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The Boeing X-37, also known as the Orbital Test Vehicle (OTV), is a reusable robotic spacecraft. It is boosted into space by a launch vehicle, then re-enters Earth's atmosphere and lands as a spaceplane. The X-37 is operated by the United States Air Force for orbital spaceflight missions intended to demonstrate reusable space technologies.[4] It is a 120%-scaled derivative of the earlier Boeing X-40.

Boeing X-37B inside payload fairing before launch.jpg
The OTV-1 X-37B in April 2010, inside its payload fairing prior to launch
Role Uncrewed spaceplane
National origin United States
Manufacturer Boeing Defense, Space & Security
First flight 7 April 2006 (first drop test)
Introduction 22 April 2010 (first spaceflight)
Primary user
Number built
  • X-37A: 1
  • X-37B: 2
Developed from Boeing X-40

The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. Its first flight was during a drop test in 2006. There have been five X-37 orbital missions. The spaceplane's first mission, USA-212, was launched in April 2010 and returned to Earth in December 2010. A second X-37 was launched on mission USA-226 in March 2011 and returned in June 2012. The third mission was USA-240, which launched in December 2012 and landed in October 2014. The fourth mission, USA-261, launched in May 2015 and landed in May 2017. The fifth and latest X-37 mission, USA-277, launched on 7 September 2017.




In 1999, NASA selected Boeing Integrated Defense Systems to design and develop an orbital vehicle, built by the California branch of Boeing's Phantom Works. Over a four-year period, a total of $192 million was spent on the project, with NASA contributing $109 million, the U.S. Air Force $16 million, and Boeing $67 million. In late 2002, a new $301-million contract was awarded to Boeing as part of NASA's Space Launch Initiative framework.[5]

1999 artist's rendering of the X-37 spacecraft

The X-37's aerodynamic design was derived from the larger Space Shuttle orbiter, hence the X-37 has a similar lift-to-drag ratio, and a lower cross range at higher altitudes and Mach numbers compared to DARPA's Hypersonic Technology Vehicle.[6] An early requirement for the spacecraft called for a total mission delta-v of 7,000 mph (3.1 km/s) for orbital maneuvers.[7] An early goal for the program was for the X-37 to rendezvous with satellites and perform repairs.[8] The X-37 was originally designed to be carried into orbit in the Space Shuttle's cargo bay, but underwent redesign for launch on a Delta IV or comparable rocket after it was determined that a shuttle flight would be uneconomical.[9]

The X-37 was transferred from NASA to the Defense Advanced Research Projects Agency (DARPA) on 13 September 2004.[10] Thereafter, the program became a classified project. DARPA promoted the X-37 as part of the independent space policy that the United States Department of Defense has pursued since the 1986 Challenger disaster.

Glide testingEdit

The vehicle that was used as an atmospheric drop test glider had no propulsion system. Instead of an operational vehicle's payload bay doors, it had an enclosed and reinforced upper fuselage structure to allow it to be mated with a mothership. In September 2004, DARPA announced that for its initial atmospheric drop tests the X-37 would be launched from the Scaled Composites White Knight, a high-altitude research aircraft.[11]

The Scaled Composites White Knight was used to launch the X-37A on glide tests.

On 21 June 2005, the X-37A completed a captive-carry flight underneath the White Knight from Mojave Spaceport in Mojave, California.[12][13] Through the second half of 2005, the X-37A underwent structural upgrades, including the reinforcement of its nose wheel supports. Further captive-carry flight tests and the first drop test were initially expected to occur in mid-February 2006. The X-37's public debut was scheduled for its first free flight on 10 March 2006, but was canceled due to an Arctic storm.[14] The next flight attempt, on 15 March 2006, was canceled due to high winds.[14]

On 24 March 2006, the X-37 flew again, but a datalink failure prevented a free flight, and the vehicle returned to the ground still attached to its White Knight carrier aircraft. On 7 April 2006, the X-37 made its first free glide flight. During landing, the vehicle overran the runway and sustained minor damage.[15] Following the vehicle's extended downtime for repairs, the program moved from Mojave to Air Force Plant 42 (KPMD) in Palmdale, California for the remainder of the flight test program. White Knight continued to be based at Mojave, though it was ferried to Plant 42 when test flights were scheduled. Five additional flights were performed,[N 1] two of which resulted in X-37 releases with successful landings. These two free flights occurred on 18 August 2006 and 26 September 2006.[16]

