'.....while the X15 used hypergolic rocket fuel and...."
I hope I'm not being too picky, but the XLR99 engine that powered the X-15 was
not hypergolic (i.e. used propellants that burn spontaneously when mixed).
The XLR99 used anhydrous ammonia and liquid oxygen which require an initial igniter.
See:
http://www.bobholland.com/me/reaction.htm for the XLR99 and
http://www.permanent.com/t-mikesc.htm for hypergolic propellants.
