The X-37 Orbital Test Vehicle is a reusable robotic spaceplane. It is a 120%-scale derivative of the Boeing X-40, measuring over 29 feet (8.8 m) in length, and features two angled tail fins. The X-37 launches atop an Atlas V version 501 rocket with a Centaur second stage. The X-37 is designed to operate in a speed range of up to Mach 25 on its reentry.
The technologies demonstrated in the X-37 include an improved thermal protection system, enhanced avionics, an autonomous guidance system and an advanced airframe. The spaceplane's thermal protection system is built upon previous generations of atmospheric reentry spacecraft, incorporating silica ceramic tiles. The X-37's avionics suite was used by Boeing to develop its CST-100 manned spacecraft. According to NASA, the development of the X-37 will "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".
The X-37 is independently powered by one Aerojet AR2-3 engine using storable propellants, providing thrust of 6,600 pounds-force (29.341 kN). 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.
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. The X-37 is the smallest and lightest orbital spaceplane flown to date; with a launch mass of around 11,000 pounds (5,000 kg), it is approximately a quarter the size of the Space Shuttle orbiter. In 2013, Guinness World Records recognised the X-37 as the world's smallest orbital spaceplane.
The Space Foundation awarded the X-37 team on 13 April 2015 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."
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 contributed to 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.
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. An early requirement for the spacecraft called for a delta-v of 7,000 mph (3.1 km/s) to change its orbit. An early goal for the program was for the X-37 to rendezvous with satellites and perform repairs. 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.
The X-37 was transferred from NASA to the Defense Advanced Research Projects Agency (DARPA) on 13 September 2004. 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.
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.
On 21 June 2005, the X-37A completed a captive-carry flight underneath the White Knight from Mojave Spaceport in Mojave, California. 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. The next flight attempt, on 15 March 2006, was canceled due to high winds.
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. 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, two of which resulted in X-37 releases with successful landings. These two free flights occurred on 18 August 2006 and 26 September 2006.
X-37B Orbital Test Vehicle
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, and was led by 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. The X-37B was designed to remain in orbit for up to 270 days at a time. 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."
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. Following their missions, X-37B spacecraft land on a runway at Vandenberg Air Force Base, California, with Edwards Air Force Base as an alternate site. In 2010, manufacturing work began on the second X-37B, OTV-2, which conducted its maiden launch in March 2011.
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. 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.
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." The primary objectives of the X-37B are twofold: reusable spacecraft technology, and operating experiments which can be returned to Earth. The Air Force states that this includes testing avionics, flight systems, guidance and navigation, thermal protection, insulation, propulsion, and re-entry systems.
Speculation regarding purpose
In 2010, Tom Burghardt wrote for Space Daily that the X-37B could be used as a spy satellite or to deliver weapons from space.The Pentagon subsequently denied claims that the X-37B's test missions supported the development of space-based weapons.In January 2012, allegations were made that the X-37B was being used to spy on China's Tiangong-1 space station module.
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. 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."
- X-37A : The X-37A was the initial NASA version of the spacecraft; the X-37A Approach and Landing Test Vehicle (ALTV) was used in drop glide tests in 2005 and 2006.
- X-37B : The X-37B is a modified version of the NASA X-37A, intended for the U.S. Air Force. It conducted multiple orbital test missions.
- X-37C : 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 is the Atlas V Evolved Expendable Launch Vehicle. In this role, the X-37C could potentially compete with Boeing's CST-100 commercial space capsule.