When launching, the fuel mixes and then ignites when it hits oxygen in our atmosphere.
How does a second stage ignite once in the vacuum of space?
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When launching the fuel mixes and then ignites when it hits oxygen in our atmosphere.
This is incorrect.
Jet airplanes do not carry an oxidizer. Jet engines rely on oxygen in the air flowing into the engine to supply the oxidizing agent. The ignition is internal to the engine.
Rocket engines do not rely on atmospheric oxygen. Chemical rocket engines either carry both oxidizing and reducing agents, use a propellant blend that contains both oxidizing and reducing agents, or use a propellant that doesn't need an oxidizing agent at all. Regardless, the ignition always occurs within the engine.
Presumably over-dubbed with audio from Apollo 11.
This is footage from Apollo 6, which was an unmanned test flight of the Saturn V. The camera systems, adapted to fit inside the second stage of the Saturn V by engineer Shelby Jacobs, were ejected and then retrieved by ship.
The two orange flames on the sides are [not] the igniters. They're ullage motors?
It's already being lit at the beginning of the shot; it doesn't become obvious that it's on until the engine shroud gets in the way. It was on before that and it's still on after that. It's been on since they said ["ignition"] "thrust is go; all engines".
The igniter was a spark plug somewhere inside the engine.