The first Aerojet-Rocketdyne AR-22 rocket engine has recently been assembled. This is a somewhat modified version of the old Space Shuttle Main Engine, meant specifically to power the first stage of the Boeing “Phantom Express” spaceplane. Thrust is 375,000 pounds and the engine is meant to be used 55 times, with servicing every 10 missions.

First Engine Assembled for DARPA and Boeing Reusable Experimental Spaceplane

The Phantom Express is meant to fly often and inexpensively… and appears to be basically an updated version of the mid-90’s Rockwell design for the X-33. It’s not clear to me that a hydrogen-burner using SSME-derived tech can compete economically with the likes of the Falcon 9, but the Phantom Express isn’t really intended to compete in the commercial market. Instead, the Phantom Express is intended as  military launch system, lobbing relatively small satellites – communications and recon, with the possibility of GPS-replacements in the event that military action takes them out. Given that any future war with a major opponent will certainly involve attacks on American space infrastructure, it’s reasonable for DARPA to want to have as many rapid response launch systems as practical. The basic concept underlying the Phantom Express is simple and straightforward enough, and likely to be somewhat more rugged and reliable than the hoverslam landing system of the Falcon series… at the cost of probably weighing more.