View source for Engineering:North American X-15

{{Short description|Rocket-powered aircraft and spaceplane operated by the US Air Force and NASA}}
{|{{Infobox aircraft begin
| name = X-15
| image = File:X-15 flying.jpg
| caption = The X-15 pulling away from its drop launch plane
| alt = Black rocket aircraft with stubby wings and short vertical stabilizers above and below tail unit
}}{{Infobox aircraft type
| type = [[Engineering:Experimental aircraft|Experimental]] high-speed [[Engineering:Rocket-powered aircraft|rocket-powered]] [[Research|research]] aircraft
| manufacturer = [[Company:North American Aviation|North American Aviation]]
| first flight = 8 June 1959 <ref name="nasa20140228" />
| introduced = 17 September 1959
| retired = December 1968 <ref name="nasa20140228" />
| status = 
| primary user = {{wipe|United States Air Force}}
| more users = [[Organization:NASA|NASA]]
| produced = 
| number built = 3 
| developed from = 
| variants with their own articles = 
}}
|}

The '''North American X-15''' is a [[Physics:Hypersonic speed|hypersonic]] [[Engineering:Rocket-powered aircraft|rocket-powered aircraft]] operated by the {{wipe|United States Air Force}} and the [[Organization:NASA|National Aeronautics and Space Administration (NASA)]] as part of the [[Engineering:List of X-planes|X-plane]] series of [[Engineering:Experimental aircraft|experimental aircraft]]. The X-15 set speed and altitude records in the 1960s, crossing the [[Earth:Kármán line|edge of outer space]] and returning with valuable data used in [[Engineering:Aircraft|aircraft]] and [[Engineering:Spacecraft|spacecraft]] design. The X-15's highest speed, {{convert|4520|mph|km/h m/s|0}},<ref name="nasa20140228">{{cite web |url=http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-052-DFRC.html |title=NASA Armstrong Fact Sheet: X-15 Hypersonic Research Program |publisher=NASA |editor-first=Yvonne |editor-last=Gibbs |date=28 February 2014 |access-date=4 October 2015 |quote= 4,520 mph (Mach 6.7 on Oct. 3, 1967,}}</ref>  was achieved on 3{{spaces}}October 1967,<ref>{{cite web|first1=Caroline|last1=Haskins|first2=Brian|last2=Anderson|first3=Jason|last3=Koebler|access-date=2019-02-05|title=Why the Piloted Flight Speed Record Hasn't Been Broken in 50 Years|url=https://motherboard.vice.com/en_us/article/qvj5vp/piloted-aircraft-flight-speed-record-1967-nasa-air-force-william-knight-x-15|date=6 October 2017}}</ref> when William J. Knight flew at [[Physics:Mach number|Mach]]{{spaces}}6.7 at an altitude of {{convert|102100|ft|m|sigfig=4}}, or 19.34{{spaces}}miles. This set the [[Physics:Flight airspeed record|official world record]] for the highest speed ever recorded by a crewed, powered aircraft, which remains unbroken.<ref name="Fastest">{{cite web |url=http://www.aerospaceweb.org/aircraft/research/x15/ |title=North American X-15 High-Speed Research Aircraft |work=Aerospaceweb.org |year=2010 |access-date=24 November 2008}}</ref><ref>{{cite web |url=https://www.cnn.com/style/article/x-15-rocket-aircraft/index.html |title=X-15: The fastest manned rocket plane ever |author=Jacopo Prisco |date=July 28, 2020 |work=CNN |access-date=November 12, 2020}}</ref>

During the X-15 program, 12{{spaces}}pilots flew a combined 199{{spaces}}flights.<ref name="nasa20140228" /> Of these, 8{{spaces}}pilots flew a combined 13{{spaces}}flights which met the Air Force [[Astronomy:Human spaceflight|spaceflight]] criterion by exceeding the altitude of {{convert|50|mi|km}}, thus qualifying these pilots as being [[Astronaut|astronaut]]s; of those 13{{spaces}}flights, two (flown by the same civilian pilot) met the [[Organization:Fédération Aéronautique Internationale|FAI]] definition ({{convert|100|km|mi}}) of [[Astronomy:Outer space|outer space]]. The 5{{spaces}}Air Force pilots qualified for military astronaut wings immediately, while the 3{{spaces}}civilian pilots were eventually awarded NASA astronaut wings in 2005, 35{{spaces}}years after the last X-15 flight.{{sfn|Jenkins|2001|p=10}}<ref name="NASA">{{cite press release |url=http://www.nasa.gov/home/hqnews/2005/aug/HQ_05233_X-15_pilots_honored.html |title=NASA Honors High Flying Space Pioneers |publisher=NASA |first1=Elvia H. |last1=Thompson |first2=Frederick A. |last2=Johnsen |date=23 August 2005 |id=Release 05-233}}</ref>

==Design and development==
[[File:X-15 in flight.jpg|thumb|right|X-15 after igniting rocket engine]]
[[File:X-15A2 NB-52B 3.jpg|thumb|right|X-15A-2, with sealed ablative coating, [[Engineering:Drop tank|external fuel tanks]], and ramjet dummy test]]
The X-15 was based on a concept study from [[Biography:Walter Dornberger|Walter Dornberger]] for the [[Organization:National Advisory Committee for Aeronautics|National Advisory Committee for Aeronautics]] (NACA) of a [[Physics:Hypersonic speed|hypersonic]] research aircraft.{{sfn|Käsmann|1999|p=105}} The [[Finance:Request for proposal|requests for proposal]] (RFPs) were published on 30{{spaces}}December 1954 for the airframe and on 4{{spaces}}February 1955 for the [[Engineering:Rocket engine|rocket engine]]. The X-15 was built by two manufacturers: [[Company:North American Aviation|North American Aviation]] was contracted for the airframe in November 1955, and [[Company:Reaction Motors|Reaction Motors]] was contracted for building the engines in 1956.

