Technology advances that should make the reusable-spaceplane launch-vehicle concept feasible this time, Darpa believes, include lower-weight, lower-cost composite airframe and tank structures; durable thermal protection; available propulsion that is reusable and affordable, and health management systems that enable aircraft-like operations. Since the previous reusable launch-vehicle programs were canceled, Boeing's X-37 reusable orbital spaceplane has flown three times, and Boeing says it plans to apply the experience gained to its XS-1 proposal.
Budget permitting, Darpa plans to award three or four $3-4 million XS-1 Phase 1 preliminary design contracts in the first quarter of 2014, followed a year later by a single design-to-cost contract worth up to $140 million to build and fly the X-plane demonstrator. If the program proceeds into Phases 2 and 3, first flight is scheduled for the third quarter of 2017, leading to an orbital flight demonstration a year later.
The program has some of the challenge characteristics of the Ansari X-Prize, won by Scaled Composites with the SpaceShipOne. The technical objectives are to fly the XS-1 10 times in 10 days, fly to Mach 10-plus at least once and launch a demonstration payload into orbit. The 10 flights in 10 days are intended to demonstrate reusability and expand the flight envelope. There is no velocity requirement for the flights, but the vehicle must take off and land each time.
Flying to Mach 10 or beyond will demonstrate the unmanned XS-1 can reach a staging speed that minimizes the size of the expendable upper stage, for which a target cost of $1-2 million has been set. There are no dynamic-pressure or load-factor requirements, but designing for Mach 10-plus will require the demonstrator to have the aero-thermal capability for space access and hypersonic testing. Similarly, there is no payload mass requirement for the launch to orbit, the objective being to demonstrate the potential for orbital flight in an operational version of the vehicle, Darpa says.
There are several possible configurations, propulsion systems and launch-and-recovery methods that could be proposed for the XS-1, but Darpa's reference X-plane is an F-15-sized, vertical-takeoff/horizontal-landing, winged spaceplane powered by two SpaceX Merlin 1D rocket motors.
Gross lift-off weight for the reference vehicle is almost 224,000 lb., compared with 190,000 lb. for a Minotaur IV carrying a 4,000-lb. payload. The expendable upper stage would weigh 15,000 lb. including payload. The design could be scaled up using Aerojet AJ26 (Russian NK33) engines, air launch or two stages, says Darpa.
The stated objective of the XS-1 program is to “break the cycle of escalating space system costs,” says the agency, pointing out that each GPS III spacecraft will cost $500 million for the satellite and $300 million for the launch, compared with $43 million and $55 million, respectively, for the first GPS in 1978.
The U.S. averages only 3-5 5,000-lb.-payload-class launches a year, Darpa says, well below the annual rate on which the XS-1's cost target of $5 million per flight is based. The agency believes a lower launch cost will grow the market, both to orbit smaller “disaggregated” satellites for the U.S. Air Force and by recapturing commercial business. But the ability of lower launch costs to stimulate demand remains to be proved.