X-37B: America’s Secret Space plane

A Continual Learner
9 min readJan 25, 2022

This article relies solely on publicly available information and does not reflect the views of the Department of Defense.

X-37B (Boeing/U.S. Air Force)

The X-37B Orbital Test Vehicle, or OTV, is an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force, now the U.S. Space Force.

“It’s a reusable spacecraft. It is autonomous, it flies without crew. It can be rapidly reconfigured to host a wide variety of experiments, and it can take off from standard launch pads on standard rockets under fairing, and it can land autonomously through public airspace.” — Jim Chilton, senior vice president of Boeing’s space and launch division

The X-37B program is currently flown under the wing of a U.S. Space Force unit called Delta 9 that supports Space Domain Awareness by conducting space-based battlespace characterization operations and also conduct on-orbit experimentation and technology demonstrations for the U.S. Space Force.

An artist’s illustration of the U.S. Air Force’s X-37B space plane in orbit (Boeing/NASA Marshall Center)

The classified X-37B program “fleet” consists of two known reusable vehicles, both of which were built by Boeing. The military space plane is 29 feet (8.8 meters) long and 9.6 feet (2.9 m) tall, with a wingspan of nearly 15 feet (4.6 m). The X-37B space plane has a payload bay of 7 feet (2.1 meters) by 4 feet (1.2 meters), a bay that can be outfitted with a robotic arm. X-37B has a launch weight of 11,000 lbs. (4,990 kilograms).

The X37B Landing (Vandenberg Space Force Base/AP Photo)

X-37B is powered by a single Aerojet engine using storable propellants. The Main Propulsion System provides a total thrust of about 700N and X-37B’s propulsion system is capable of providing a total delta-V (ΔV) of nearly 3,100 m/s over the course of a mission. Delta-V is the measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver in-space orbital maneuver. Power generation is accomplished with a solar array consisting of by gallium-arsenide solar cells with lithium-ion batteries.

Recovery crew members process the X-37B Orbital Test Vehicle at Vandenberg SFB (Boeing)

Like many spacecraft, the X-37B uses hydrazine or a hypergolic nitrogen tetroxide and hydrazine mixture for maneuvering and other purposes. These propellants are extraordinarily toxic and require class 2 full body hazmat suits upon entry.

Layout of X37B

The thermal protection system of the X-37B, called toughened uni-piece fibrous reinforced oxidation-resistant composite, or TUFROC, was developed by NASA Ames and was built upon previous generations of atmospheric entry spacecraft using silica ceramic tiles to protect the vehicle from the harsh re-entry environment. On its belly the X-37B carries toughened uni-piece fibrous insulation, or TUFI, tiles similar to those flown on the space shuttle for 15 years. TUFI is more durable and provides a better barrier against water absorption when rained on.


X-40A, the 85-percent scale version of the X-37 experimental space-access technology demonstrator (NASA)

NASA’s original X-37 program began in 1999 from X-40 design and was transferred to DARPA in 2004. NASA envisioned building two vehicles, an Approach and Landing Test Vehicle, or ALTV, and an Orbital Vehicle. DARPA completed the ALTV portion of the X-37 program in 2006, executing a series of captive carry and free flight tests.

DARPA successfully validated the flight dynamics and extended the flight envelope beyond the low speed/low altitude tests previously conducted by NASA on the X-40A, a sub-scale version of the X-37 developed by Air Force Research Labs. NASA’s X-37 Orbital Vehicle was never built, but it’s design was the starting point for the Air Force’s X-37B Orbital Test Vehicle program.


The missions of the X-37B space planes are carried out under the authority of the Space Rapid Capabilities Office and mission control for OTV flights are handled by the 3rd Space Experimentation Squadron under Delta 9 at Schriever Space Force Base in Colorado.

While the X-37 and X-37B documents report the orbital vehicle launch mass as 5000 kg, the Atlas V 501 (5 meter fairing, 0 solid rocket boosters, 1 engine Centaur) rocket used in all OTV missions has the capability to lift heavier payloads to the designated 200–1000 km orbits.

Atlas V 501 (United Launch Alliance)

Assuming an intermediate altitude orbit of 500km, ULA calculates the Atlas V 501 capable of lifting the X-37B plus an additional 3000kg to a 28.5° orbit. Whether this is truly unused excess capacity or if this translates to additional payload or propellant on board the operational X-37B remains classified.

X37B Payload Fairing (U.S. Air Force)

X-37B OTV-1 was launched on 22 April, 2010 from Cape Canaveral on an Atlas-5 rocket. OTV-1 launched on its mission, also known as USA-212, meaning USA series of military spacecraft had no orbital information released. Although orbital data was classified, the OTV-1 was found in an 401 km × 422 km, 40° orbit, inclination of 39.99º, and period of 90 minutes by amateur observations.

During the mission, the vehicle made a number of orbit adjustments changing its altitude and ground coverage. During a short period in the mission, OTV-1 repeated its ground track every two days and late into the flight, the orbit of the vehicle was lowered to 281 to 292 Kilometers. The possible payloads aboard the X-37B include ELINT/SIGINT (electronic/signals intelligence). It landed successfully on December 3, 2010 after 225 days in space.

X-37B OTV-2 (USA-226) was launched on 5 March, 2011 on an Atlas-5 rocket. This time it stayed 469 days in space, more than doubling the endurance of the first mission. Over the course of the mission, the orbit was modified and varied between altitude of 278.5 km x 289.3 km, inclination of 41.9°, and period of 90.2 minutes.

