WASHINGTON — Boeing has decided to no longer continue development of an experimental suborbital spaceplane for the Defense Advanced Research Projects Agency, the latest setback for DARPA’s long-running efforts in space access.

In a Jan. 22 statement to SpaceNews, 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.

“Following a detailed review, Boeing is ending our role in the Experimental Spaceplane (XSP) program immediately,” Boeing spokesman Jerry Drelling said. “We will now redirect our investment from XSP to other Boeing programs that span the sea, air and space domains.”

DARPA selected Boeing in May 2017 for Phases 2 and 3 of what was originally 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, then scheduled for 2020.

Boeing beat out Masten Space Systems and Northrop Grumman for that award, which DARPA valued at $146 million and with unspecified funding contributions by Boeing. All three companies received Phase 1 study contracts from DARPA in 2014.

Boeing’s concept, called Phantom Express, would take off vertically, powered by a single Aerojet Rocketdyne AR-22 engine, a variant of the Space Shuttle Main Engine. The vehicle, 30 meters long and with a wingspan of 19 meters, would fly a suborbital trajectory at speeds up to Mach 10 before gliding to a runway landing. The vehicle was designed to carry an expendable upper stage for placing small satellites into orbit.

DARPA announced the XS-1 program in 2013 as a way of supporting the development of responsive, reusable launch vehicles. One major goal of the program was to perform 10 flights of the vehicle in 10 days, with at least one of those flights going to Mach 10, to demonstrate its rapid turnaround.

In July 2018, Aerojet Rocketdyne showed that its AR-22 engine was capable of such a high flight rate when the company performed 10 static-fire tests of the same engine in a 240-hour period. At the time, DARPA called that test series “a significant go/no-go milestone for us” to continue with the program. Neither DARPA nor Boeing, though, provided many updates on the status of the program after that series of tests.

DARPA cited that series of AR-22 engine tests as one of the major achievements of the program. “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,” Adams said. “Through XSP, DARPA 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.”

“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,” Drelling said.

Boeing’s decision, which effectively ends the XSP program, adds another chapter to DARPA’s history of unsuccessful launch vehicle development efforts. In the early 2000s, DARPA’s Responsive Access, Small Cargo, Affordable Launch (RASCAL) program supported initial development of an air-launch system using a high-speed aircraft by a small California startup, Space Launch Corp. DARPA terminated the RASCAL program in 2005 while that concept was still in its early design phases.

DARPA then embarked on Force Application and Launch from Continental U.S. (FALCON) program to develop both a hypersonic testbed vehicle and a small launch system. FALCON included study contracts to several companies, such as Lockheed Martin, Orbital Sciences and SpaceX. Another startup, AirLaunch LLC, proposed development of a small launch vehicle that would be deployed from a C-17 cargo aircraft, conducting a drop test to demonstrate the feasibility of their concept. DARPA, though, elected to focus the FALCON program on a hypersonic testbed.

Coincident with the XS-1 program was DARPA’s Airborne Launch Assist Space Access (ALASA), which sought to develop a small rocket that could be launched from a fighter with just 24 hours’ notice and for no more than $1 million. Boeing won a contract in 2014 to develop a rocket using an unusual “mixed monopropellant” of nitrous oxide and acetylene, called NA-7, that could be launched from an F-15.

However, DARPA ended plans to perform a flight demonstration with ALASA in November 2015 after discovering that NA-7 was too volatile to be safely handled. The agency did continue ground tests of some technologies related to the program.

More recently, DARPA shifted from funding specific systems to promoting industry innovation for responsive launch. The DARPA Launch Challenge, announced in 2018, offered a top prize of $10 million to the company able to perform two launches of a small launch vehicle from two different sites on short notice. DARPA announced last April it selected three finalists for the competition — Vector, Virgin Orbit and a “stealth” company — for launches then scheduled for early 2020.

However, Vector dropped out of the competition after suspending operations in August, and the company has since filed for bankruptcy protection. Virgin Orbit announced in October it would no longer participate in the competition, preferring instead to focus on upcoming launches for government and commercial customers. The company expects to perform a first launch of its LauncherOne system early this year.

DARPA plans to proceed with the competition with that single stealth competitor, widely believed to be a company that identifies itself as Astra Space in license applications with the Federal Aviation Administration. As of last October, DARPA said the first launch in the competition would take place in February, but the agency has not announced a formal date or location for that launch.