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The Boeing X-37B Orbital Test Vehicle is a spaceplane which originated in NASA design studies in the late 1990s. Although the original concept was to be a technology testbed experiment used to study technologies and systems for an eventual shuttle replacement, NASA dropped the program. In the early 2000s, however, the program was resurrected and shifted from the auspices of the civil space administration to the USAF and DARPA - and information about it promptly dried up. The development and construction of the vehicle were assigned to Boeing's "Phantom Works" division, where the company carries out highly-classified aerospace programs.
With little advance notice, the X-37B was announced to be flightworthy, and was launched on April 22, 2010 for a mission of 'indeterminate length' intended, the public was told, to test the vehicle's systems. It was carried to space atop an Atlas 5 booster and deployed using a Centaur upper stage. Serving as a magnet for curious space enthusiasts and satellite spotters, the X-37 did in fact spend several months in space, changing its orbit often enough to sometimes vanish entirely from public eye. It landed at Vandenberg AFB on December 3, 2010 after a 220 day mission - the second spaceplane ever to land fully autonomously. The first? Well, that would be the Soviet Buran Space Shuttle clone - which landed, unmanned, using its autopilot the first and only time it flew - in 1986.
The X-37B does look a bit like a small, streamlined Space Shuttle. It is 29 feet, 3 inches in length (8.9 meters) and 9 feet, 6 inches tall (2.9 meters) sitting on its undercarriage. Unlike the Shuttle, instead of a traditional vertical stabilizer it has twin tails in a V configuration along with stubby delta wings (14 feet, 11 inches in span, or 4.5 meters) which look like those on its older brother. Also like the shuttle, it has a large cargo bay along its spine with bay doors intended to open in flight to serve as radiators and solar collectors - the power demand is low enough that it can rely on solar panels, extending the endurance (the Shuttle uses fuel cells). It has a single large engine at the rear, presumably for orbital maneuvering and de-orbit burn, and attitude thrusters at various spots. Although the Air Force won't give out specs on that engine, at one point in the system's long history the design called for a vehicle with up to 7,000 miles per hour of delta-V - around 3,130 meters/second. It is designed to operate in Low Earth Orbit, at apogees ranging from 110 to 500 miles, for missions of up to 270 days in duration.
The X-37B is called an 'Orbital Test Vehicle' because the Air Force (and Boeing, and everyone else involved) swear that the spaceplane is not an 'operational' asset, but merely serves as a testbed for various technologies. Whether or not you believe that is up to you. It has a cargo bay which is said to be roughly the size of that of a pickup truck which can be opened to space via the bay doors mentioned above. Its payload capacity isn't given, but a total launch weight of 11,000 pounds (4,990 kg) is listed on its Air Force fact sheet, which is a suspiciously round number. Other stories indicate that the vehicle can carry payloads weighing a up to four hundred kilograms for exposure or deployment in orbit.
The main advantage of the X-37 over the Space Shuttle is that being unmanned, it doesn't have to waste payload (and, hence, launch capability) on anything whose purpose is to keep humans alive and comfortable. It has no cabin, no windscreen, no life support, no chairs, no john, no airlock, nothing. It is essentially a space UAV, and there are those who suspect that is what its 'operational' mission would be.
Tellingly, the Air Force office leading the program is called the 'Air Force Rapid Capabilities Office.' The program was accelerated several years ago when the notion of 'Prompt Global Strike' was popular. Although it is unlikely the X-37B could be used as a 'space bomber,' it would be just perfect for the quick and quiet deployment of spy satellites into previously-unused orbits. It is intended to be rapidly turned around and quickly launched if necessary - from alert to launch in as little as 5 days, if a booster and launch facility are available. In addition, it has proven that it can remain on orbit for several months, changing orbits several times. It could be launched in times of tension so as to have new satellites available in case of need.
The Air Force admits that the following technologies are being tested on the X-37B - new thermal protection systems (tiles, like the shuttle), avionics, navigation & control systems, reusable insulation, and lightweight electromechanical flight systems. Those last are another significant difference from the Shuttle, which uses hydraulics with their associated weight and fragility.
Its first mission has apparently been successful, and the Air Force tells us that they have contracted for a second vehicle to be built. One interesting bit of data to watch for will be to see if they perform a rapid turnaround and launch of the vehicle for two back-to-back successful missions, and if so, how long the turnaround time is.
Various commentators (and nations) have asserted (or complained) that the X-37B is evidence of the United States attempting to gain a lead in the weaponization of space. While this is certainly possible, there is no evidence that the Air Force is testing systems on the X-37 which in fact have any offensive strike capability. It's quite likely that they are, in fact, testing flight systems and the ability to deploy satellite packages quickly and quietly from a lurking vehicle. Of course, since they have shrouded the vehicle and its mission in several layers of secrecy, it's difficult to be sure that they aren't doing anything nefarious - and since this is the United States military, such suspicions come naturally. Geoffrey Forden, a physicist known for commenting on arms control and military tech, offers the hypothesis that one observed orbit of the X-37B on its maiden flight would be appropriate for deploying TACSAT satellites, intended for battlefield observation. These are small enough that a fairly large number could be carried by a single X-37B flight, allowing enough to be sown to provide reasonable coverage of a specific area despite the low dwell time available in LEO.
It should be pointed out that the United States Air Force maintained a separate astronaut program from NASA on a few occasions, including the Manned Orbiting Laboratory program and the Manned Spaceflight Engineer program which trained Air Force personnel to fly on Space Shuttle DoD missions. There is no doubt the X-37B technologies will be applied to future spaceflight initiatives, both manned and unmanned, both civil/commercial and military.