Minimize X-37B OTV

X-37B OTV (Orbital Test Vehicle) a classified program of the USAF

Spacecraft    Background   References

On May 7, 2017, the USAF X-37B Orbital Test Vehicle mission 4 (OTV-4), the Air Force's unmanned, reusable space plane, landed at NASA's Kennedy Space Center Shuttle Landing Facility, Fla. Managed by the Air Force Rapid Capabilities Office, the X-37B program is the newest and most advanced reentry spacecraft that performs risk reduction, experimentation and concept of operations development for reusable space vehicle technologies. 1)


Figure 1: The X-37B Orbital Test Vehicle-4 landed at Kennedy Space Center (image credit: USAF)

"Today marks an incredibly exciting day for the 45th Space Wing as we continue to break barriers," said Brig. Gen. Wayne Monteith, the 45th SW commander. "Our team has been preparing for this event for several years, and I am extremely proud to see our hard work and dedication culminate in today's safe and successful landing of the X-37B."

The OTV-4 conducted on-orbit experiments for 718 days during its mission (May 20, 2015 to May 7, 2017), extending the total number of days spent on-orbit for the OTV program to 2,085 days.

"The landing of OTV-4 marks another success for the X-37B program and the nation," said Lt. Col. Ron Fehlen, X-37B program manager. "This mission once again set an on-orbit endurance record and marks the vehicle's first landing in the state of Florida. We are incredibly pleased with the performance of the space vehicle and are excited about the data gathered to support the scientific and space communities. We are extremely proud of the dedication and hard work by the entire team."

"The hard work of the X-37B OTV team and the 45th Space Wing successfully demonstrated the flexibility and resolve necessary to continue the nation's advancement in space," said Randy Walden, the director of the Air Force Rapid Capabilities Office. "The ability to land, refurbish, and launch from the same location further enhances the OTV's ability to rapidly integrate and qualify new space technologies."


Figure 2: Alternate view of the X-37B OTV-4 mini shuttle after landing at KSC, FL (image credit: USAF)

The stubbed-winged orbiter will be towed later today from the Florida runway to a modified NASA shuttle hangar, left over from retirement of the civilian program, for post-flight deservicing. 2)

Early this morning, the craft received the ground-issued command to return to Earth, initiating a fully autonomous sequence of events to fire its propulsion system and brake from orbit. Plunging through the atmosphere for a super-hot reentry, the spaceplane executed a series of turns to dissipate speed as it flew towards the Cape.

With Runway 15 fast approaching the glider dropped its landing gear with dinner plate-sized wheels for a tire-smoking touchdown some time around 8 a.m. EDT (1200 GMT) while flying on a sophisticated autopilot fed with GPS navigation.



Some background of the X-37 Program

The Boeing X-37, also known as the OTV (Orbital Test Vehicle), is a reusable unmanned spacecraft. It is boosted into space by a launch vehicle, then reenters Earth's atmosphere and lands as a spaceplane. The X-37 is operated by the USAF (United States Air Force) for orbital spaceflight missions intended to demonstrate reusable space technologies. 3) It is a 120%-scaled derivative of the earlier Boeing X-40. 4)

The X-37 began as a NASA project in 1999, before being transferred to the U.S. Department of Defense in 2004. It conducted its first flight as a drop test on 7 April 2006, at Edwards Air Force Base, California. The spaceplane's first orbital mission, USA-212, was launched on 22 April 2010 using an Atlas V rocket. Its successful return to Earth on 3 December 2010 was the first test of the vehicle's heat shield and hypersonic aerodynamic handling. A second X-37 was launched on 5 March 2011, with the mission designation USA-226; it returned to Earth on 16 June 2012. A third X-37 mission, USA-240, launched on 11 December 2012 and landed at Vandenberg AFB on 17 October 2014. The fourth X-37 mission, USA-261, launched on 20 May 2015 and landed on 7 May 2017 at Kennedy Space Center.

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.

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 ΔV of 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-4 or comparable rocket after it was determined that a shuttle flight would be uneconomical. — NASA's X-37 Orbital Vehicle was never built: but its design was the starting point for the Air Force's X-37B Orbital Test Vehicle program.

The X-37 project was transferred from NASA to the DARPA (Defense Advanced Research Projects Agency) on 13 September 2004. 5) 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.

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 OTV (Orbital Test Vehicle). 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 AFRL (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. 6) 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". 7)



Spacecraft design:

The X-37 Orbital Test Vehicle is a reusable robotic spaceplane. It is a 120%-scale derivative of the Boeing X-40, measuring 8.8 m in length, 2.9 m in height and features two angled tail fin with a wingspan of 4.5 m. Power is provided by GaAs (Gallium Arsenide) solar cells with Li-ion batteries. The launch mass of OTV-1 is 4,990 kg. The X-37 launches atop an Atlas-V version 501 rocket of ULA with a Centaur second stage.

X-37B is powered by a single Aerojet engine using storable propellants. The Main Propulsion System provides a total thrust of about 700 N and X-37B's propulsion system is capable of providing a total ΔV of nearly 3.1km/s over the course of a mission. Power generation is accomplished with a solar array consisting of Gallium Arsenide solar cells.

The spacecraft features an autonomous guidance system as well as modern avionics and flight computers. OTV is a testbed for advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, reentry and landing. According to Boeing, X-37B is capable of operating in orbits with altitudes from 200 to 925 km.