X-37B Orbital Test VehicleEdit

On 17 November 2006, the U.S. Air Force announced that it would develop its own variant from NASA's X-37A. The Air Force version was designated the X-37B Orbital Test Vehicle (OTV). The OTV program was built on earlier industry and government efforts by DARPA, NASA, and the Air Force under the leadership of the U.S. Air Force Rapid Capabilities Office in partnership with NASA and the Air Force Research Laboratory. Boeing was the prime contractor for the OTV program.[7][17][18] The X-37B was designed to remain in orbit for up to 270 days at a time.[19] The Secretary of the Air Force stated that the OTV program would focus on "risk reduction, experimentation, and operational concept development for reusable space vehicle technologies, in support of long-term developmental space objectives".[17]

The X-37B was originally scheduled for launch in the payload bay of the Space Shuttle, but following the 2003 Columbia disaster, it was transferred to a Delta II 7920. The X-37B was subsequently transferred to a shrouded configuration on the Atlas V rocket, following concerns over the unshrouded spacecraft's aerodynamic properties during launch.[20] Following their missions, X-37B spacecraft primarily land on a runway at Vandenberg Air Force Base, California, with Edwards Air Force Base as a secondary site.[21] In 2010, manufacturing work began on the second X-37B, OTV-2,[22] which conducted its maiden launch in March 2011.[23]

On 8 October 2014, NASA confirmed that X-37B vehicles would be housed at Kennedy Space Center in Orbiter Processing Facilities (OPF) 1 and 2, hangars previously occupied by the Space Shuttle. Boeing had said the space planes would use OPF-1 in January 2014, and the Air Force had previously said it was considering consolidating X-37B operations, housed at Vandenberg Air Force Base in California, nearer to their launch site at Cape Canaveral. NASA also stated that the program had completed tests to determine whether the X-37B, one-fourth the size of the Space Shuttle, could land on the former Shuttle runways.[24] NASA furthermore stated that renovations of the two hangars would be completed by the end of 2014; the main doors of OPF-1 were marked with the message "Home of the X-37B" by this point.[24]

Most of the activities of the X-37B project are secret. The official U.S. Air Force statement is that the project is "an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force".[4] The primary objectives of the X-37B are twofold: reusable spacecraft technology, and operating experiments which can be returned to Earth.[4] The Air Force states that this includes testing avionics, flight systems, guidance and navigation, thermal protection, insulation, propulsion, and re-entry systems.[25]

Speculation regarding purposeEdit

In January 2012, allegations were made that the X-37B was being used to spy on China's Tiangong-1 space station module.[26] Former U.S. Air Force orbital analyst Brian Weeden later refuted this claim, emphasizing that the different orbits of the two spacecraft precluded any practical surveillance fly-bys.[27]

In October 2014, The Guardian reported the claims of security experts that the X-37B was being used "to test reconnaissance and spy sensors, particularly how they hold up against radiation and other hazards of orbit".[28]

In November 2016, the International Business Times speculated that the U.S. government was testing a version of the EmDrive electromagnetic microwave thruster on the fourth flight of the X-37B.[29] The X-37B was actually testing a Hall-effect thruster system for Aerojet Rocketdyne.[30]


Processing for the X-37 is done inside Bays 1 and 2 of the Orbiter Processing Facility (OPF) at Kennedy Space Center in Florida, where the vehicle is loaded with its top-secret payload. The X-37 is then placed inside a fairing along with its stage adapter and transported to the launch site. Previous launch sites have included SLC-41 and Kennedy Space Center LC-39A.[citation needed]

Landing is done at one of three sites across the US: the Shuttle Landing Facility at Kennedy Space Center, Vandenberg Air Force Base, or Edwards Air Force Base. To return to Kennedy Space Center, the X-37 is placed into a payload canister and loaded into a Boeing C-17 cargo plane. Once at Kennedy, the X-37 is unloaded and towed to the OPF, where it is prepared for its next flight.


The X-37 (far right) is the smallest and lightest orbital spaceplane yet flown. Both the North American X-15 and SpaceShipOne were suborbital. Of the spaceplanes shown, only the X-37 and Buran conducted uncrewed spaceflights.

