In the history of aerospace engineering in the United States, the X-15 is lauded as a critical and necessary step on the path to manned spaceflight. Throttle-able chemical rocket propulsion systems, reaction controls, and pressurized flight suits were all major design features of the X-15 that eventually found their way into future successful spacecraft.

Given the obvious resources the USSR put into the space race and their numerous successful missions, why did they never build a hypersonic space-plane akin to the X-15?

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    $\begingroup$ As a rule of thumb, be slightly skeptical of the merits of any device when they are being loudly praised by the nation that built it in the first place. Doubly so for Cold War toys. $\endgroup$ Feb 3, 2020 at 8:31
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    $\begingroup$ Given the technological components you mention, if you have no answer here you may consider asking on space.SE. $\endgroup$
    – Manu H
    Feb 3, 2020 at 8:41
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    $\begingroup$ It's a spaceplane, arguably it belongs in both $\endgroup$
    – Bryson S.
    Feb 3, 2020 at 9:04
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    $\begingroup$ I disagree with the close vote, this thing barely reached space. Not trying to discredit the engineering behind it, though, but the fact that most of its flight was in atmosphere qualifies it for this site in my opinion $\endgroup$ Feb 3, 2020 at 9:35
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    $\begingroup$ That's generally frowned upon @AEheresupportsMonica $\endgroup$
    – GdD
    Feb 3, 2020 at 9:44

5 Answers 5


I like the X-15, it was certainly an amazing airplane, but the truth is there were few benefits to the space program from the X-15. It was far from a critical or necessary step:

  • The Mercury space suit was a direct derivative of the BF Goodrich Navy Mark IV, which had been in use for years. It wasn't developed for the X-15
  • The rockets used in the space program were completely different from the X-15. They were orders of magnitude stronger for one thing, used different fuels and completely different designs. Throttle-able rocket propulsion systems weren't new
  • The X-15 thermal protection system was of no use to the space program, which had to deal with far higher temperatures

There were some benefits:

  • It was a good source of astronauts: several pilot from the program went on to the NASA programs, including Neil Armstrong
  • Data from the X-15 missions were used in the X-20 Dyna Soar spaceplane which never flew, but it eventually morphed into the space shuttle

As to why didn't the Soviets make one, they didn't need it. Their manned space program built off of their ICBM development efforts which were successful. If the US hadn't built the X-15 the manned space program would most likely never gone the space shuttle route, but that wouldn't have been any sort of deal breaker.

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    $\begingroup$ I don't @BrysonS. A lot of Soviet space development is still hidden. It's very possible they learned it from the US Government's own published materials, pictures of the X-15 and subsequent spacecraft designs. $\endgroup$
    – GdD
    Feb 3, 2020 at 15:05
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    $\begingroup$ @BrysonS. iirc detached shock wave location can be predicted nicely from the theory, I don't think there was a need for a prototype except to validate the model. $\endgroup$ Feb 3, 2020 at 15:08
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    $\begingroup$ @GdD: So, I used my language joker and googled a bit. Russian Wikipedia (duh) says the shape of Souz was chosen "by March 1963", but lamets that it has not so good aerodynamic quality that leads to overheating. It is stable, however. In buran.ru/htm/cliper02.htm they just state that the shape was derived from the re-entry ability. There is some scan under akf.bmstu.ru/images/…, they list various shapes in "Рис. 3". Hope that helps. $\endgroup$ Feb 3, 2020 at 20:18
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    $\begingroup$ 'extremely successful' : Soviet space program, Notable firsts $\endgroup$
    – Mazura
    Feb 4, 2020 at 0:21
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    $\begingroup$ @GdD yes, they did. And it was under constant threat from the navy's and army's programs that were merged into NACA's program when NACA became NASA. X-15 was basically the last part of the AF's program to see the light of day as an independent (rather than tri-service or in conjunction with NASA) program. $\endgroup$
    – jwenting
    Feb 4, 2020 at 11:23

I'll complement GdD's answer from a slightly different perspective.

In the history of aerospace engineering...

Wait, there is a problem right there. Due to various historical reasons, there was no aerospace industry in the USSR, at least the way it is known in the West. The very word "aerospace" was almost never used before the 90s. Aeronautics and space were much more separated than in the US. There were no companies that dealt with both the way Boeing or Lockheed did. (Though some were forcibly switched from aviation to space in the late 50s).

Space industry, especially at the early stages, had more affinity with artillery than with aviation. This involves not only the formal bureaucratic subordination, but the whole mentality, from design to testing. In this scheme of things, rocket designers wouldn't even think of building an 'aeroplane' (and X-15 is undeniably an aircraft) for testing. They would rather shoot a few more rockets. Especially given the preferrential supply of resources to the space industry.

There were some later attempts from the aeronautics industry to get involved, but none were remarkable (until, perhaps, Buran).

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    $\begingroup$ I think the MiG-105 program was also part of a conscious effort to incorporate lifting reentry into a space vehicle $\endgroup$
    – Bryson S.
    Feb 4, 2020 at 5:58
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    $\begingroup$ Yes, and that's exactly what aviation could potentially contribute. $\endgroup$
    – Zeus
    Feb 4, 2020 at 6:41
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    $\begingroup$ @CalinCeteras that's in no small part because the USSR collapsed early in its test program, they simply ran out of money for more flights while several Buran craft were still under construction. It certainly showed promise. $\endgroup$
    – jwenting
    Feb 4, 2020 at 8:59
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    $\begingroup$ The shape of the Buran was heavily based on space shuttle plans gained through espionage. They even copied the programmed after launch roll, even though they didn't know why. The Buran project was certainly a technical achievement, but there's a big bucket full of 'why?' that accompanies it. $\endgroup$
    – GdD
    Feb 4, 2020 at 9:12
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    $\begingroup$ There some great info on the Buran here: falsesteps.wordpress.com/tag/buran-shuttle $\endgroup$
    – GdD
    Feb 5, 2020 at 21:14

Great question, but no, the X-15 was not a "critical and necessary step on the path to manned space flight" at all, it was used to test the feasibility of sustained and controlled hypersonic flight of an aircraft at very high altitudes and speeds. The X-15 was an extension of the X program, started in the 1940s, to continuously push the speed envelope of aircraft through the sound "barrier", and the thermal "barrier" beyond it.