Figure 3: The first X-37B Orbital Test Vehicle waits in the encapsulation cell of the EELV (Evolved Expendable Launch Vehicle) April 5, 2010, at the Astrotech facility in Titusville, Fla. Half of the Atlas-V 5 m fairing is visible in the background. The OTV-1 launch on April 22, 2010 on a ULA vehicle from KSC, FL (image credit: USAF)

As of early May 2015, the Air Force has successfully flown three X-37B missions, OTV-1 through OTV-3, beginning with its first launch on April 22, 2010 from Cape Canaveral Air Force Station, Florida. All three have landed successfully at Vandenberg AFB, California, with the latest landing of OTV-3 occurring on Oct. 17, 2014, after 674 days in orbit. the first three OTV missions have spent a total of 1,367 days in orbit, successfully checking out the x-37B's reusable flight, reentry and landing technologies.

Flight mission

Launch date

Mission designation

Mission duration

Return date

Short description


22 April 2010


224 days 9 hours

Dec. 3, 2010

- First American autonomous orbital runway landing
- First X-37B flight


05 March 2011


468 days 14 hours

June 16, 2012

- First flight of 2nd X-37B vehicle


11 December 2012


674 days 22 hours

Oct. 17, 2014

- Second flight of 1st X-37B vehicle


20 May 2015


717 days 20 hours

My 07, 2017

- Second flight of 2nd X-37B vehicle
- First landing on the Shuttle Landing Facility at KSC, FL

Table 1: As of May 2017, the two operational X-37Bs have completed four orbital missions, spending a combined 2,086 days in space.

Orbit of OTV-1, altitude of 401 km x 422 km (mean altitude of 410 km), inclination of 39.99º, period of 90 minutes.

Orbit of OTV-2, altitude of 278.5 km x 289.3 km, inclination of 41.9°, period of 90.2 minutes.

Orbit of OTV-3, altitude of 320 km x 333 km, inclination of 43.5, period of 90.93 minutes.

Orbit of OTV-4, altitude of ~310 km (initial altitude), later it raised its altitude to 350 km where it spent the majority of its mission. On Feb. 5, 2017, the spaceplane maneuvered into an orbit of 307 x 312 km.


Secondary payloads on the OTV-4 flight (that were deployed from the spaceplane):

• LightSail-A, a 3U CubeSat of The Planetary Society, Pasadena, CA.

• GEARRS-2, a 3U CubeSat built by NearSpace Launch in collaboration with the AFRL (Air Force Research Laboratory).

• OptiCube-1, -2, -3, three 3U CubeSats developed by Cal Poly to provide on-orbit targets for ground assets to calibrate sensors for orbital debris studies and small-object tracking improvements.

• USS (Unix Space Server) Langley, a 3U CubeSat built by US Naval Academy.

• BRICSat-P, a 1.5 CubeSat of the USNA (US Naval Academy) and George Washington University.

• AeroCube-8A and AeroCube-8B, a 2U CubeSat mission of the Aerospace Corporation of El Segundo, CA.

• ParkinsonSat, a 1.5U CubeSat of USNA.


Payloads aboard the OTV-4:

• According to Aerojet Rocketdyne, the XR-5A Hall Thruster has completed initial in-orbit validation testing aboard the X-37B space plane. XR-5A is an electric propulsion experiment to enable in-space characterization of the design modifications that are intended to improve performance to the units onboard the AEHF (Advanced EHF (Extremely-High Frequency) for RF communications), a military communications spacecraft. 8)

- The XR-5A Hall Thruster is an enhanced version of the Aerojet Rocketdyne XR-5 Hall Thruster. Both thrusters are 5 kW class Hall Thrusters; however, the XR-5A incorporates modifications that improve performance and operating range. Aerojet Rocketdyne has manufactured and delivered 16 XR-5 Hall Thrusters and flown 12 to date. As with most new product introductions, Aerojet Rocketdyne is introducing a product upgrade to incorporate improvements identified after the initial low-rate production and flight programs.


Figure 4: Aerojet Rocketdyne's upgraded XR-5A Hall thruster demonstrates successful on-orbit operation (image credit: Aerojet Rocketdyne)

• METIS (Materials Exposure and Technology Innovation in Space) experiment of NASA to expose nearly 100 different materials samples to the space environment for more than 200 days. METIS is based on the MISSE (Materials on International Space Station Experiment), which flew more than 4,000 samples in space from 2001 to 2013.


Figure 5: Illustration of the X-37B Orbital Test Vehicle elements (image credit: NASA, ULA) 9)


Figure 6: Artist's rendition of the X-37B spaceplane with the deployed solar panels (image credit: Boeing Phantom Works)


1) "X-37B Orbital Test Vehicle-4 lands at Kennedy Space Center," U.S. Air Force, May 7, 2017, URL:

2) Justin Ray, "X-37B spaceplane returns to Earth and makes autopilot landing in Florida," Spaceflight Now, May 7, 2017, URL:

3) "X-37B Orbital Test Vehicle," USAF, April 15, 2015, URL:

4) "Boeing X-37," Wikipedia, URL:

5) Brian Berger, "NASA Transfers X-37 Project to DARPA," Space News, Sept. 15, 2004, URL:

6) Stephen Clark, "Air Force X-37B spaceplane arrives in Florida for launch," Spaceflight Now, Feb. 25, 2010, URL:

7) Leonard David, "U.S. Air Force Pushes For Orbital Test Vehicle," Space.Com, November 17, 2006, URL:

8) "Aerojet Rocketdyne's Modified XR-5 Hall Thruster Demonstrates Successful On-Orbit Operation," Aerojet Rocketdyne, July 1, 2015, URL:

9) Leonard David, "Air Force's Mysterious X-37B Space Plane Wings by 600 Days in Orbit,", January 10, 2017, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (

Spacecraft    Background   References    Back to top