Now We Know at Least Two Payloads on the X-37B

But most everything else about the Air Force spaceplane’s current mission remains secret.

X-37B at Vandenberg.jpg
A little mystery: The Air Force X-37B mini-shuttle gets serviced after its landing at Vandenberg AFB in California last October.

The Air Force recently parted—very slightly—the veil of secrecy surrounding its enigmatic X-37B unmanned spaceplane by announcing two of the experiments launched last week on the X-37B’s fourth long-duration flight.

The mini-spaceplane’s official purpose is to demonstrate reusable spaceflight technologies for the Air Force. In a published fact sheet, the Air Force says the two X-37B spacecraft are testing avionics, flight systems, guidance and navigation, thermal protection and insulation, propulsion, and re-entry systems—nearly every aspect of spacecraft design.

However, in the five years since the first X-37B launch, the Air Force has shared very few details about what the spaceplane actually does on its months-long orbital missions. That’s why it came as something of a surprise when two of the payloads included on the current mission—an Air Force ion thruster and a NASA materials experiment—were revealed ahead of the launch.

It’s still not clear what prompted those revelations now; an Air Force spokesperson declined to comment on whether the announcements signal that the program is starting to open up. It is plausible, though unconfirmed, that other, undisclosed payloads were included on this flight, which is expected to last at least 200 days.

The Air Force Research Laboratory is taking this opportunity to test a Hall thruster, a type of ion engine that uses a magnetic field to accelerate charged particles of a gas, usually xenon, out the back of the thruster. Ion acceleration is slow but steady. It generates enough thrust for orbital maneuvers, if you’re willing to wait longer than chemical propulsion maneuvers require. The big advantage is that ion engines use much less fuel than conventional thrusters, leaving more launch mass for payloads.

The experimental Hall thruster on this flight “will impart a small thrust on the spacecraft during operation, which we will measure as part of the experiment in order to assess performance of the thruster,” according to a public affairs representative for the 88th Air Base Wing. The Air Force hasn’t said just how much thrust, so it remains unclear whether the X-37B is using the experimental engine to maneuver on this flight.

According to the Air Force, the test thruster is a modified version of Hall thrusters already in use aboard Advanced Extremely High Frequency military communications satellites. The three AEHF satellites now in orbit represent the first half of a planned network of six satellites, which is scheduled to be operational later this year.

The X-37B also carries about 100 samples of NASA-supplied spacecraft materials, which will be exposed to space for several months to see how they hold up. It’s a follow-up to previous tests on board the International Space Station, which apparently caught the Air Force’s attention.

“For this METIS investigation, the Air Force approached NASA and offered the flight because of previous interest in data gathered through MISSE investigations,” said a public affairs officer at NASA’s Marshall Space Flight Center. This is the first NASA materials science experiment to fly on the X-37B, according to the agency.

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