Of all the "might have beens" of the space race sparked by the launch of Sputnik
on October 4th, 1957, the X-20 (or Dyna-Soar) space craft ranks among those which came closest to becoming reality.
Sponsored by the United States Air Force, the X-20 was conceived as a reusable military vehicle from the get-go, with missions that included reconnaisance, delivering nuclear bombs and satellite inspection/destruction. The spacecraft's specifications and design evolved from as it moved from concept to the drawing board, and by the time the project was cancelled, the basic idea was that the X-20 itself would not itself be an operational military vehicle, but a prototype for military versions to follow in relatively short order.
Originally, the X-20 was designed to be a pure boost-glide vehicle. This meant that the vehicle would be boosted into space by a regular expendable booster rocket. The X-20 would then fly over its target and return to base in a single orbit. In this mode of operation no retro rockets were required as the vehicle actually glided along a trajectory which was essentially a simple suborbital ballistic flight path -- just one whose range happened to equal the circumference of the Earth.
The X-20 was initially annouced under the Dyna-Soar name (short for Dynamic Soaring, a reference to a atmospheric skipping re-entry technique intially proposed by Eugene Sanger) by the Air Force a few days after Sputnik was launched. Internally, the Dyna-Soar program was created by merging three pre-existing advanced projects known as Brass Bell, RoBo and HYWARDS.
NASA, which was still NACA at the time, signed an agreement to participate in program on May 20, 1958. The development schedule at the time expected suborbital testing of a prototype vehicle to begin in 1966, with the final military version becoming available in 1974.
However, specifications and schedules began to change almost immediately, even as the project was out to competitive tender. However on November 9, 1959 a team comprised of the Boeing and Chance-Vought aerospace companies was awarded a contract to deliver 10 prototypes between 1965 and 1967 for flight and static testing. Martin-Marietta was awarded a contract to develop a human rated version of the Titan booster (which featured distinctive large guidance fins at its base) for the project.
In late 1961 it was decided to give the X-20 true orbital capability. Apart requiring an upgraded guidance system for the X-20, the Titan II booster had to be replaced with an even bigger Titan, the Titan IIIC, which featured two strap on rockets and a fourth stage. This stage would insert the X-20 into orbit, and provide the burn required for deorbiting.
The final version of the X-20 was designed to around a basic delta planform, 10.76 meters long, with a wingspan of 6.22 meters and a dry mass of 4,923 kg.
But even as pilots were selected to fly the X-20, funding problems were arising. The military value of the vehicle, compared to regular planes and missiles, was unclear to the U.S. Department of Defence. As a testing ground for military technologies, the X-20 faced stiff competition from Blue Gemini, a militarised version of NASA's Gemini spacecraft intended to service the planned Manned Orbiting Laboratory (MOL), an orbiting spy space station. Blue Gemini had the advantange that most of the R&D had already been done (and paid for) by someone else and that it could seat two people, while the X-20 could only seat one (although later plans included a two seat version).
As a result on December 10, 1963 the project was cancelled and the funding transferred to the Manned Orbiting Laboratory (the MOL itself would never fly either, losing its raison d'etre as spy satellites became capable of first deorbiting photograph film capsules and then radioing picture back to Earth).
However, the basic idea of the X-20 has not gone away. A similar design was proposed for the stillborn French-led ESA spacecraft Hermes and several current post-Challenger shuttle replacement proposals also feature vehicles similar to the Dyna-Soar concept.