so the answer given here was not available on that date. Epistemic State lists each statement that postdates it.
The Source Base
This article rests on a narrow base of primary documents and it says how narrow. Everything about the allocations comes from one compiler’s reconstruction, Andreas Parsch’s, obtained from the Department under the Freedom of Information Act, read at its current version and at four archived versions. Everything about the rule comes from one published instruction and two earlier editions, the 2005 edition as the X-67 article read it and the 1994 edition read directly here. Everything about motive comes from four press releases, one quoted statement, one set of released emails, and, for the X-50A precedent, one NASA monograph quoting the contractor’s programme manager.
What was read directly for this article, rather than inherited, is the current register and its archived copies of 19 August 2025, 10 December 2025, 15 January 2026, 1 February 2026 and 12 February 2026, the missing-designations page at its current version and as archived on 12 November 2025, 12 February 2026, 20 February 2026 and 17 May 2026, DARPA’s announcement, the contractor’s release, the Army’s release, the Air Force Reserve Command’s announcement of the F-47, and the released public affairs emails in full. A further ten documents were read directly as well. They are the 1994 instruction’s text, DARPA’s announcement of the XRQ-73, the Great Horned Owl programme page, the semiquincentennial statute, the National Archives transcript of the Declaration, the National Security Act of 1947, the Army’s history of 14 June 1775, the Air Force Reserve Command’s announcement for the President’s words, NASA’s inventory of the X-vehicles read at its X-39 to X-50 entries, and Boeing’s release on the X-50A. The registry records of thirteen research papers were read directly too, and three of them carry an abstract that the prose follows. The instruction’s clauses are quoted from the copy the X-67 article read, and the quotations match that text.
The research survey holds 1,232 research records drawn from Crossref’s registry of scholarly works, NASA’s Technical Reports Server and the Defense Technical Information Center, and thirteen of them are research primaries cited in the argument above. Of them, 22 are report primaries, being 1.8 percent, by the identifier rule set out under The Definition of Primary, 16 from NASA’s Technical Reports Server and 6 from the Defense Technical Information Center. The median publication year is 2017, and 701 of the 1,169 dated records, being 60.0 percent, date from 2015 or later.
The report-server share is small, as expected for a subject that is not engineering. Those servers hold the X-plane programme literature and NASA’s own inventory of the X-vehicles, while the fields this question sits in publish mostly elsewhere. Two cited documents from the Defense Technical Information Center are not retrievable, an edition of the designating instruction catalogued in 1997 and the Navy’s 2012 report on naming vessels, because the registry’s document server returns a refusal page. Both are cited as research works and neither informs a claim.
The Definition of Primary
Primacy is a property of the document, and this survey derives it from the identifier. A record is a report primary when its address is on the NASA Technical Reports Server or it carries a digital object identifier under the 10.21236 prefix, which Crossref registers for Defense Technical Information Center reports. Most of this article’s primary evidence is not in the survey at all, being the register, the instruction, the releases and the emails listed above, which are cited as references rather than as research records. The rule does not reach the thirteen research primaries either, which are journal papers and book chapters such as Ruggles and Brodie on wartime serial numbers and Azaryahu on street names, so 1.8 percent is a lower bound on how much of the survey is first-hand.
Thin Subjects
Five subjects that this article’s argument depends 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.
| Subject | Title terms counted | Records |
|---|---|---|
| a meaningful number in a designation or numbering system | designation, designations, design number, design numbers, numbering, numbering plan, serial number, serial numbers, version number, version numbers, identifier, identifiers together with commemoration, commemorative, commemorate, commemorating, anniversary, centennial, bicentennial, semiquincentennial, founding year | 0 |
| a passed, reserved or reused number | the numbering terms of the row above together with skip, skipped, skipping, reserved, reservation, reuse, reused, retired, retirement, reclaim, reclaimed, reassignment, recycling, recycled | 1 |
| estimation from serial numbers | German tank, serial number, serial numbers | 7 |
| a founding year as a chosen number | founding year, founding years, year of founding, foundation year | 0 |
| rules set against discretion | rules versus discretion, rule versus discretion, rules rather than discretion, rules and discretion, rules vs discretion | 6 |
No title in the survey joins a designation or numbering term to a commemoration, so no study of commemorative design numbers in any designation system was found. The nearest work concerns names rather than numbers. It includes the Navy’s report to Congress on its policies and practices for naming vessels, in the heritage cluster [Department Of The Navy Washington Dc 2012], and a study of commemoration practices in special operations organisations, in the commemoration cluster [Bineham 2013]. The nearest by title is a study called Numerical Commemoration, about collective remembrance in Israel [Zerubavel 2014]. Its title names the phenomenon studied here, and it has not been read for this survey, so it is cited for no more than its title says.
