The DARPA XS-1 is a planned experimental spaceplane/booster designed to deliver small satellites into orbit for the U.S. Military. It is intended to be reusable as frequently as 10 times in 10 days. The XS-1 is to directly replace the “first stage” of a multistage rocket that will be capable of flying at hypersonic speed at suborbital altitude, enabling one or more expendable upper stages to separate and deploy a payload into low Earth orbit. The XS-1 would then return to Earth, where it could be serviced fast enough to repeat the process at least once every 24 hours.
DARPA is already working with three groups on designs for XS-1. In early 2017, DARPA is expected to select one group to move forward with the construction of an XS-1 prototype for flight testing.
The XS-1 program has four primary technical goals, according to DARPA. The first is a plane that can do 10 flights in 10 days, and demonstrate “aircraft-like access to space.” Second, the plane must be able to deliver a payload into low Earth orbit, which means it has to be able to move very fast. It must be able to launch a payload weighing up to 1,500 lbs. (680 kilograms) and have the capability to upgrade to 3,000-lb. (1,360 kg) payloads. And, each flight of the space plane, even with its heaviest payload, can’t cost more than $5 million.
The XS-1 program began in 2013, and initially DARPA aimed to make the first test flights in 2018. More recent estimates put the first flights sometime in 2019 or 2020.
The military has another reusable space plane in operation: The X-37B space plane, which hops a ride aboard a rocket and can then orbit the Earth for months at a time. Built by Boeing, the two X-37Bs have launched on a total of four missions in six years. Just what they’re doing up there is a mystery; most X-37B payloads are classified.