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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 spee

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.
  - Retric 8y ago
    > Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. With the current design and mission profile yes they need to reduce speed via thrust. But, a large chunk of this is Falcon 9's aerodynamic profile. Even if heat's not an issue long cylinders are not stable but you could build a rocket with a different profile. Remember, it's just fine without a heat shield (other than the upper stage / engine area) when being used as a rocket and peak velocity is at separation. So, I agree with this design that burn is necessary, my point is in terms of design space it's one of many options. Further, to be clear it's not a meaningful option when you start talking about orbital velocity. The shuttle kind of gave heat shields a bad rap, but they can be really light and simple. The shuttle's problem was trying to build a reusable heat shield at the limit of what was possible vs a huge range of much simpler designs.
    - jessriedel 8y ago
      > Remember, it's just fine without a heat shield... when being used as a rocket and peak velocity is at separation. This is a useful point for me to keep in mind, thanks!

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.
- Retric 8y ago
  > Also, to be clear, the re-entry burns are definitely necessary to avoid damage to the Falcon 9 first stage. With the current design and mission profile yes they need to reduce speed via thrust. But, a large chunk of this is Falcon 9's aerodynamic profile. Even if heat's not an issue long cylinders are not stable but you could build a rocket with a different profile. Remember, it's just fine without a heat shield (other than the upper stage / engine area) when being used as a rocket and peak velocity is at separation. So, I agree with this design that burn is necessary, my point is in terms of design space it's one of many options. Further, to be clear it's not a meaningful option when you start talking about orbital velocity. The shuttle kind of gave heat shields a bad rap, but they can be really light and simple. The shuttle's problem was trying to build a reusable heat shield at the limit of what was possible vs a huge range of much simpler designs.
  - jessriedel 8y ago
    > Remember, it's just fine without a heat shield... when being used as a rocket and peak velocity is at separation. This is a useful point for me to keep in mind, thanks!