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Spaceplanes have always been thought to be a SSTO system with flight from ground to orbit and back, and while some concepts demonstrated suborbital capabilities, like the XCOR Lynx none ever made it. How many concepts were actually analysed and taken up for a realistic project. As far as I know, Skylon and VentureStar seem to be prominently worked upon, but are there more hidden gems, especially in the golden age of space(?)

Note: I read through a significant number of SSTO questions on the stack and feel this maybe different, but if you guys feel this has been addressed before, let me know and I shall delete it.

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    $\begingroup$ Related: space.stackexchange.com/q/12552/195 $\endgroup$
    – Russell Borogove
    Oct 13, 2018 at 21:48
  • $\begingroup$ Some issues with your examples: SS2 is suborbital and calling the aerial launch platform a "first stage" is questionable, XCOR Lynx was suborbital and had only a single stage. VentureStar's quite dead, and Skylon's only getting funding for propulsion research. $\endgroup$
    – Christopher James Huff
    Oct 16, 2018 at 11:54
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    $\begingroup$ @ChristopherJamesHuff SS2 why wouldn't the aerial launch patform qualify as a stage? The carrier aircraft is essentially part of the system and lifting it at quite a height to redue atmospheric effects and thus liftoff essentially has reduced loads on the vehicle(?). But agreed that these were suborbital. $\endgroup$
    – Rajath Pai
    Oct 16, 2018 at 12:36
  • $\begingroup$ It's not that much altitude and very little speed compared to what's required. It's relatively significant for SS2 since it only does a short vertical hop, but even so, you'd never build a rocket first stage that gives such a minor boost to later stages. The plane in air-launched systems is better understood as an airborne launch platform. $\endgroup$
    – Christopher James Huff
    Oct 16, 2018 at 21:47

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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.

My list:

scale model:

enter image description here

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: enter image description here

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.

enter image description here

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):

enter image description here

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