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rocket sled launch
A rocket sled launch, also known as "ground based launch assist", "catapult launch assist", and "sky ramp launch", is a proposed method for launching space vehicles. With this concept the launch vehicle is supported by an eastward pointing rail or maglev track that goes up the side of a mountain while an externally applied force is used to accelerate the launch vehicle to a given velocity. Using an externally applied force for the initial acceleration reduces the propellant the launch vehicle needs to carry to reach orbit. This allows the launch vehicle to carry a larger payload and reduces the cost of getting to orbit. When the amount of velocity added to the launch vehicle by the ground accelerator becomes great enough, single-stage-to-orbit flight with a reusable launch vehicle becomes possible.
For hypersonic research in general, tracks at Holloman Air Force Base have tested, as of 2011, small rocket sleds moving at up to (Mach 8.5).〔''U.S. Air Force: 〕
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.〔(【引用サイトリンク】title=Transformational Technologies to Expedite Space Access and Development )〕
==Overview of the problem==
NASA studies have shown that the Space Shuttle used more than a third of its fuel just to reach .〔''NASA: (【引用サイトリンク】title=Space Shuttle Basics )〕 If a rocket were already moving at launch, with corresponding reduced propellant needs, a greater fraction of liftoff mass could have been payload and hardware.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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