Rocket sled launch

Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit.

For hypersonic research in general, tracks at Holloman Air Force Base have tested, as of 2011, small rocket sleds moving at up to 6453 mph (2,885 m/s; Mach 8.5).

[1] Effectively a sky ramp would make the most expensive, first stage of a rocket fully reusable since the sled is returned to its starting position to be refueled, and may be reused in the order of hours after use.

Among other possibilities, a larger single-stage-to-orbit (SSTO) could be reduced in liftoff mass by 35% with such launch assist, dropping to 4 instead of 6 engines in one case considered.

It is a system with low marginal costs dominated by initial capital costs[3] Although a fixed site, it was estimated to provide a substantial net payload increase for a high portion of the varying launch azimuths needed by different satellites, with rocket maneuvering during the early stage of post-launch ascent (an alternative to adding electric propulsion for later orbital inclination change).

[5] Rocket sled launch helps a vehicle gain altitude, and proposals commonly involve the track curving up a mountain.

[8] Another advantage to high altitude launches is that it eliminates the need to throttle back the engine when the max Q limit is attained.

A launch from high altitude need not throttle back at max Q as it starts above the thickest portion of the Earth's atmosphere.