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I'm guessing it is the heat resistance of the missiles leading edge material. Hypersonic cruise missiles all seem to attain Mach 6 and below. The boost-glide weapons fly faster but up much higher. I think the Russians have ABMs that make it to Mach 10, but that is also intended for high altitude intercept. The X-15 attained about Mach 6, again up around 100,000 feet.

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    $\begingroup$ You might need to factor in the power available vs the power needed to overcome air resistance. Consider why rockets start straight up, and gradually bend their trajectory to the horizontal as they get above atmosphere. $\endgroup$
    – jamesqf
    Oct 21, 2021 at 4:06
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    $\begingroup$ The limit is imposed by the maximum dynamic pressure max-q the aircraft can sustain. Rockets are usually slowed down during their ascent, before reaching the max-q point, in order to stay within structural limits. $\endgroup$
    – mins
    Oct 21, 2021 at 17:48
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    $\begingroup$ Consider that a Saturn V burns all its 1st stage fuel in about 2.5 minutes in order to accelerate to about 6,000 mph (at 42 miles altitude). A bit of searching suggests only about 1/5 of a rocket's energy is used to increase altitude, the rest is to accelerate to orbital velocity: space.stackexchange.com/questions/35858/… So achieving sustained high speeds in level flight, especially at lower altitudes, takes a heck of a lot of fuel. $\endgroup$
    – jamesqf
    Oct 22, 2021 at 2:53
  • $\begingroup$ @jamesqf That's comparing apples with oranges. The main reason that the early stages burns so much fuel is because it's accelerating the rest of the stages, not the low altitude. Orbital velocity is Mach 23 at sea level... $\endgroup$
    – Sanchises
    Oct 22, 2021 at 18:17
  • $\begingroup$ @Sanchises: So have you got better figures for fuel consumption at high speed & low altitude? AFAIK rocket launches are the only practical example that comes close - at least that's not classified secret :-) $\endgroup$
    – jamesqf
    Oct 22, 2021 at 20:13

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