The X-37 Orbital Test Vehicle is a reusable robotic spaceplane. It is an approximately 120%-scale derivative of the Boeing X-40,[5][21] measuring over 29 feet (8.8 m) in length, and features two angled tail fins.[4][31] The X-37 launches atop an Atlas V 501[4][18] or a SpaceX Falcon 9 rocket.[32] The spaceplane is designed to operate in a speed range of up to Mach 25 on its reentry.[33][34]

The technologies demonstrated in the X-37 include an improved thermal protection system, enhanced avionics, an autonomous guidance system and an advanced airframe.[9] The spaceplane's thermal protection system is built upon previous generations of atmospheric reentry spacecraft,[35] incorporating silica ceramic tiles.[36] The X-37's avionics suite was used by Boeing to develop its CST-100 crewed spacecraft.[37] The development of the X-37 was to "aid in the design and development of NASA's Orbital Space Plane, designed to provide a crew rescue and crew transport capability to and from the International Space Station", according to a NASA fact sheet.[38]

The X-37 for NASA was to be powered by one Aerojet AR2-3 engine using storable propellants, providing thrust of 6,600 pounds-force (29.4 kN).[39] The human-rated AR2-3 engine had been used on the dual-power NF-104A astronaut training vehicle and was given a new flight certification for use on the X-37 with hydrogen peroxide/JP-8 propellants.[40] This was reportedly changed to a hypergolic nitrogen-tetroxide/hydrazine propulsion system.[20][41]

The X-37 lands automatically upon returning from orbit and is the second reusable spacecraft to have such a capability, after the Soviet Buran shuttle.[42] The X-37 is the smallest and lightest orbital spaceplane flown to date; it has a launch mass of around 11,000 pounds (5,000 kg) and is approximately one quarter of the size of the Space Shuttle orbiter.[43] In 2013, Guinness World Records recognised the X-37 as the world's smallest orbital spaceplane.[44]

On 13 April 2015, the Space Foundation awarded the X-37 team with the 2015 Space Achievement Award "for significantly advancing the state of the art for reusable spacecraft and on-orbit operations, with the design, development, test and orbital operation of the X-37B space flight vehicle over three missions totaling 1,367 days in space".[45]

Operational historyEdit

As of July 2019, the two operational X-37Bs have completed four orbital missions and one of them has been in orbit since 7 September 2017; they have spent a combined 2756 days in space.

Flight Vehicle Launch date Launcher Mission[46] Duration Notes
OTV-1 1 22 April 2010
23:52 UTC
Atlas V 501 USA-212 224 days and 9 hours
  • First launch of Atlas V 501 configuration
  • First American autonomous orbital runway landing
  • First X-37B flight
OTV-2 2 5 March 2011
22:46 UTC
Atlas V 501 USA-226 468 days and 14 hours
  • First flight of second X-37B
OTV-3 1 11 December 2012
18:03 UTC
Atlas V 501 USA-240 674 days and 22 hours
  • Second flight of first X-37B
OTV-4 2 20 May 2015
15:05 UTC
Atlas V 501 USA-261 717 days and 20 hours
OTV-5 ? 7 September 2017
14:00 UTC
Falcon 9 USA-277 Elapsed: 670 days and 15 hours
  • First launch of an X-37B on SpaceX's Falcon 9 vehicle


OTV-1 sits on the runway after landing at Vandenberg AFB at the close of its USA-212 mission on 3 December 2010.

The first X-37B launched on its first mission - OTV-1/USA-212 - on an Atlas V rocket from Cape Canaveral Air Force Station, Florida, on 22 April 2010 at 23:52 UTC. The spacecraft was placed into low Earth orbit for testing.[18] While the U.S. Air Force revealed few orbital details of the mission, a worldwide network of amateur astronomers claimed to have identified the spacecraft in orbit. On 22 May 2010, the spacecraft was in an inclination of 39.99 degrees, circling the Earth once every 90 minutes on an orbit 249 by 262 miles (401 by 422 km).[47][48] OTV-1 reputedly passed over the same given spot on Earth every four days, and operated at an altitude that is typical for military surveillance satellites.[49] Such an orbit is also common among civilian LEO satellites, and the spaceplane's altitude was the same as that of the ISS and most other crewed spacecraft.