The passed number has no literature. The one title that joins a numbering term to one of these words concerns a designation system for recycled carbon fibres, where what is recycled is the material. Whether a passed number stays available, which is the question the section headed The Next Research Number Is a Test leaves open, is answered in the record only by the X-49 precedent.
Serial-number estimation is a small and settled literature. All 7 titles concern estimation from serial numbers. The estimator they share treats numbers as issued in sequence without choice, which is the assumption a chosen number breaks.
No title names a founding year. The founding-year class therefore rests on the three sponsors’ own statements and on the register, and not on any study of the practice.
The survey holds the opposition of rules and discretion thinly, and none of it concerns allocation. Six titles set rules against discretion, one of them Kydland and Prescott’s, in economic planning, capital regulation, teacher hiring, flexible regulation and family law, and none of them concerns the allocation of a number or a name. The 199 records of the discretion cluster study discretion as administrators exercise it, which is the other half of the instruction’s pairing of a rule with a discretion to depart from it.
What the Survey Admits
The research survey admits a record only when a person reading its title finds it on this article’s subject, which is the set of fields named at the head of the section headed The Contemporary Literature. Many words these fields depend on also name something else, and a record that shares only such a word is excluded. Licence plates are chosen numbers when they are auctioned and something else in plate recognition and traffic policy. A symbolic number in numerical cognition is a numeral rather than a symbol, the German tank problem shares a word with real tanks, round numbers are also integers in school mathematics and smooth numbers in number theory, lucky numbers are also a sieve, a graph labelling and a protein, and a version number is also a field in a network protocol. Registration also names the alignment of images in guided surgery, an identifier is also a radiation instrument, and a namespace is also a feature of operating-system containers. Tributes to individual scientists and notices of the anniversaries of journals, societies, laboratories and academies are excluded as well, since they use a centennial only as a label. Correction notices, peer-review reports, meeting and festival notices, editorials introducing an issue or a collection, obituaries, reviews of books, exhibitions and recordings, dictionary, glossary and definition entries, encyclopedia entries of two pages or fewer, bibliography entries, and the figure and object records registered for parts of works are excluded too, because a survey counts research works and those are parts of works, reference aids or editorial events rather than works, and so is a second deposit of a document already counted, such as a preprint beside its journal version or one chapter on two publishers’ platforms. Several neighbouring literatures share this subject’s words and not its question, and they are excluded as well. Configuration management in general is excluded while the configuration identification of items is kept, numerical taxonomy applied to one group of organisms or products is excluded while its method is kept, and the psychology of military identity is excluded while military naming and tradition are kept. Naming in the sense of defining a phenomenon, such as naming violence or naming corruption, is excluded while the naming of places, people, parties and things is kept, and a review of a field or a history that an anniversary merely dates is excluded while the observance of the anniversary is kept.
Some records of doubtful relevance are kept, 122 of the 1,232 in all. They include personal naming traditions, kept as naming as a political act, exhibits and buildings of the 1876 exhibition, kept as records of that observance, the bicentennial programmes of professions and libraries, kept as observances of the national year, textbook chapters on the discrete uniform distribution, kept as the model the serial-number estimator assumes, and a report on a festival of persistent identifiers and the registration of plant cultivar names, kept as the practice of identifier registries. Titles reading only Lucky numbers or Number Thirteen, whose subject the title does not show, are among them, and a reader may weigh each accordingly. Auctions of licence plates are kept as well, as prices paid for chosen numbers.
Every research title has been read for relevance, so the off-topic share that remains is a matter of reading judgement, and no rate is claimed for it. A reading of titles cannot show that a work is relevant in substance, so this is a statement about titles only.
Epistemic State
The Article’s Own Vantage Point
The article is dated 14 December 2025 and written on 2 October 2026. The allocations it is about happened inside its dateline. Their publication and their explanation did not. The register first showed both rows between 15 January and 1 February 2026, and DARPA’s statement of the reason is dated 9 March 2026. The article writes from current knowledge and lists what postdates its date.
Historical Fact, From Primary Documents
- The X-68A was allocated on 20 August 2025 and the X-76A on 20 October 2025, both to DARPA, and no research row carries any number from 69 to 75.
- The XRQ-72A, XRQ-73A and YMV-75A carry 72, 73 and 75 in other basic missions.
- The 2020 instruction refuses requests in reverse or skipped sequences and reserves to the allocating office the authority to skip at its discretion.
- DARPA stated that the X-76 designation is a deliberate nod to the revolutionary spirit of 1776.
- The Army stated that the 75 in MV-75 commemorates its founding year, 1775.
