X-Planes: X-39, Reserved but Never Assigned

Every other article in this series is about an aircraft. This one is about a number that never became one. The X-39 designation was reserved on 23 April 1997 for the Future Aircraft Technology Enhancements programme of the Air Force Research Laboratory, and it was never assigned to anything, because the written request that would have assigned it was never submitted. This is the fortieth 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, and the X-38.

A gap in a numbered series invites a particular kind of reading. The X-designations are a public register of experimental aircraft, so a missing number looks like withheld information. Missing numbers in the military series have drawn that speculation, as the F-19 and the numbers F-112 to F-116 did, and at least some of the latter were assigned to secret aircraft [Missing USAF and DOD Aircraft Designations]. The X-39 does not reward that reading. Its gap was produced by two documents that were never written. The more consequential of the two is not the request that would have created the designation but the cancellation that would have released the number for somebody else.

This article’s finding concerns the register, and aviation is incidental to it. The numbering is not a record of aircraft. It is a record of correspondence about aircraft, and the two come apart in a way that the X-39 makes unusually legible.

This article carries every section of the series order, although there is no vehicle here to dimension and no keystone question to derive. Padding a short article would misrepresent the record as richer than it is, so the sections on sizing, dependent systems and flight say that they are empty and why, and stop there. The absence of a vehicle does not excuse a survey, and this article surveys the literature that bears on its subject in place of the literature that surrounds its series. The designations come from the tri-service system adopted in 1962, which remains the framework for every number in this series [United States Department of Defense Aerospace Vehicle Designation].

The Research Question

The X-39 had no research question of its own, because there was never a vehicle to carry one. The number was held for a demonstrator that the Future Aircraft Technology Enhancements programme expected to need, and that demonstrator was never designed. What the record supports is a question the programme asked before any vehicle existed, which was whether its candidate technologies needed flight validation on a new airframe at all [Cupstid 1997].

This article can settle a different question, one that concerns the register. It asks what a gap in a numbered series of experimental aircraft means when no aircraft, no refusal and no secret stands behind it. The sections that follow answer that question from the governing instructions and the designation surveys, and they leave the aviation question where the programme left it, unanswered by flight.

Programme Origin

The reservation is dated precisely and the source is explicit about its status. Andreas Parsch’s survey of missing United States Air Force and Department of Defense designations records that X-39 was reserved on 23 April 1997 for the Future Aircraft Technology Enhancements programme of the Air Force Research Laboratory, and that no formal written request to allocate X-39 to that programme was ever put forward, so the designation remained officially unassigned [Missing USAF and DOD Aircraft Designations].

Future Aircraft Technology Enhancements, abbreviated FATE, was a technology programme rather than an aircraft programme, and it was a subset of the national Fixed Wing Vehicle effort. It was sponsored by Wright Laboratory at Dayton, Ohio, which was merged into the Air Force Research Laboratory in 1997 [Wright Laboratory]. Its stated purpose was to develop technologies that would become the foundation for the next generation of combat aircraft, and the candidate technologies named in the public description include affordable low-observable data systems, the active aeroelastic wing, robust composite sandwich structures, advanced compact inlets, photonic vehicle management systems, self-adaptive flight controls, and electric flight control actuation [X-39, Federation of American Scientists] [Jenkins, Landis and Miller, American X-Vehicles].

The programme was organised in three phases and only the first was ever executed. Phase I of FATE I was to identify which technologies required flight validation. Phase II of FATE I was to produce preliminary vehicle design concepts, a demonstrator system, and demonstration plans. FATE II was to build and flight test the demonstrator [Lockheed Martin FATE briefing]. The X-39 designation attached to that third phase, since a research designation is for a vehicle and the vehicle was FATE II’s product.

Two contractors executed Phase I between June and October 1997. Lockheed Martin performed its study from June through October 1997 under contract F33615-97-C-3804 [Lockheed Martin FATE briefing]. Boeing Phantom Works performed a parallel study whose period of performance was 30 June through 30 October 1997, with Jim Cupstid as its programme manager [Boeing FATE Block 1 Phase 1 briefing]. The study was reported in November 1997 as Future Aircraft Technology Enhancements, Block I, held by the Defense Technical Information Center under accession number ADA367849 [Cupstid 1997]. That report states that the study identified technologies with the highest potential against the Fixed Wing Vehicle goals, identified the subset that needed flight test on a new vehicle, suggested a modular vehicle that could flight validate them, and prepared a rough-order-of-magnitude cost estimate for a follow-on programme to build and fly it.

The Lockheed Martin briefing survives in full at fifty-eight slides and is the most detailed public account of what Phase I concluded [Lockheed Martin FATE briefing, full deck]. A ranking exercise selected five technologies as those requiring flight demonstration, being the compact inlet, the conformal fluidic nozzle, tailless technologies, the continuous aerodynamic control surface and the lambda wing [FATE, five technologies selected]. The concluding slide is the one that matters for the designation. It reports that the five selected technologies achieve three of the six Fixed Wing Vehicle goals, that further technologies would be needed to meet the cost goals, and that integrating them into an existing vehicle would require substantial modification [FATE, conclusions and recommendations].

That last finding is the technical case for a new airframe, and therefore for a new design number. A technology that can be demonstrated on an existing aircraft needs no new designation. Both contractors concluded that these could not be, Boeing recommending a modular vehicle and Lockheed Martin reporting that an existing one would need substantial modification. Hence the reservation made sense in April 1997, and hence it became worthless when the Air Force declined to build the vehicle.

Note the order of events. The designation was reserved on 23 April 1997 [Missing USAF and DOD Aircraft Designations]. The Boeing study that was to determine whether a demonstrator was needed at all began its period of performance on 30 June 1997, sixty-eight days later [Boeing FATE Block 1 Phase 1 briefing]. The number was spoken for before the study that would justify it had started.

The demonstrator was then cut. The Air Force dropped the Fixed Wing Vehicle demonstrator element of FATE, and the reason reported in the trade press was that it would duplicate work already planned under the Uninhabited Combat Air Vehicle project of the Defense Advanced Research Projects Agency. The programme was to continue as technology development making increased use of demonstrator aircraft that already existed, among them the X-36 [US Air Force cuts fixed wing FATE demonstrator plans], and Phase I of FATE I was used as a starting point for the Uninhabited Combat Air Vehicle Advanced Technology Demonstration, which replaced the FATE activity altogether [Lockheed Martin FATE briefing] [Boeing FATE briefing, transition to UCAV-ATD]. The Unmanned Combat Air Vehicle demonstration that followed did receive a research designation, the X-45 [Boeing X-45] [Jenkins, Landis and Miller, American X-Vehicles].

So the vehicle the number was held for was never designed, let alone built. Phase 2 of the Fixed Wing Vehicle programme would have addressed the last four of its seven objectives, and Wright Laboratory chose not to proceed with it in favour of the transition to the Uninhabited Combat Air Vehicle Advanced Technology Demonstration [Boeing FATE briefing, transition to UCAV-ATD]. The article infers from that cut, and from the absence of any trace in the sources consulted, that there was no airframe, no wind-tunnel campaign, no contractor mock-up and no cancelled production order. There is a reserved number, a set of technology studies, and a decision not to proceed.

Why the Designation Was Never Assigned

The governing instruction is unambiguous about what makes a designation real, and the X-39 never met the test. Air Force Joint Instruction 16-401, issued on 9 September 1994 by the Secretaries of the Air Force and the Navy and therefore the version in force in April 1997, states at paragraph 3.1 that the Military Departments must submit a written request for assignment of a distinctive Mission Design Series designator as early as possible in the development cycle, that requests travel through the department single point of contact to the Aeronautical Systems Center, which assigns the designator and transmits the request onward for processing and approval, as paragraph 3.2 sets out. Its statement of responsibilities places approval with the office serving as the Department of Defense executive agent. Paragraph 3.1 also settles the matter with the sentence do not use Mission Design Series designators before approval [Air Force Joint Instruction 16-401].

The instruction implements Department-level policy and does not stand alone [Air Force Joint Instruction 16-401]. The naming and designating authority is established at Department of Defense level [DoD Instruction 4120.15], and the approved designators themselves were published in a single Department-wide list, DoD 4120.15-L, whose stated purpose is to list approved designators and popular names [DoD 4120.15-L].

That list can be checked, and checking it is better than asserting from it. The edition of 12 May 2004, which the current instruction still lists among its references, carries an appendix of approved aircraft and unmanned aerial vehicle designators running to more than sixty pages. Its X-series entries include X-35A, X-36A, X-37A, X-38A, X-40A and X-40B, each with a manufacturer, an engine count and a one-line description, and they appear in a single contiguous passage. There is no X-39 entry anywhere in the document [DoD 4120.15-L].

That absence corroborates the non-assignment without demonstrating it, and the difference should be kept. X-33 and X-34 are also absent from the same list, and both of those were substantial programmes that NASA cancelled in March 2001 before either vehicle had flown free [Jenkins, Landis and Miller, American X-Vehicles], so absence from the 2004 edition is consistent with more than one history. What the list establishes is that the X-39 is not among the approved designators of record while its immediate neighbours on both sides are. The demonstrative evidence that no request was ever submitted remains Parsch’s, and this is independent corroboration of it from the primary list.

No such request was submitted for the X-39, so the instruction’s process never began [Missing USAF and DOD Aircraft Designations]. The reason is not mysterious and does not require a hypothesis about secrecy. The request would have been made when the demonstrator entered development, and the demonstrator never entered development. The programme was cut between the Phase I studies and the Phase II work on its remaining objectives, and the paperwork that would have converted a reservation into a designation had no occasion to be written [Boeing FATE briefing, transition to UCAV-ATD].

This distinguishes the X-39 sharply from the other anomalies in the series, and the distinction is one of kind rather than degree. The X-23 was requested and the request was disapproved in 1965 on the ground that the system covered manned aircraft only. The X-52 was requested in 2006 and refused because of possible confusion with the B-52 series, and the X-53 designation was assigned instead [Missing USAF and DOD Aircraft Designations]. Those are decisions, and a decision leaves a record and a reason. The X-39 was never refused by anybody. It was never asked for, and an omission leaves neither a record nor a reason. Every source accordingly reduces its entry to a date, a programme name, and a negative.

Sizing From First Principles

There was no vehicle to size, and this section records that absence without inventing one. Sizing in this series derives a vehicle’s dimensions from the quantity it existed to measure. The FATE demonstrator never reached the Phase II work that was to produce preliminary vehicle design concepts, so no mass, wing area, thrust or envelope was ever stated for it [Lockheed Martin FATE briefing].

