4 ms·

The shuttle didn't do any re-entry engine burns. It fully relied on the friction.

Retric 8y ago
It's not really friction that's slows stuff on reentry. The heat is from compressing the air. Which is why the heat spikes away from the vehicle. Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.
- foota 8y ago
  Why not? Isn't that what they currently do for the falcon 9s? Sure, the rocket equation makes it more difficult since it's an exponential increase in fuel, but that's not impossible, right?
  - balfirevic 8y ago
    > Isn't that what they currently do for the falcon 9s? It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity. It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.
- LeifCarrotson 8y ago
  It takes much, much less energy to slow down a nearly-empty shell than one that was full of fuel and oxidizer.
  - manicdee 8y ago
    You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.
    - twtw 8y ago
      Yes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.
      - Retric 8y ago
        No, they are using a heat shield which is unnecessary if you slow down before reentry. What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
        manicdee 8y ago
        You can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.
    - ygra 8y ago
      It eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).
      - Retric 8y ago
        Falcon uses air to slow down most of the way, they even use parachutes to get as slow as possible, before turning on the engine for the past possible second.
        ygra 8y ago
        Falcon 9 doesn't have parachutes as far as I know. The fairings do, but not the first stage.
  - Retric 8y ago
    An empty rocket can’t fire the engines. And even an empty rocket is heavy so you need a significant amounts of fuel to slow down from orbital velocity, but you need even more fuel to lift what amounts to cargo aka the fuel used to slow down into orbit. Or, you can just use a heat shield which is really light weight when you are only returning an empty rocket. PS: Compare the amount lifted to LEO with the weight of an empty rocket. You find the weight of the rocket is significant which is why they use multi stage rockets even though this means lifting multiple sets of engines.
jessriedel 8y ago
I don't think BFR is going to do any substantial re-entry burns. The reason is that it's completely impractical to slow down from orbital or interplanetary speeds with rockets, so you have to rely on your heat shield. BFR will only do very small de-orbit burns in order to bring it's trajectory into the atmosphere, just like the shuttle did. In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.
- Retric 8y ago
  The falcon 9 first stage is not just suborbital, it’s vastly slower than LEO at just Mach 10. It’s also still in very thin atmosphere at 80 km, it’s also spending more time in very thin air as it’s sperates while going up, which is why they don’t need heat shields. It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO. PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.
  - jessriedel 8y ago
    Everything you've said is agreeing with me except maybe the clarification that the Falcon 9 first stage doesn't quite cross the the Karman line, right? In particular, I think "sub-orbital velocity" is commonly taken to include trajectories that are substantially below orbital velocity https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7. https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosphere-on-their-return-to-earth https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp... (You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.

It's not really friction that's slows stuff on reentry. The heat is from compressing the air. Which is why the heat spikes away from the vehicle. Anyway, the rocket equation means if you wanted to slow down before reentry you would effectively need to accelerate to twice the final speed 0 to X then X to 0. That's simply not going to happen.

foota 8y ago
Why not? Isn't that what they currently do for the falcon 9s? Sure, the rocket equation makes it more difficult since it's an exponential increase in fuel, but that's not impossible, right?
- balfirevic 8y ago
  > Isn't that what they currently do for the falcon 9s? It's not (not even close). Falcon 9 first stage isn't reentering from orbital velocity. It's essentially impossible, and if it weren't it would be terribly wasteful. Orbital velocity is going to be primarily shed by atmosphere.
LeifCarrotson 8y ago
It takes much, much less energy to slow down a nearly-empty shell than one that was full of fuel and oxidizer.
- manicdee 8y ago
  You have to take the slowing-down fuel with you on the speeding-up part of the trip though, which means it takes much more energy to speed up in the first place.
  - twtw 8y ago
    Yes. SpaceX surely knows this, and reckons that the cost is worth full reusability with reduced refurbishment cost.
    - Retric 8y ago
      No, they are using a heat shield which is unnecessary if you slow down before reentry. What confuses people is you need to bleed off a little energy to start decent, and another tiny amount to land. But, between those points 99% of the energy is bleed off via air.
      - manicdee 8y ago
        You can compare the weight of the heat shield required for atmospheric reentry braking to the weight of the fuel you need for propulsive reentry braking: if the fuel weighs more, use the heat shield instead. This could be the difference, for example, between 100t of fuel versus 15t of heatshield.
  - ygra 8y ago
    It eats into the payload, of course. That's why they have different landing trajectories (RTLS, barge in the ocean) and sometimes don't land at all, depending on the payload mass. The propellant itself doesn't cost much (Elon once noted that fully fueling a Falcon 9 is about 200k, which is nothing compared to the 62M that a launch costs).
    - Retric 8y ago
      Falcon uses air to slow down most of the way, they even use parachutes to get as slow as possible, before turning on the engine for the past possible second.
      - ygra 8y ago
        Falcon 9 doesn't have parachutes as far as I know. The fairings do, but not the first stage.
- Retric 8y ago
  An empty rocket can’t fire the engines. And even an empty rocket is heavy so you need a significant amounts of fuel to slow down from orbital velocity, but you need even more fuel to lift what amounts to cargo aka the fuel used to slow down into orbit. Or, you can just use a heat shield which is really light weight when you are only returning an empty rocket. PS: Compare the amount lifted to LEO with the weight of an empty rocket. You find the weight of the rocket is significant which is why they use multi stage rockets even though this means lifting multiple sets of engines.

jessriedel 8y ago

I don't think BFR is going to do any substantial re-entry burns. The reason is that it's completely impractical to slow down from orbital or interplanetary speeds with rockets, so you have to rely on your heat shield. BFR will only do very small de-orbit burns in order to bring it's trajectory into the atmosphere, just like the shuttle did. In contrast, the Falcon 9 first stages are at suborbial speeds and don't have a heat shield, so they need to do a substantial re-entry burns to bring down their speed before they hit the atmosphere to avoid damage.

Retric 8y ago
The falcon 9 first stage is not just suborbital, it’s vastly slower than LEO at just Mach 10. It’s also still in very thin atmosphere at 80 km, it’s also spending more time in very thin air as it’s sperates while going up, which is why they don’t need heat shields. It’s true they fire the engines briefly at the top, but doing so to slow slightly from Mach 10 is very different vs slowing from Mach 10 to zero or Mach 22.7 to Mach 10. And this still significantly reduces the cargo they can take to LEO. PS: Remeber kenetic energy is velocity ^ 2 so your bleeding off 1/5 the energy. Further, adiabatic heating is from the compression of gas, lower speeds means lower compression.
- jessriedel 8y ago
  Everything you've said is agreeing with me except maybe the clarification that the Falcon 9 first stage doesn't quite cross the the Karman line, right? In particular, I think "sub-orbital velocity" is commonly taken to include trajectories that are substantially below orbital velocity https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight https://en.m.wikipedia.org/wiki/Sub-orbital_spaceflight like the Mercury-Redstone mission Freedom 7 that achieved a peak speed of ~5,100 mph, less than Mach 7. https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 https://en.m.wikipedia.org/wiki/Mercury-Redstone_3 Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosphere-on-their-return-to-earth https://www.quora.com/Why-dont-rockets-burn-up-in-the-atmosp... (You don't explicitly say otherwise, but one could misread your comment as suggesting that the re-entry burns are optional or negligible.) My main point was that the re-entry burns are about reducing the velocity significantly (30%, or whatever), not just for steering the trajectory into the atmosphere like for de-orbit burns.