The interstage of the Saturn V rocket, which connected the first and second stages of the rocket, was jettisoned after the first stage was spent. The interstage was designed to be jettisoned by explosive bolts, which would ignite as they were detonated, creating the appearance of fire. This was not an actual fire, but rather the result of the ignition of the explosive bolts and the release of gases from the rocket. The interstage was designed to break apart and fall away from the rocket after it was jettisoned to reduce aerodynamic drag and weight on the rocket, allowing the second stage to continue its ascent.
Why does the interstage of Saturn V seem to have fire in it after it was jettisoned?
Because the J-2 engines are already running, these engines burn hydrogen and oxygen with an almost perfectly invisible flame.
But that flame is still very, very hot, and when the hot exhaust plume hits the paint, wiring, and contaminants in the interstage, they vaporize and add color to the flame.
What the onboard film cameras during the Apollo 4 and 6 test flights videos showing the engineering footage from the camera pods recovered from the early Apollo flights, and notice a few things:
The shots past the engines are looking down through the interstage toward the first stage.
In the vacuum of space, rocket plumes spread out and can even climb the sides of the rocket.
As the first stage drops aware, glowing fire can be seen pouring out of the interstage.
This is hot exhaust from the second stage engines, which has rebounded into the interstage because the first stage is nearby.
As the first stage falls away, the pressure drops, and this backwash clears.
As the first stage falls, you can see unburnt propellant venting beneath it.
When the interstage is jettisoned, it passes through the second stage exhaust plum (remember that the view is through a wide-angle lens) and adds color only where it is in contact with the engine plume.
Sunlit views from deeper inside are from the upper stage, looking up at the S-IVB third stage.
The initial flame is from the three APS pods pulling the third stage clear.
Like hydrogen and oxygen, the hypergolic used by these motors are almost invisible in space, but here are in contact with and therefore given color by the spacecraft.
As the S-IVB climbs away, you can see three constellations of glittering debris rising not quite as fast.
This is debris from the prytos that just separated the stages and is blown back toward the camera, and out of frame one, the single J-2 of the S-IVB ignites.
The initial cloudiness occurs when that engine ignites the unburnt hydrogen pumped through it during pre-chill.
This is quickly gone, leaving only the glow of the combustion chamber. The plume itself is invisible.
Although the flame was invisible, the second stage’s five J-2 engines were already firing at full thrust by the time the interstage ring fell away.
As the ring fell through the plume, the rocket exhaust burned the paint, wiring insulation, and other volatile components of the interstage.
This only lasted for a second or two, but since it was shot with a high-speed camera (for better image analysis).
The footage is usually seen in slow motion, making it seem like the fire must be coming from some source on the ring itself.