I am not going to invoke the K-word in this question!

Hypersonic long-range craft have to fly pretty high to travel circa mach 20. Would they need to travel into space to do so efficiently?

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    $\begingroup$ Which demarcation are you using as your criterion for judging what is space and what isn’t? Ultimately, your question can’t be answered without some standard for comparison. $\endgroup$
    – Paul
    Commented Dec 27, 2018 at 15:37
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    $\begingroup$ Can you provide any reference to the craft under discussion? $\endgroup$ Commented Dec 27, 2018 at 16:20
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    $\begingroup$ Also, apophasis much? $\endgroup$ Commented Dec 27, 2018 at 16:22
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    $\begingroup$ Does this recently tested craft have a name? I'm unfamiliar with it and it's hard to search for it without a moniker, even if it's in Russian that's fine. Also, +1 for censoring the Kerbal Line, I have no idea why it has become so controversial but I am sick of reading about it heh. (+1 because I want to know more too) $\endgroup$ Commented Dec 27, 2018 at 16:43
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    $\begingroup$ The Uhoh Limit has been invoked! $\endgroup$ Commented Dec 27, 2018 at 17:06

1 Answer 1


ICBM’s don’t typically fly at hypersonic speeds for the entire duration of their flight. They do typically cruise at altitudes above 100km to take advantage of reduced drag, but they do so without engines turned on. They follow suborbital trajectories with velocities that decrease up to apoapsis and increase upon reentry. This particular missile (avangard hypersonic missile) uses a supersonic combustion ramjet engine (scramjet), which would not work in high knudsen flow (rarefied gas dynamics). So while it won’t accelerate while in space, it can accelerate to suborbital trajectories that traverse space.

Cruise Missiles, on the other hand, usually stay well below 100km in altitude. But they don’t tend to travel at hypersonic speeds either. It would be interesting to know the actual altitude that the missile flies at, but I suspect there may be a strategic reason for not disclosing it publicly.

  • $\begingroup$ Is there enough air around and above 100 km for operation of a scramjet engine? $\endgroup$
    – Uwe
    Commented Dec 27, 2018 at 20:54
  • $\begingroup$ This one is more similar to a Cruise missile though. $\endgroup$
    – SF.
    Commented Dec 27, 2018 at 21:04
  • $\begingroup$ @Uwe: According to Aviation SE, no. $\endgroup$
    – Paul
    Commented Dec 27, 2018 at 21:37
  • $\begingroup$ @S.F., if it is truly a cruise missile, then it wouldn’t go any where near 100km. $\endgroup$
    – Paul
    Commented Dec 27, 2018 at 22:00

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