Technicians inspect an X-37B in June 2012 after it had completed the program’s second mission. (USAF/Boeing)

X-37B OTV-3 (USA-240) was launched on 11 December, 2012 on an Atlas-5 rocket. The satellite was probably in altitude of 320 km x 333 km, inclination of 43.5, and period of 90.93 minutes. It returned on 17 October 2014 to the Vandenberg AFB after 675 days in space.

X-37B OTV-4 (USA-261) was launched on 20 May, 2015 from Cape Canaveral Space Force Station. OTV-4 had altitude of roughly 310 km (initial altitude), and 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.

It conducted an Hall thruster electric propulsion experiment to enable in-space characterization of design modifications that are intended to improve performance to the units onboard the Advanced Extremely High Frequency (AEHF) military communications spacecraft.

AEHF Satellite (Air Force Technology)

Produced by Aerojet Rocktdyne, the AEHF satellites provide secure communications for military application. The XR-5A Hall Effect thrusters are 4.5-kilowatt units that use electricity and xenon to produce thrust for maneuvering satellites in space. The on-orbit test plans for the experiment were developed by Air Force Research Laboratory and administered by the Rapid Capabilities Office, which runs the X-37B program.

NASA’s Test for MISSE (NASA)

Also on board was NASA’s METIS (Materials Exposure and Technology Innovation in Space) experiment 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. METIS flew a variety of materials including polymers, composites and coatings. The mission ended after 718 days with a landing at Cape Canaveral AFS on 7 May 2017.

X-37B OTV-5 (USA-277) was launched on 7 September, 2017 from Cape Canaveral on a Falcon-9 rocket. The mission carries the Air Force Research Laboratory (AFRL) Advanced Structurally Embedded Thermal Spreader (ASETS-11) to test experimental electronics and oscillating heat pipes in the long duration space environment.

ASETS-II payload (Air Force Research Laboratory)

Developed by the U.S. Air Force Research Laboratory (AFRL), this cargo is testing experimental electronics and Oscillating Heat Pipes for long duration stints in the space environment. The OHP provides lightweight and highly efficient temperature control on higher power yet smaller spacecraft. According to AFRL, the payload’s three primary science objectives are to measure the initial on-orbit thermal performance, to measure long duration thermal performance, and to assess any lifetime degradation.

ThermAvant Technologies and Maxar Technologies Oscillating Heat Pipes (OHP) (AFRL)

The spacecraft remained undetected in orbit for a long time, but was eventually found in a 54.5° orbit at 356 km altitude. It had deployed three small satellites in orbit, which remained unidentified and unregistered until end of the mission. The vehicle returned to earth after 780 days landing at Cape Canaveral SFS on 27 October 2019.

Ralf Vandebergh photographed OTV-5. The Images were taken through a 10 inch F/4,8 aperture Newtonian telescope with an Astrolumina ALccd 5L-11 mono CMOS camera.

X-37B OTV-6 (USA-299) was launched on 17 May, 2020 on an Atlas-5 rocket. Contrary to the earlier mission, a service module, on the aft of the vehicle has been attached for this mission to host more experiments. The U.S. Naval Research Laboratory included the PRAM (Photovoltaic Radio-frequency Antenna Module) experiment to transform solar power into radio frequency microwave energy which could then be transmitted to the ground. PRAM is where one side receives solar energy with a photovoltaic panel, electronics in the middle convert that direct current to a radiofrequency (RF), and the other side has an antenna to beam power away.

Naval Research Laboratory (NRL) has pioneered “sandwich” modules that are used in space solar power experiments. (NRL/Jamie Hartman)

This mission did also deploy the FalconSat-8, a small satellite developed by the U.S. Air Force Academy and sponsored by the Air Force Research Laboratory to conduct several experiments on orbit. Some of the research include Magneto gradient Electrostatic Plasma Thruster (MEP), which uses the Lorentz force (the force on a charged particle by an electromagnetic field) to generate thrust. The thruster works by using a gaseous material that is ionized feeds into an acceleration chamber, where the magnetic and electrical fields are created using a power source.

(a) Photograph of the helicon plasma thruster at Tohoku University. (b) The measured plasma pressure profile and the particle dynamics relating to the loss of axial momentum loss. (Kazunori Takahashi)

Another research focused on Meta-Material Antenna (MMA), a low size, weight, power antenna with phased-array like performance, and Carbon nanotube experiment (CANOE), a RF cabling with carbon nanotube braiding flexed using shape-memory alloy. Furthermore, the research included Attitude Control and Energy Storage (ACES), a Commercial reaction wheel modified into a flywheel for energy storage and release, and SkyPad, an Off-the-shelf cameras and GPUs integrated into low-SWAP (size, weight and power) package.

Also onboard were two NASA experiments, one to study the effects of the space environment on a materials sample plate and a payload of seeds to test radiation effects. OTV-6 was the first to use a service module to host experiments. The service module was an attachment to the aft of the vehicle that allowed additional experimental payload capability to be carried to orbit.

Tracking OTV-6 on n2yo

Currently, OTV-6 has perigee of 341 km and apogee of 344 km, with inclination of 45.0 ° and period of 91.2 minutes. It can be tracked with Orbitron or n2yo. Air Force officials have suggested that the X-37B could in the future share information with the service’s fifth-generation fighters, the F-22 and F-35, giving them “the ability to operate from all domains.”



A Continual Learner

With passion for technology, military, and economics, In Bok Lee has created A Continual Learner with the purpose of sharing knowledge.