For decades, U.S. government agencies, both civil and military, have sought to develop a reusable launch vehicle (RLV), seeing it as a critical tool for lowering the cost of space access. The space shuttle is the best known such effort, but it’s hardly the only one: the National Aerospace Plane, Delta Clipper, X-33, X-34 and Space Launch Initiative all tried to develop reusable launchers — and all failed.

“We’ve been pursuing this low-cost, aircraft-like access to space literally since the 1960s, and seriously since the 1980s, and we have had failure after failure after failure,” said Jess Sponable, a program manager at the Defense Advanced Research Projects Agency, in an April 7 talk at the Space Access ’16 conference, a low-profile gathering of space entrepreneurs and enthusiasts in Phoenix.

Sponable is managing the latest government effort to develop an RLV, called Experimental Spaceplane 1, or XS-1. Announced by DARPA in 2013, XS-1 is in many respects less ambitious than many of those previous efforts: rather than build a large, single-stage-to-orbit reusable vehicle, XS-1 is focused instead on a smaller reusable first stage that, coupled with an expendable upper stage, could place payloads weighing up to a couple thousand kilograms into orbit.

And unlike those earlier programs, XS-1 is still alive and well. With an initial phase of study contracts awarded in 2014 wrapping up, DARPA plans to release a solicitation for the next phase of the program in May. At stake is about $140 million for the development of a prototype and a flight test program that will emphasize the ability to fly, and fly often.

“We want to push the industry to the point where we can fly a lot more often,” Sponable said. A goal from the beginning of the program to demonstrate that capability is to fly XS-1 10 times in 10 days, demonstrating its reliability and low-cost operations.

That central goal has been a major challenge for some companies working on the initial XS-1 studies. “Ten flights in 10 days for a rocketplane is a stressing requirement,” said Doug Young, the Northrop Grumman vice president who is leading that company’s XS-1 work. “Even turning around a new airplane in its initial stages of its flight test program every day is a big stretch.”

Other parts of XS-1, though, have evolved since its introduction. DARPA originally sought to have one of those 10 test flights go to Mach 10, but Sponable suggested DARPA was now more flexible about that requirement. “We left it up to industry to tell us how fast they want to go,” Sponable said. “Honestly, I’ve got people all over the map.”

Also, while XS-1 has the goal of launching payloads weighing at least 1,360 kilograms for less than $5 million, the flight test program wi l l have the requirement of only launching a single “representative payload” weighing as little as 410 kilograms. That could be achieved with any number of readily available upper stages, Sponable said, but companies will have to show they can “rapidly transition” to a larger upper stage that meets the original goal.

Assuming DARPA releases the solicitation as scheduled, Sponable said he expects the agency to make an award relatively soon after the U.S. government’s fiscal year 2017 begins Oct. 1. That would, he said, allow flight tests to take place around 2020.

Sponable acknowledged that the funding DARPA has set aside for XS-1 won’t be enough to fully fund the winning vehicle’s development and flight tests. “It’s enough to pick someone and go. It’s probably not enough to fully fund what we have envisioned,” he said.

DARPA will make the XS-1 award using its Other Transaction Authority, which is more flexible than a traditional contract. “That implies cost share,” he said, with the winning company expected to contribute its own money to some degree to fully fund the vehicle’s development.

Company plans

While DARPA issued XS-1 study contracts in 2014 to Boeing, Masten Space Systems and Northrop Grumman, DARPA is will accept proposals for the next phase from any interested companies.

“We feel that a lot of these technologies, particularly the reusable ones, have come a long ways since we started the program a couple of years ago,” said Pamela Melroy, deputy director of DARPA’s Tactical Technology Office, during an April 25 presentation to the National Academies’ Aeronautics and Space Engineering Board. “So we’re throwing this to a full and open competition for phase two in the hope that we bring in some fresh ideas.”

Any new entrant, though, will be at a disadvantage to the three original companies, whose studies are roughly equivalent to a preliminary design review. “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,” Sponable said.

The three phase one companies are gearing up for the upcoming competition with varying degrees of enthusiasm. “We’re excited. This is really an amazing time,” said Northrop’s Young in an interview during the 32nd Space Symposium in Colorado Springs, Colorado, April 14.

Young said Northrop’s design has changed little from what it disclosed in 2014 when it won the XS-1 study contract: a winged vehicle that would take off vertically and glide to a runway landing. “We’ve made some refinements in the overall aerodynamic performance and the overall mission design,” he said. “We feel pretty good about the path we’ve started on.”

Northrop Grumman is also keeping the same industry team, which includes Scaled Composites — wholly owned by Northrop — to handle fabrication and assembly, and Virgin Galactic to plan for the eventual transition of XS-1 to commercial operations.

While XS-1 is a relatively small program for Northrop Grumman, it’s a big deal for Masten Space Systems, a small company in Mojave, California. Prior to its XS-1 award, it was best known for winning a NASA-funded lunar lander prize competition in 2009 (sponsored, ironically, by Northrop Grumman.)

Work on XS-1 has helped Masten’s workforce to more than double in the last year, but the company still has fewer than 40 employees, company founder Dave Masten said in a Space Access ’16 presentation. The company and its team of contractors were wrapping up work on their phase one contract and preparing for phase two. The company is also working to raise a $50 million financing round to support that effort, and plans to raise even more should it win the competition.

