DARPA’s Experimental Spaceplane (XSP) has takan a further step toward its goal for rapid turnaround and on-demand launch with the completion of a series of tests on the spaceplane’s AR-22 Experimental Spaceplane Main Engine.

The engineering team successfully fired the engine 10 times in just under 240 hours. All firings lasted at least 100 seconds. The AR-22 engine is a variant of the RS-25 Space Shuttle Main Engine (SSME).

Darpa says that the 10x10 test series, with its sequence of 10 discrete, long-duration firings in rapid succession, is unprecedented for large hydrogen-fueled liquid rocket engines. The test is an early check for one of the Experimental Spaceplane programme’s most critical subsystems required to achieve aircraft-like operations.

This first-of-its-kind test series took place at NASA’s Stennis Space Center in Stennis, Mississippi.

One significant challenge of the series was rapidly drying the engine between tests. The propellants – liquid hydrogen and liquid oxygen – combine to create liquid water that accumulates internally.

If the engine is chilled down and restarted with excessive internal water, damage could result. our turnaround, the team introduced new processes to cut the drying time first to eight hours and then down to six.

One ingredient in the test program's success was its use of a new main engine controller, originally developed for the RS-25 variant that will power NASA's Space Launch System rocket. The controller, designed around modern avionics standards, provided significant flexibility and advanced capabilities to the program.

The Experimental Spaceplane programme is a public-private partnership between DARPA and Boeing. Boeing teamed with Aerojet Rocketdyne for the AR-22. The program is in the second of three phases, the final of which is a flight test targeted for early 2021.