Manned space flight, requiring rockets with much more thrust for far longer, simply is in another league, and was founded in the ICBM programs of both the US and USSR, which owed much of its foundation to the pioneering work of Goddard, and the development and scale up (including the all important turbo pump) of team von Braun.

Recovery systems from these flights were, and are, capsules. It is rather dumb to haul an airplane into space when payload to orbit costs are measured in thousands of $ per pound.

The Space Shuttle was an awesome technology demonstrator, but never succeeded as a low cost replacement for ordinary rockets to move cargo to orbit, but remains a viable option as a means to return cargo and crew from orbit.

So, although the X-15 did reach the edge of space on some of its flights, and did use control thrusters, it was much more an airplane, and should be more than welcome as a subject for readers to enjoy on Aviation Stack Exchange.


There were three competing ways to space pursued in the US while in the USSR everything was centrally managed, so they followed only the way that Konstantin Tsiolkovsky had first proposed.

What were those three ways?

  1. The US Navy used a home-grown team to develop a rocket at the Naval Research Laboratory. Their Vanguard rocket failed several times, however, before it worked as designed.
  2. The US Army relied on a bunch of German engineers who had worked for the Wehrmacht artillery before 1945. Fittingly, they were housed in the Redstone arsenal grounds of Huntsville, Alabama. Their Juno rocket was used for the first US satellite launch after being placed in storage for almost a year so the Navy team could try first.
  3. The US Air Force wanted to use a winged craft to get into space. For them, rockets were a temporary shortcut and several, rocket-powered test aircraft were developed in order to get closer to spaceflight. While also developing the Atlas rocket, the US Air Force in cooperation with NACA, later NASA, ordered several X-planes to test access to space and re-entry with a winged vehicle (X-2, X-15, X-17, X-20, X-23, X-24, X-30, X33, X-34, X-37, X-38, X-40, X-41 and the list goes on).

The planners in the USSR never planned something like the X-15. Also, when the X-15 flew, they saw no need to do something similar.

  • $\begingroup$ Opposed to this answers FIRST sentence - although soviet russia used to central manage everything .. their approach to space flight was a "competition of science bureaus" - at least 2 teams lead by Sergej Koroljov (OKB-1) vs Valentin Glushko / Vladimir Chelomei - both teams stood in heavy competition over the design principles - while the U.S .. the land of the free founded the NASA - ONE (large) organization hat should henceforth coordinate and manage ALL space activities. So in hindsight NASA approach appears far more centrally managed $\endgroup$
    – eagle275
    Feb 6, 2020 at 11:20
  • $\begingroup$ "NASA appears to be more centrally managed", until Wernher left the house. The X-37B shuttle is in use today with the Air Force. Accuracy of capsule recovery seems to be improving. Interestingly, the Gemini program did seem to have some thoughts about gliding in with a Rogallo wing, and included ejection seats with the capsule. $\endgroup$ Feb 6, 2020 at 17:23
  • $\begingroup$ @eagle275: So one of the bureaus didn't follow Tsiolkovsky's approach? And I thought all were working on multi-stage rockets. So you say they weren't? And I thought that OKB 456 designed the rocket engines for Korolev and Chelomei tried to fill the void Korolev had left with his early death. The limited competition earlier was due to the trick of using Khrushchev's relatives which undermined central planning. $\endgroup$ Feb 6, 2020 at 17:43
  • $\begingroup$ Well, to be fair, there was quite a bit of competition, sometimes fierce. But it was less of an open competition for who does better; rather, more of a competition for (bureaucratic) power: who convinces the authorities that his way is the 'right' one. Glushko, for one, was never happy making engines for Korolyov: earlier he was Korolyov's boss and struggled to accept being No.2; also he was a fan of UDMH while Korolyov pushed for LOX/Kerosene. The same in aviation. It's a more complicated picture than 'centrally managed'. $\endgroup$
    – Zeus
    Feb 6, 2020 at 23:36
  • $\begingroup$ @Zeus: The point is that the plan focused on rockets and rocket-powered aircraft were not considered. Given the priority of rocket development, the bureaucracy tolerated some competition – in the end the USSR competed with the US as well. $\endgroup$ Feb 7, 2020 at 0:03

Most of the X-15 data, like most NASA research studies are made available to the public. Why spend millions of Rubles on a space plane testbed when you can examine the research data for free after the other guy spent the cash and built one?

And, apparently, as both the US and USSR space shuttles bear a VERY similar appearance to each other, it appears that both sides came to similar conclusions as to the best configuration for a space plane based on hypersonic research data from the X-15 program.

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    $\begingroup$ When was it made public? $\endgroup$ Feb 4, 2020 at 15:10
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    $\begingroup$ I'm pretty sure this data was classified at least in the 50s and 60s. $\endgroup$
    – Simon
    Feb 5, 2020 at 15:06

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