The surviving statements about the intended vehicle are qualitative. Boeing suggested a modular vehicle that could flight validate the selected technologies [Cupstid 1997], and Lockheed Martin reported that integrating them into an existing vehicle would require substantial modification [FATE, conclusions and recommendations]. Neither statement carries a dimension.

The only quantity in this article with a binding constraint is the design number. That constraint is administrative in character, and What the Data Changed states it formally.

Dependent Systems

No dependent systems exist, since nothing was built for them to depend on. The record names the technologies a demonstrator would have carried, and Phase I ranked five of them, the compact inlet, the conformal fluidic nozzle, tailless technologies, the continuous aerodynamic control surface and the lambda wing, as requiring flight demonstration [FATE, five technologies selected]. Those are candidate payloads for a vehicle, not systems of one, and no source describes how any of them would have been integrated, powered or controlled on a specific airframe.

The nuclear turbojet that shares the string is not a dependent system of this designation. It was a General Electric engine built from the J47 [General Electric J47] for a different aircraft, the X-6 [Bernier 2025], and the article treats it only to show that the same string names unrelated hardware.

The Flight Test Record

The flight test record of the X-39 is empty. No vehicle was assigned the designation, so no vehicle flew under it. The Fixed Wing Vehicle demonstrator was cut before Phase II, and the work it was to perform passed to the Uninhabited Combat Air Vehicle Advanced Technology Demonstration and to demonstrator aircraft that already existed [US Air Force cuts fixed wing FATE demonstrator plans]. Whatever those programmes flew, they flew under other designations.

The engines that carried the same string did not fly either. The museum account places them on a ground test stand in 1956, intended for the X-6, which would have flown them had its programme survived [Bernier 2025].

Comparison With Ground Prediction

The ground predictions are all from the Phase I studies, and no flight data exist against which to test them. Lockheed Martin’s ranking concluded that the five selected technologies would achieve three of the six Fixed Wing Vehicle goals, that further technologies would be needed to meet the cost goals, and that an existing vehicle would need substantial modification to carry them [FATE, conclusions and recommendations]. Boeing prepared a rough-order-of-magnitude cost estimate for a follow-on programme to build and fly a demonstrator [Cupstid 1997].

None of those claims met a flight test under this designation. The demonstrator was cut, so neither the goal attainment nor the cost estimate was ever checked against a flown vehicle. This article cannot report agreement or disagreement, only that the comparison was never made.

The one prediction the record does resolve is administrative. The reservation of 23 April 1997 assumed that a vehicle would follow and would need a number. The vehicle did not follow, and the consequences of that failed assumption for the register are taken up under What the Data Changed.

What the Data Changed

The Reservation Had No Standing in the Instruction

The regulation that governs designations does not describe the act that consumed the X-39. Air Force Joint Instruction 16-401 describes requesting, assigning, and approving [Air Force Joint Instruction 16-401]. It does not describe a programme office reserving a design number in advance of a request. Parsch’s account of the allocation process in practice supplies the missing step, and describes it as an informal one, in which somebody from the programme office contacts the Department of Defense control point for discussion, and if a designation is tentatively established then the Mission Design Series is regarded as reserved [Allocation of Official Aerospace Vehicle MDS Designations].

The same source states the consequence of stopping there. It records that if the informal step is never followed by the written request, the result is a reserved designation that is never officially requested, let alone assigned, and that this has happened for the design numbers A-11, C-30 and C-36, V-17, V-19 and V-21, and X-39, effectively producing gaps in the lists of allocated designations [Allocation of Official Aerospace Vehicle MDS Designations].

So the X-39 was removed from circulation by an act the written procedure does not provide for. The instruction uses the word differently, and the two senses need separating. The joint instruction as reissued on 14 April 2005 provides at paragraph 5 that the assigning authority will assign and reserve the next available consecutive design number within each basic mission for new vehicles [Air Force Joint Instruction 16-401, 2005]. That reservation is performed by the assigning authority as part of handling a written request. The reservation that consumed the X-39 was initiated before any written request existed, by the party that would have submitted one [Allocation of Official Aerospace Vehicle MDS Designations]. The two acts share a word and nothing else.

The instruction has been reissued since, and the changes answer three questions this article would otherwise have to leave open. The current joint instruction is dated 3 November 2020 and carries the Army and Navy numbers alongside the Air Force one [Air Force Instruction 16-401, 2020].

Reserving a design number in advance of a request is still not a procedure. The word appears in the current text for the approving authority’s retained powers and for the Reserve components, and nowhere as something a programme office may do to a number [Air Force Instruction 16-401, 2020]. Twenty-three years after the X-39 was lost to it, the act remains undescribed.

Skipping a number, by contrast, is now explicitly authorised. The current instruction states at attachment 2 that the approving authority reserves the authority to skip design number at discretion [Air Force Instruction 16-401, 2020]. What was an accident of correspondence in 1997 is now a stated discretionary power. The change concerns the character of the gaps and does not alter their number.

A retirement procedure exists, but it is not a release mechanism. The current instruction carries a section on the retirement and reactivation of a designator, with eligibility conditions for each, and states that a retired designator cannot be used again to identify another aerospace vehicle [Air Force Instruction 16-401, 2020]. The 1994 instruction already placed a designator on a retired list once all of its vehicles had left the inventory [Air Force Joint Instruction 16-401], and the 2005 edition added reactivation for vehicles returned to that designator [Air Force Joint Instruction 16-401, 2005]. No edition provides a way to give up a reserved number so that another vehicle can take it. Retirement applies to designators that were assigned, and the X-39 never was. The newer document repeats the same gap.

One further change bears on the check made above. The current instruction records the cancellation of DoD 4120.15-L as the publicly accessible database of designators, replaced by a listing on a Department web service [Air Force Instruction 16-401, 2020]. The 2004 edition examined here is therefore the last public snapshot of the approved list, a status that justified checking the absence of the X-39 from it directly.

The System Provided for Reserving Names and Not Numbers

The asymmetry is explicit in the text and it runs the wrong way. The same joint instruction states at paragraph 16 that manufacturers or military services may reserve a set of names for future models for their exclusive use [Air Force Joint Instruction 16-401, 2005]. There is no corresponding sentence anywhere for design numbers.

A popular name is a marketing asset and a design number is a structural index. The instruction anticipated that a manufacturer would want to hold a name against future use and wrote a provision for it, while the numbering, consumed consecutively and impossible to replenish, was left with no reservation provision at all. The result was not that numbers went unreserved. It was that they were reserved anyway, through an informal channel with no rule attached, and therefore with no rule governing release.

The Numbering Has a Binding Constraint and It Can Be Written Down

The numbering has a binding constraint, and the instruction states it. Design numbers run consecutively from 1 to 999 within a basic mission, and the assigning authority assigns the next available consecutive number [Air Force Joint Instruction 16-401, 2005]. A reserved number can be reused only if its reservation is cancelled before the next number is allocated [Allocation of Official Aerospace Vehicle MDS Designations]. It is a finite resource consumed in one direction, and the argument about permanence is built on it, so the relations below write it out in symbols.

Let $t$ denote time, let $A(t)$ be the set of design numbers already allocated within a basic mission at time $t$, and let $R(t)$ be the set reserved but not yet allocated. A number is unavailable when it lies in either set, so the unavailable set is their union.

\[U(t) = A(t) \cup R(t)\]

The instruction directs the assigning authority to issue the next available consecutive design number, so the number it would issue at time $t$, written $m(t)$, is the smallest natural number not currently unavailable.

\[m(t) = \min \left\{ n \in \mathbb{N} : n \notin U(t) \right\}\]

One property comes from the practice and cannot be derived from that definition, and the distinction matters. Read literally, the minimum would fall again whenever a lower number was released, as happened to C-16. What the documented practice adds is that a release helps only while the sequence still stands at the number, so once a higher number has been issued the authority does not come back for the one it passed [Allocation of Official Aerospace Vehicle MDS Designations]. The rule is empirical, a description of how the office behaves, and the definition does not entail it. The argument below depends on it.

For a design number $r$, let $\tau_r$ be the moment the sequence passes it, being the first time the next issue exceeds it.

\[\tau_r = \inf \left\{ t : m(t) > r \right\}\]

Let $t_c(r)$ be the time at which a reservation of $r$ is cancelled by its original requester, taken as infinite when no cancellation is ever filed. Recovery is possible exactly when the cancellation precedes the passage. The relation below rewrites Parsch’s stated rule in one line.

\[\operatorname{recoverable}(r) \iff t_c(r) < \tau_r\]

The asymmetry of the previous subsection now yields a formal consequence. The instruction supplies a route by which numbers enter $R$ and prescribes no procedure for removing them, so in the absence of an informal cancellation the reserved set only grows.

\[R(t_1) \subseteq R(t_2) \quad \text{whenever} \quad t_1 \leq t_2\]

The permanent gaps are the reserved numbers the sequence has already gone past, and that set inherits the same monotonicity.

\[G(t) = \left\{ n \in R(t) : n < m(t) \right\}\]

A register built this way accumulates holes and has no mechanism for closing them. The gaps at A-11, C-30, C-36, V-17, V-19, V-21 and X-39 are not a list of unrelated administrative accidents. They are the elements of $G$, and the set has no rule that could ever remove one.

The Gap Is Permanent Because a Second Document Was Also Missing

This part of the record changes how the gap should be read, and it concerns recovery, where earlier subsections concerned creation. A reservation that leads nowhere does not have to be permanent. A number can be recovered, and the condition is precise. Parsch records that if a reservation is explicitly cancelled by the original requester before the next number is allocated, the number can effectively be reused, and gives the C-16 designation as a case where that happened [Allocation of Official Aerospace Vehicle MDS Designations].

Nobody cancelled the X-39 reservation, and the window in which cancelling it would have helped was short [Allocation of Official Aerospace Vehicle MDS Designations]. The X-40A designation was allocated in 1997, initially to the Space Maneuver Vehicle and soon reassigned to the Integrated Technology Test Bed, the ninety percent scale unpowered vehicle that carried it into flight [Boeing X-37 and X-40]. The FATE demonstrator was cut after the Phase I studies concluded on 30 October 1997, and the cut was reported on 17 December 1997 [US Air Force cuts fixed wing FATE demonstrator plans]. So the next number was allocated in the same year the X-39 was reserved, and the demonstrator that justified the reservation was cut in that year too.

