The engine development from 2009 to 2015 was funded exclusively through private investment by SpaceX, and not as a result of any funding from the US government.[27][2] In January 2016, SpaceX did agree with the US Air Force to take US$33.6 million 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 US$67.3 million on the same upper-stage development project, on a minimum 2:1 private-to-government funding basis.[3][4]