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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 parti

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

> 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!