The US Air Force has been testing a mysterious unmanned space plane for the last few years known at the X-37B. It has spent more than a thousand days in orbit over just a handful of missions, and now the Department of Defense has released the first details about what it’s doing up there.
On the upcoming mission, expected to launch between May 6th and 20th, the X-37B will test an experimental Hall thruster, a type of ion engine that operates on the principle of electric propulsion. These low-thrust engines aren’t enough to take the plane into orbit — that’s going to be handled by a ULA Atlas V rocket. Ion engines like this one are, however, very good at moving small objects a very long way without wasting too much space on fuel.
The X-37B’s Hall thrusters work by ionizing and accelerating xenon atoms to produce thrust. These engines operate at 4.5 kilowatts and can produce 0.06 pounds of thrust. That’s not very much, but when you spread that continuous force out over hours or days, you can alter an object’s orbit using very little fuel. It does require a lot of power, so the X-37B is covered in gallium arsenide solar cells.
Ion engine technology has been used for decades in smaller satellites and space probes, but the X-37B is quite a lot larger. It’s 29 feet in length and weighs about 11,000 pounds. Hall thrusters could be invaluable for satellites, which could change orbit and remain in space for much longer. Conventional engines offer more thrust in a shorter time, but they consume a lot of heavy fuel.
The X-37B also shows how far ion engines have come, but we don’t know what else the Air Force expects to do with it. There are probably military applications that remain classified. In the past it has been speculated the X-37B could be an experimental satellite killer or some other weapons platform. That’s all speculation at this point, though.