Apollo 11 as viewed from an Air Force EC-135N plane, photographed using a 70mm Airborne Lightweight Optical Tracking System (ALOTS) camera. Noting how the exhaust plume seems to engulf the lower part of the rocket, <i>Apollo Flight Journal</i> contributor Paul Fjeld states, "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/U.S.  Air Force
Taken on 16 July 1969
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