DARPA has announced Phase 2 of its Experimental Spaceplane (XS-1) programme, which seeks to design and fabricate an experimental unmanned spaceplane for more routine, responsive and affordable space operations. The reusable XS-1 would demonstrate the potential for low-cost and “aircraft-like” high-ops-tempo space flight, enabling a host of critical national security options while helping to launch a new and potentially fruitful commercial sector.

“We intend to break the cycle of escalating DoD space system launch costs, catalyze lower-cost satellite architectures, and prove that routine and responsive access to space can be achieved at costs an order of magnitude lower than with today’s systems,” explains DARPA programme manager Jess Sponable.

XS-1 envisions a fully reusable unmanned booster vehicle that would fly to high speeds at a suborbital altitude. At that point, one or more expendable upper stages would separate, boost and deploy a satellite into low Earth orbit (LEO). The reusable first stage would then return to earth, land and be prepared for the next flight. Although relatively small by conventional aircraft standards, the bizjet-sized XS-1 flight booster would be sufficient to validate credible scaling to larger reusable launch systems.

XS-1 has four primary technical goals:

• Fly 10 times in a 10-day period (not including weather, range and emergency delays) to demonstrate aircraft-like access to space and eliminate concerns about the cost-effectiveness and reliability of reusable launch.
• Achieve flight velocity sufficiently high to enable use of a small (and therefore low-cost) expendable upper stage.
• Launch a 900-1,500lb representative payload to demonstrate an immediate responsive launch capability able to support both DoD and commercial missions. The same XS-1 vehicle could eventually also launch future 3,000+lb payloads by using a larger expendable upper stage.
• Reduce the cost of access to space for 3,000+lb payloads, with a goal of approximately $5m per flight for the operational system, which would include a reusable booster and expendable upper stage(s).

Successful design would require integrating state-of-the-art technologies, processes and system approaches to deliver routine aircraft-like operability, reliability and cost efficiency. In particular, incorporation of autonomous technology and operations promises to significantly decrease the logistical footprint and enable rapid turnaround between flights. Structures made of advanced materials, cryogenic tanks, durable thermal protection, and modular subsystems would make possible a vehicle able to launch, fly to high speeds and then land in a condition amenable to rapid turnaround and launch with the next payload. Reusable, reliable propulsion would also be essential for a low-cost and recurring flight capability.

In Phase 1 of XS-1, DARPA sought to evaluate the technical feasibility and methods for achieving the program’s goals. To achieve that, it awarded prime contracts to three companies, each working in concert with a commercial launch provider: Boeing (working with Blue Origin); Masten Space Systems (working with XCOR Aerospace); and Northrop Grumman (working with Virgin Galactic).