Like many X-series aircraft, the X-15 was designed to be carried aloft and [[Engineering:Drop test|drop launched]] from under the wing of a [[Engineering:Boeing B-52 Stratofortress|B-52]] [[Engineering:Mother ship|mother ship]].  Air Force NB-52A, "The High and Mighty One" (serial 52-0003), and NB-52B, "The Challenger" (serial 52-0008, also known as ''[[Engineering:Balls 8|Balls 8]]'') served as carrier planes for all X-15 flights.  Release of the X-15 from NB-52A took place at an altitude of about {{convert|8.5|mi|km}} and a speed of about {{convert|805|km/h|mph|order=flip|sp=us}}.<ref name="E-4942">{{cite web |url=http://www.dfrc.nasa.gov/Gallery/Photo/X-15/HTML/E-4942.html |title=X-15 launch from B-52 mothership |publisher=Armstrong Flight Research Center |date=6 February 2002 |id=Photo E-4942}}</ref> The X-15 fuselage was long and cylindrical, with rear [[Engineering:Aircraft fairing|fairings]] that flattened its appearance, and thick, dorsal and ventral wedge-fin stabilizers. Parts of the fuselage (the outer skin<ref name=NASA-FS>{{Cite web|url=http://www.nasa.gov/centers/armstrong/news/FactSheets/FS-052-DFRC.html|title=NASA Dryden Fact Sheets - X-15 Hypersonic Research Program|first=Yvonne|last=Gibbs|date=13 August 2015|website=NASA}}</ref>) were heat-resistant [[Chemistry:Nickel|nickel]] [[Chemistry:Alloy|alloy]] ([[Chemistry:Inconel|Inconel]]-X{{spaces}}750).{{sfn|Käsmann|1999|p=105}} The retractable landing gear comprised a nose-wheel carriage and two rear skids. The skids did not extend beyond the ventral [[Biology:Fin|fin]], which required the pilot to jettison the lower fin just before landing. The lower fin was recovered by parachute.

===Cockpit and pilot systems===
[[File:North American X-15A-2 cockpit 2 USAF.jpg|thumb|left|Cockpit of an X-15]]

The X-15 was the product of developmental research, and changes were made to various systems over the course of the program and between the different models. The X-15 was operated under several different scenarios, including attachment to a launch aircraft, drop, main engine start and acceleration, ballistic flight into thin air/space, re-entry into thicker air, unpowered glide to landing, and direct landing without a main-engine start. The main rocket engine operated only for a relatively short part of the flight but boosted the X-15 to its high speeds and altitudes. Without main engine thrust, the X-15's instruments and control surfaces remained functional, but the aircraft could not maintain altitude.

As the X-15 also had to be controlled in an environment where there was too little air for aerodynamic flight control surfaces, it had a [[Engineering:Reaction control system|reaction control system]] (RCS) that used rocket thrusters.<ref name=paul /> There were two different X-15 pilot control setups: one used three joysticks, the other, one joystick.<ref name=jarvis />

The X-15 type with multiple control sticks for the pilot placed a traditional center stick between a left 3-axis joystick that sent commands to the Reaction Control System,<ref name=paul2 /> and a third joystick on the right used during high-G maneuvers to augment the center stick.<ref name=paul2 /> In addition to pilot input, the X-15 "Stability Augmentation System" (SAS) sent inputs to the aerodynamic controls to help the pilot maintain [[Astronomy:Spacecraft attitude control|attitude control]].<ref name=paul2 /> The Reaction Control System (RCS) could be operated in two modes&nbsp;– manual and automatic.<ref name=jarvis /> The automatic mode used a feature called "Reaction Augmentation System" (RAS) that helped stabilize the vehicle at high altitude.<ref name=jarvis>{{cite book |url=http://www.nasa.gov/centers/dryden/pdf/87709main_H-364.pdf |title=Operational Experience With the X-15 Reaction Control and Reaction Augmentation Systems |publisher=NASA |first1=Calvin R. |last1=Jarvis |first2=Wilton P. |last2=Lock |year=1965 |id=TN D-2864 |oclc=703664750}}</ref> The RAS was typically used for approximately three minutes of an X-15 flight before automatic power off.<ref name=jarvis />

The alternative control setup used the MH-96 flight control system, which allowed one joystick in place of three and simplified pilot input.<ref name=goleta>{{cite web |url=http://www.air-and-space.com/x-15%20forty%20years%20later%20200202.htm |title=Forty Years ago in the X-15 Flight Test Program, November 1961–March 1962 |publisher=Goleta Air & Space Museum |access-date=3 October 2011}}</ref> The MH-96 could automatically blend aerodynamic and rocket controls, depending on how effective each system was at controlling the aircraft.<ref name=goleta />

Among the many controls were the rocket engine throttle and a control for jettisoning the ventral tail fin.<ref name=paul2 /> Other features of the cockpit included heated windows to prevent icing and a forward headrest for periods of high deceleration.<ref name=paul2 />

The X-15 had an ejection seat designed to operate at speeds up to {{convert|4|Mach|altitude_ft=120000}} and/or {{convert|120000|ft|km}} altitude, although it was never used during the program.<ref name=paul2>{{cite web |url=http://www.sierrafoot.org/x-15/pirep2.html |title=X-15 Pilot Report, Part 2: X-15 Cockpit Check |work=SierraFoot.org |first=Paul |last=Raveling |access-date=1 October 2011}}</ref> In the event of ejection, the seat was designed to deploy fins, which were used until it reached a safer speed/altitude at which to deploy its main parachute.<ref name=paul2 /> Pilots wore pressure suits, which could be pressurized with nitrogen gas.<ref name=paul2 /> Above {{convert|35000|ft|km}} altitude, the cockpit was pressurized to {{convert|3.5|psi|kPa atm|abbr=on}} with nitrogen gas, while oxygen for breathing was fed separately to the pilot.<ref name=paul2 />

===Propulsion===
[[File:NASM-SI-2008-5570.jpg|thumb|right|X-15 tail with XLR-99]]

The initial 24 powered flights used two Reaction Motors XLR11 [[Engineering:Liquid-propellant rocket|liquid-propellant rocket]] engines, enhanced to provide a total of {{convert|16000|lb-f|kN}} of thrust as compared to the {{convert|6000|lb-f|kN}} that a single XLR11 provided in 1947 to make the [[Engineering:Bell X-1|Bell X-1]] the first aircraft to fly faster than the [[Physics:Speed of sound|speed of sound]]. The XLR11 used ethyl alcohol and [[Physics:Liquid oxygen|liquid oxygen]].

