The US Air Force's most public secret, the X-37B unmanned spaceplane, is now a little less top secret. The Air Force has revealed that when the Orbital Test Vehicle (OTV) 4 mission lifts off from Cape Canaveral AFB on May 20, it will be carrying a Hall thruster as part of an experiment to improve the design for use on Advanced Extremely High Frequency (AEHF) military communications spacecraft.

The X-37B, which resembles a trimmed down Space Shuttle, is a piece of space technology that's received a lot of press, even though very little is known about what it does or what it's for. Two were built by Boeing Phantom Works, and they have flown three missions so far, clocking up 1,367 days in orbit. The purpose of those missions has been shrouded in secrecy, with the Air Force discussing their role in the development of reusable space vehicles only in the most general terms.

The Hall thrusters that the X-37B will carry on its next mission are ion engines similar to the one on the Dawn deep space probe, which uses electrons in a magnetic field to ionize atoms of a noble gas, such as xenon, and accelerates the ions for thrust. Though this thrust is equivalent to just the weight of a sheet of paper, the thruster is extremely efficient and can fire for years on end, so eventually it can produce very high velocities.

X-37B prior to liftoff (Photo: US Air Force)

The Hall thruster experiment is a partnership between the Air Force Research Laboratory (AFRL), Space and Missile Systems Center (SMC), and Rapid Capabilities Office (CRO) and is based on the thrusters used on the first three AEHF satellites.

Once in operation, the experiment will use telemetry to record thruster performance and the thrust it puts on the spacecraft. The Air Force says that the results will be used to improve thruster and environmental models, and to better extrapolate ground test results to actual on-orbit performance.

A Hall thruster in operation (Photo: NASA/JPL)

"Space is so vitally important to everything we do," says Major General Tom Masiello, AFRL commander. "Secure comms, ISR, missile warning, weather prediction, precision navigation and timing all rely on it, and the domain is increasingly contested. A more efficient on-orbit thruster capability is huge. Less fuel burn lowers the cost to get up there, plus it enhances spacecraft operational flexibility, survivability and longevity."

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