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Phantom Express XS-1 space plane
An artist’s conception shows Boeing’s Phantom Express XS-1 space plane in flight. (Boeing Illustration)

The Pentagon’s Defense Advanced Research Projects Agency says Boeing is dropping out of its Experimental Spaceplane Program immediately, grounding the XS-1 Phantom Express even though technical tests had shown the hypersonic space plane concept was feasible.

“The detailed engineering activities conducted under the Experimental Spaceplane Program affirmed that no technical showstoppers stand in the way of achieving DARPA’s objectives, and that a system such as XSP would bolster national security,” DARPA said in a statement issued today.

In a follow-up statement, Boeing confirmed that it’s ending its role in the program after a detailed review.

“We will now redirect our investment from XSP to other Boeing programs that span the sea, air and space domains,” Boeing said. “We’re proud to have been part of a DARPA-led industry team that collaborated to advance launch-on-demand technology. We will make it a priority to harvest the significant learnings from this effort and apply them as Boeing continues to seek ways to provide future responsive, reusable access to space.”

DARPA initiated the space plane development program in 2013, and chose Boeing over Northrop Grumman and Masten Space Systems to become the lead contractor for Phases II and III in 2017. The public-private effort stood to receive as much as $146 million in support from DARPA.

The Experimental Spaceplane Program was aimed at developing a launch system for military and commercial applications that had aircraft-like operability, including flights on demand, rapid turnaround, a low footprint for ground infrastructure and low recurring costs.

A key 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 the course of 10 days, with each firing lasting 100 seconds at full throttle.

The two-stage Phantom Express launch system was designed to be capable of conducting 10 liftoffs in 10 days, with each launch delivering 3,000 pounds of payload to low Earth orbit at a cost of less than $5 million a flight. An initial demonstration flight was scheduled for 2021.

DARPA said the program “identified evidence that present-day liquid rocket propulsion systems are capable of supporting XSP objectives, remain of interest, and may be explored in separate efforts.”

Hypersonic vehicles — capable of traveling more than five times the speed of sound — are becoming a high priority for weapons development, due to their theoretical ability to evade air defenses. China and Russia are developing hypersonic weapons, and the U.S. military is scrambling to keep pace.

Just this week, Seattle-based Stratolaunch confirmed that it would be working on the development and testing of hypersonic vehicles.

Phantom Express isn’t Boeing’s only entry in the hypersonic race: The company’s fleet of X-37B space planes has taken on five hush-hush orbital test missions for the U.S. Air Force, and a sixth launch is planned this year. Also, Boeing’s HorizonX venture investment arm is a backer of Reaction Engines, a British aerospace startup that’s working on a hybrid rocket-jet propulsion technology for hypersonic vehicles.

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