I am curious if fighters in stgs use rocket engines to leave the atmosphere. One cannot use jet engines to leave the orbit. See the post http://wiki.answers.com/Q/Why_can%27t_I ... into_space.
I am not exactly sure what the exact propulsion type would be considered. IT clearly cannot be a jet engine however because a jet can't work in outer space. It would have to be some type of engine that provides both a fuel to burn and an oxidizer to provide oxygen to support combustion. I can't say for certain that they are rocket engines like the ones we currently use, but they are not jet engines for sure.
I would probably guess they use some form of propulsion that combines scram jet technology with standard rocket thrust. Such a unit would have all of the necessary components for transitional flight.
Lucan's suggestion sounds very reasonable to me.