The ordering of those events is not established, because the calendar is not. The record accessible here dates the X-40A allocation to 1997 without a month, so this article does not claim that the X-39 reservation displaced the X-40 allocation, and the relative order of those two events within 1997 is left open. The same missing month leaves open whether the next design number was allocated before or after the demonstrator was cut, since the cut falls between 30 October and 17 December 1997. The conclusion does not depend on either ordering, because recovering the number needed a cancellation and none was ever filed.

The instantiation is short enough to check by eye. For the X-39 the design number is $r = 39$, and no cancellation was ever filed, so the cancellation time is infinite and the recovery condition fails against any finite passage time whatever.

\[t_c(39) = \infty \quad \implies \quad \operatorname{recoverable}(39) = \text{false}\]

The dated part of the argument is the ordering of the passage against the cut of the programme, and the record bounds both without ordering them. The X-40A allocation places the passage inside 1997, and the cut came after the Phase I studies ended on 30 October 1997 and no later than the trade report of 17 December 1997, at a time written $t_{\text{cut}}$.

\[\tau_{39} \leq \text{31 December 1997}, \qquad \text{30 October 1997} < t_{\text{cut}} \leq \text{17 December 1997}\]

The two bounds overlap, leaving the order of the two events undetermined. If the sequence passed 39 before the cut, the number was lost while the demonstrator was still a live proposal, and a release filed after the cut would have achieved nothing. If the cut came first, a release filed between the cut and the passage would have recovered the number. No release was filed, so the number was lost either way, and the missing month decides only whether anyone ever had the chance to save it once the reservation had become pointless.

The bounds are silent on one point. They do not place the reservation time $t_{\text{res}}$ of 23 April 1997 relative to $\tau_{39}$, because the X-40A allocation is dated to 1997 without a month. Both orderings remain open and the conclusion holds under either, since a cancellation time of infinity fails the condition regardless of when the passage occurred.

The Designator Escaped the System That Never Issued It

The instruction says not to use a designator before approval, and the X-39 has been in continuous public use for nearly thirty years without ever being approved. The Federation of American Scientists published a page headed X-39 under a military aircraft heading, which correctly reports that the designator is apparently unassigned while nonetheless presenting it as an entry alongside aircraft that exist [X-39, Federation of American Scientists]. The number is in public use, the very use the instruction forbids before approval.

Here the divergence stated at the top of this article takes its sharpest form. The register of designations and the population of aircraft are different things, maintained by different processes, and neither is a reliable index of the other. This series has now met the divergence from both ends. The X-35 was a real aircraft that flew a competition, and the X-35 article finds its literature under its own designation thin because the work continued under the production aircraft’s name. The X-39 is the mirror image, a designation with a substantial public footprint and no aircraft behind it at all.

The Same String Names an Engine That Ran

An X-39 did exist, but it was not an aircraft. General Electric applied the designation to J47 turbojets rebuilt to take their heated compressed air from a reactor heat exchanger rather than from burning fuel. Those engines ran. The Smithsonian National Air and Space Museum records that General Electric successfully operated its X-39 engines on a ground test stand in 1956, and that the Convair X-6 was to have carried four of them beneath its belly under the reactor [Bernier 2025]. They were operated against three successive reactors, the Heat Transfer Reactor Experiments numbered one, two and three, under the Aircraft Nuclear Propulsion programme established by the Air Force and the Atomic Energy Commission in 1951 [General Electric J47] [Aircraft Nuclear Propulsion]. The X-6 is the seventh article in this series, and had the programme survived it would have flown them.

The two sources disagree on what counted as one engine and the disagreement is left standing. The museum account has the X-6 carrying four X-39 engines, while the encyclopaedia describes the X-39 as consisting of two modified J47 units [General Electric J47]. Both can be true if the designation names a unit of two, but no source consulted here says so, so no count is asserted.

Two registers therefore contain the same string and disagree completely about what it denotes. In the aircraft designation series, X-39 names nothing and never did. In the engine designation series, X-39 names hardware that operated for years and is documented in detail. The two have no relationship beyond the coincidence, since the engine string comes from a General Electric programme and the aircraft number from the military designation system.

The encyclopaedia records the collision without resolving it. There is no article for the aircraft designation at all. What exists is a disambiguation page listing two senses, the nuclear turbojet and the reserved aircraft number, with the latter pointing back to the technology programme rather than to any vehicle [X39]. A designation that was never assigned has been reduced to one line on a page whose purpose is to warn readers that the term is ambiguous. The page fairly summarises the designation’s standing.

A reader who treats the gaps in the X-series as evidence of concealment is reading the wrong artefact. The gaps record the failure modes of a correspondence process. Some of them, such as the X-52, record a decision. This one records an informal contact that was never followed by a letter, and a second letter that was never sent to undo the first.

The Contemporary Literature

Cluster	Records
Recordkeeping, retention and provenance	220
Archival silence and the colonial archive	161
Persistent identifiers and link rot	105
Authority control and registers of names	62
Numbering plans, number portability, domain names and address space	51
The argument from silence and absence of evidence	51
Archival appraisal and selection	46
Nuclear propulsion of aircraft	40
Classification and administrative categories	29
Military designation and nomenclature systems	2
The Future Aircraft Technology Enhancements programme	1
Total	768

This article’s subject is not an aircraft, so its survey is not of the aerospace literature. A survey of that literature would not bear on whether a letter was written in 1997. The subject is what a gap in an official register means, and there is a large and current literature on that question, spread across archival science, infrastructure studies, identifier administration and the logic of inference from absence. Two smaller clusters hold the literature of the programmes the number and its string belonged to.

The survey holds 768 records in eleven clusters. Two of them, the Boeing Phase I report and the museum account of the nuclear turbojet, are among the argument’s sources. The rest are not cited as evidence for any claim about the X-39, and no paper among them has been read beyond its title, although every title has been read so that records sharing only a word with the subject are excluded. They are a map of the surrounding literature, offered so that a reader who wants to pursue the general question has somewhere to start.

Archival Silence and the Record That Was Never Made

Archival science has a name for what the X-39 is, and a literature about it. A silence is a place where the record is absent, and the field’s central claim is that archives are made by the same exercise of power that leaves their gaps, so that a silence carries information about the institution rather than merely lacking it [Schwartz and Cook 2002] [Harris 2002] [Halidu 2025]. The literature studies the destruction of records that were made [Dolganov 2025], and treats a refusal to record as an act with consequences [Goeman-Shulsky 2022]. This article makes the same claim about a designation gap.

The Argument From Silence, as Method and as Fallacy

Inferring from an absence is a recognised move with a recognised failure mode. The argument from silence is treated in historiography [Lange 1966], in biblical and religious-historical scholarship [Duncan 2012] [Vinzent 2021], and in informal logic as the argument from ignorance [Cummings 2015] [Lisanyuk and Khamidov 2021]. The literature asks when an absence of evidence becomes evidence of absence [Thompson and Scurich 2018], and treats the question as one of rational and Bayesian inference [Hsu et al 2016] [Taroni et al 2019]. The condition this article relies on is that an absence supports an inference only where the record would be expected to speak. The X-39 meets that condition, since an approved designator would appear in a list whose purpose is to list approved designators.

Classification Systems as Infrastructure

A designation system is a classification, and there is a field that studies classifications as infrastructure. Its founding study, Sorting Things Out, holds that classification has consequences beyond the sorting it performs, and treats classifications as large-scale infrastructures [Classification and Large-Scale Infrastructures 2000] [Categorical Work and Boundary Infrastructures 1999]. Its residual categories, the places where whatever does not fit is put, receive separate study [Star and Bowker 2007] [Star 2010], and administrative categories are studied as instruments that act on whoever is sorted into them [Vetters 2007]. A gap in a numbering series is a residual of this kind, an empty place in the scheme.

Identifier Assignment, Persistence and Reuse

The rule that removed the X-39 from use is a live design question in identifier systems. The persistent identifier literature names uniqueness and persistence among the requirements of a trusted identifier [Lehnert et al 2019], and measures what happens to references when persistence fails [Zittrain and Albert 2013] [Sadatmoosavi et al 2026]. Reuse is rarely the subject of a title, and the titles that address it describe a reuse mechanism in one system [Yuan Shi et al 2006] and a market in re-registering expired domain names [Salvador and Nogueira 2011]. The Mission Design Series arrives at permanence by a different route, through a practice with no written rule attached.

Administering a Finite Number Space

The X-39 is one case of a general problem, the issue of a finite space of names that cannot be replenished. Internet address space and telephone numbering are administered under that constraint. The literature gathered here treats the exhaustion of Internet address space [Boucadair et al 2009] [Beeharry and Nowbutsing 2016], the administration of numbering [Price Controls and Numbering Administration 2005], and the portability of a number from one holder to another [Gans et al 2001] [Wu 2013]. Reservation and reclamation, the two acts the X-39 turns on, are almost absent from its titles, as the count under Thin Subjects shows.

Recordkeeping, Retention and the Administrative Trace

The article’s evidence is correspondence, and there is a discipline devoted to what correspondence does and does not preserve. Records management studies retention and disposition [Drewry 1955] [Records Retention Schedules 2009], the distance between what is recorded and what happened [Trace 2002], and the consequences when recordkeeping fails [Moret 2024]. The X-39 is a case where the act left no trace because the trace was the act, a reservation being nothing but the record of itself.

Maintaining a Register Over Decades

A register is not a document but a process, and the maintenance literature is about that process. In the titles gathered here, the operation with a sustained literature is authority control, the work that keeps one name attached to one person or body across a catalogue [Authority Control 2012], together with catalogue maintenance [Oldfield 1988] and the upkeep of a master index of persons [Williams et al 2006]. No title names the deprecation, withdrawal or retirement of an entry, the operation the X-39 needed. The Mission Design Series instruction has provided for retiring an assigned designator since at least 1994, and no edition has provided for releasing a reserved number [Air Force Joint Instruction 16-401] [Air Force Instruction 16-401, 2020].

Military Designation and Nomenclature Practice

The narrowest literature is the one closest to the subject, and it is genuinely small. Two records treat military or government designation as a system, a 1997 Air Force issuance on designating and naming defense military aerospace vehicles [Department of the Air Force Washington Dc 1997] and an appendix on aircraft nomenclature in a textbook of aircraft performance [Appendix A Aircraft Nomenclature 1997]. Dictionary and handbook entries that define a designation system are reference entries rather than research works, and the survey excludes them. That thinness is also a finding, and it explains why the authoritative account of the X-39 is a privately maintained web survey rather than a paper.

