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Northrop XS-1
Northrop Grumman says it’s made only minor changes to its proposed design of the XS-1 reusable vehicle from this concept it released in 2014. (credit: Northrop Grumman)

XS-1 prepares for liftoff

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Last week, the Defense Advanced Research Projects Agency (DARPA) formally released the solicitation for the next phase of the Experimental Spaceplane 1 (XS-1) program. The solicitation was not a surprise to the industry: DARPA announced more than a month earlier of its intent to move ahead with the XS-1 program, and it held an industry day about its plans at its Arlington, Virginia, headquarters late last month.

“It’s enough to pick someone and go. It’s probably not enough to fully fund what we have envisioned,” Sponable said of the XS-1 funding.

XS-1 is the latest—and, potentially, last—effort by the US government to spur the development of reusable launch vehicles. The goal of the XS-1 program is to develop a reusable first stage that, coupled with an expandable upper stage, launch payloads weighing up to a few thousand kilograms into orbit for no more than $5 million a launch: a fraction of the price of current vehicles in that mass class.

Announced in 2013, DARPA awarded three “Phase One” study contracts in 2014 to teams led by Boeing, Masten Space Systems, and Northrop Grumman. Those companies have all wrapped up, or are about to wrap up, those studies, bringing their designs to roughly the level of a preliminary design review.

The outcome of those studies led DARPA to press ahead with the next phase, which calls for development of a prototype followed by flight demonstrations. “DARPA’s been very good to me, and I can tell you, officially now, that we have a funded flight demonstration and experimental program for the next phase of the XS-1 program,” said Jess Sponable, DARPA XS-1 program manager, in a talk at the Space Access ’16 conference last month in Phoenix timed to the DARPA announcement about its plans to move ahead with XS-1.

DARPA expects all three companies that won Phase One contracts to submit proposals for Phase Two and Three (representing prototype development and flight demonstrations, respectively), with those proposals due by July 22. Other companies are also able to submit proposals, but Sponable suggested that they will be at a significant disadvantage unless their designs are at a similar level of maturity. “We are looking for a level of detail in the response that will make it very difficult for some people to come in off the fly and respond,” he said.

Another factor that may deter companies from competing is that DARPA expects the winning company—Sponable said he anticipates only a single award—to share the cost of developing the XS-1 prototype. DARPA will provide up to $140 million to the winning company, but expects them to pitch in a significant amount of money to cover development. “It’s enough to pick someone and go. It’s probably not enough to fully fund what we have envisioned,” he said of the funding.

Sponable said at Space Access that the award will be done through DARPA’s “other transactional authority,” a more flexible approach than standard government contracts that is analogous to NASA’s funded Space Act Agreements for commercial cargo and crew. “That implies cost share,” he said.

“We were using a new propellant technology that greatly simplified the design of the rocket engine,” said Melroy of ALASA’s problems. “It’s highly energetic, and in all of our flow testing we continually had unplanned energetic events.”

The response from the companies with Phase One agreements to that cost-sharing arrangement varies. In an interview during the 32nd Space Symposium in Colorado Springs a few days after Space Access, Doug Young, the Northrop Grumman vice president responsible for XS-1, said his company was looking forward to competing in the next phase of the competition.

“We’re excited. This is really an amazing time,” he said. The company has made some refinements to the design it unveiled in 2014, he said, but has not made any major changes. He added he was planning to keep intact the team of companies involved in the Phase One XS-1 study, including Scaled Composites and Virgin Galactic.

Masten Space Systems, a company a tiny fraction the size of Northrop, is also looking ahead to the next phase of the XS-1 competition. Company founder Dave Masten said at Space Access said the company was sticking to its approach of having the vehicle make a vertical landing, rather than gliding to a runway landing, based on the company’s experience with vertical takeoff and landing.

Masten also said his company was in the midst of raising $50 million, a far larger financing round than the company has sought. That funding, he said, would go to winning the XS-1 program “and be able to actually work that program,” which requires growing the company beyond its current size of about 40 employees. Winning Phase Two, he added, would likely requiring raising more funding beyond the current $50-million round.