By November 1960, Reaction Motors delivered the XLR99 rocket engine, generating {{convert|57000|lb-f|kN}} of thrust. The remaining 175{{spaces}}flights of the X-15 used XLR99 engines, in a single engine configuration. The XLR99 used [[Chemistry:Ammonia|anhydrous ammonia]] and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered propellants to the engine.<ref name=paul /> It could burn {{convert|15000|lb|kg|0}} of propellant in 80{{spaces}}seconds;<ref name=paul /> Jules Bergman titled his book on the program ''Ninety Seconds to Space'' to describe the total powered flight time of the aircraft.<ref name="gale196110">{{cite magazine |url=https://archive.org/stream/Galaxy_v20n01_1961-10#page/n173/mode/2up |title=Galaxy's 5-Star Shelf |magazine=Galaxy Magazine |first=Floyd C. |last=Gale |volume=20 |issue=1 |page=174 |date=October 1961}}</ref>

The X-15 [[Engineering:Reaction control system|reaction control system]] (RCS), for maneuvering in the low-pressure/density environment, used [[Chemistry:High-test peroxide|high-test peroxide]] (HTP), which decomposes into water and oxygen in the presence of a catalyst and could provide a [[Physics:Specific impulse|specific impulse]] of {{convert|140|isp|abbr=on}}.<ref name="jarvis" />{{sfn|Davies|2003|p=8.28}} The HTP also fueled a turbopump for the main engines and [[Engineering:Auxiliary power unit|auxiliary power unit]]s (APUs).<ref name=paul>{{cite web |url=http://www.sierrafoot.org/x-15/pirep1.html |title=X-15 Pilot Report, Part 1: X-15 General Description & Walkaround |work=SierraFoot.org |first=Paul |last=Raveling |access-date=30 September 2011}}</ref> Additional tanks for helium and liquid nitrogen performed other functions; the fuselage interior was [[Astronomy:Purging (gas)|purged]] with helium gas, and liquid nitrogen was used as coolant for various systems.<ref name=paul />

===Wedge tail and hypersonic stability===
[[File:X-15 and B-52 Mother ship.jpg|thumb|X-15 attached to its B-52 mother ship with a [[Engineering:Northrop T-38 Talon|T-38]] flying nearby]]

The X-15 had a thick wedge tail to enable it to fly in a steady manner at hypersonic speeds.<ref name=wendell>{{cite book |url=http://www.hq.nasa.gov/office/pao/History/SP-60/cover.html |title=X-15 Research Results: With a Selected Bibliography |publisher=NASA |first=Wendell H. |last=Stillwell |year=1965 |id=NASA SP-60 |oclc=44275779 |access-date=4 May 2003 |archive-date=13 April 2022 |archive-url=https://web.archive.org/web/20220413203342/http://www.hq.nasa.gov/office/pao/History/SP-60/cover.html |url-status=dead }}</ref> This produced a significant amount of base drag at lower speeds;<ref name=wendell /> the blunt end at the rear of the X-15 could produce as much drag as an entire [[Engineering:Lockheed F-104 Starfighter|F-104 Starfighter]].<ref name=wendell />
{{Blockquote|A wedge shape was used because it is more effective than the conventional tail as a stabilizing surface at hypersonic speeds. A vertical-tail area equal to 60 percent of the wing area was required to give the X-15 adequate directional stability.|Wendell H. Stillwell|[https://history.nasa.gov/SP-60/ch-3.html X-15 Research Results (SP-60)]}} Stability at hypersonic speeds was aided by side panels that could be extended from the tail to increase the overall surface area, and these panels doubled as air brakes.<ref name=wendell />

==Operational history==
[[File:1959-06-11 X-15 Aloft.ogv|thumb|thumbtime=17|Newsreel showing a test flight of the X-15 in 1959]]

Before 1958, {{wipe|United States Air Force}} (USAF) and NACA officials discussed an orbital X-15 [[Astronomy:Spaceplane|spaceplane]], the '''X-15B''' that would launch into [[Astronomy:Outer space|outer space]] from atop an [[Engineering:SM-64 Navaho|SM-64 Navaho]] missile. This was canceled when the NACA became [[Organization:NASA|NASA]] and adopted [[Astronomy:Project Mercury|Project Mercury]] instead.

By 1959, the [[Engineering:Boeing X-20 Dyna-Soar|Boeing X-20 Dyna-Soar]] space-glider program was to become the USAF's preferred means for launching military crewed spacecraft into orbit. This program was canceled in the early 1960s before an operational vehicle could be built.{{sfn|Jenkins|2001|p=10}} Various configurations of the Navaho were considered, and another proposal involved a Titan I stage.<ref name=wade/>

Three X-15s were built, flying 199{{spaces}}test flights, the last on 24{{spaces}}October 1968.

The first X-15 flight was an unpowered glide flight by Scott Crossfield, on 8{{spaces}}June 1959. Crossfield also piloted the first powered flight on 17{{spaces}}September 1959, and his first flight with the XLR-99 [[Engineering:Rocket engine|rocket engine]] on 15{{spaces}}November 1960. Twelve test pilots flew the X-15. Among these were Neil Armstrong, later a NASA [[Astronaut|astronaut]] and the first man to set foot on the Moon, and Joe Engle, later a commander of [[Engineering:Space Shuttle program|NASA Space Shuttle]] missions.