Recordkeeping, retention and provenance

Records management and the archival theory of the record. Retention and disposition schedules, recordkeeping requirements and practice in organisations of every kind, electronic records management, and provenance and original order as the principles that keep a record’s context.

220 records. [1998, RECORDKEEPING REQUIREMENTS] [1998, Retention and Disposition of Records] [1999, Management of Active Records] [2000, Recordkeeping] [2000, Retention of Records] [2003, Records management] [2005, CHAPTER THREE. The corporate memory and Records Management] [2006, Identity, trust, evidence and the recordkeeping nexus] [2006, Legal and social relationships and the recordkeeping nexus] [2006, OSHA Recordkeeping] [2006, Recordkeeping participants legal and ethical responsibilities] [2006, Recordkeeping regulatory models in the web environment] [2006, The recordkeeping-ethics-law nexus and recordkeeping regulatory models] [2007, Records and recordkeeping] [2007, Records Management and Records Management Policies] [2009, Government Records Management/Recordkeeping] [2009, Provenance of Archival Materials] [2009, Records Retention Schedules] [2011, Office and records management] [2012, Challenges of the dynamic e-environment for records management] [2012, Critical Considerations for Records Management and Retention] [2012, Recordkeeping Requirements] [2012, Scheduling Records for Retention] [2013, Data Management and Records Retention] [2014, Documentation and Recordkeeping] [2015, Sustainable Electronic Court Records Risk Management Conceptions and Development of Framework] [2016, CHAPTER 34. CDS Records Retention] [2016, Records Management] [2016, The records management function] [2017, The Management of Native Estate Records in the NAN] [2018, Administrative Records and the Courts] [2019, Sources of Law Governing the Management of Police Records] [2021, Records Management nach ISO 15489] [2026, Medical Records as Legal Evidence Challenges in Documentation, Disclosure, and Retention in Healthcare Practice] [Abdulrahim and Zayyana 2025] [Abuzawayda et al 2012] [Adriansyah et al 2024] [Akussah and Asamoah 2015] [Al-Alawi 2024] [Allegrezza 2020] [An and Cook 2003] [An 2005] [Anderson 2024] [Asamoah et al 2018] [Ataullah et al 2008] [Ataullah and Tompa 2009] [Bachell and Barr 2014] [Bae et al 2016] [Bahmer 1943] [Bailey 1999] [Bailey 2009] [Baker 2003] [Bantin and Bernbom 1996] [Barry 1997] [Bastian 2006] [Bastian 2020] [Bezdrabko 2019] [Bhatia et al 2020] [Bhatia and Wright de Hernandez 2019] [Boiteau 2024] [Borglund Erik A.M. and Anderson Karen 2011] [Brooks 2019] [Brunskill 2012] [Brunskill 2012] [Chalmers 1989] [Chaputula 2022] [Chartaby 1991] [Ciechanowski 2021] [Courtney 2022] [Cox 1997] [Cox 2006] [Crookston 2011] [Cumming and Findlay 2010] [Cumming 2007] [Cunliffe and Miller 1989] [Dearstyne 2006] [Dearstyne 2006] [Demb 2012] [Drewry 1955] [Duranti 1999] [Egbuji 1999] [Ellis 2005] [Evans et al 2024] [Fan 2018] [Farrell 2024] [Foscarini et al 2021] [Franks 2015] [Franks 2020] [Friedrich 2021] [Frings-Hessami and Henderson 2025] [Frings-Hessami and Oliver 2022] [Fuzeau 2005] [Fuzeau 2014] [Garaba 2026] [Ghaddar 2025] [Gill and Mitchell 1963] [Gilliland-Swetland 1991] [Golding 2019] [Gooda 2011] [Grant 2017] [Grondona et al 2024] [Guercio et al 2020] [Guimarães et al 2025] [Guimarães and Tognoli 2015] [Gutterman 2023] [Hartzell 1946] [Hasan et al 2008] [Hasan and Winslett 2010] [Henderson 2019] [Hirsch 2010] [Hofman and Katuu 2022] [Hossain et al 2025] [Hurley et al 2024] [Hurley 2005] [Hussin and Johare 2014] [Huvila 2008] [Ivanov 2017] [Johnson 1996] [Joseph et al 2012] [Joseph 2016] [Kafka 2009] [Kallberg 2010] [Kang and Rieh 2016] [Katuu 2023] [Katuu 2025] [Kautto and Hotti 2024] [Kavishe and Garaba 2025] [Kemoni et al 2007] [Kemoni 1998] [Kerr 2020] [Kikki 2000] [Klareld 2018] [Xie 2015] [lacovino 2005] [Larkin 1942] [Lee 2008] [Lee 2015] [Lee 2016] [Lee-Gulley and Drudi 2022] [Lemieux 2017] [Li and Fan 2018] [Lips and Rapson 2010] [Luyombya 2011] [Machado et al 2020] [Madayag 2025] [Malatji and Mojapelo 2025] [Manase et al 2024] [Manoharan 2025] [Marini 2024] [Marutha 2020] [McCracken and Hogan 2021] [McDonald 1989] [McDonald 2010] [McKemmish et al 1999] [Michetti 2016] [Min 2015] [Mitchell 1966] [Mokhtar and Yusof 2017] [Moret 2024] [Nesmith 2007] [Oladejo and Hadžidedić 2021] [Oladejo and Hofman 2023] [Overly 2021] [Pan 2017] [Park 2004] [Park 2017] [Parvez 2017] [Pestova 2019] [Piggott 2012] [Piggott 2012] [Pires et al 2019] [Popovici 2015] [Pymer 2025] [Qaddoumi 2025] [Raas 1999] [Rajh 2021] [Rebuglio et al 2025] [Reed et al 2018] [Reed 1997] [Reed 2008] [Rees Jones 2024] [Remize 2014] [Rolan and Lewis 2024] [Rolan 2017] [Ryan 1966] [Saglam 2024] [Seol and Lee 2016] [Smoczynski 2024] [Stuart and Bromage 2010] [Sutton 2015] [Svärd 2017] [Svärd 2017] [Svärd 2017] [Sweeney 2008] [Syaffieq et al 2026] [Tanimu et al 2016] [Tennis 2015] [Thibodeau 2016] [Thornborow 2007] [Todrović Bilić 2025] [Trace 2002] [Trace 2024] [Uhl 2001] [Upward et al 2013] [Upward 2000] [Vasseur 2020] [Velloso de Oliveira and Panisset 2025] [Viegas et al 2019] [Vincent et al 2024] [Wallace 1993] [Watson 1994] [Welt 1995] [Wendelken 2023] [Wickman 2011] [Wilczek 2026] [Xie 2016] [Yusof and Chell 1999] [Zhang 2014] [Zinner Henriksen and Viborg Andersen 2008] [Zou and Park 2024]

Archival silence and the colonial archive

Absences, gaps, erasures and silences in archives, and the archive as a constructed and selective object. Of these titles, 62 name a silence and 39 name the colonial or imperial archive, which is the literature’s main case of an archive whose omissions record the institution that made it.

161 records. [1994, THREE Archival Memory and the Destruction of the Past] [2002, Six. The Imperial Archive Colonial Knowledge and Colonial Rule] [2007, 4. Collecting Colonial Postcards Gender and the Visual Archive] [2007, Introduction Of Soft and Savage Bodies in the Colonial Domestic Archive] [2009, Chapter 7. Confessing in the absence of recording linguistic and extralinguistic evidence of coercion in a police interrogation] [2013, Alienated memories migrants and the silences of the archive] [2013, Anxieties of Authorship in the Colonial Archive] [2014, Whose Lessons? Subjects of the Colonial Archive] [2015, 11 Epilogue The Colonial Gaze, History and the Archive] [2021, Archival Memory and the Destruction of the Past] [2022, 1 Colonial Listening and the Making of a Sound Archive] [2024, 4. Silences Take Two Gendered Archival Lacunae] [2024, CHAPTER TWO Silence, Perseverance, and Survival in State-Run Philippine Film Archives] [2024, Gaps in the Archive] [2025, 4 Archival Silence, Sexual Violence] [2026, Archival Absences, Silences and Fragments The Unmade and Film History] [2026, Navigating Archival Silences Through Art] [Achim 2023] [Adelusi-Adeluyi 2020] [Al-Amoudi et al 2022] [Alexander 2005] [Araluen Corr 2018] [Arondekar 2005] [Basu 2025] [Bezdrabko 2026] [Bhowmik 2021] [Black 2024] [Blouin 1999] [Bridger and Hazan 2022] [Brooks 2005] [Bruns 2024] [Buckley 2005] [Buenrostro 2021] [Bunjun 2018] [Burke 2007] [Burke 2014] [Burke 2015] [Burton 2007] [Butler 2009] [Carlson 2021] [Chakravorty 2021] [Chatterjee 1997] [Conor and Lydon 2011] [Coombes 2016] [de Melo Fischer and Novella 2026] [Decker 2013] [Delaney 2026] [Desrochers-Turgeon 2022] [Ditzler 2014] [Diver 2024] [Dolganov 2025] [Dragojlovic 2018] [Dralle 2022] [Dunsworth 2018] [Durepos and Thurlow 2025] [Durepos and Thurlow 2025] [Durepos and Thurlow 2025] [Durepos and Thurlow 2025] [Durepos and Thurlow 2025] [Durepos and Thurlow 2025] [Dávila-Freire 2020] [Eadon 2019] [Ellis et al 2026] [Emberley 1990] [Falzetti 2014] [Fithri and Basuki 2026] [Fraccaroli and Simonetto 2025] [Gauba 2024] [Glanville 2017] [Goeman-Shulsky 2022] [Golding 2022] [Gray 2019] [Gualtieri 2019] [Guberek and Hedstrom 2017] [Hager 2021] [Hagström Molin 2025] [Halidu 2025] [Han et al 2024] [Hardy 2021] [Harris 2002] [Harris 2002] [Hernández 2024] [Hodder 2017] [Hogue 2020] [Horn 2020] [Huang 2020] [Hunt-Kennedy 2021] [Hyman and Mundy 2023] [Johnes 2020] [Jokic 2012] [Ketelaar 2005] [Ketzmerick-Calandrino 2024] [Khoruzhenko 2023] [Klein 2013] [Kortenaar 2025] [Koziura 2026] [Kros 2017] [Landman 2009] [Lange 2018] [Lavine and Thomas 2024] [Lee 2012] [Lihoma 2021] [Marouf 2025] [Marraffa 2026] [Martin 2015] [Martínez 2016] [Masibi et al 2023] [McWilliam and Shepherd 2022] [Meyer 2008] [Mills 2000] [Mithal and Paul 2024] [Mithal and Paul 2024] [More 2020] [Muñiz 2021] [Namhila 2004] [Netshakhuma 2019] [Niang 2026] [Nii Owoo 2025] [Olson 2023] [Papatheodorou 2021] [Patil 2024] [Pennamon and Coates 2021] [Percel 2024] [Perry 2005] [philip and Bertacco 2022] [Pigorsch and Rusinek 2026] [Prasad 2022] [Ragni 2020] [Rawat 2016] [Ray 2023] [Regidor 2023] [Rogers 2026] [Rosén 2008] [Saikia 2004] [Schwartz and Cook 2002] [Sherman et al 2024] [Sizemore-Barber 2017] [Soares 2024] [Souchen 2021] [Sousa 2026] [Spieker 2026] [Stoler 2002] [Szikszai 2026] [Tali 2017] [Taoua 2003] [Thelin 2009] [Thompson 2019] [Tough 2011] [Urbec 2023] [Valerio 2023] [VanHaitsma 2020] [Vicente 2018] [Vizcaya 2008] [Voli and Virtù 2023] [Wanhalla 2024] [Wevers et al 2025] [Williams 2021] [Wolf 2018] [Zeman 2023] [Özdemir and İcimsoy 2021] [Рамазанова, 2025, Types of evidence in inheritance cases in the absence of civil status records based on examples from judicial practice]

