4 ms·

Off topic but why is hypersonic Mach 5 and above? Anything special about that threshold, or was it chosen arbitrarily?

rocqua 3y ago
If I recall scott Manley correctly, mach 5 is roughly the speed only reachable by ramjets. And also roughly the speed where aerodynamic heating starts generating plasma around the airframe. Hence getting to mach 5 requires solving two problems that don't really need to be solved for slower speeds.
- stavros 3y ago
  "You can't reach this speed easily" doesn't sound like a problem that's specific to Mach 5+.
  - geraldhh 3y ago
    yet, here we are
    - stavros 3y ago
      Yet here we are, at a problem that's not specific to Mach 5+?
  - vGPU 3y ago
    It’s not reaching it, it’s staying there. The heat and plasma represent enormous challenges. Comms are incredibly difficult to maintain, for example.
    - stavros 3y ago
      Isn't this the same with any speed, though? We've always had trouble breaking the next record.
      - dotnet00 3y ago
        The point isn't specifically having trouble breaking the record, the point is that having to deal with the plasma makes it a completely different flight regime with completely different design considerations, exactly like how sustained supersonic flight requires all sorts of considerations that are very different from what subsonic flight requires.
        stavros 3y ago
        Yes, that's one problem. The GP said there are two problems, the other one being "we can't reach this speed easily".
        PhilippGille 3y ago
        I think what they mean is: Sonic speeds / Mach 1 brings a set of problems. Mach 2, 3, 4 don't add new ones. But Mach 5 does, hence warranting a different naming with hypersonic. At least that's how I understood the conversation.
        obmelvin 3y ago
        I believe the second issue is that engines which work well at supersonic speeds don't work well at slower speeds and visa versa "A typical air-breathing DMRJ propulsion system can only begin operating when the vehicle achieves supersonic speeds greater than Mach 3. In fact, since ramjet (and scramjet) engines don’t have rotating parts (compressor and turbine) as turbojet and turbofan engines, they rely on specially designed air inlets to compress the air to the pressure required for the combustion. The efficiency, however, only gets sufficiently high when the speed reaches at least Mach 2, due to the higher temperature and pressure obtained in the combustion chamber. This is why ramjets usually need other means to reach the minimum speed for ignition, such as rocket boosters or, in the SR-71 Blackbird, dual cycle turbo-ramjet engines." from https://theaviationist.com/2023/12/16/ge-aerospace-demonstrates-hypersonic-dual-mode-ramjet/ https://theaviationist.com/2023/12/16/ge-aerospace-demonstra...
    - rocqua 3y ago
      Thank god for the comms problem. Without it, hypersonic missles probably aren't far from being able to take out any aircraft carrier. That would be bad for Taiwan.
      - vGPU 3y ago
        A barrage of regular missiles would easily be able to take out any aircraft carrier. You don’t need hypersonic. This is due to the simple fact that you can only carry so much AA ammunition and restocking ammunition on a ship takes significantly longer than it does on land. This isn’t even touching on drones, both flying and boats, as well as torpedos and mines. Taking out an aircraft carrier isn’t difficult. It’s just that luckily, no country with the capacity to do so has been crazy enough to start ww3.
        deleted 3y ago
        [deleted]
      - geraldhh 3y ago
        fwiw european diesel-electric submarines routinely sink supercarriers in exercises
  - numpad0 3y ago
    *using air breathing engine, I think. Earth escape velocity is Mach 22, 7.9km/s, 5 mi/sec., 67k mph, etc. Clearly there are faster guided man-made objects.
  - rocqua 3y ago
    "Turbofans stop working, and you start needing to deal with plasma from air resistance" are the specific points here. Moreover, if you can handle these issues, you can probably use similar tech to go even faster.
avereveard 3y ago
" Typical speeds for hypersonic aircraft are greater than 3000 mph and Mach number M greater than five, M > 5. We are going to define a high hypersonic regime at M > 10 to account for re-entry aerodynamics. The chief characteristic of hypersonic aerodynamics is that the temperature of the flow is so great that the chemistry of the diatomic molecules of the air must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At high hypersonic speeds, the molecules break apart producing an electrically charged plasma around the aircraft. Large variations in air density and pressure occur because of shock waves, and expansions. " https://www.grc.nasa.gov/www/k-12/airplane/lowhyper.html https://www.grc.nasa.gov/www/k-12/airplane/lowhyper.html
sandworm101 3y ago
It is speeds where air starts forming plasma when hitting an object, which triggers all sorts of issues. In practical aerospace, hypersonic generally means anything significnantly faster than the sr71, the mach 4-5 range being the low end and orbital velocities the high (mach 10).
deleted 3y ago
[deleted]

