Hypersonic vehicles are becoming a high priority for weapons development due to their theoretical ability to evade air defences. China and Russia are developing hypersonic weapons, and the U.S. military is scrambling to keep pace.
Defence Advanced Research Projects Agency (DARPA) announced the XS-1 program in 2013 as a way of supporting the development of responsive, reusable launch vehicles. DARPA selected Boeing in May 2017 for Phases 2 and 3 of what was initially called the XS-1 program. Phase 2 covered the development of the vehicle, while Phase 3 called for up to 15 flight tests of the vehicle.
DARPA spokesman Jared Adams said that Boeing had notified the agency of its decision to exit the Experimental Spaceplane Program “immediately.” DARPA didn’t state why Boeing was dropping out of the program.
A vital test of the propulsion technology came in 2018 when Boeing and Aerojet Rocketdyne successfully conducted a series of 10 firings of the space plane’s intended AR-22 rocket engine over ten days, with each firing lasting 100 seconds at full throttle.