Persistent identifiers and link rot

Persistent identifier schemes and the decay of references when they fail. Digital object identifiers, researcher identifiers, handles and similar schemes, their adoption, and link rot. Of these titles, 36 name the Digital Object Identifier, 30 name ORCID and 8 name link or reference rot.

105 records. [1999, Executive briefing The Digital Object Identifier*] [2009, Digital Object Identifier DOI® System] [2009, ORCID] [2011, Digital Object Identifier DOI® System] [2015, PID Service an advanced persistent identifier management service for the Semantic Web] [2015, Your name is not good enough introducing the ORCID researcher identifier at Imperial College London] [2017, ORCID the challenge ahead] [Arroyo-Machado et al 2025] [Arunachalam and Madhan 2021] [Awojobi 2011] [Bangert and Frances 2017] [Bellini et al 2014] [Bellini 2019] [Berber and Yahyapour 2017] [Bordons et al 2024] [Briguglio et al 2014] [Bygrave et al 2009] [Carter‐Templeton et al 2021] [Celko 2010] [Ceravolo and Bellini 2010] [Chandrakar 2006] [Craigle et al 2022] [Cronk and Pitblado 2026] [Damasio 2013] [Dappert et al 2017] [De Waard 2019] [Dhiman 2015] [Dreyer et al 2019] [Dvořák et al 2024] [Fenner et al 2011] [Fenner 2011] [Ferreira et al 2015] [Foley and Kochalko 2012] [Francis 2013] [Freire et al 2023] [Goddard 2021] [Goldenberg 2014] [Gould and Praetzellis 2021] [Gould 2022] [Gómez-Ontiveros 2017] [Habermann et al 2026] [Hardisty et al 2019] [Hardisty et al 2021] [Heredia and Marín-Arraiza 2022] [Heusse and Cabanac 2022] [Hisseine et al 2024] [Islam et al 2022] [Jain et al 2023] [Juty et al 2013] [Kansa and Kansa 2022] [Klein and Balakireva 2021] [Krauskopf and Salgado 2023] [Król 2019] [Kumar et al 2015] [Kurakawa and Takeda 2012] [Lehnert et al 2019] [Lena Pavlovska 2017] [Liu 2021] [Liu 2021] [Margolis 2018] [Markwell and Brooks 2003] [Marín-Arraiza and Heredia 2021] [Marín-Arraiza and Heredia 2024] [Mayernik and Maull 2017] [Meadows et al 2019] [Mondal and Mondal 2023] [Morgan 1998] [Nasrullah et al 2025] [Nazaré et al 2018] [Okune and Chan 2023] [Owango 2025] [Park et al 2011] [Park et al 2024] [Paskin 1999] [Paskin 2005] [Paul and Shivraj 2014] [Pires et al 2017] [Porter et al 2025] [Pérez-Yarza 2001] [Reuss et al 2023] [Risher and Rosenblatt 1998] [Sadatmoosavi et al 2026] [Saepuloh et al 2026] [Saltzer and Kaashoek 2009] [Sayão 2007] [Scott and Thomas 2018] [Teixeira da Silva et al 2025] [Teixeira da Silva 2024] [Testal 2001] [Thompson and Tong 2018] [Treacy and Okrent 2023] [Uchida 1999] [Van de Sompel et al 2014] [Vasile et al 2020] [Wang et al 2017] [Wang et al 2017] [Weigel et al 2014] [Wojtkiewicz 2025] [Yatskiv 2016] [Yuan Shi et al 2006] [Zherebchuk 2024] [Zittrain and Albert 2013] [Марушко, 2016, International identifier ORCID and its role in the global scholarly communication] [إِیمان عزیز خضر and عمار عبداللطیف زین العابدین, 2022, المعرف الرقمی للوصول إِلى المعلومات Digital Object Identifier DOI - دراسة فی ماهیته ومکوناته وفوائده للباحثین والدوریات] [عبد الحمید, 2022, “المعرف المفتوح للباحثین والمساهمین ORCID دراسة تحلیلیة لواقع استخدامه من جانب أعضاء هیئة التدریس بأقسام المكتبات والمعلومات” “ Open Researcher and Contributor ID ORCID Analytical study of the reality of its use by faculty members in library and information departments]

Authority control and registers of names

Keeping one entry per person or body in a catalogue or archive. Name authority control, archival authority-record standards, catalogue maintenance, and registers of persons.

62 records. [2008, Chapter 5 Issues of Name Authority Control in the Cataloging of Theses and Dissertations] [2012, Authority Control Experiences and Projects] [2012, Authority Control in the Context of Bibliographic Control in the Electronic Environment] [2012, Authority Control of Creators and the Second Edition of ISAAR CPF, International Standard Archival Authority Record for Corporate Bodies, Persons, and Families] [2012, Authority Control State of the Art and New Perspectives] [2017, Name Authority Control] [2024, Name Authority Considerations] [Beaubien and Head 2011] [Bourdon 1992] [Burke and Shorten 2010] [Chávez-Aragón et al 2009] [Cunningham 2008] [Danskin et al 2011] [Delsey 2024] [Diaz-Valenzuela et al 2010] [Dickson and Zadner 2024] [DiLauro et al 2001] [Dragon 2009] [Dryden 2018] [Dunn 2015] [El-Sherbini 2013] [Emanuel 2011] [Fenly 1985] [Folkner and Glackin 2009] [Goldman and Smith 2024] [Harwood 1986] [Hobart 2019] [Hong et al 2004] [Hu et al 2004] [Kirby 2024] [Knyazeva et al 2016] [Leuner 2021] [Lopes 1993] [MacEwan et al 2013] [Mak 2011] [Messmer 2004] [Mihwa Lee 2012] [Neatrour and Myntti 2019] [Niu 2013] [Oldfield 1988] [Orihuela 2009] [Rankine et al 2008] [Reid and Simmons 2018] [Ridgeway 2024] [Salo 2009] [Savoja and Vitali 2018] [Snyman and Jansen van Rensburg 1999] [Snyman and Jansen van Rensburg 2000] [Snyman and van Rensburg 2000] [Spink and Leatherbury 1994] [Szary 2006] [Taylor 2024] [Thompson 2016] [Tillett 2024] [Van Pulis 2006] [Wanigasooriya 2023] [Warnner and Brown 2001] [Waugh et al 2014] [Williams et al 2006] [Xia and Liu 2018] [Yamada and Inoue 2021] [Yu et al 2019]

Numbering plans, number portability, domain names and address space

The administration of finite name and number spaces as a regulatory act. Telephone numbering plans, the portability of a number between carriers as policy, domain-name registration and the governance of the Domain Name System, and the exhaustion and allocation of Internet address space. Of these titles, 25 name number portability and 11 name domain names or the Domain Name System.

51 records. [2003, Number portability] [2005, Price controls and numbering administration Cartesian-bureaucratic issue-processing episodes] [2009, The Internet Address Space] [2012, Domain name registration systems in OECD countries surveys, illustrations and analysis] [2012, Liberalization, consumer protection and growth the tug and pull relationship in the domain name registration system] [2012, The domain name registration system an overview] [2019, A global ‘private’ regime governing the Domain Name System DNS] [Beeharry and Nowbutsing 2016] [Boucadair et al 2009] [Buehler and Haucap 2004] [Buehler et al 2006] [Chen et al 2007] [Dobrovolskis 2003] [Gans et al 2001] [Garcia‐Murillo 2007] [Ghalumyan 2017] [Ghalumyan 2018] [Gjika and Bushati 2023] [Guillermo de Jesús Valdivia-Medina 2023] [Hu et al 2017] [Keevers 1966] [Kim 2019] [Kimutai et al 2013] [LeeInHo 2013] [Li and Zhang 2017] [Lindsay 2016] [Merrill 2016] [Mockapetris and Dunlap 1988] [Mueller 2010] [Murugesan and Ramadass 2009] [Musiani 2016] [Musyarri 2025] [Myers 1961] [Nunn 1952] [Nunn 1952] [Nuredin Ahmed 2026] [Osborn and Alan 2023] [Pande and Pawar 2012] [Pitt et al 1996] [Rizal and Ali Muayyadi 2013] [Robinson 2003] [Salvador and Nogueira 2011] [Sujarittanonta 2017] [Sutherland 2007] [Tebepah 2026] [Tournier 2023] [Turnbull et al 2010] [Usero Sánchez and Asimakopoulos 2012] [Wu 2013] [Yi-Binq Lin et al 2003] [Zhou 2009]

The argument from silence and absence of evidence

Inference from absence, as historiography, as logic and as statistics. The argument from silence, the argument from ignorance, and the general question of when an absence of evidence is evidence of absence.

