[ianstallings]

My understanding is that a lot of the energy used to get to space these days is to get through the lower atmosphere. The idea of a space plane is to fly higher to the thinner atmosphere and then begin the ascent to space, avoiding the energy loss used burning through the entire troposphere. If you can fly to 5-6km you just took off a significant amount of total energy cost. Then you crank these babies up and fly to mach 5 and launch your payload or maybe the craft itself using rockets. Because you already got 1700 m/s already to start with.

And to answer the obvious question yes I have been playing entirely too much Kerbal Space Program.