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Supersonic vs Suborbital planes?

  1. Jun 29, 2017 #1
    So I'm watching a documentary on supersonic flight, and the challenges they describe seem SO daunting that I couldn't help but wonder: Wouldn't it just be easier to fly above (most of) the atmosphere? There must be an altitude where sonic booms no longer reach the ground and wind pressure no longer superheats the surface of the plane. Would this be in the mesosphere? There is still oxygen there, so bringing it along would not be necessary.

    I'm aware of SpaceShipOne, but with only three people on board, it looks more like a thrill ride then a transportation solution.
     
  2. jcsd
  3. Jun 29, 2017 #2
    A few notes:
    1) Much of the suborbital flight would be under weightless or near weightless conditions.
    2) The speeds would be very high. For any worthwhile distance, certainly over Mach 6.
    3) The engines would need to be rocket engines - since they would not survive heating if they were trying to ingest enough sparse Mach 6 air to create thrust. I would guess that there would also be an issue of drag when using an air inlet of that size.
    4) This is already being done for short distances (about 200 miles) by Elon Musk.
    5) Of course, it was also done by the NASA Shuttles.
     
  4. Jun 29, 2017 #3
    I'm probably misusing the term "suborbital" here. Wings can't generate lift in the mesosphere? If they could then something like Mach 3 might generate more manageable temperatures?
     
  5. Jun 30, 2017 #4
    As the Shuttle came out of the bottom of the mesosphere (50Km), it had already transited the blackout zone and was travelling about Mach 9.
    The chart below shows this happening for both the nominal case (t=1240, v=3200 Mach 9.3) and for the specific case of STS-5 (t=1170, v=3100 Mach 9.0).
    sts_reentry_trajectory.gif
    The graph also shows that the rate of descent (slope of altitute line) is constant, indicating that it was generating about 1g of lift. The velocity is also a fairly constant slope showing a decrease of 1000M/s over 105 seconds or roughly 1G of drag as well - so 1.4G's total.

    Obvious, there are more aerodynamic shapes, but the shuttle needed to contend with very high reentry temperatures. Your craft could do better. Still, skirting the bottom of the mesosphere would require an airspeed of roughly Mach 7 or 8 and substantial thrust to sustain that velocity.
     
  6. Jun 30, 2017 #5
    Correction:
    The graph also shows that the rate of descent (slope of altitude line) is constant, indicating that it was generating about 2/3 G's of lift - with the remaining 1/3 G's from the centrifugal force of that trans-orbital velocity. The velocity is also a fairly constant slope showing a decrease of 1000M/s over 105 seconds or roughly 1G of drag as well - so about 1.2G's total.

    Note that this correction does not affect any of the remaining estimates. Your craft will get a similar weight relief from its velocity.
     
  7. Jul 7, 2017 #6
    So back to basics. How high do you have to fly so that a sonic boom won't reach the ground? The best I've been able to find is that 70,000 ft isn't high enough, and SST designers in the '60s gave up on trying to go higher. There ARE people talking about flying on Mars, so winged travel at 100,000 feet ought to be possible. (Same air pressure.)
     
  8. Jul 7, 2017 #7
    Three times the gravity though...
     
  9. Jul 7, 2017 #8
    The quote I recall was "If you can fly at 100,000 feet, then you can fly on mars." So it seemed like they took that into account. And since the Mezzoplane doesn't have to escape earth's gravity, it can be bigger.
     
  10. May 13, 2018 #9
    Air pressure halves roughly every 18,000 feet. U2 spy plane routinely flew at 70,000 feet, but when doing so, it's "Never exceed" speed was only about 10 mph above it's stall speed. Apparently it was not trivial to fly in this regime. To fly at 100,000 your air pressure is going down by another factor of 3-4.

    The XR-71 blackbird once got to 85,000 feet, and routinely operated at 80000 feet, at mach 3.2 Maintenance was a big issue. https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird

    Added to that: Speed of sound in a gas is essentially determined by the temperature and average atomic weight. As you get colder the speed drops, so you are flying closer to the mach barrier. It gets increasingly attractive to fly supersonic. However hitting that air hard takes energy. And the friction heats up the skin of the aircraft. We need a method to keep the air off the skin of the plane.

    ***

    While the sonic boom will be attenuated with distance, the right design of canard and wing may make it possible to partially cancel them out.
     
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