In a 1962 proposal, NASA considered using the B-52/X-15 as a launch platform for a Blue Scout rocket to place satellites weighing up to {{convert|150|lb|kg}} into orbit.<ref name="wade">{{cite encyclopedia |url=http://www.astronautix.com/lvs/x15scout.htm |title=X-15/Blue Scout |encyclopedia=Encyclopedia Astronautica |first=Mark |last=Wade |access-date=30 September 2011 |url-status=dead |archive-url=https://web.archive.org/web/20111011061550/http://astronautix.com/lvs/x15scout.htm |archive-date=11 October 2011 |df=dmy-all}}</ref><ref name="cis20120321">{{cite web |url=http://www.citizensinspace.org/2012/03/historical-note-blue-scout-x-15/ |title=Historical note: Blue Scout / X-15 |work=Citizensinspace.org |date=21 March 2012}}</ref>

In July and August 1963, pilot [[Biography:Joseph A. Walker|Joe Walker]] exceeded {{nowrap|100 km}} in altitude, joining NASA astronauts and Soviet [[Biography:Cosmonaut|cosmonaut]]s as the first human beings to cross that line on their way to [[Astronomy:Outer space|outer space]]. The USAF awarded astronaut wings to anyone achieving an altitude of {{convert|50|mi|km}}, while the [[Organization:Fédération Aéronautique Internationale|FAI]] set the [[Earth:Kármán line|limit of space]] at {{convert|100|km|mi|1|sp=us}}.

On 15{{spaces}}November 1967, U.S. Air Force test pilot Major Michael J. Adams was killed during [[Engineering:X-15 Flight 3-65-97|X-15 Flight 191]] when X-15-3, {{nowrap|AF Ser. No. 56-6672}}, entered a hypersonic spin while descending, then oscillated violently as aerodynamic forces increased after re-entry. As his aircraft's flight control system operated the control surfaces to their limits, acceleration built to {{convert|15|g0|lk=on|abbr=on}} vertical and {{convert|8.0|g0|lk=on|abbr=on}} lateral.  The airframe broke apart at {{convert|60000|ft|km}} altitude, scattering the X-15's wreckage across {{convert|50|sqmi|km2}}.  On 8{{spaces}}May 2004, a monument was erected at the cockpit's locale, near Johannesburg, California.<ref name="xpress2004">{{cite journal |url=http://www.nasa.gov/centers/dryden/news/X-Press/stories/2004/073004/ppl_adams.html |title=Michael Adams: Remembering a Fallen Hero |journal=The X-Press |first=Peter W. |last=Merlin |volume=46 |issue=6 |date=30 July 2004}}</ref> Major Adams was posthumously awarded Air Force astronaut wings for his final flight in X-15-3, which had reached an altitude of {{convert|81.1|km|mi|order=flip|sp=us}}. In 1991, his name was added to the [[Astronomy:Space Mirror Memorial|Astronaut Memorial]].<ref name="xpress2004" />

[[File:X-15 Crash at Mud Lake, Nevada - GPN-2000-000120.jpg|thumb|X-15-2 crash at Mud Lake, Nevada]]

The second plane, X-15-2, was rebuilt <ref>{{harvnb|Evans|2013a|p=183}}</ref> after a landing accident on 9{{spaces}}November 1962 which damaged the craft and injured its pilot, John McKay.<ref>{{harvnb|Evans|2013a|p=143}}</ref> The new plane renamed '''X-15A-2''', had a new 28 -in. fuselage extension to carry liquid hydrogen.<ref name="nasa20140228" /> It was lengthened by {{convert|2.4|ft|cm}}, had a pair of auxiliary fuel tanks attached beneath its [[Engineering:Fuselage|fuselage]] and wings, and a complete heat-resistant [[Astronomy:Atmospheric entry#Ablative|ablative]] coating was added. It took flight for the first time on 25{{spaces}}June 1964. It reached its maximum speed of {{convert|7274|km/h|mph|order=flip|sp=us}} in October 1967 with pilot William "Pete" Knight of the {{wipe|U.S. Air Force}} in control.

Five principal aircraft were used during the X-15 program: three X-15 planes and two modified "nonstandard" [[Engineering:Boeing B-52 Stratofortress|NB-52]] bombers:
* {{nowrap|'''X-15-1''' –}} ''56-6670'', 81 free flights
* {{nowrap|'''X-15-2''' (later '''X-15A-2''') –}} ''56-6671'', 31 free flights as X-15-2, 22 free flights as X-15A-2; 53 in total
* {{nowrap|'''X-15-3''' –}} ''56-6672'', 65 free flights, including the [[Engineering:X-15 Flight 3-65-97|Flight 191]] disaster
* {{nowrap|'''NB-52A''' –}} ''52-003'' nicknamed ''The High and Mighty One'' (retired in October 1969)
* {{nowrap|'''NB-52B''' –}} ''[[Engineering:Balls 8|52-008]]'' nicknamed ''The Challenger'', later ''Balls 8'' (retired in November 2004)

Additionally, [[Engineering:North American F-100 Super Sabre|F-100]], [[Engineering:Lockheed F-104 Starfighter|F-104]] and [[Engineering:Douglas F5D Skylancer|F5D]] chase aircraft and [[Engineering:Lockheed C-130 Hercules|C-130]] and [[Engineering:Douglas C-47 Skytrain|C-47]] transports supported the program.<ref>{{cite book |url=https://www.nasa.gov/connect/ebooks/aero_x15_detail.html |title=X-15: Extending The Frontiers of Flight |publisher=NASA |last=Jenkins |first=Dennis R. |date=2010 |isbn=978-1-4700-2585-4}}</ref>

The 200th flight over Nevada was first scheduled for 21{{spaces}}November 1968, to be flown by William "Pete" Knight. Numerous technical problems and outbreaks of bad weather delayed this proposed flight six times, and it was permanently canceled on 20{{spaces}}December 1968. This X-15 (56-6670) was detached from the B-52 and then put into indefinite storage. The aircraft was later donated to the Smithsonian Air & Space Museum for display.

<gallery widths="180px">
File:Boeing NB-52A carrying X-15.jpg|NB-52A (s/n 52-003), permanent test variant, carrying an X-15, with mission markings; horizontal X-15 silhouettes denote glide flights, diagonal silhouettes denote powered flights.
File:X-15 launched bw.jpg|X-15 just after release.
File:NorthAmericanX-15600.jpeg|X-15 touching down on its skids, with the lower ventral fin jettisoned.
File:X15A2 with tanks.jpg|X-15A-2 (56-6671) with external fuel tanks
</gallery>

==Current static displays==
[[File:X-15 at the Mary Baker Engen Restoration Hangar.jpg|thumb|X-15 in the Mary Baker Engen Restoration Hangar. ]]
[[File:17 14 083 X15.jpg|thumb|X-15 at the USAF Museum]]
* X-15-1 (AF Ser. No. 56-6670) was on display in the National Air and Space Museum "Milestones of Flight" gallery, [[Place:Washington, D.C.|Washington, D.C.]], but is now in possession of the Mary Baker Engen Restoration Hangar at the Steven F. Udvar-Hazy Center.
* X-15A-2 (AF Ser. No. 56-6671) is at the National Museum of the United States Air Force, at Wright-Patterson Air Force Base, near Dayton, Ohio. It was retired to the museum in October 1969.{{sfn|USAF Museum Guidebook|1975|p=73}} The aircraft is displayed in the museum's Research and Development Gallery alongside other "X-planes", including the [[Engineering:Bell X-1|Bell X-1]]B and [[Engineering:Douglas X-3 Stiletto|Douglas X-3 Stiletto]].

