5 ms·

One question I've never been able to figure out with this concept is what happens when this fast moving launch vehicle hits the stationary air. Would it be like

One question I've never been able to figure out with this concept is what happens when this fast moving launch vehicle hits the stationary air. Would it be like hitting a wall? I'm a bit physics ignorant.

fredley 8y ago
I assume you'd expose it to gradually increasing air pressure on launch somehow.
- AlanSE 8y ago
  You're talking about minimizing jerk: https://en.wikipedia.org/wiki/Jerk_(physics) https://en.wikipedia.org/wiki/Jerk_(physics) To my knowledge, there is no reason to minimize jerk for machinery. Only the maximum acceleration value matters.
  - loeg 8y ago
    Is https://en.wikipedia.org/wiki/Max_Q https://en.wikipedia.org/wiki/Max_Q relevant?
  - sandworm101 8y ago
    >> To my knowledge, there is no reason to minimize jerk for machinery. Maybe not for a blob of homogeneous metal parts, but with complex structures that jerk can multiply into bad things. As the nose hits air (the sudden high G-load) a shockwave moves down the vehicle at the speed of sound (sound through metal, not air). Shockwaves can do funny things in complex structures. Imagine that this thing had a solid rocket motor for circulization. As the shockwave moving down the metal walls at one speed, slower through the fuel, the fuel might crack in ways that are not-good for a rocket. Fluid-filled things like pipes can also be subject to water hammer-like effects when g-loading changes abruptly as opposed to a gradual onset. The fluid gets moving quickly as the pipe stretches at one end, then must stop abruptly as the pipe hits its limit.
  - owenversteeg 8y ago
    That's actually a very interesting question. Are there electronics or machinery affected by jerk? How would that work?
    - AlanSE 8y ago
      Another comment mentioned "hammer-like effects"... I can see it. Mentally, I picture a half-filed milk jug being pushed around the counter. Smooth acceleration changes cause the water level to safely assume the new steady-state, but high jerk values can cause the container to collide with the water hard. If you have hardened electronics, hopefully none of the components are loose.
TylerE 8y ago
You'd probably have a stationary launch ramp exit, built up the side of a mountain. If you can put the top of the tube at 10-15k ft that will reduce air drag _substantially_. Even a 5000ft exit would be in substantially thinner air.
- Merad 8y ago
  I have a hard time believing that the reduction is anywhere near substantial enough. Friction heating was a major issue for high speed aircraft like the SR-71, and they "only" fly at ~3500 km/h at 20+ km altitudes.
  - TylerE 8y ago
    The world record top speed at sea level is less than 1000mph. It's like flying in soup by comparison.

fredley 8y ago

I assume you'd expose it to gradually increasing air pressure on launch somehow.

AlanSE 8y ago
You're talking about minimizing jerk: https://en.wikipedia.org/wiki/Jerk_(physics) https://en.wikipedia.org/wiki/Jerk_(physics) To my knowledge, there is no reason to minimize jerk for machinery. Only the maximum acceleration value matters.
- loeg 8y ago
  Is https://en.wikipedia.org/wiki/Max_Q https://en.wikipedia.org/wiki/Max_Q relevant?
- sandworm101 8y ago
  >> To my knowledge, there is no reason to minimize jerk for machinery. Maybe not for a blob of homogeneous metal parts, but with complex structures that jerk can multiply into bad things. As the nose hits air (the sudden high G-load) a shockwave moves down the vehicle at the speed of sound (sound through metal, not air). Shockwaves can do funny things in complex structures. Imagine that this thing had a solid rocket motor for circulization. As the shockwave moving down the metal walls at one speed, slower through the fuel, the fuel might crack in ways that are not-good for a rocket. Fluid-filled things like pipes can also be subject to water hammer-like effects when g-loading changes abruptly as opposed to a gradual onset. The fluid gets moving quickly as the pipe stretches at one end, then must stop abruptly as the pipe hits its limit.
- owenversteeg 8y ago
  That's actually a very interesting question. Are there electronics or machinery affected by jerk? How would that work?
  - AlanSE 8y ago
    Another comment mentioned "hammer-like effects"... I can see it. Mentally, I picture a half-filed milk jug being pushed around the counter. Smooth acceleration changes cause the water level to safely assume the new steady-state, but high jerk values can cause the container to collide with the water hard. If you have hardened electronics, hopefully none of the components are loose.

TylerE 8y ago

You'd probably have a stationary launch ramp exit, built up the side of a mountain. If you can put the top of the tube at 10-15k ft that will reduce air drag _substantially_. Even a 5000ft exit would be in substantially thinner air.

Merad 8y ago
I have a hard time believing that the reduction is anywhere near substantial enough. Friction heating was a major issue for high speed aircraft like the SR-71, and they "only" fly at ~3500 km/h at 20+ km altitudes.
- TylerE 8y ago
  The world record top speed at sea level is less than 1000mph. It's like flying in soup by comparison.