51 records. [2023, Introduction Absence of Evidence or Evidence of Absence?] [Alderson 2004] [Altman and Bland 1995] [Altman and Bland 1996] [Bassetti 2017] [Batra and Rubman 2019] [Binder and Roberts 2023] [Bunce et al 2014] [Cummings 2015] [Duncan 2012] [Fina and Hofacker 2026] [Friedland 2025] [Gardner and Gardner 1991] [Garvey 2010] [Goddard 2019] [Hartstone‐Rose 2025] [Hartung et al 1983] [Hsu et al 2016] [Jaykaran et al 2011] [Jensen 2023] [Kang 2019] [Kačar 2021] [Klingbiel et al 2017] [Kraemer 2020] [Lange 1966] [Lisanyuk and Khamidov 2021] [Loosley Leeming 2018] [McGrew 2013] [Mesicek 2014] [Michon 2015] [Nadal-Ginard et al 2014] [Pacheco et al 2019] [Pawel et al 2024] [Raman and Jawitz 2021] [Rolón 2019] [Sedgwick 2011] [Sedgwick 2014] [Shah 2023] [Shaw 2006] [Shrader-Frechette 1980] [Slyomovics 2005] [Smith 2008] [Sober 2008] [Sánchez-Ramón et al 2015] [Taroni et al 2019] [Thompson et al 2017] [Thompson and Scurich 2018] [Vinzent 2021] [Wu 2011] [Zarefsky 2014] [Zimmerman 2022]

Archival appraisal and selection

The decision about what is kept, and so about what will later be absent. Appraisal theory, appraisal practice in national, state and community archives, and its methods.

46 records. [2004, CHAPTER FOUR. Archival appraisal briefly reviewed in historical context] [2004, CHAPTER TWO. Appraisal and archival appraisal in modern organizations purpose, participants, roles and responsibilities] [2009, Archival Appraisal and Acquisition] [2017, Acquisition and archival appraisal] [2019, Effectiveness of Appraisal of Public Records in Tanzania Findings from President’s Office Records and Archives Management Department PO-RAMD] [Al-Samadi 2025] [Anderson et al 2015] [Bauer and Wrisley 2020] [Bayeck et al 2022] [Beneito Arias 2008] [Bezdrabko 2019] [Black 2019] [Boyko 2024] [Brown 1999] [Cook 2011] [Cox 2001] [Cox 2002] [Cox 2003] [Cox 2003] [Cox 2003] [Duranti 1994] [Edquist 2023] [He and Tian 2018] [Henttonen and Packalén 2023] [Hodge et al 2022] [Hosker and Richmond 2006] [Kim and Ross 2013] [Kim 2008] [Kim 2010] [Kretzschmar 2005] [Mancuso 2013] [Ngoepe 2022] [Ngulube 2001] [Nilsson 2016] [Niu 2012] [Nunes da Silva and Parrela 2022] [Rhee 2012] [Rhee 2014] [Rhee 2016] [Roberts 2007] [Sucha-xaya 2024] [Suderman 2006] [Wallot 2006] [Walters 1996] [Wiśniewska-Drewniak 2022] [Xu et al 2010]

Nuclear propulsion of aircraft

The programme whose General Electric engines carried the X-39 string. Aircraft nuclear propulsion at the level of the programme, its engines and powerplants, its reactor experiments and its aircraft concepts, with later studies of nuclear-powered aircraft.

40 records. [Na 1957] [Aircraft Nuclear Propulsion Project 1959] [Aircraft Nuclear Propulsion Project 1960] [Bernier 2025, The World Wasn’t Ready for Nuclear-Powered Bombers] [Comassar 1962] [Cottrell 1951] [Cottrell 1952] [Cottrell 1953] [Dibble 1958] [Calculated performance of a 1951] [Preliminary analysis of three 1950] [Jordan et al 1957] [Larson 1958] [Larson 1958] [Larson 1959] [Larson 1959] [Larson 1959] [Larson 1959] [Larson 1960] [Larson 1960] [Larson 1960] [Larson 1960] [Larson 1960] [Larson 1960] [Larson 1960] [Layman 1962] [Leverett 1959] [Lewis 2024] [Liao et al 2025] [Lubarsky and Greenstreet 1953] [Noyes 1961] [Perry 1958] [Puthoff 1970] [Robson and Peters 1972] [Rom 1970] [Schmickrath 1960] [Schwartz et al 1950] [Thornton and Rothstein 1962] [Weinberg 1950] [Wild 1967]

The General Electric direct-air-cycle programme’s own comprehensive technical reports of 1961 and 1962 are here, among them its programme summary [Thornton and Rothstein 1962] and a section on the XNJ140E nuclear turbojet [Layman 1962], together with powerplant studies of nuclear turbojets for Pratt and Whitney engines. No title among them names the X-39 engines or the Heat Transfer Reactor Experiments, so the engine’s identity and its ground-test history in this article rest on the museum account and the encyclopaedia, as the section headed Epistemic State records.

Classification and administrative categories

Classification schemes as artefacts. The consequences of classification, residual categories, and the categories a bureaucracy constructs and applies.

29 records. [1992, Classification system Old and new categories] [1999, Categorical Work and Boundary Infrastructures Enriching Theories of Classification] [2000, Classification and Large-Scale Infrastructures] [Alpaten and Khurdianto 2026] [Auther 2002] [Barron et al 2016] [Cornwell 2019] [De Zwart 2005] [Delaisse et al 2025] [Dousa 2017] [Goldstein 2005] [Hickman and Hill 2010] [Horváth 2006] [Jansen 2013] [Karlander 2017] [Klaes 2003] [Li 2025] [Lyna and Bulten 2021] [Mennerat François 1997] [Moss 1964] [Moynihan et al 2023] [Olson 2007] [Parotte 2021] [Rajaram and Arendas 2013] [Star and Bowker 2007] [Star 2010] [Vetters 2007] [Williams 2003] [Willis 1993]

Military designation and nomenclature systems

Military and government designation and nomenclature treated as systems.

2 records. [1997, Appendix A Aircraft Nomenclature] [Department of the Air Force Washington Dc 1997]

The Future Aircraft Technology Enhancements programme

The programme the X-39 was reserved for.

1 record. [Cupstid 1997, Future Aircraft Technology Enhancements, Block I]

The one record is the Boeing Phase I report that the argument cites. NASA’s Technical Reports Server returns 33 records for the programme’s name and none of their titles names the programme, and the leading results of the same search of Crossref and of the Defense Technical Information Center reports that Crossref registers name it in no title either. The rest of what is known about the programme comes from the briefing and the trade report that the earlier sections cite.

Where the Framing Breaks Down

The series framing assumes that a number names a vehicle, and here that assumption fails completely. Every other article in the series begins from an aircraft and asks what it was built to measure. The X-39 offers no aircraft, so the framing’s research question, sizing and flight record are all empty, and the sections carrying those titles in this article say so and leave them empty.

The framing also invites the wrong reading of the gap. A register of experimental aircraft suggests that a missing number hides an aircraft. The record shows instead a reservation that was never followed by a request and never cancelled [Missing USAF and DOD Aircraft Designations], so the gap is part of the history of correspondence and not of flight.

The replacement framing that this article offers has a weak joint, and the joint is named here. The permanence argument depends on a rule that a release stops helping once the sequence has passed the number, and that rule is drawn from Parsch’s description of practice rather than from any regulation [Allocation of Official Aerospace Vehicle MDS Designations]. Epistemic State labels it as assumed.

The designation string does not stay inside one register. The same X-39 names a nuclear turbojet that ran on a ground stand [General Electric J47], so any framing that treats a designation as a unique name for one thing breaks at this number.

The Source Base

The eighteen sources that carry the argument and the 768 research records that map the field are different things, although two belong to both.

The evidentiary base is small because the subject is. Eighteen entries carry the argument, being two designation surveys, three versions of the joint instruction, two Department-level issuances, four parts of the Lockheed Martin Phase I briefing, two parts of the Boeing Phase I briefing, the Boeing technical report, one trade report of the cancellation, one account of the X-40 allocation and two of the nuclear turbojet. Twelve of those eighteen are primary documents, which is an unusually high share and a consequence of the subject being administrative. The governing instructions are not commentary on the evidence. They are the evidence.

The research survey holds 768 records, drawn from the scholarly registry Crossref, which also registers Department of Energy and Defense Technical Information Center reports, and from NASA’s Technical Reports Server, together with one report at the Defense Technical Information Center’s own address and one museum article. Of them, 34 are report primaries, being 4.4 percent, by the rule set out under The Definition of Primary. The median publication year is 2015. No paper in the survey has been read beyond its title, and apart from the Boeing report and the museum article, which the argument cites, no survey record is cited in support of any claim about the X-39. The article would say exactly the same things if the rest of the survey were deleted.

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 it is a NASA Technical Reports Server record, a Defense Technical Information Center report at that centre’s own address or under the 10.21236 prefix that Crossref registers for it, or a Department of Energy report under the 10.2172 prefix that Crossref registers for the Office of Scientific and Technical Information. By that rule 30 primaries are Department of Energy reports, two are NASA reports and two are Defense Technical Information Center reports, and every one of them belongs to the clusters on military designation and nomenclature systems, nuclear propulsion of aircraft and the Future Aircraft Technology Enhancements programme. The rule does not reach the instructions, the briefings or the designation surveys, which are curated references rather than research records, so 4.4 percent describes the survey and not the argument, twelve of whose eighteen sources are primary.

Thin Subjects

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

Subject	Title terms counted	Records
designation systems for military and aerospace vehicles	designation, designations, designator, designators, designating, nomenclature together with military, aircraft, aerospace, electronic equipment, electronics type, band	2
reserving a number or identifier before it is assigned	reserve, reserved, reservation, reserving together with number, numbers, identifier, identifiers, address, addresses, designation, domain name, domain names	0
releasing, reclaiming or reusing a number or identifier	reuse, reused, reclaim, reclamation, recycle, recycled, recycling, reassign, reassigned, re-registration, release together with number, numbers, identifier, identifiers, address, addresses, domain name, domain names	3
retiring or withdrawing an entry from a register	deprecation, deprecated, withdrawal, withdrawn, retire, retired, retirement, obsolete	0
the X-39 engine and the Heat Transfer Reactor Experiments	X-39, X39, Heat Transfer Reactor, HTRE	0
the Future Aircraft Technology Enhancements programme	Future Aircraft Technology Enhancements, Fixed Wing Vehicle	1

Designation as a system has two records, on aircraft nomenclature and the 1997 issuance on designating and naming military aerospace vehicles. Neither concerns the gaps in a designation series.