===Mockups===
* Dryden Flight Research Center, [[Organization:Edwards Air Force Base|Edwards AFB]], California, United States (painted with {{nowrap|AF Ser. No. 56-6672}})
* Pima Air & Space Museum, adjacent to Davis-Monthan AFB, Tucson, Arizona (painted with {{nowrap|AF Ser. No. 56-6671}})
* Evergreen Aviation & Space Museum, McMinnville, Oregon (painted with {{nowrap|AF Ser. No. 56-6672}}). A full-scale wooden mockup of the X-15, it is displayed along with one of the rocket engines.

===Stratofortress mother ships===
[[File:Boeing NB-52B takeoff 061127-F-1234S-007.jpg|thumb|right|NB-52B ''Balls 8'' takes off with an X-15]]
* NB-52A (AF Ser. No. 52-003) is displayed at the Pima Air & Space Museum adjacent to {{wipe|Davis–Monthan Air Force Base}} in Tucson, Arizona. It launched the X-15-1 30{{spaces}}times, the X-15-2, 11{{spaces}}times, and the X-15-3 31{{spaces}}times (as well as the [[Engineering:Northrop M2-F2|M2-F2]] four times, the [[Engineering:Northrop HL-10|HL-10]] 11{{spaces}}times and the X-24A twice).
* NB-52B (AF Ser. No. [[Engineering:Balls 8|52-008]]) is on permanent display outside the north gate of [[Organization:Edwards Air Force Base|Edwards AFB]], California. It launched the majority of X-15 flights.

==Record flights==

[[File:X15 speed altitude.jpg|thumb|Key speed and altitude benchmarks of the X-15]]

===Highest flights===

During 13 of the 199 total X-15 flights, eight pilots flew above {{convert|264,000|ft|mi km}}, thereby qualifying as astronauts according to the US Armed Forces definition of the space border. All five Air Force pilots flew above 50{{spaces}}miles and were awarded military astronaut wings contemporaneously with their achievements, including Adams, who received the distinction posthumously following the flight{{spaces}}191 disaster.<ref>Jenkins (2000), Appendix 8, p. 117.</ref> However the other three were NASA employees and did not receive a comparable decoration at the time. In 2004, the Federal Aviation Administration conferred its first-ever commercial astronaut wings on Mike Melvill and Brian Binnie, pilots of the commercial [[Engineering:SpaceShipOne|SpaceShipOne]], another spaceplane with a flight profile comparable to the X-15's. Following this in 2005, NASA retroactively awarded its civilian astronaut wings to Dana (then living), and to McKay and Walker (posthumously).<ref>{{cite web |url=https://www.nasa.gov/missions/research/X-15_wings.html |title=X-15 Pioneers Honored as Astronauts |last=Johnsen |first=Frederick A. |date=23 August 2005 |website=NASA}}</ref><ref>{{cite web |url=https://www.space.com/1465-nasa-15-pilots-awarded-astronaut-wings.html |title=Former NASA X-15 Pilots Awarded Astronaut Wings |last=Pearlman |first=Robert Z. |date=23 August 2005 |website=space.com}}</ref> Forrest S. Petersen, the only Navy pilot in the X-15 program, never took the aircraft above the requisite altitude and thus never earned astronaut wings.

Of the thirteen flights, only {{nowrap|two{{tsp}}{{mdash}}}}{{tsp}}flights 90 and 91, piloted by {{nowrap|Walker{{tsp}}{{mdash}}}}{{tsp}}exceeded the {{convert|100|km|abbr=on}} altitude used by the FAI to denote the [[Earth:Kármán line|Kármán line]].

