They don't have a hybrid propulsion system in it apparently. To slow down they have to shut the engine off which means they cant get it restarted once it dips below a few mach numbers since they are probably trying to use a scram jet or something similar. So from mach 5 or whatever when they shutdown it can travel another 200 miles before its slow enough that it could land. Seems kind of like a contrived problem that they cant solve because the vehicle needs to have better capabilities for the mission they are trying to perform with it.