The company’s XS-1 design, like previous vehicles it has developed, takes off and lands vertically, although it has stubby wings to allow it to fly back to its launch site to make that vertical landing. “We don’t know what wheels, landing gear or runways are,” he quipped.

Masten is not deterred by XS-1’s requirement of 10 flights in 10 days, based on the company’s experience during frequent flight tests of its low-altitude test vehicles. “We know it’s possible because we’ve been doing it,” he said. “The question for us is, can we scale up to an orbital vehicle system and still maintain that?”

While Masten and Northrop sounded enthusiastic about the next phase of XS-1, the third company to win a study contract, Boeing, has sounded a more cautionary note. Working with Blue Origin, Boeing’s vehicle concept is similar to Northrop’s, including gliding to a runway landing.

But during a company roundtable at Space Symposium, Boeing officials suggested that the cost sharing requirement might lead them to seek changes in the scope of the next phase of XS-1.

“There was always going to be company participation, but the amount of company participation will be a little different” because the funding for phase two is less than what was previously expected, said Alex Lopez, vice president for global sales and marketing at Boeing Network and Space Systems.

He suggested Boeing might ask DARPA to change some of the requirements to reflect that reduced funding. “One way or another we’re going to go forward with this program,” he said, “but we’re working closely with DARPA to work on the expectations for the next phase.”

What’s different this time

Despite the enthusiasm of at least some of the companies working on XS-1 concepts, there’s still the poor track record of past attempts by NASA and the Defense Department to develop RLVs. “Haven’t I been to this movie before?” Sponable asked in his Space Access presentation, bringing up those past, failed efforts.

He argued that those earlier programs suffered from immature technologies coupled with demanding requirements. “Is there really any surprise that we ran into serious problems as we pursued these programs, and they ended up being something different than when we started them?” he said.

By contrast, XS-1 will use technologies further along in their development: at a technology readiness level of at least five, which on the one-to-nine TRL scale means at least some components of key technologies have been validated. Sponable said that earlier efforts often used technologies with TRLs of three or less.

But if DARPA fails with XS-1, it might not get another chance to work on an RLV program for years to come, and it’s unlikely anyone else in the federal government would, either. NASA is working on the heavy-lift, but expendable, Space Launch System. The Air Force is now focused on developing a new engine to replace the Atlas 5’s RD-180. Neither has any clear interest in, or funding for, RLVs.

Moreover, the private sector is catching up to government RLV development efforts. The day after Sponable spoke at Space Access, attendees watched a webcast of a SpaceX Falcon 9 launch, including the successful landing of the rocket’s first stage on a ship. Blue Origin has already reflown its New Shepard suborbital vehicle several times, and plans to use that technology on an orbital vehicle the company expects to start launching around the end of the decade.

Sponable, though, thinks there’s still something XS-1 can show the private sector. “We think flying 10 times in 10 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.

XS-1 is Sponable’s latest effort involving RLVs, but hardly his first. A quarter-century ago, as a major in the U.S. Air Force, he was the program manager for Delta Clipper Experimental (DC-X), a technology demonstrator for a proposed, but never built, RLV. He later worked for Universal Space Lines, a company founded by some members of the DC-X team to try to develop a commercial RLV.

“I never thought,” he said, “it would take this long to solve the access to space problem.”


“We want to push the industry to the point where we can fly a lot more often.”

– Jess Sponable

Northrop Grumman, with Scaled Composites and Virgin Galactic, is developing a preliminary design for DARPA’s Experimental Spaceplane XS-1. Credit: Northrop Grumman artist's concept

Northrop Grumman, with Scaled Composites and Virgin Galactic, is developing a preliminary design for DARPA’s Experimental Spaceplane XS-1. Credit: Northrop Grumman artist’s concept

Boeing's XS-1 design. Credit: Boeing artist's concept

Boeing’s XS-1 design. Credit: Boeing artist’s concept

Masten, building reusable rockets for years, was selected by DARPA to bring their expertise to bear for the XS-1 program. Credit: Masten artist's concept

Masten, building reusable rockets for years, was selected by DARPA to bring their expertise to bear
for the XS-1 program. Credit: Masten artist’s concept

Blue Origin’s New Shepard suborbital vehicle after its Nov. 23 suborbital launch and landing. Credit: blue origin

Blue Origin’s New Shepard suborbital vehicle after its Nov. 23 suborbital launch and landing. Credit: blue origin

SpaceX landed its first-stage Falcon 9 rocket booster on a drone ship for the first time April 8. It repeated the feat May 6. Credit: SpaceX

SpaceX landed its first-stage Falcon 9 rocket booster on a drone ship for the first time April 8. It repeated the feat May 6. Credit: SpaceX

Illustration of NASA and Lockheed’s X-33 RLV demonstrator. Credit: NASA artists's concept

Illustration of NASA and Lockheed’s X-33 RLV demonstrator. Credit: NASA artists’s concept

Photo of NASA and Orbital Sciences’ X-34 RLV demonstrator. Credit: NASA

Photo of NASA and Orbital Sciences’ X-34 RLV demonstrator. Credit: NASA