XS-1 project is under development by by three contractors along with their partners: Boeing, Northrop Grumman and Masten Space Systems chosen by DARPA. On April 7, 2016, it is entering in next phase.
XS-1 program of new unmanned spaceplane was started in 2013 as alternative launch vehicle for military purposes. At the beginning following companies joined program and managed to achieve contracts to build prototypes in July 2014: Boeing in the cooperation with Blue Origin, Masten Space Systems in the cooperation XCOR Aerospace, and Northrop Grumman with Virgin Galactic. After one year, on August 2015, companies were awarded with $6.5 million each for further development of their projects. All three companies are required to present prototypes which will meet following demands described by DARPA: Spaceplane should be able to perform hypersonic flight with speed at least of 10 Ma with 1800 kg payload (400 kg in prototype) to reach trajectory orbit. It should be able to perform at least one flight per day in ten days one after another; spacecplane should be launched on atop of reusable booster (it is acceptable to use upper stages). Cost of the one flight of the XS-1 should not exceed $5 million. DARPA considers 2020 as possible date of the first flight of the XS-1.
Program was divided into three phases: first phase (which was actually consisted of Phase 1A and Phase 1B) was covering design concept propositions. Phase 2 covers final design, manufacturing and integration, assembling prototype and tests. Phase 3 is flight test campaign and preparing for manufacturing chosen spaceplane. Phase 2 was announced on April 7, 2016 with a proposer's day on April 29. Since now it is known that Masten proposition was basing on VTVL, Northrop Grumman and Boeing presented VTHL conceptions but no further details were unveiled.
Idea of using spaceplane for delivering payload to orbit is present in American space industry since project of X-20 developed in late fifties. In spite of various spaceplanes which were only experimental vehicles like X-15, some of them became reality. Space Shuttle was some kind of spaceplane - maybe not in exact meaning of this word but for sure its conception was closer to spaceplane than anything else before X-37 (unmanned military spaceplane by Boeing). One of most iconic spaceplane projects with capability of manned flights and performing engine flight in atmosphere as ordinary jet plane was suspended X-33 by Lockheed, which was developed during eighties and early nineties as cooperation between NASA and US Air Force. Projects were representing different approaches but goal of every Spaceplane program was similar- reducing cost of launch and increasing frequency of launches. It seems that for the moment we will see another unmanned robotic spaceplane than American version of Skylon (read more here), but maybe this is just the beginning of the era of robotic launch vehicles where crew is totally separated from their vehicle.
Another aspect of the getting by military another medium sized Spaceplane (as X-37 is in service) is clear signal that DoD is rather considering miniaturization and reducing size of satellites in future. Relying on large satellites, which are for sure easy target for ASAT weapon is not reasonable, not to mention that single satellite in case of failure will be replaced not in days but probably in months. If USA would possess number of medium spaceplanes it would be quite useless without satellites which could be delivered to orbit. It is possible that change constellation of satellites (like remote sensing, reconnaissance, and weather or electronic warfare spacecrafts) from small number of large spacecrafts to smaller satellites is ultimate goal and next spaceplane program is just sign of future change. It is worth to remind that in published in the end of December 2015 report of U.S. - China Economic and Security Review Commission places threat of Chinese progress in ASAT weapon as one of the more important threats for USA security (read more here). It should not surprise that Department of Defense is intensifying effort to develop solution – cost effective, reusable spaceplane for lifting satellites to orbit is just necessary.