{| class="wikitable sortable"
|+ X-15 flights higher than 50 miles<ref name="harvnb|Evans|2013b|p=12">{{harvnb|Evans|2013b|p=12}}</ref>{{sfn|X-15 First Flight|1991|loc=Appendix A}}
|- style="background:#efefef;"
! Flight
! Date
! Top speed{{efn|name=speedmach|The [[Physics:Speed of sound#Altitude variation and implications for atmospheric acoustics|speed of sound]] in the atmosphere varies with altitude, so a comparatively lower airspeed (measured in mph or km/h) can correspond to a higher [[Physics:Mach number|Mach number]].<ref name="harvnb|Evans|2013b|p=12">{{harvnb|Evans|2013b|p=12}}</ref>}}
! Altitude
! Pilot
|-
| '''Flight 91'''
| 22 August 1963
|align="right"| {{convert|3794|mph|km/h|0|abbr=on}} (Mach 5.58)
|align="right"| {{convert|67.1|mi|km|abbr=on}}
| Joseph A. Walker
|-
| '''Flight 90'''
| 19 July 1963
|align="right"| {{convert|3710|mph|km/h|0|abbr=on}} (Mach 5.50)
|align="right"| {{convert|65.9|mi|km|abbr=on}}
| Joseph A. Walker
|-
| Flight 62
| 17 July 1962
|align="right"| {{convert|3832|mph|km/h|0|abbr=on}} (Mach 5.45)
|align="right"| {{convert|59.6|mi|km|abbr=on}}
| Robert M. White
|-
| Flight 174
| 1 November 1966
|align="right"| {{convert|3750|mph|km/h|0|abbr=on}} (Mach 5.46)
|align="right"| {{convert|58.1|mi|km|abbr=on}}
| William H. "Bill" Dana
|-
| Flight 150
| 28 September 1965
|align="right"| {{convert|3732|mph|km/h|0|abbr=on}} (Mach 5.33)
|align="right"| {{convert|56.0|mi|km|abbr=on}}
| John B. McKay
|-
| Flight 87
| 27 June 1963
|align="right"| {{convert|3425|mph|km/h|0|abbr=on}} (Mach 4.89)
|align="right"| {{convert|54.0|mi|km|abbr=on}}
| Robert A. Rushworth
|-
| Flight 138
| 29 June 1965
|align="right"| {{convert|3432|mph|km/h|0|abbr=on}} (Mach 4.94)
|align="right"| {{convert|53.1|mi|km|abbr=on}}
| Joe H. Engle
|-
| Flight 190
| 17 October 1967
|align="right"|  {{convert|3856|mph|km/h|0|abbr=on}} (Mach 5.53)
|align="right"| {{convert|53.1|mi|km|abbr=on}}
| William J. "Pete" Knight
|-
| Flight 77
| 17 January 1963
|align="right"| {{convert|3677|mph|km/h|0|abbr=on}} (Mach 5.47)
|align="right"| {{convert|51.5|mi|km|abbr=on}}
| [[Biography:Joseph A. Walker|Joseph A. Walker]]
|-
| Flight 143
| 10 August 1965
|align="right"| {{convert|3550|mph|km/h|0|abbr=on}} (Mach 5.20)
|align="right"| {{convert|51.3|mi|km|abbr=on}}
| Joe H. Engle
|-
| Flight 197
| 21 August 1968
|align="right"| {{convert|3443|mph|km/h|0|abbr=on}} (Mach 5.01)
|align="right"| {{convert|50.7|mi|km|abbr=on}}
| William H. Dana
|-
| Flight 153
| 14 October 1965
|align="right"| {{convert|3554|mph|km/h|0|abbr=on}} (Mach 5.08)
|align="right"| {{convert|50.5|mi|km|abbr=on}}
| Joe H. Engle
|-
| Flight 191
| 15 November 1967
|align="right"|  {{convert|3570|mph|km/h|0|abbr=on}} (Mach 5.20)
|align="right"| {{convert|50.4|mi|km|abbr=on}}
| Michael J. Adams<sup>†</sup>
|}<sup>†</sup> fatal

===Fastest recorded flights===

{| class="wikitable sortable"
|+ X-15 ten fastest flights<ref>{{harvnb|Evans|2013b}}</ref>{{sfn|X-15 First Flight|1991|loc=Appendix A}}
|- style="background:#efefef;"
! Flight
! Date
! Top speed{{efn|name=speedmach}}
! Altitude
! Pilot
|-
| Flight 188
| 3 October 1967
|align="right"| {{convert|4520|mph|km/h|0|abbr=on}} (Mach 6.70)
|align="right"| {{convert|19.3|mi|km|abbr=on}}
| William J. "Pete" Knight
|-
| Flight 175
| 18 November 1966
|align="right"| {{convert|4250|mph|km/h|0|abbr=on}} (Mach 6.33)
|align="right"| {{convert|18.7|mi|km|abbr=on}}
| William J. "Pete" Knight
|-
| Flight 59
| 27 June 1962
|align="right"| {{convert|4104|mph|km/h|0|abbr=on}} (Mach 5.92)
|align="right"| {{convert|23.4|mi|km|abbr=on}}
| [[Biography:Joseph A. Walker|Joseph A. Walker]]
|-
| Flight 45
| 9 November 1961
|align="right"| {{convert|4093|mph|km/h|0|abbr=on}} (Mach 6.04)
|align="right"| {{convert|19.2|mi|km|abbr=on}}
| Robert M. White
|-
| Flight 97
| 5 December 1963
|align="right"| {{convert|4018|mph|km/h|0|abbr=on}} (Mach 6.06)
|align="right"| {{convert|19.1|mi|km|abbr=on}}
| Robert A. Rushworth
|-
| Flight 64
| 26 July 1962
|align="right"| {{convert|3989|mph|km/h|0|abbr=on}} (Mach 5.74)
|align="right"| {{convert|18.7|mi|km|abbr=on}}
| Neil A. Armstrong
|-
| Flight 137
| 22 June 1965
|align="right"| {{convert|3938|mph|km/h|0|abbr=on}} (Mach 5.64)
|align="right"| {{convert|29.5|mi|km|abbr=on}}
| John B. McKay
|-
| Flight 89
| 18 July 1963
|align="right"| {{convert|3925|mph|km/h|0|abbr=on}} (Mach 5.63)
|align="right"| {{convert|19.8|mi|km|abbr=on}}
| Robert A. Rushworth
|-
| Flight 86
| 25 June 1963
|align="right"| {{convert|3911|mph|km/h|0|abbr=on}} (Mach 5.51)
|align="right"| {{convert|21.2|mi|km|abbr=on}}
| Joseph A. Walker
|-
| Flight 105
| 29 April 1964
|align="right"| {{convert|3906|mph|km/h|0|abbr=on}} (Mach 5.72)
|align="right"| {{convert|19.2|mi|km|abbr=on}}
| Robert A. Rushworth
|}

==Pilots==
<gallery mode=packed heights=300>
File:X-15 Pilots - GPN-2000-000143.jpg|The X-15 flight crew, left to right: Air Force Captain Joseph H. Engle, Air Force Major Robert A. Rushworth, NASA pilot John B. "Jack" McKay, Air Force Major William J. "Pete" Knight, NASA pilot Milton O. Thompson, and NASA pilot William H. Dana.
File:X-15 Pilots.jpg|The X-15 pilots clown around in front of the #2 aircraft. From left to right: Joseph Engle, Robert Rushworth, John McKay, William Knight, Milton Thompson, and William Dana.
</gallery>

