Boeing Cancels Phantom Express Launch Vehicle 2 Years After Winning XS-1 Contract from DARPA

The XS-1 program was a DARPA project to develop a rapidly reusable launch vehicle which could also be used for hypersonic research. Boeing, Northrop Grumman and Masten Space Systems were seen as the main contenders, but Boeing eventually won with a design based on the RS-25 Space Shuttle Main Engine.
After 2 years working on the project Boeing stopped, essentially killing the project and leaving another hypersonic aircraft project unfinished.

Masten XS-1 Aerothermal Calculations


Colonial Viper CFD:

DARPA