Per ExtremeTech, All of the launch vehicles in service today and planned in the near future are based on multi-stage rockets of some sort. Some private space firms like SpaceX are trying to improve on the traditional disposable rocket by recovering the first stage. But DARPA is moving forward with an ambitious plan to develop a spaceplane-based launch system called the Experimental Spaceplane (XS-1). With Phase 1 planning complete, the time has come to start building.
The XS-1 is not intended to be a new Space Shuttle. Unlike that vehicle, the main body of the XS-1 would be designed only for sub-orbital flight. It’s also an unmanned vehicle. After getting airborne, the XS-1 would gain altitude and accelerate to near escape velocity. Instead of heading into space itself, an upper stage would separate and boost up into orbit to deliver its cargo. This stage would be expendable, but small and inexpensive. All the heavy lifting would be accomplished by the spaceplane, which glides back down for a soft landing after the upper stage detaches. It is believed that such a vehicle could be serviced for another launch much faster than a rocket.
The first phase consisted of contracting with three firms to design different versions of the XS-1 vehicle. Those companies were Boeing, Masten Space Systems, and Northrop Grumman, but only one will be awarded a contract for Phase 2 of the project to actually start building a prototype. The test platform will be about the size of a commercial airliner, and DARPA has a whole list of qualities it wants to see integrated into the design. It will include technologies like advanced heat-resistant composites and modular systems that will reduce turnaround time.
Perhaps the most important aspect of the XS-1 is its ability to relaunch quickly. DARPA wants the Phase 2 prototype to be capable of flying 10 missions in 10 days, assuming weather cooperates. This would demonstrate high cost-effectiveness compared with rocket launches, which are often scheduled years in advance. The test vehicle also needs to show it can reach sufficient speeds in the upper atmosphere to successfully launch a smaller second stage that would boost into low-Earth orbit. Small doesn’t mean really small, though. DARPA wants a payload capacity of 900-1,500 pounds. That would be enough to accommodate most satellites for both government and commercial uses. A future version is aimed at lifting 3,000 pounds to low-Earth orbit.
DARPA believes that reaching these goals will reduce that launch cost for that larger payload to as little as $5 million. Being targeted at low-Earth orbit, the XS-1 won’t be as versatile as something like the SpaceX Falcon 9. Still, it might quickly surpass SpaceX in number of launches if it really can operate on a daily basis.