In January 2016, SpaceX did agree with the US Air Force to take in defense department funding in order to develop a particular Raptor model: a prototype of a new upper-stage variant of the Raptor engine designed for potential use as an upper stage on Falcon 9 and Falcon Heavy, with SpaceX agreeing to fund at least on the same upper-stage development project, on a minimum 2:1 private-to-government funding basis.