No title names the reservation of a number or identifier before it is assigned. The reservations that Crossref does hold under that vocabulary are Internet standards documents that reserve blocks of addresses, which the survey excludes as standards documents rather than works about reservation, so the act that consumed the X-39 has no literature here under its own name.

Of the three titles that name reuse, reclamation or re-registration together with an identifier, a number or a domain name, two concern reusing the identifier itself, one in a single system and one in the market for expired domain names. The third concerns the reuse of data.

No title names deprecating, withdrawing or retiring an entry from a register, although that is the operation whose absence made the X-39 permanent.

No title names the X-39 engines or the Heat Transfer Reactor Experiments, although the survey holds the programme’s own reports, as the cluster on nuclear propulsion of aircraft records.

The programme the number was reserved for has one record, the Boeing Phase I report that the argument cites.

What the Survey Admits

A record is admitted only when a person reading its title finds it on this article’s subject, which is how an official record or register is made, kept, gapped, reused, retired or released, and how an absence in a record is reasoned from, together with the two programmes that the number and its string belonged to. A shared word is not enough, and neither is the use of a technique in a distant field.

Excluded on that ground are many records that share this subject’s words. They include archival storage media, engineering infrastructure and its technical standards, category theory in mathematics, machine-learning classification, boundary objects in organisational coordination, the nomenclature and designation systems of alloys, teeth, fasteners and other objects, identifiers in source code, network engineering and security of the Domain Name System, trademark disputes over domain names and the procedures that settle them, protocol engineering of the transition between the two versions of the Internet Protocol, algorithmic assignment of cell identifiers and spectrum inside a network, the allocation, licensing and auction of radio spectrum, which is a physical resource and not a register of names or numbers, routing engineering of address allocation, market studies of number portability whose subject is competition, the curation of biological databases, clinical, laboratory and manufacturing documentation, clinical questions about the records that patients hold, the application of a classification scheme to the documents or resources of one subject, records used only as a data source, and studies of artworks, novels, films and performances that take the archive as their theme. Applications of the maxim that absence of evidence is not evidence of absence to one drug, disease, species or site are excluded as well, because their subject is the finding and not the inference. So are naval reactors, nuclear rockets and other nuclear subjects outside aircraft propulsion. Reviews of a single book, announcements, event notices, news items, columns, editorials, appendices of rules, dictionary, glossary and handbook reference entries, standards documents, and a second deposit or edition of the same document are excluded because they are not research works. A signed comment, discussion, reply or letter on a work within this subject is admitted, because it argues about a result, while an unsigned item of that kind and a referee’s report on a manuscript are not.

Some records of doubtful relevance are kept by judgement. They are chiefly historical and critical studies whose titles name the colonial archive without naming a silence, regulatory studies of number portability in a named country, and adoption studies of researcher identifiers. Each treats the making of a record, a number held under a rule or the take-up of an identifier, and a reader may weigh it accordingly.

No cluster is a residual. The largest, on recordkeeping, retention and provenance, holds 28.6 percent of the survey and is a named literature. A record that answers two clusters is counted once, so the cluster sizes are a partition of the survey and not a measure of how much literature addresses each theme.

Every research title has been read for relevance, so the off-topic share that remains is a matter of reading judgement. A title can mislead, and no paper has been read beyond its title, so the article claims a clean corpus nowhere.

Epistemic State

Historical Fact

The X-39 designation was reserved on 23 April 1997 for the Future Aircraft Technology Enhancements programme of the Air Force Research Laboratory, and no formal written request to allocate it was ever submitted, so it remained officially unassigned [Missing USAF and DOD Aircraft Designations]. FATE was a subset of the national Fixed Wing Vehicle programme, sponsored by Wright Laboratory at Dayton, Ohio, and structured in three phases of which only the first was executed. Lockheed Martin performed a Phase I study from June to October 1997 under contract F33615-97-C-3804 [Lockheed Martin FATE briefing], and Boeing Phantom Works performed a parallel Phase I study from 30 June to 30 October 1997 [Boeing FATE Block 1 Phase 1 briefing], reported in November 1997 [Cupstid 1997]. The Air Force subsequently cut the Fixed Wing Vehicle demonstrator, and the Uninhabited Combat Air Vehicle Advanced Technology Demonstration replaced the FATE activity [US Air Force cuts fixed wing FATE demonstrator plans] [Boeing FATE briefing, transition to UCAV-ATD]. The X-40A designation was allocated in 1997 [Boeing X-37 and X-40]. Air Force Joint Instruction 16-401 of 9 September 1994 required a written request for assignment and directed that designators not be used before approval [Air Force Joint Instruction 16-401].

Separately and unrelatedly, General Electric applied the designation X-39 to modified J47 turbojets adapted to run on reactor heat under the Aircraft Nuclear Propulsion programme. Those engines were operated on a ground test stand with the Heat Transfer Reactor Experiments one, two and three, and were intended for the Convair X-6 [General Electric J47]. This is an engine designation and has no connection to the aircraft designation beyond the shared string.

Verified Against the Primary List

The absence of the X-39 from the approved-designator list was verified against the document. The 12 May 2004 edition of DoD 4120.15-L was retrieved and read, and its aircraft appendix carries X-35A, X-36A, X-37A, X-38A, X-40A and X-40B with no X-39 entry anywhere in the document [DoD 4120.15-L].

The check is corroboration, and the article does not present it as demonstration. X-33 and X-34 are also absent from the same edition, and both were real programmes cancelled in March 2001 [Jenkins, Landis and Miller, American X-Vehicles], so absence from a 2004 list is consistent with several histories including retirement after cancellation. The list shows that the X-39 is not among the approved designators of record. It cannot by itself show that it never was.

Analysis

The formal treatment of the numbering restates the sources and adds no new fact. The set definitions, the next-issue rule and the recovery condition encode what the joint instruction and Parsch’s account of the practice already say in prose. Their value is that an ordering argument becomes checkable, and the conclusion for the X-39 follows from a cancellation time of infinity without needing any date at all.

One step in that treatment is assumed and is labelled as assumed in the text. Taken literally, the minimum of the available set would fall again whenever a lower number was released, and the C-16 case shows that a release can recover a number. The rule that a release stops helping once the sequence has passed the number is empirical, drawn from Parsch’s description of the practice, and the permanence argument leans on this step hardest. No regulation quoted here states it.

Inference

That the X-39 request was never submitted because the demonstrator was cancelled is an inference, not a statement any source makes directly. The sources establish that the reservation was for the FATE programme, that a research designation attaches to a vehicle, that the vehicle was FATE II’s product, that FATE II was cut, and that no request followed. The causal link between the last two is the natural reading, but no document records it. An alternative reading, that the request was forgotten while the programme was still alive, is not excluded by anything found here.

Whether the reuse window had closed before the cancellation is not settled by the dated facts, which are the allocation of X-40A during 1997 and a cut that fell between 30 October and 17 December 1997. Either answer depends on the reuse rule as Parsch states it, a description of practice for which no regulatory text has been found.

What the Record Does Not Settle

No source accessible here gives the month of the X-40A allocation, so the relative order of the X-39 reservation and the X-40 allocation within 1997 remains open, and no claim about displacement is made.

The exact date of the FATE demonstrator cancellation is not established here, but it is bounded. The Phase I studies ran to 30 October 1997 and their results fed the successor programme, so the cut came after that date, and Flight International reported on 17 December 1997 that the Air Force had scrapped the demonstrator [US Air Force cuts fixed wing FATE demonstrator plans]. The Federation of American Scientists page agrees [X-39, Federation of American Scientists]. It is marked as updated on 24 December 1998, although its text speaks of early 1999, and it describes the three phases while stating that Phase I was used to start the Uninhabited Combat Air Vehicle Advanced Technology Demonstration, which will replace the FATE activity. The NASA inventory of X-vehicles of 2003 describes the rest of FATE as seemingly cancelled before being started [Jenkins, Landis and Miller, American X-Vehicles].

A finer date would need a document that is not available here. A research and development budget justification would show the programme element and the year its funding stopped, and none has been found for this programme. The cut is bounded to seven weeks, and the month of the X-40A allocation is the missing fact that would order it against the passage of 39.

Who reserved the number is unknown. The sources name the programme and the laboratory, not an office or an individual, and the instruction’s routing implies a system programme office that no accessible source identifies.

Whether the reservation was ever formally released is not established. The absence of a cancellation is inferred from the number remaining unallocated and from Parsch listing X-39 among the reservations that produced permanent gaps, rather than from any document recording that no cancellation was filed.

The current instruction has not codified reservation, and that question is settled. The 1994, 2005 and 2020 versions of the joint instruction were all read directly, the 2020 text from a copy held on the designation survey site.

The nuclear turbojet is now sourced to a museum account as well as an encyclopaedia, and neither is primary. The Smithsonian article supplies the 1956 ground-test date and the intended installation on the X-6. No separate encyclopaedia article exists for the engine, and the account there sits within the article on the J47 from which it was derived. The two disagree on how many J47 units constituted one X-39, so no count is asserted. The reactor pairings should be treated as secondary reporting, and the engine is used here only to establish that the designation string was applied to hardware in a different series, which both sources support. The survey holds the General Electric programme’s own comprehensive technical reports, read by title only, and no title among them names the X-39 engines or the Heat Transfer Reactor Experiments.

One source conflict is recorded and left unresolved. The encyclopaedia article on FATE describes the programme as run by the Air Force Research Laboratory and the Defense Advanced Research Projects Agency jointly [Future Aircraft Technology Enhancements], while the contemporaneous sources here describe FATE as an Air Force programme whose demonstrator was cut because it would have duplicated a separate agency project. The second reading is better supported, but the conflict is left standing.

Out of Scope

A survey of the aerospace literature is omitted, because it would measure nothing relevant to whether a letter was written in 1997. The article’s subject is not an aircraft. It is what a gap in an official register means, and a large current literature addresses that question, so that literature is surveyed and the aerospace field is not.

The survey is therefore chiefly of archival science, classification, identifier and number-space administration and the logic of inference from absence, with two small clusters on the programmes that the number and its string belonged to. Apart from the Boeing Phase I report and the museum account of the nuclear turbojet, which the argument cites, no survey record is cited in support of any claim about the X-39.