{| class="wikitable sortable"
|+ X-15 pilots and their achievements during the program
|- style="background:#efefef;"
! Pilot
! Organization
! Year assigned<br />to X-15<ref name="whoswho">{{Cite book|url=https://archive.org/details/whoswhoinspace00cass|title=Who's Who in Space|last=Cassutt|first=Michael|date=November 1998|publisher=Macmillan Library Reference|isbn=9780028649658|edition=Subsequent|location=New York|language=en|url-access=registration}}</ref><ref name=X15FlightLog>{{cite web |url=http://www.mach25media.com/Resources/X15FlightLog.pdf |title=The X-15 Rocket Plane: Flying the First Wings Into Space-Flight Log |last=Evans |first=Michelle |date=2013 |pages=5, 32, 33 |website=Mach 25 Media}}</ref>
! Total<br />flights
! USAF<br />space<br />flights
! FAI<br />space<br />flights
! Max<br />Mach
! Max<br />speed<br />(mph)
! Max<br />altitude<br />(miles)
|-
| Michael J. Adams<sup>†</sup>
| U.S. Air Force
| 1966
|align="center"| 7
|align="center"| 1
|align="center"| 0
|align="center"| 5.59
|align="right"| 3,822
|align="right"| 50.3
|-
| Neil A. Armstrong
| NASA
| 1960<ref>{{cite web |last1=Gibbs |first1=Yvonne |title=Neil Armstrong with X-15 #1 After Flight |url=https://www.nasa.gov/centers/dryden/multimedia/imagegallery/X-15/E-USAF-Armstrong-X-15.html |website=NASA |access-date=10 September 2023 |date=2 June 2015 |quote=Armstrong was actively engaged in both piloting and engineering aspects of the X-15 program from its inception. He completed the first flight in the aircraft equipped with a new flow-direction sensor (ball nose) and the initial flight in an X-15 equipped with a self-adaptive flight control system. He worked closely with designers and engineers in development of the adaptive system, and made seven flights in the rocket plane from December 1960 until July 1962.}}</ref>
|align="center"| 7
|align="center"| 0
|align="center"| 0
|align="center"| 5.74
|align="right"| 3,989
|align="right"| 39.2
|-
| Scott Crossfield
| North American Aviation
| 1959
|align="center"| 14
|align="center"| 0
|align="center"| 0
|align="center"| 2.97
|align="right"| 1,959
|align="right"| 15.3
|-
| William H. Dana
| NASA
| 1965
|align="center"| 16
|align="center"| 2
|align="center"| 0
|align="center"| 5.53
|align="right"| 3,897
|align="right"| 58.1
|-
| Joe H. Engle
| U.S. Air Force
| 1963
|align="center"| 16
|align="center"| 3
|align="center"| 0
|align="center"| 5.71
|align="right"| 3,887
|align="right"| 53.1
|-
| William J. Knight
| U.S. Air Force
| 1964
|align="center"| 16
|align="center"| 1
|align="center"| 0
|align="center"| 6.7
|align="right"| 4,519
|align="right"| 53.1
|-
| John B. McKay
| NASA
| 1960
|align="center"| 29
|align="center"| 1
|align="center"| 0
|align="center"| 5.65
|align="right"| 3,863
|align="right"| 55.9
|-
| Forrest S. Petersen
| U.S. Navy
| 1958
|align="center"| 5
|align="center"| 0
|align="center"| 0
|align="center"| 5.3
|align="right"| 3,600
|align="right"| 19.2
|-
| Robert A. Rushworth
| U.S. Air Force
| 1958
|align="center"| 34
|align="center"| 1
|align="center"| 0
|align="center"| 6.06
|align="right"| 4,017
|align="right"| 53.9
|-
| [[Biography:Milton Orville Thompson|Milton O. Thompson]]
| NASA
| 1963
|align="center"| 14
|align="center"| 0
|align="center"| 0
|align="center"| 5.48
|align="right"| 3,723
|align="right"| 40.5
|-
| [[Biography:Joseph A. Walker|Joseph A. Walker]]<sup>††</sup>
| NASA
| 1960<ref>{{cite web |last1=Conner |first1=Monroe |title=Joseph A. Walker |url=https://www.nasa.gov/centers/armstrong/about/biographies/pilots/joseph-walker.html |website=NASA |access-date=10 September 2023 |date=23 June 2020}}</ref>
|align="center"| 25
|align="center"| 3
|align="center"| 2
|align="center"| 5.92
|align="right"| 4,104
|align="right"| 67.0
|-
| Robert M. White
| U.S. Air Force
| 1957
|align="center"| 16
|align="center"| 1
|align="center"| 0
|align="center"| 6.04
|align="right"| 4,092
|align="right"| 59.6
|-
|}
<sup>†</sup> Killed in the crash of X-15-3<br>
<sup>††</sup> Died in a group formation accident on June 8, 1966.

==Specifications==
[[File:North American X-15 3-view.svg|right|300px|thumb|Three view drawing]]
Other configurations include the Reaction Motors XLR11 equipped X-15, and the long version.

{{Aircraft specs
|ref=<ref name=X15FlightLog/><!-- reference -->
|prime units?=imp
<!--
        General characteristics
-->
|crew=One
|length ft=49
|length in=2
|length note={{efn|
* 56’ 1.5” With nose boom and XLR-11 rocket engine
* 55’ 2.5” With nose boom and XLR-99 rocket engine
* 50’ 1” With Q-Ball nose and XLR-11 rocket engine
* 49’ 2” With Q-Ball nose and XLR-99 rocket engine
* 51’ 11” Modified 66671 aircraft (X-15A-2)
}}
|span ft=22
|span in=4
|span note={{efn|
* 22’ 4” Standard aircraft
* 23’ 8” With wing tip pods
}}
|height ft=13
|height in=1
|height note={{efn|
* 13’ 1” Standard aircraft
* 11’ 6” Without lower ventral fin and with landing gear extended
}}
|wing area sqft=200
|empty weight lb=14600
|empty weight note={{efn|
* Burn-out weight: 14,500 lb Standard aircraft
* Landing weight: 13,800 lb Standard aircraft
}}
|gross weight lb=33500
|more general=
<!--
        Powerplant
-->
|eng1 number=1
|eng1 name=Reaction Motors XLR99-RM-2
|eng1 type=[[Engineering:Liquid-propellant rocket|liquid-fuelled]] [[Engineering:Rocket engine|rocket engine]]
|eng1 lbf=70400
|more power=
<!--
        Performance
-->
|max speed mph=4520
|max speed mach=<!-- supersonic aircraft -->
|range miles=280
|ceiling ft=354330
|climb rate ftmin=60000
|thrust/weight=2.07
|more performance=
}}

