<<A rocket sled launch is a method of launching space vehicles. A rail or maglev track and a rocket or jet booster is used to accelerate a sled holding a vehicle up an eastward facing mountain slope. Substantial fuel savings can be gained since the spacecraft does not need to use its engines for the initial acceleration, allowing a single-stage-to-orbit reusable vehicle.
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. Present launch vehicles have performance-driven costs of thousands of dollars per kilogram of dry weight; sled launch would aim to reduce performance requirements and amortize hardware expenses over frequent, repeated launches. Designs for mountain based inclined rail 'rocket' sleds often use jet engines or rockets to accelerate the spacecraft mounted on it. Electromagnetic methods (such as Bantam, Maglifter, and StarTram) are another technique investigated to accelerate a rocket before launch, potentially scalable to greater rocket masses and velocities than air launch.
NASA studies have shown that the Space Shuttle uses more than a third of its fuel just to reach 1,000 mph. If a rocket was already moving at launch, with corresponding reduced propellant needs, a greater fraction of liftoff mass could be payload and hardware.
Due to factors including the exponential nature of the rocket equation and higher propulsive efficiency than if a rocket takes off stationary, a NASA Maglifter study estimated that a 600 mph launch of an ELV rocket from a 3000 meter altitude mountain peak could increase payload to LEO by 80% compared to the same rocket from a conventional launch pad. Mountains of such height are available within the mainland U.S. for the easiest logistics, or nearer to the Equator for a little more gain from Earth's rotation. Among other possibilities, a larger SSTO could be reduced in liftoff mass by 35% with such launch assist, dropping to 4 instead of 6 engines in one case considered.
At an anticipated efficiency close to 90%, electrical energy consumed per launch of a 500-ton rocket would be around 30 GJ, 8000 kilowatt hours (each kilowatt-hour costing a few cents at the current cost of electricity in the United States), aside from any additional losses in energy storage. It is a system with low marginal costs dominated by initial capital costs 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). Maglev guideway costs were estimated as $10 – $20 million per mile in the 1994 study, which had anticipated annual maglev maintenance costs on the order of 1% of capital costs.>>