There is no physical mathematics here, and the little that appears is not decoration. Engineering analysis of an aircraft derives a keystone relationship and dimensions dependent systems against it. A designation has no mass and no area, so there is nothing to size. There is nonetheless a binding constraint, since the numbering is a finite resource issued consecutively and never replenished, and the case for permanence depends on that mechanism. The eight relations above state that mechanism and nothing else. None of them is a physical law and none of them is dimensional, and they earn their place only by making an ordering argument checkable that prose states loosely.

The evidentiary base is small and is not padded, and the reference list is large for a different reason. Eighteen of the entries below carry the argument. They are the two designation surveys, the three versions of the joint instruction, the two Department-level issuances, the four parts of the Lockheed Martin briefing, the two parts of the Boeing briefing, the Boeing technical report, the trade report of the cancellation, the account of the X-40 allocation, and the two accounts of the nuclear turbojet. A further seventeen name, define or give context rather than bear the argument’s weight. Apart from the related posts in this series, everything beyond those thirty-five is the reference survey, which also holds the two research records among the eighteen, and which is otherwise a map of the surrounding field and not evidence, as the Source Base section above sets out.

Twelve of those eighteen are primary documents, being the three instructions, the two Department-level issuances, the six briefing parts and the technical report. That share is higher than a subject this thin would normally support, and the administrative nature of the subject explains it. The governing documents are the evidence here, rather than commentary on the evidence.

Conclusion

The X-39 is a gap in a public register that records nothing about aviation and something precise about administration. A number was informally set aside on 23 April 1997 for a demonstrator that a study beginning about ten weeks later was supposed to justify. The study finished, the demonstrator was cut in favour of work already being done elsewhere, and the written request that would have made the designation real was never written. The number was never refused, because it was never requested.

What made the gap permanent was a second absence rather than the first. Recovering a reserved number requires the original requester to cancel the reservation before the next number is allocated, and the next number was allocated in the same year the reservation was made, without any cancellation. The X-39 was never released, so it was never recovered.

The system that lost it had written down a procedure for reserving names and none for reserving numbers. The reservation happened anyway, through a channel with no rule attached, which meant there was no rule for releasing it either. A register maintained this way is a record of correspondence, and reading it as a record of aircraft, which is what a gap invites, gets the artefact wrong.

The sharpest evidence for that is a coincidence of naming. An X-39 does exist and did run. It denotes the nuclear-heated J47 turbojets built for the X-6 under a different designation series entirely. One register holds a number with no hardware and the other holds hardware under the same number, and nothing connects them except the string.

The next article returns to a vehicle that was built and flown.

References

Reference

• Active Aeroelastic Wing
• Aeronautical Systems Center
• Air Force Instruction 16-401, 2020
• Air Force Joint Instruction 16-401, 1994
• Air Force Joint Instruction 16-401, 2005
• Air Force Research Laboratory
• Aircraft Nuclear Propulsion
• Allocation of Official Aerospace Vehicle MDS Designations
• Boeing FATE Block 1 Phase 1 briefing
• Boeing FATE briefing, transition to UCAV-ATD
• Boeing Phantom Works
• Boeing X-37 and X-40
• Boeing X-40
• Boeing X-45
• Defense Advanced Research Projects Agency
• Defense Technical Information Center
• DoD 4120.15-L, Model Designation of Military Aerospace Vehicles
• DoD Instruction 4120.15, Designating and Naming Military Aerospace Vehicles
• FATE, conclusions and recommendations
• FATE, five technologies selected for study
• Future Aircraft Technology Enhancements
• General Electric J47
• Jenkins, Landis and Miller, American X-Vehicles
• Lockheed Martin FATE briefing
• Lockheed Martin FATE briefing, full deck
• Missing USAF and DOD Aircraft Designations
• United States Department of Defense Aerospace Vehicle Designation
• United States Tri-Service Aircraft Designation System of 1962
• Unmanned Combat Aerial Vehicle
• US Air Force cuts fixed wing FATE demonstrator plans
• Wright Laboratory
• X-39, Federation of American Scientists
• X39

Related Post

• X-Planes: Aerojet X-8 Aerobee
• X-Planes: Bell X-1
• X-Planes: Bell X-14
• X-Planes: Bell X-16
• X-Planes: Bell X-2
• X-Planes: Bell X-22
• X-Planes: Bell X-5
• X-Planes: Bell X-9 Shrike
• X-Planes: Bensen X-25
• X-Planes: Boeing X-20 Dyna-Soar
• X-Planes: Boeing X-32
• X-Planes: Boeing X-37
• X-Planes: Convair X-11
• X-Planes: Convair X-12
• X-Planes: Convair X-6
• X-Planes: Curtiss-Wright X-19
• X-Planes: Douglas X-3 Stiletto
• X-Planes: Framing and the Research Aircraft Model
• X-Planes: Grumman X-29
• X-Planes: Hiller X-18
• X-Planes: Lockheed Martin X-33
• X-Planes: Lockheed Martin X-35
• X-Planes: Lockheed X-17
• X-Planes: Lockheed X-27
• X-Planes: Lockheed X-7
• X-Planes: Martin Marietta X-23 PRIME and a Contested Assignment
• X-Planes: Martin Marietta X-24
• X-Planes: McDonnell Douglas X-36
• X-Planes: North American X-10
• X-Planes: North American X-15
• X-Planes: Northrop X-21
• X-Planes: Northrop X-4 Bantam
• X-Planes: Orbital Sciences X-34
• X-Planes: Osprey X-28 Sea Skimmer
• X-Planes: Rockwell X-30 and the National Aero-Space Plane
• X-Planes: Rockwell-MBB X-31
• X-Planes: Ryan X-13 Vertijet
• X-Planes: Scaled Composites X-38
• X-Planes: Schweizer X-26 Frigate

Research

• 1992, Classification system Old and new categories
• 1994, THREE Archival Memory and the Destruction of the Past
• 1997, Appendix A Aircraft Nomenclature
• 1998, RECORDKEEPING REQUIREMENTS
• 1998, Retention and Disposition of Records
• 1999, Categorical Work and Boundary Infrastructures Enriching Theories of Classification
• 1999, Executive briefing The Digital Object Identifier*
• 1999, Management of Active Records
• 2000, Classification and Large-Scale Infrastructures
• 2000, Recordkeeping
• 2000, Retention of Records
• 2002, Six. The Imperial Archive Colonial Knowledge and Colonial Rule
• 2003, Number portability
• 2003, Records management
• 2004, CHAPTER FOUR. Archival appraisal briefly reviewed in historical context
• 2004, CHAPTER TWO. Appraisal and archival appraisal in modern organizations purpose, participants, roles and responsibilities
• 2005, CHAPTER THREE. The corporate memory and Records Management
• 2005, Price controls and numbering administration Cartesian-bureaucratic issue-processing episodes
• 2006, Identity, trust, evidence and the recordkeeping nexus
• 2006, Legal and social relationships and the recordkeeping nexus
• 2006, OSHA Recordkeeping
• 2006, Recordkeeping participants legal and ethical responsibilities
• 2006, Recordkeeping regulatory models in the web environment
• 2006, The recordkeeping-ethics-law nexus and recordkeeping regulatory models
• 2007, 4. Collecting Colonial Postcards Gender and the Visual Archive
• 2007, Introduction Of Soft and Savage Bodies in the Colonial Domestic Archive
• 2007, Records and recordkeeping
• 2007, Records Management and Records Management Policies
• 2008, Chapter 5 Issues of Name Authority Control in the Cataloging of Theses and Dissertations
• 2009, Archival Appraisal and Acquisition
• 2009, Chapter 7. Confessing in the absence of recording linguistic and extralinguistic evidence of coercion in a police interrogation
• 2009, Digital Object Identifier DOI® System
• 2009, Government Records Management/Recordkeeping
• 2009, ORCID
• 2009, Provenance of Archival Materials
• 2009, Records Retention Schedules
• 2009, The Internet Address Space
• 2011, Digital Object Identifier DOI® System
• 2011, Office and records management
• 2012, Authority Control Experiences and Projects
• 2012, Authority Control in the Context of Bibliographic Control in the Electronic Environment
• 2012, Authority Control of Creators and the Second Edition of ISAAR CPF, International Standard Archival Authority Record for Corporate Bodies, Persons, and Families
• 2012, Authority Control State of the Art and New Perspectives
• 2012, Challenges of the dynamic e-environment for records management
• 2012, Critical Considerations for Records Management and Retention
• 2012, Domain name registration systems in OECD countries surveys, illustrations and analysis
• 2012, Liberalization, consumer protection and growth the tug and pull relationship in the domain name registration system
• 2012, Recordkeeping Requirements
• 2012, Scheduling Records for Retention
• 2012, The domain name registration system an overview
• 2013, Alienated memories migrants and the silences of the archive
• 2013, Anxieties of Authorship in the Colonial Archive
• 2013, Data Management and Records Retention
• 2014, Documentation and Recordkeeping
• 2014, Whose Lessons? Subjects of the Colonial Archive
• 2015, 11 Epilogue The Colonial Gaze, History and the Archive
• 2015, PID Service an advanced persistent identifier management service for the Semantic Web
• 2015, Sustainable Electronic Court Records Risk Management Conceptions and Development of Framework
• 2015, Your name is not good enough introducing the ORCID researcher identifier at Imperial College London
• 2016, CHAPTER 34. CDS Records Retention
• 2016, Records Management
• 2016, The records management function
• 2017, Acquisition and archival appraisal
• 2017, Name Authority Control
• 2017, ORCID the challenge ahead
• 2017, The Management of Native Estate Records in the NAN
• 2018, Administrative Records and the Courts
• 2019, A global ‘private’ regime governing the Domain Name System DNS
• 2019, Effectiveness of Appraisal of Public Records in Tanzania Findings from President’s Office Records and Archives Management Department PO-RAMD
• 2019, Sources of Law Governing the Management of Police Records
• 2021, Archival Memory and the Destruction of the Past
• 2021, Records Management nach ISO 15489
• 2022, 1 Colonial Listening and the Making of a Sound Archive
• 2023, Introduction Absence of Evidence or Evidence of Absence?
• 2024, 4. Silences Take Two Gendered Archival Lacunae
• 2024, CHAPTER TWO Silence, Perseverance, and Survival in State-Run Philippine Film Archives
• 2024, Gaps in the Archive
• 2024, Name Authority Considerations
• 2025, 4 Archival Silence, Sexual Violence
• 2026, Archival Absences, Silences and Fragments The Unmade and Film History
• 2026, Medical Records as Legal Evidence Challenges in Documentation, Disclosure, and Retention in Healthcare Practice
• 2026, Navigating Archival Silences Through Art
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