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Just from what I've read - At 1st stage separation the F9 is going Mach 10 at an altitude of 80km. There's no hard and fast definition of how high/fast you nee

by gedmark 12y ago

Just from what I've read - At 1st stage separation the F9 is going Mach 10 at an altitude of 80km. There's no hard and fast definition of how high/fast you need to be going for it to count as a "re-entry", but that certainly qualifies.

jessriedel 12y ago
Hey, thanks much. That's useful. For everyone else's information, low-earth orbit is ~8km/s, or Mach 24. According to Wikipedia, "Thermal control becomes a dominant design consideration" above Mach 10. https://en.wikipedia.org/wiki/Mach_number https://en.wikipedia.org/wiki/Mach_number If anyone knows anything about the how the first stage is shielded, I'd be very interested to know. In the artistic video, you can see a quick peak of the nose of the first stage. It's colored tan/yellow like the heat shield on the second stage, but there aren't many details. (Presumably, this was purposefully vague.)
- lholden 12y ago
  The orange you are seeing at the top of the bottom stage may be either a tank or some sort of shielding for the tanks from the upper stage. I know the upper stage looks like it will use it's own heat shield... but it will also be re-entering from a much higher orbit. The engines on a rocket are designed to withstand huge amounts of heat. It may be the intent is to have the rocket re-enter engine first. I'd love to see more information on that part of the rocket myself. Certainly interesting :D
  - lutorm 12y ago
    If you don't enter engines-first, the engines aren't pointing in the right direction to slow the rocket down...
    - lholden 12y ago
      The engines do not burn for the majority of descent. You turn the engines forward and make a short burn. This reduces your velocity and causes the orbit of your ship to eventually enter the atmosphere. Atmospheric drag is then going to provide most of your deceleration, slowing you down to terminal velocity. You then make another burn at the very end of the flight to slow down from terminal velocity and stop. The second stage is a good example. After making a deceleration burn, it will be re-orienting so that the heat shield at the top is facing the direction of movement. It will then keep this orientation until it is safe to orient the vehicle around for landing. You could use your engines for most of the deceleration... but it would consume a very large amount of fuel. No point in doing so when you already have atmospheric drag to do the work for you.
      - lutorm 12y ago
        I didn't mean the engines were on the whole way down. But if they aren't pointing in the right direction to begin with, you need to flip the entire 70m-long stage around somewhere in the atmosphere. Even at terminal velocity it's moving pretty fast, and I doubt it will survive going sideways at 100m/s through the air, never mind that you need some thrusters that can even develop enough force to flip it around. That animation for the second stage reentry is an animation. Recovering the second stage will be extremely difficult and, if it is ever done successfully, I don't think it'll involve doing a 180 flip in the lower atmosphere.

by gedmark 12y ago

jessriedel 12y ago

Hey, thanks much. That's useful. For everyone else's information, low-earth orbit is ~8km/s, or Mach 24. According to Wikipedia, "Thermal control becomes a dominant design consideration" above Mach 10. https://en.wikipedia.org/wiki/Mach_number https://en.wikipedia.org/wiki/Mach_number If anyone knows anything about the how the first stage is shielded, I'd be very interested to know. In the artistic video, you can see a quick peak of the nose of the first stage. It's colored tan/yellow like the heat shield on the second stage, but there aren't many details. (Presumably, this was purposefully vague.)

lholden 12y ago
The orange you are seeing at the top of the bottom stage may be either a tank or some sort of shielding for the tanks from the upper stage. I know the upper stage looks like it will use it's own heat shield... but it will also be re-entering from a much higher orbit. The engines on a rocket are designed to withstand huge amounts of heat. It may be the intent is to have the rocket re-enter engine first. I'd love to see more information on that part of the rocket myself. Certainly interesting :D
- lutorm 12y ago
  If you don't enter engines-first, the engines aren't pointing in the right direction to slow the rocket down...
  - lholden 12y ago
    The engines do not burn for the majority of descent. You turn the engines forward and make a short burn. This reduces your velocity and causes the orbit of your ship to eventually enter the atmosphere. Atmospheric drag is then going to provide most of your deceleration, slowing you down to terminal velocity. You then make another burn at the very end of the flight to slow down from terminal velocity and stop. The second stage is a good example. After making a deceleration burn, it will be re-orienting so that the heat shield at the top is facing the direction of movement. It will then keep this orientation until it is safe to orient the vehicle around for landing. You could use your engines for most of the deceleration... but it would consume a very large amount of fuel. No point in doing so when you already have atmospheric drag to do the work for you.
    - lutorm 12y ago
      I didn't mean the engines were on the whole way down. But if they aren't pointing in the right direction to begin with, you need to flip the entire 70m-long stage around somewhere in the atmosphere. Even at terminal velocity it's moving pretty fast, and I doubt it will survive going sideways at 100m/s through the air, never mind that you need some thrusters that can even develop enough force to flip it around. That animation for the second stage reentry is an animation. Recovering the second stage will be extremely difficult and, if it is ever done successfully, I don't think it'll involve doing a 180 flip in the lower atmosphere.