When the Falcon 9 experiences stage separation, why isn't the first stage severely damaged by the blast of the second stage?

When separation occurs, there seems to be some mechanical force to push the stages apart but the relative velocity looks to be only a few meters per second at best. Is this enough to gain enough clearance in time to safely ignite the second stage without damaging the first?

Does the first stage perform a flip before second stage ignition such that the first-stage engines act as a shield?


1 Answer 1


They do not complete the flip before igniting the second stage:

The first stage actually goes through worse during its reentry burn:

For that matter, the Falcon Heavy side boosters have it worse after separation, since they have 9 engines pointed at them that are already running when they separate:

The air's thin up there, so the exhaust spreads out and isn't terribly effective at heating solid objects up quickly. They're just built to handle the brief heating...notably, they had to change the grid fins from aluminum to titanium because the aluminum ones had bits melt out of them on some reentries.

  • 3
    $\begingroup$ The fact that the 1st stage and FH boosters go through worse without damage don't get us closer to an answer---they just beg the question still more... To say that the interstage is built to handle the brief heating is also stating the obvious---clearly they're built for it. The question is how do they handle that brief heating, or at least how do they avoid the punishment of that heating :D $\endgroup$
    – user36480
    Jun 11, 2020 at 8:03
  • $\begingroup$ I think it boils down to that rapidly expanding rocket exhaust in a diffuse atmosphere isn't terribly damaging when it only impinges for a very short time. $\endgroup$
    – ikrase
    Dec 6, 2020 at 22:45
  • $\begingroup$ The temperature of the rocket exhaust drops enormously as it is allowed to expand. To the extent that heat is not really a problem. Nor is the "sandblasting" from 3km/s exhaust gases a problem, as the exhaust is a pretty uniform diffuse gas by then. The only significant danger is the several hundred to a few thousands of Newtons of force being applies to the first stage from a very unusual direction. But this force is being applies to a structure that until seconds previously had been supporting the many tons of the seconds stage under 5g of acceleration, so it is way, way, way overbuilt $\endgroup$ May 7, 2021 at 13:41
  • $\begingroup$ @PcMan there's expansion cooling, but there's also rapid compression that reheats it when it encounters an obstacle. You can see some signs of this in the video of the grid fins and upper stage opening while in the exhaust. The main factor is just that it's very thin gas and exposure time is limited, and as you say, the structure's way overbuilt for what little heating it can receive from the second stage exhaust. $\endgroup$ May 7, 2021 at 14:07

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