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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    $\begingroup$ Igniting any rocket, of whichever stage, has very little to do with atmosphere. $\endgroup$ Commented Feb 27, 2022 at 5:02
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    $\begingroup$ As mentioned here "Rocket vehicles carry their own oxidizer, unlike most combustion engines, so rocket engines can be used in a vacuum..." Even solid rocket engines carry their own oxidizer. $\endgroup$
    – AJN
    Commented Feb 27, 2022 at 5:49
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    $\begingroup$ You might also be interested in the process of ignition and the mechanisms used to start it. $\endgroup$
    – AJN
    Commented Feb 27, 2022 at 5:58
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    $\begingroup$ If the second stage would need the oxygen of the air for ignition, how should it burn without air? $\endgroup$
    – Uwe
    Commented Feb 27, 2022 at 15:49
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    $\begingroup$ How would either stage burn? There's no air intakes, and air's certainly not getting in past the pillar of fire blasting out the back. $\endgroup$ Commented Feb 27, 2022 at 17:50

2 Answers 2


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.

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    $\begingroup$ Small bit of pedantry: for hydrolox engines, which burn very fuel-rich, the excess superheated hydrogen in the exhaust does ignite and burn when it hits the oxygen in the external air, but by this point it's no longer contributing to the rocket's thrust anyways. $\endgroup$
    – Vikki
    Commented Feb 28, 2022 at 8:33

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.

enter image description here


  • $\begingroup$ What "orange flames on the sides" are you referring to? Surely you don't mean those features that are completely outside of the engines. $\endgroup$ Commented Feb 28, 2022 at 0:26
  • $\begingroup$ Suggest you check out page 69 here, and rewrite this. chrispennello.com/j2/J-2%20Engine%20Fact%20Sheet.pdf $\endgroup$ Commented Feb 28, 2022 at 0:35
  • $\begingroup$ "After first stage burnout and initial separation, eight rocket motors attached equidistantly around the interstage are fired for approximately 4 seconds. These motors, called ullage motors (an old brewer's term referring to the gaseous zone in a tank above the liquid), provide positive acceleration and therefore pressure to force the stage's propellants into the feed lines to the J-2 engines." $\endgroup$
    – Mazura
    Commented Feb 28, 2022 at 0:56
  • $\begingroup$ "This is called the ullage maneuver. The interstage is separated from the second stage approximately 30 seconds after it separates from the first stage. The two-step separation of the interstage is called dual-plane separation." $\endgroup$
    – Mazura
    Commented Feb 28, 2022 at 0:56
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    $\begingroup$ The ullage motors are not the second stage ignition system. The intent of ullage engines is twofold: To prevent the first stage from colliding with the second, and to make the second stage propellants (both fuel and oxidizer) settle toward the bottom of the propellant tanks. $\endgroup$ Commented Feb 28, 2022 at 5:45

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