- The Air Force stated that the 47 has three meanings, and its public affairs office stated that General Allvin made the decision.
- The X-50A’s number was asked for because of its meaning, by DARPA in the compiler’s and NASA’s accounts and by Boeing in its programme manager’s, and the X-49 it passed was issued later, as NASA’s 2003 monograph predicted.
- The Semiquincentennial Commission Act of 2016 fixes 4 July 2026 as the 250th anniversary of the founding, as marked by the Declaration of Independence.
- DARPA’s announcement calls the XRQ-73 an X-plane.
Measured From the Documents Rather Than Asserted by Them
- The pointer walk of 22 steps and its single advance of eight.
- The dates on which the X-68A and X-76A rows first appear in archived copies of the register.
- The insertion and removal of the compiler’s sentence about the X-77, dated by archive capture rather than by the page’s stamp.
- The 24 allocation events since November 2020 and the six that follow the rule as written, or seven if the RQ-170 row is set aside.
- The classification of the reasons given for 24 designations that are out of sequence or that caused a skip, 25 assignments in all, which is this article’s reading of the compiler’s notes.
Engineering Analysis, Derived Here and Checkable
- The allocation rate, the Poisson tail of $1.83 \times 10^{-10}$, and the equivalence of seven numbers to 8.36 years of research allocation.
- The ordinal bound of 64 for the X-76A.
- The count of 23 two-digit numbers left above the research pointer.
Inference, Stated as Such
- That the compiler removed his X-77 sentence because of DARPA’s statement. The timing fits and the page does not say so.
- That the YMV-75A and X-76A respond to the 250th anniversaries of 1775 and 1776. Both were allocated in the run-up to them and the record states no common policy. The F-47A’s stated meanings include a president’s number as well as a founding year, so it joins the class by its year and not by an anniversary.
- That the X-76 was chosen as the F-47 was. Nothing in the record says who chose it.
Statements in This Article That Postdate Its Own Date
- The register’s publication of the X-68A and X-76A rows, between 15 January and 1 February 2026.
- The compiler’s missing-designations versions stamped 3 January 2026, with and without the X-77 sentence.
- DARPA’s and the contractor’s releases of 9 March 2026.
- The compiler’s revised note on the X-76A quoting DARPA.
- The observation that no X-69 or X-77 is public as of 2 October 2026.
- The register’s copy last updated 23 August 2026, from which every row count is taken, with the counts of the version current on this article’s date stated beside them where they differ.
What the Record Does Not Settle
- Who chose the number 76. The instruction gives the skip to the allocating office and the statement gives the motive to nobody in particular. The X-50’s number was never settled on this point either, its three accounts naming two different requesters.
- Whether 69 through 75 can still be requested. The instruction’s text says no and the X-49 precedent says yes.
- Whether the block was ever reserved. No record of a reservation exists, and an unrecorded reservation would leave none.
- Whether the founding-year class was a policy or a coincidence of three sponsors. The record shows three statements and no directive.
Out of Scope
- The X-76 as an aeroplane, which is the subject of the next article in this series.
- The F-47 and MV-75 as aircraft, which belong to no research series.
- The unmanned series’ departures from sequence, which the X-58 and X-67 articles measured.
Conclusion
The leapfrogged block is seven skips made at once so that one number could be chosen. The register shows a single allocation passing seven numbers, the series’ rate gives seven unseen allocations in the gap a probability of about two in ten billion, the instruction provides no way to reserve a block, and the sponsor later said the number was a deliberate nod to 1776.
What the anomaly reveals is that design numbers have started to carry messages. Three of them in eleven months were chosen at least in part to match founding years. The rule the instruction states describes less than a third of the allocations made under it. And the rule’s refusal of skipped sequences, if it is applied as written, makes every chosen number permanent and every number it passes lost. Whether it is applied as written will be known when the next research designation is issued, as either the X-69A or the X-77A.
