DARPA will soon select a company to build its robotic XS-1 space plane. After downselect, a critical design review shuold take place in 2018, and a series of flights could be made as early as 2020.
The XS-1 space plane will consist of a reusable booster vehicle and an expendable upper stage. According to the DARPA website, the XS-1 program has four primary technical goals:
1. Fly 10 times in a 10-day period, to demonstrate efficient, aircraft-like access to space.
2. Fly fast enough to allow the use of a small (and therefore cheap) expendable upper stage.
3. Launch a 900-lb. to 1,500-lb. (408 to 680 kilograms) payload, to demonstrate a launch capability that could support both military and commercial missions. The same XS-1 vehicle could eventually also launch future payloads in excess of 3,000 lbs. (1,360 kg), by using a larger upper stage.
4. Reduce the cost of access to space to about $5 million per flight for payloads of at least 3,000 lbs.