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I remember in the late 90's and early 00's it seemed like SSTO launch vehicles were all the rage and they were even building the X-33, then suddenly nobody wanted to talk about SSTO vehicles anymore. What happened that made nobody interested in single stage to orbit launch vehicles anymore?

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closed as primarily opinion-based by GdD, Jan Doggen, Machavity, uhoh, JCRM Feb 15 '18 at 9:56

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ *cough* $\endgroup$ – Nathan Tuggy Feb 12 '18 at 8:18
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    $\begingroup$ SpaceX's BFR/BFS concept may obviate the need for SSTO entirely - a fully reusable TSTO gives you all the economic benefits of reusability along with all the performance benefits of staging. They key appears to be thinking big - having excess capability coming out of your ears. $\endgroup$ – John Bode Feb 12 '18 at 15:52
  • $\begingroup$ @NathanTuggy I think the X-33 is a better example. The project wasn't a dead idea, the engineers worked hardly until the last time, and the major "reason" of the killing of the project was that the carbon fiber oxygen tank were damaged on the prototype testing. The engineers could solve the problem, and produced a good tank. But somehow it didn't change the decision. $\endgroup$ – peterh Feb 13 '18 at 13:12
  • $\begingroup$ I think the question is not opinion-based, for example, the NASA had its communicated reason to close the X-33. It may be a matter of opinion, if it is an acceptable reason (in my opinion, it is not), but how the NASA reasoned it, that is an objective fact and is not matter of opinion. $\endgroup$ – peterh Feb 13 '18 at 13:14
  • $\begingroup$ because they're too expensive, Skylon would cost around $20 billion for it's first GTO launch $\endgroup$ – JCRM Feb 15 '18 at 3:58
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Interest was lost mostly because R&D money dried up, and also because the shuttle proved that reusability was difficult and expensive.

SSTOs generally have worse payload fractions than staged launchers, with no inherent advantage in exchange. In the later portion of ascent, after burning most of its fuel, an SSTO is carrying significant mass in now-empty fuel tankage and now-overpowered engines; a two-stage rocket has left that mass behind.

If you're designing a reusable launcher, SSTOs are attractive because you only have to recover one part, but then you're adding the mass penalties for a reusable on top of the mass penalties for SSTO -- thermal shielding and either aero surfaces for a horizontal landing spaceplane (like X-33) or reserve fuel for powered vertical landing (like SpaceX). Cost-effective reusability is challenging enough to design as it is, without the physical constraints of SSTO.

By contrast, a two-stage fully reusable launcher needs far less thermal shielding on the slower-moving bottom stage than on the upper, so you only carry the heavy thermal solution on a small part of the entire launcher, and you get to drop some dead weight along the way. As SpaceX demonstrates, you can also develop this capability one piece at a time, but there's no such thing as a partly reusable SSTO.

Skylon may be able to offset some of the SSTO mass penalties by taking oxidizer from the air, but to make the most use of the air-breathing mode, it has to fly a lower ascent trajectory -- which means more heating, thus more mass penalty in its thermal design.

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  • $\begingroup$ I think your post has an attitude that you suspect it was a rational, reasonable decision, and the relevant decision-makers did their tasks well. But I don't think it was so. I see a revolutionary project which was killed on pity nuances. Btw, also two-stage fully reusable launcher didn't happen until now, even the second-stage reusability of the new SpaceX rocket is a questionable thing, "sometimes". Killing a revolutionary project X saying that "Y will be better", and then doing no X and no Y, arises serious doubts about the real intention behind the killing X. $\endgroup$ – peterh Feb 13 '18 at 13:06

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