Skyramp

A skyramp (also known as an inclined rocket sled or an inclined jet sled) uses a rail or maglev track and rocket or jet boosters to accelerate a sled holding a rocket up an eastward facing mountain slope. By starting 2000 meters above sea level and accelerating to Mach 0.8 to Mach 3 substantial fuel savings can be gained allowing a single stage to orbit reusable vehicle.
This will require no new engineering as the test tracks at Holloman Air force base have tested Rocket Sleds moving at above Mach 3. NASA studies of maglev sleds concluded that using current technology they could not magnetically accelerate rockets fast enough. But most designs for skyramps use jet engines or rockets to accelerate the spacecraft mounted on it. Effectively a sky ramp makes the first stage of a rocket fully reusable since the sled is returned to its starting position, refueled and may reused at once.
Overview of the Problem
NASA studies have shown that the Space Shuttle uses half of its fuel just to reach 1000 mph (Mach 1.3) . If the rocket was already moving Mach 1.3 it would not be able to double its playload (since the extra payload would need more fuel to accelerate it into orbit) but a substantial savings would be made. This would allow either a smaller spacecraft with less fuel or a more massive payload.
High altitude launches
An advantage to any launch system that starts from high altitudes will reduce gravity drag (the cost of lifting fuel in a gravity well). The thinner air will reduce air resistance and allow more efficient engine geometries. (Rocket nozzles have different shapes to maximize thrust at different air pressures. NASA's aerospike engine for the Lockheed Martin X-33 was designed to change geometry to remain efficient at a variety of different pressures, but the aerospike engine was heavy and complex and the project was eventually canceled.)
For example, the air is 39% thinner at 2500 meters. The more efficient rocket plume geometry and the reduced air friction allows the engine to be 5% more efficient per amount of fuel burned.
Debora A. Grant and James L. Rand in: "The Balloon Assisted Launch System - A Heavy Lift Balloon" wrote: "It was established some time ago that a ground launched rocket capable of reaching 20 km would be able to reach an altitude of almost 100km if it was launched from 20km." They suggest that small rockets are lifted above the majority of the atmosphere by balloon in order to avoid the problems discussed above.
Reusable launch vehicle
Rocket sleds at China Lake testing ground have reached Mach 4 while carrying 60,000 kg masses. An inclined skyramp of this size & 4 km long would be enough to make the X-33 RLV (Reusable Launch Vehicle) a workable proposition.
Other technologies used with a skyramp
Note that a skyramp can be combined with other technologies to further reduce the cost to orbit. For example, a skyramp can launch a spacecraft at Mach 3, which is moving fast enough to allow a scramjet to operate. A small laser launch system could be used at the end of a skyramp to further superheat the exhaust and improve the Specific impulse (Isp). A large skyramp could launch craft to reach the teather end of a rotating .
Skyramps in fiction
*A skyramp was used in the movie When Worlds Collide to help launch the Ark to Bronson Beta.
*Robert A. Heinlein used a skyramp in Space Cadet.
*Dean Ing used a similar system in his 1988 novel "The Big Lifters".
Summary
A skyramp that gave a Mach 2 launch assist would reduce the fuel to orbit by 40%. Thus a skyramp angled at 45 degrees to vertical on a tall mountain would allow a single stage to orbit reusable vehicle with no new technology. This would reduce the price to orbit by about 20 fold.

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