==In popular culture==

==See also==
{{aircontent|
|see also=
* [[Engineering:SpaceShipOne|SpaceShipOne]]
* [[Engineering:SpaceShipTwo|SpaceShipTwo]]
|related=
|similar aircraft=
* [[Engineering:Bell X-2|Bell X-2]]
* [[Engineering:Douglas D-558-2 Skyrocket|Douglas D-558-2 Skyrocket]]
|lists=
* [[Engineering:List of rocket aircraft|List of rocket aircraft]]
* List of X-15 flights
* [[Astronomy:List of spaceflight-related accidents and incidents|List of spaceflight-related accidents and incidents]]
}}

==Notes==
{{notelist}}

==References==
{{Reflist}}

==Bibliography==
{{Refbegin}}
* {{cite book |url=https://books.google.com/books?id=lkSM9rmWQBgC&pg=SA8-PA28 |title=The Standard Handbook for Aeronautical and Astronautical Engineers |publisher=McGraw-Hill |location=New York |editor-first=Mark |editor-last=Davies |pages=8–28 |year=2003 |isbn=978-0-07-136229-0 }}
* {{cite book |title=The X-15 Rocket Plane, Flying the First Wings into Space |publisher=University of Nebraska Press |location=Lincoln, Nebraska |first=Michelle |last=Evans |year=2013a |isbn=978-0-8032-2840-5 }}
* {{cite web |url=http://www.mach25media.com/Resources/X15FlightLog.pdf |title=The X-15 Rocket Plane: Flying the First Wings Into Space - Flight Log |last=Evans |first=Michelle |date=2013b |website=Mach 25 Media}}
* {{cite book |title=X-15: The NASA Mission Reports |publisher=Apogee Books |location=Burlington, Ontario |editor-first=Robert |editor-last=Godwin |year=2001 |isbn=1-896522-65-3 }}
* {{cite book |chapter=Saga of the Rocket Ships |title=Air Enthusiast Six |publisher=Pilot Press |location=Bromley, Kent, UK |first=Richard P. |last=Hallion |editor1-first=William |editor1-last=Green |editor2-first=Gordon |editor2-last=Swanborough |year=1978 }}
* {{cite book |title=Space Shuttle: The History of the National Space Transportation System: The First 100 Missions |edition=3rd |publisher=Voyageur Press |location=Stillwater, Minnesota |first=Dennis R. |last=Jenkins |year=2001 |isbn=0-9633974-5-1 |url-access=registration |url=https://archive.org/details/spaceshuttlehist0000jenk }}
* {{cite book |url=https://web.archive.org/web/20200425225303/https://history.nasa.gov/monograph31.pdf |title=American X-Vehicles: An Inventory – X-1 to X-50 |series=Monographs in Aerospace History No. 31 |publisher=NASA |first1=Dennis R. |last1=Jenkins |first2=Tony |last2=Landis |first3=Jay |last3=Miller |date=June 2003 |id=SP-2003-4531 |oclc=68623213 }}
* {{cite book |url=https://www.nasa.gov/connect/ebooks/aero_x15_detail.html |title=X-15: Extending the Frontiers of Flight |publisher=NASA |last1=Jenkins |first1=Dennis R. |last2=Dana |first2=William H.| year=2010 |isbn=978-0-16-079285-4 |id=SP-2007-562 }}
* {{cite book |title=Die schnellsten Jets der Welt: Weltrekord-Flugzeuge |trans-title=The Fastest Jets in the World: World Record Aircraft |publisher=Aviatic Verlag |location=Kolpingring, Germany |language=de |first=Ferdinand C. W. |last=Käsmann |year=1999 |isbn=3-925505-26-1 }}
* {{cite book |url=http://www.sierrafoot.org/x-15/documents/nasa_cr-82003.html |title=Design Report – Thermal Protection System, X-15A-2 |publisher=Martin Marietta Corporation |location=Denver, Colorado |first=A. B. |last=Price |date=12 January 1968 |id=NASA CR-82003 }}
* {{cite book |url=https://www.nasa.gov/centers/dryden/pdf/88211main_H-1622.pdf|title=Proceedings of the X-15 First Flight 30th Anniversary Celebration |publisher=NASA |date=January 1991 |id=NASA CP-3105 |ref={{harvid|X-15 First Flight|1991}} }}
* {{cite book |title=At the Edge of Space: The X-15 Flight Program |publisher=Smithsonian Institution Press |location=Washington, D.C. |first=Milton O. |last=Thompson |year=1992 |isbn=1-56098-107-5 |url-access=registration |url=https://archive.org/details/atedgeofspacex1500thom }}
* {{cite book |title=X-15 Diary: The Story of America's First Space Ship |publisher=iUniverse |location=Lincoln, Nebraska |first=Richard |last=Tregaskis |year=2000 |isbn=0-595-00250-1 }}
* {{cite book |title=United States Air Force Museum Guidebook |publisher=Air Force Museum Foundation |location=Wright-Patterson AFB, Ohio |year=1975 |ref={{harvid|USAF Museum Guidebook|1975}} }}
* {{cite book |url=http://www.nasa.gov/centers/dryden/pdf/87756main_H-524.pdf |title=Flight Experience With Shock Impingement and Interference Heating on the X-15-2 Research Airplane |publisher=NASA |first=Joe D. |last=Watts |date=October 1968 |id=NASA-TM-X-1669 }}
{{Refend}}

==External links==
;NASA
*[https://history.nasa.gov/x15/cover.html ''X-15: Hypersonic Research at the Edge of Space'']
*[https://history.nasa.gov/monograph18.pdf ''Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane'']
*{{Internet Archive short film | gov.archives.arc.45005 | Research Project X-15 }}

;Non-NASA
*[https://web.archive.org/web/20160820191926/http://www.astronautix.com/x/x-15a.html X-15A] at Encyclopedia Astronautica
*[http://www.sierrafoot.org/x-15/documents/X-15_design_proposal.pdf ''X-15: Advanced Research Airplane''], design summary by North America Aviation

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{{Sourceattribution|North American X-15}}