Only Boeing seemed to hedge its bets about XS-1. Company officials, speaking at Space Symposium, suggested the cost-sharing requirement was less than expected, and thus was causing them to rethink their plans. (Others in industry said they had long anticipated that DARPA would provide no more than about $140 million for XS-1.)

“One way or another we’re going to go forward with this program, but we’re working closely with DARPA to work on the expectations for the next phase,” Alex Lopez, vice president for global sales and marketing at Boeing Network and Space Systems, said at the symposium.

But despite the commitment of the three companies, and perhaps others who, despite the long odds, will also submit proposals, what’s the future of the XS-1? Two factors are working against its success, or at least its relevance.

One factor is DARPA’s checkered history with launch vehicle development. DARPA started XS-1 around the time it was working on a separate concept, called the Airborne Launch Assist Space Access (ALASA). That program sought to create a small launch vehicle that would be air-launched from an F-15 for placing small satellites into orbit.

DARPA, though, cancelled plans for flight tests of ALASA late last year after its prime contractor, Boeing, ran into problems with the unusual propellant combination it selected for the launch vehicle.

“Ten flights in ten days for a rocketplane is a stressing requirement,” said Young. “Even turning around a new airplane in its initial stages of its flight test program every day is a big stretch.”

“We were using a new propellant technology that greatly simplified the design of the rocket engine,” said Pam Melroy, deputy director of DARPA’s Tactical Technology Office, during an April presentation to a National Academies committee. “It’s highly energetic, and in all of our flow testing we continually had unplanned energetic events.” Or, in other words, it exploded.

Before ALASA, DARPA also worked on launch vehicle programs in the early 2000s, with an air launch system called Responsive Access, Small Cargo, Affordable Launch (RASCAL) and a small launcher effort called Force Application and Launch from Continental United States (FALCON). Neither made it into flight testing.

DARPA emphasizes the experimental nature of the XS-1 effort in the Phase Two solicitation. The document lists five factors that it will use for evaluating proposals, with “overall scientific and technical merit” the most important. The two least important ones were “cost realism” and “schedule realism,” which raises questions about the ability to get a vehicle developed (with significant fiscal resources provided by the winning company) and into flight testing by 2020, as desired by DARPA.

Adding to that challenge are some of the technical requirements in place for the program. DARPA has scaled back some aspects of XS-1’s performance: an initial launch can carry a payload weighing as little as 410 kilograms, so long as the company can demonstrate a path to launching larger ones. It also dropped a requirement that XS-1 perform at least one test flight to Mach 10, a speed that raised issues for some in industry. While a Mach 10 flight remains an “objective” for the program, the minimum threshold for success is a single flight to at least Mach 3.

Still in place, though, is the requirement to fly ten times in ten days, a central goal of the overall XS-1 program to demonstrate fast turnaround. That is a challenge for companies used to more incremental flight test programs.

“Ten flights in ten days for a rocketplane is a stressing requirement,” said Northrop’s Young. “Even turning around a new airplane in its initial stages of its flight test program every day is a big stretch.”

“We want to push the industry to the point where we can fly a lot more often,” Sponable said at Space Access, making the case for the reflight requirement.

That push is also another issue affecting the odds of success for XS-1. Once the nearly exclusive domain of government agencies, both civil and military, reusable launch vehicle development is now being led by industry. SpaceX has now repeatedly landed Falcon 9 first stages—three in a row after a May 27 launch—with plans to refly one of those stages later this year. Blue Origin has reflown its New Shepard propulsion module on several suborbital flights.

Sponable acknowledged those efforts in his Space Access talk, but suggested XS-1 could distinguish itself from those commercial efforts by emphasizing a high flight rate. “We think flying ten times in ten days is something well outside the capability of either SpaceX or Blue Origin at this time, and we hope to spur them along to get them to think about how they can increase their flight rates,” he said.

Those companies, and others, are no doubt already thinking about increased flight rates (it’s been nearly two months since New Shepard’s last flight, for example.) XS-1 could provide some answers for an increased flight rate by the time flight tests of the vehicle begin in 2020—if the program remains on track, and if those companies haven’t figured out how to do it themselves by then.