The revised version would have used a unique rotating annular aerospike engine: the engine and base of the launch vehicle would spin at high speed (720 rpm) to pump fuel and oxidizer to the rim by the rotation. Unlike the landing rotor, due to the shallow angle of the nozzles in the base rotor, the rotation speed self limited and required no control system. Since the density of the LOX (liquid oxygen) was higher than that of the kerosene, extra pressure was available with the LOX, so it would have been used to cool the engine's throat and other components, rather than using the kerosene as the coolant as in a conventional LOX/kerosene rocket. However, at the high G levels at the outer edge of the rotating engine block, clarity on how LOX would work as a coolant was both unknown and difficult to validate. That added one layer of risk.