The U.S. Air Force announced a 3–6 December landing on 30 November 2010.[50][51] As scheduled, the X-37B was de-orbited, reentered Earth's atmosphere, and landed successfully at Vandenberg AFB on 3 December 2010, at 09:16 UTC,[52][53][54] conducting the first US autonomous orbital landing onto a runway. This was the first such landing since the Soviet Buran shuttle in 1988. In all, OTV-1 spent 224 days and 9 hours in space.[18][52] OTV-1 suffered a tire blowout during landing and sustained minor damage to its underside.[22]


The second X-37B launched on its inaugural mission, designated OTV-2/USA-226,[55] aboard an Atlas V rocket from Cape Canaveral on 5 March 2011 at 22:46 UTC.[56] The mission was classified and described by the U.S. military as an effort to test new space technologies.[57] On 29 November 2011, the U.S. Air Force announced that it would extend USA-226 beyond the 270-day baseline duration.[58] In April 2012, General William L. Shelton of the Air Force Space Command declared the ongoing mission a "spectacular success".[59]

On 30 May 2012, the Air Force stated that the X-37B would land at Vandenberg AFB in June 2012.[60][61] The spacecraft landed autonomously on 16 June 2012, having spent 468 days and 14 hours in space.[56][1][62]


The second flight of the first X-37B, OTV-3 was originally scheduled to launch on 25 October 2012[63][64] before being successfully launched from Cape Canaveral on 11 December 2012 at 18:03 UTC.[43][65][66] Once in orbit, the spacecraft was designated USA-240.[67][68] Landing occurred at Vandenberg AFB on 17 October 2014 at 16:24 UTC, after a total time in orbit of 674 days and 22 hours.[65][2][69][70]


OTV-4 stationary at the Shuttle Landing Facility

The fourth X-37B mission, OTV-4, was codenamed AFSPC-5 and designated as USA-261 in orbit. It was the second flight of the second X-37B vehicle.[20] The X-37B launched an Atlas V rocket from Cape Canaveral Air Force Station on 20 May 2015 at 15:05 UTC.[71] Objectives included a test of Aerojet Rocketdyne's XR-5A Hall-effect thruster in support of the Advanced Extremely High Frequency communications satellite program,[30][72] and a NASA investigation of the performance of various materials in space[20][45][73] for at least 200 days.[20] The vehicle spent what was then a record-breaking 717 days and 20 hours in orbit before landing at Kennedy Space Center's Shuttle Landing Facility on 7 May 2017 shortly before 12:00 UTC.[3][74]


The fifth X-37B mission, designated USA-277 in orbit,[46] was launched on 7 September 2017 at 14:00 UTC, just before the arrival of Hurricane Irma.[75][76] The launch vehicle was a Falcon 9 rocket flying from Kennedy Space Center Launch Complex 39A,[77] and number of small satellites also shared the ride.[78] The spacecraft was inserted at a higher inclination orbit than previous missions, further expanding the X-37B's envelope.[78] During the flight, the spacecraft modified its orbit, using an on-board propulsion system.[79] While the complete payload for OTV-5 is unknown, the Air Force announced that one experiment flying is the Advanced Structurally Embedded Thermal Spreader II (ASETS-II), which measures the performance of an oscillating heat pipe.[80]


The sixth X-37B mission, OTV-6, is planned to fly on an Atlas V 501 rocket in December 2019.[46]



The X-37A Approach and Landing Test Vehicle (ALTV) was an initial NASA version of the spacecraft used in drop glide tests in 2005 and 2006.[13][81]


The X-37B is a modified version of the NASA X-37A, built for the U.S. Air Force.[4] Two were built and have been used for multiple orbital missions.[65]


In 2011, Boeing announced plans for a scaled-up variant of the X-37B, referring to it as the X-37C. The X-37C spacecraft would be between 165% and 180% of the size of the X-37B, allowing it to transport up to six astronauts inside a pressurized compartment housed in the cargo bay. Its proposed launch vehicle was the Atlas V.[82] In this role, Boeing's X-37C could potentially compete with the corporation's CST-100 Starliner commercial space capsule.[83]


Three-views of the X-37B


Data from USAF,[4][35] Boeing,[84] Air & Space Magazine,[81] and PhysOrg.[85]

General characteristics

  • Crew: none
  • Length: 29 ft 3 in (8.92 m)
  • Wingspan: 14 ft 11 in (4.55 m)
  • Height: 9 ft 6 in (2.90 m)
  • Max takeoff weight: 11,000 lb (4,990 kg)
  • Electrical power: Gallium arsenide solar cells with lithium-ion batteries[4]
  • Payload bay: 7 × 4 ft (2.1 × 1.2 m)[84]


See alsoEdit

Aircraft of comparable role, configuration and era


  1. ^ Source of flights: mission markings posted on side of White Knight aircraft.
  2. ^ This figure is based on pre-launch design estimates; it does not reflect the spacecraft's actual performance capacity. During its 2012–2014 test mission, the OTV-3 X-37B spent over 670 days in space.


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