Aerojet Rocketdyne [AJRD] has successfully tested its AR-22 booster rocket engine’s ability to re-fire 10 times within 10 days, a critical milestone needed for its use on Boeing’s [BA] Phantom Express reusable military space plane launch system, company officials said on Tuesday.

The AR-22 was tested June 26-July 10 during which engineers were able to get the turn-around time for the engine down from 17 hours to 6 hours, as officials ready the system for use on Phantom Express demonstration flights in 2021.

The AR-22 engine built by Aerojet Rocketdyne during hot-fire testing at NASA Stennis on June 27, 2018. The engine was built for Boeing as part of the U.S. Defense Advanced Research Projects Agency Experimental Spaceplane program. This new Boeing spaceplane, called Phantom Express, is intended to demonstrate a new paradigm for more routine, responsive and affordable space access. Photo courtesy of NASA.
The AR-22 engine built by Aerojet Rocketdyne during hot-fire testing at NASA Stennis on June 27, 2018. Photo courtesy of NASA.

“Aerojet Rocketdyne has continued to refine the reusable engine technology we originally developed for the Space Shuttle program. With the AR-22 we are taking reusability to the next level and have demonstrated that daily, affordable access to space is within reach,” Eileen Drake, CEO of Aerojet Rocketdyne, said in a statement.

Jeff Haynes, program manager for Aerojet Rocketdyne’s AR-22, told reporters Tuesday that the engine was able to fire up in 100 seconds and then re-fire seamlessly 24 hours later. The process was then repeated for 10 days in a row.

“We scored a perfect 10 last week,” said Haynes, who explained the tests were conducted to determine the feasibility of being able to re-launch the Phantom Express in successive days.

The AR-22 tests were conducted by Aerojet Rocketdyne, Boeing and DARPA as part of the Experimental Spaceplane program.

Scott Wierzbanowski, DARPA program manager for the spaceplane effort, told reporters the Phantom Express would be capable of reaching low Earth orbit and delivering up to 3,000 pounds of cargo. Each flight would cost approximately $5 million.

Boeing’s Phantom Express would be used to deliver small satellites into low-earth orbit, with potential flights being taken in successive days.

DARPA has contributed $146 million to the project, according to Wierzbanowski.