References
Reference
- Air Force Instruction 16-401(I), Designating and Naming Defense Military Aerospace Vehicles, 14 April 2005
- Air Force Joint Instruction 16-401, Designating and Naming Defense Military Aerospace Vehicles, 9 September 1994
- Air Force public affairs correspondence on the F-47, March 2025, released under the Freedom of Information Act
- Air Force Reserve Command, Air Force Awards Contract for Next Generation Air Dominance Platform, F-47, 21 March 2025
- Air Force, General David W. Allvin, biography, as archived on 27 November 2024
- Army, 14 June 1775, 250th Anniversary of the Infantry and the U.S. Army, 26 March 2025
- Bell, Bell Completes Critical Design Review on DARPA SPRINT X-Plane Program, Receives Official X-Plane Designation, 9 March 2026
- Boeing, The Revolutionary Boeing Canard Rotor Wing Aircraft Begins Flight Testing, 4 December 2003
- Centennial Exposition
- DARPA, Announcing DARPA’s Newest X-Plane, X-76, 9 March 2026
- DARPA, Meet DARPA’s Newest X-plane, XRQ-73, 24 June 2024
- Declaration of Independence, National Archives transcript
- Department of the Air Force Instruction 16-401, Designating and Naming Defense Military Aerospace Vehicles, 3 November 2020
- Parsch, DOD 4120.15-L Addendum
- Parsch, DOD 4120.15-L Addendum, as archived on 1 February 2026
- Parsch, DOD 4120.15-L Addendum, as archived on 10 December 2025
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- Parsch, DOD 4120.15-L Addendum, as archived on 19 August 2025
- EASA, Type-Certificate Data Sheet IM.E.010, General Electric CT7-series engines, Issue 10, 19 December 2025
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- Missing USAF and DOD Aircraft Designations, as archived on 12 February 2026
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- Missing USAF and DOD Aircraft Designations, as archived on 20 February 2026
- National Security Act of 1947, Public Law 80-253, 61 Stat. 495
- Program Executive Office Aviation, Army designates MV-75 as mission design series for Future Long Range Assault Aircraft, 27 May 2025
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- X-Planes: Northrop X-4 Bantam
- X-Planes: Bell X-5
- X-Planes: Convair X-6
- X-Planes: Lockheed X-7
- X-Planes: Aerojet X-8 Aerobee
- X-Planes: Bell X-9 Shrike
- X-Planes: North American X-10
- X-Planes: Convair X-11
- X-Planes: Convair X-12
- X-Planes: Ryan X-13 Vertijet
- X-Planes: Bell X-14
- X-Planes: North American X-15
- X-Planes: Bell X-16
- X-Planes: Lockheed X-17
- X-Planes: Hiller X-18
- X-Planes: Curtiss-Wright X-19
- X-Planes: Boeing X-20 Dyna-Soar
- X-Planes: Northrop X-21
- X-Planes: Bell X-22
- X-Planes: Martin Marietta X-23 PRIME and a Contested Assignment
- X-Planes: Martin Marietta X-24
- X-Planes: Bensen X-25
- X-Planes: Schweizer X-26 Frigate
- X-Planes: Lockheed X-27
- X-Planes: Osprey X-28 Sea Skimmer
- X-Planes: Grumman X-29
- X-Planes: Rockwell X-30 and the National Aero-Space Plane
- X-Planes: Rockwell-MBB X-31
- X-Planes: Boeing X-32
- X-Planes: Lockheed Martin X-33
- X-Planes: Orbital Sciences X-34
- X-Planes: Lockheed Martin X-35
- X-Planes: McDonnell Douglas X-36
- X-Planes: Boeing X-37
- X-Planes: Scaled Composites X-38
- X-Planes: X-39, Reserved but Never Assigned
- X-Planes: Boeing X-40
- X-Planes: X-41 Common Aero Vehicle
- X-Planes: Orbital Sciences X-42
- X-Planes: Micro-Craft X-43 Hyper-X
- X-Planes: X-44, One Designation and Two Aircraft
- X-Planes: Boeing X-45
- X-Planes: Boeing X-46
- X-Planes: Northrop Grumman X-47
- X-Planes: Boeing X-48
- X-Planes: Piasecki X-49 SpeedHawk
- X-Planes: Boeing X-50 Dragonfly
- X-Planes: Boeing X-51 Waverider
- X-Planes: X-52, the Designation Refused
- X-Planes: Boeing X-53 Active Aeroelastic Wing
- X-Planes: Gulfstream X-54
- X-Planes: Lockheed Martin X-55 ACCA
- X-Planes: Lockheed Martin X-56
- X-Planes: ESAero X-57 Maxwell
- X-Planes: X-58, the Slot Taken by XQ-58
- X-Planes: Lockheed Martin X-59 Quesst
- X-Planes: Generation Orbit X-60
- X-Planes: Dynetics X-61 Gremlins
- X-Planes: Lockheed Martin X-62 VISTA
- X-Planes: ABL Space Systems X-63
- X-Planes: Invocon X-64
- X-Planes: Aurora Flight Sciences X-65 CRANE
- X-Planes: Boeing X-66
- X-Planes: X-67, the Slot Taken by XQ-67A
- X-Planes: General Atomics X-68 LongShot
Referenced by
- X-Planes: Synthesis and What the Designation Became 2025-12-16
- X-Planes: Bell Textron X-76 SPRINT 2025-12-15