If I recall scott Manley correctly, mach 5 is roughly the speed only reachable by ramjets. And also roughly the speed where aerodynamic heating starts generating plasma around the airframe. Hence getting to mach 5 requires solving two problems that don't really need to be solved for slower speeds.

stavros 3y ago
"You can't reach this speed easily" doesn't sound like a problem that's specific to Mach 5+.
- geraldhh 3y ago
  yet, here we are
  - stavros 3y ago
    Yet here we are, at a problem that's not specific to Mach 5+?
- vGPU 3y ago
  It’s not reaching it, it’s staying there. The heat and plasma represent enormous challenges. Comms are incredibly difficult to maintain, for example.
  - stavros 3y ago
    Isn't this the same with any speed, though? We've always had trouble breaking the next record.
    - dotnet00 3y ago
      The point isn't specifically having trouble breaking the record, the point is that having to deal with the plasma makes it a completely different flight regime with completely different design considerations, exactly like how sustained supersonic flight requires all sorts of considerations that are very different from what subsonic flight requires.
      - stavros 3y ago
        Yes, that's one problem. The GP said there are two problems, the other one being "we can't reach this speed easily".
        PhilippGille 3y ago
        I think what they mean is: Sonic speeds / Mach 1 brings a set of problems. Mach 2, 3, 4 don't add new ones. But Mach 5 does, hence warranting a different naming with hypersonic. At least that's how I understood the conversation.
        obmelvin 3y ago
        I believe the second issue is that engines which work well at supersonic speeds don't work well at slower speeds and visa versa "A typical air-breathing DMRJ propulsion system can only begin operating when the vehicle achieves supersonic speeds greater than Mach 3. In fact, since ramjet (and scramjet) engines don’t have rotating parts (compressor and turbine) as turbojet and turbofan engines, they rely on specially designed air inlets to compress the air to the pressure required for the combustion. The efficiency, however, only gets sufficiently high when the speed reaches at least Mach 2, due to the higher temperature and pressure obtained in the combustion chamber. This is why ramjets usually need other means to reach the minimum speed for ignition, such as rocket boosters or, in the SR-71 Blackbird, dual cycle turbo-ramjet engines." from https://theaviationist.com/2023/12/16/ge-aerospace-demonstrates-hypersonic-dual-mode-ramjet/ https://theaviationist.com/2023/12/16/ge-aerospace-demonstra...
  - rocqua 3y ago
    Thank god for the comms problem. Without it, hypersonic missles probably aren't far from being able to take out any aircraft carrier. That would be bad for Taiwan.
    - vGPU 3y ago
      A barrage of regular missiles would easily be able to take out any aircraft carrier. You don’t need hypersonic. This is due to the simple fact that you can only carry so much AA ammunition and restocking ammunition on a ship takes significantly longer than it does on land. This isn’t even touching on drones, both flying and boats, as well as torpedos and mines. Taking out an aircraft carrier isn’t difficult. It’s just that luckily, no country with the capacity to do so has been crazy enough to start ww3.
      - deleted 3y ago
        [deleted]
    - geraldhh 3y ago
      fwiw european diesel-electric submarines routinely sink supercarriers in exercises
- numpad0 3y ago
  *using air breathing engine, I think. Earth escape velocity is Mach 22, 7.9km/s, 5 mi/sec., 67k mph, etc. Clearly there are faster guided man-made objects.
- rocqua 3y ago
  "Turbofans stop working, and you start needing to deal with plasma from air resistance" are the specific points here. Moreover, if you can handle these issues, you can probably use similar tech to go even faster.

avereveard 3y ago

" Typical speeds for hypersonic aircraft are greater than 3000 mph and Mach number M greater than five, M > 5. We are going to define a high hypersonic regime at M > 10 to account for re-entry aerodynamics. The chief characteristic of hypersonic aerodynamics is that the temperature of the flow is so great that the chemistry of the diatomic molecules of the air must be considered. At low hypersonic speeds, the molecular bonds vibrate, which changes the magnitude of the forces generated by the air on the aircraft. At high hypersonic speeds, the molecules break apart producing an electrically charged plasma around the aircraft. Large variations in air density and pressure occur because of shock waves, and expansions. " https://www.grc.nasa.gov/www/k-12/airplane/lowhyper.html https://www.grc.nasa.gov/www/k-12/airplane/lowhyper.html

sandworm101 3y ago

It is speeds where air starts forming plasma when hitting an object, which triggers all sorts of issues. In practical aerospace, hypersonic generally means anything significnantly faster than the sr71, the mach 4-5 range being the low end and orbital velocities the high (mach 10).

deleted 3y ago

[deleted]