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I was reading this question about Falcon 9's re-entry Max Q, which states that the Falcon 9 has a re-entry pressure 3 times higher than during launch.

I was shocked, because SpaceX takes precautions against launch Max Q — specifically a conical fairing. But during re-entry the leading surface is less than aerodynamic — the 9 engine bells are basically sails. I'm surprised they don't get torn off during re-entry, but not only are they still there, they're usable!

What does SpaceX do to protect the Falcon 9's engines during re-entry?

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    $\begingroup$ Note that on ascent, the rocket is more fragile and unstable - longer, multiple pieces, with center of mass far behind center of pressure. On the reentry it's a single stubby and relatively rigid piece, with heavy engines and propellants at the leading end and grid fins on the trailing end -- much more stable. $\endgroup$ – Russell Borogove Apr 6 '17 at 2:55
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    $\begingroup$ Remember that the nozzles are built to whist and the tremendous pressure and heat of liquid fuel combustion in the first place. $\endgroup$ – Antzi Apr 7 '17 at 1:19
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SpaceX has been using supersonic retro-propulsion to slow down the stages enough to not cause significant damage to the engines. This is the same method that will be used to land the Red Dragon probes on the Martian surface. In order for supersonic retro-propulsion to work you need to have very high chamber pressures to effectively push the air out of the combustion chamber. Since the engines are in front during reentry, this means the brunt of reentry is in the engines. But they are made to withstand high temperatures and pressures. The reentry burn SpaceX uses slows the stage down enough so that no extensive damage happens to the stage.

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SpaceX does a series of burns during first stage landing.

There is a boostback burn, done to initially cancel the forward velocity, and if going back to RTLS at LZ-1, then to actually change direction back to the launch site. On ASDS landings this is not usually done.

Then during reentry itself, they initiate a reentry burn. This is done to slow the stage down, enough that the stage can survive entering the atmosphere.

Thus they run the engines near the point at which they need to slow down, and as the engines are running they are protected from the air incoming due to the pressure they put out as running engines.

Now it is worth noting only 1 or 3 engines are used at this point, but the other engines are clearly strong enough to survive them.

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  • $\begingroup$ Just a question IF Fuel was not an issue(as in a fuel supply in space intended to provide enough fuel for the desired landing) how would that change things ...specifically in regards to the stress and g'forces experienced by a Human. Range the age of that Human from a Baby to a 90 year old. $\endgroup$ – Enigma Maitreya Apr 6 '17 at 19:52
  • $\begingroup$ A fuel supply in space doesn't make fuel not an issue; you still have to carry the fuel -- and no one is interested in building a fuel depot to cater to 90-year-old passengers. A healthy 77-year-old can handle an unpowered descent with 3g stress reasonably well in any case. $\endgroup$ – Russell Borogove Apr 7 '17 at 0:47

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