The Skylon spaceplane is a proposal by Reaction Engines Limited — a British company based in Oxfordshire — to build an orbital launch vehicle which could take off from a conventional runway, accelerate to Mach 5.5 using a turbojet engine, then close its air intake, switch operation to that of a rocket engine, and accelerate to typical orbital speed of Mach 23.
It would then release a 13 ton payload, reenter the Earth's atmosphere, and land again on a conventional runway. By using the SSTO (single-stage-to-orbit) approach, Skylon's designers hope to radically reduce launch costs and open up the high frontier to private enterprise. Skylon has not yet been built, but detailed plans have been created. Reaction Engines Limited is pursuing funding to build a prototype.
The most important and unique component of Skylon is its specially designed engine, SABRE (Synergetic Air-Breathing Rocket Engine). SABRE is an air-breathing, bi-modal engine fueled by liquid hydrogen. The engine uses a pre-cooler before vaporizing and expelling fuel, allowing low-weight alloys to be used in construction, in turn increasing the overall payload-to-fuel ratio. The incoming air is used as an oxidizer, decreasing fuel mass. At around Mach 5.5, the incoming air starts to become too hot to be used usefully. The intake valve is shut off and the rocket engine is ignited, taking Skylon the rest of the way.
The Skylon design has a length of 82 m (269 ft), about 40% longer than the Space Shuttle. It has a fuselage diameter of 6.25 m (20.5 ft) and a wingspan of 25 m (82 ft). The chassis is long, sleek, and reminiscent of the SR-71 Blackbird. Its unladen mass is 41,000 kg (90,400 lbs), with a fuel mass of 220,000 kg (485,000 lbs or 242.5 tons), and a maximum payload mass of 12,000 kg (26,450 lbs). The entire craft can be thought of as similar to a SR-71 Blackbird, except about twice the size and equipped with a rocket engine.
The Skylon spaceplane would be a reusable vehicle, ready to re-launch after only two days of maintenance and capable of taking 200 trips to orbit before requiring replacement. Its advocates hope that the Skylon could decrease cost to orbit by a factor of 10-20 or more. The Skylon spaceplane would be a nice stepping stone for launch methods, something cheaper and better than our current rockets, to get the space economy started while we're still getting started on building a space elevator.