There is a long list of SSTO projects here.
I'm going to list some concepts that serious work was done on (as in years of engineering effort spent by established aerospace companies).
- were they realistic? Well, SSTO is still on the edge of what's technically feasible. Every SSTO project ran into trouble because it needed new technologies that weren't ready, so the project budget ballooned with having to develop these new technologies. It's like going from the DC-6 to the Boeing 747 in one step.
- were their designs reliable? We have no idea, because all these projects were cancelled before the end product ever flew.
By 1989, the outlook for HOTOL had become bleak; from the onset of the project, support between the British government and industrial partners had been uneven, while the United States had emerged as the only foreign nation that showed willingness to contribute to the programme,4 in part because of the secrecy surrounding the program. There was little prospect for European involvement, the ESA having elected to pursue development of what would become the Ariane 5, a conventional space launch system.4 Rolls-Royce withdrew from the project, judging the eventual market for the engine was unlikely to be large enough to repay the development costs.
Alan Bond, one of the engineers behind HOTOL, went on to incorporate Reaction Engines which kept working on the idea, leading to today's Skylon and SABRE engine.
- National Aero-Space Plane, US project. $500 M spent on development. Was planned to use a scramjet engine (which didn't exist at the start of the program), and needed new, more heat-resistant materials. A technology demonstrator called X-30 was planned.
Despite progress in the necessary structural and propulsion technology, NASA still had substantial problems to solve. The Department of Defense wanted the X-30 to carry a crew of two and even a small payload. The demands of being a human-rated vehicle, with instrumentation, environmental control systems and safety equipment, made the X-30 larger, heavier, and more expensive than required for a technology demonstrator. The X-30 program was terminated amid budget cuts and technical concerns in 1993.
Model of the X-30:
This was a concept for a vertical takeoff, vertical landing vehicle. DC-X was a technology demonstrator, an SSTO was planned as a follow-on of this. DC-X is notable in that it made it to the hardware stage and was flown.
SSTO using an aerospike engine. A subscale demonstrator, the X-33 was built but never flew due to problems with a carbon fiber tank for hydrogen, the tank's irregular shape made it very difficult to build. $1B spent.
X-33 (left) and Venturestar (right):