July 16, 1969. The moon-bound Apollo 11 as viewed from an Air…

July 16, 1969. The moon-bound Apollo 11 as viewed from an Air Force EC-135N plane. Paul Fjeld writes, “It’s called ‘plume recirculation.’ Because the F-1 nozzles are underexpanded for that altitude, a very small bit of the escaping burn mass actually goes forward from the lip of the nozzle. Because there are neighbor nozzles doing the same thing there is even more pinging ‘upwards.’ Couple that with a suction behind the detached part of the supersonic shock wave forming on the side of the S1-C stage and you get that dirty fire creeping up the rocket. It was expected and protected against. You can see the same phenomenon on shuttle launches just before the solids kick off – it looks like the bottom of the External Tank is on fire!”

(NASA)

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