RLVs: The Numbers Just Don’t Add Up

The Simple Truth about Reusable Launchers Is Not So Simple
Space Daily

There are several key factors that have retarded progress in this area. An ideal RLV would be: a single stage vehicle; inexpensive to operate and able to be turned around quickly. Thanks to NASA’s failed billion-dollar experience trying to build a scaled down technology demonstrator, the X-33, we can say that single-stage RLVs are beyond the current state of technology. The fundamental reason has to do with the energy needed to achieve orbit and the lack of a propulsion system that can deliver the required vehicle velocity at a high enough efficiency.

As a result, using the best propulsion technology available, a single-stage vehicle would have to lift off with a propellant mass fraction approaching 90 percent. In other words, the maximum mass left to reach orbit would be only about 10 percent of the gross liftoff mass.

This may not seem to be a show-stopper, but that 10 percent must include everything that is not ascent propellant. This includes all of the structure, propulsion system, propellant tanks and plumbing, avionics, reentry heat shields, residual and maneuvering propellants and payload. Current technology limits us to a minimum mass fraction of about 18 percent. Alas, we are about 80 percent overweight on the amount of mass to orbit.

Read the full analysis.