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Comment by jessriedel | Hacker News Reader

Do you know how fast the Falcon 9 1st stage gets? Judging by the video animation of their long term re-usability plan, the 1st state separates before it achieves a velocity necessitating re-entry.

https://www.youtube.com/watch?v=kJrFwxE3lzI

Is this video incomplete? Will both the 1st and 2nd stages will actually require re-entry shielding?

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Everything that isn't going into orbit requires "re-entry". The typical friction re-entry is used on capsules because it is cheap (all you need is a heat shield and some RCS engines) a "powered" re-entry can be done at well below thermal stress levels, but hasn't been in the past because the fuel used means you're not putting as much mass into orbit as you could have (the mass fraction).

If you know the first stage mass, relative to the lift off mass, you can compute the energy needed to decelerate it to 0 from what ever velocity it reaches at separation. Because fuel is a significant component of the launch weight, you need significantly less fuel to return the nearly empty stage than you did to put it up there to begin with.

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.

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

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.)

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

If you don't enter engines-first, the engines aren't pointing in the right direction to slow the rocket down...

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.

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.

This [1] forum post on nasaspaceflight.com says at stage 1 cutoff, velocity is 2,531 m/s (mach 6.76) and altitude is 84.1 km. The actual deceleration burn starts 12 seconds after that, when the stage has coasted up to nearly 100km and has slowed (slightly) to 2,479 m/s.

I don't think (?) this altitude and speed requires heat shielding for the 1st stage. The 2nd stage is jettisoned far too fast and high to make recovery feasible (for now!).

[1] http://forum.nasaspaceflight.com/index.php?topic=27748.690

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