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Both the Saturn V and the Falcon 9 use TEA-TEB to ignite their kerosene-fueled engines. TEA-TEB is pyrophoric, igniting spontaneously on contact with air. This poses handling issues; it must be stored in nitrogen.

It's also an expendable resource which puts limits on the number of times an engine can be started in flight; the Falcon 9 only loads TEA-TEB for restarts on those engines that will be restarted on a given mission, reasonably enough, but this makes it susceptible to engine failures in the reentry and landing burns that could otherwise be recovered from by using different engines.

Some other engines, such as the hydrogen-fueled J-2 and RL-10, use electric spark ignition rather than a chemical starter. Both engines are reliable and capable of multiple restarts after launch.

Can kerosene-LOX engines use electric ignition? If so, why consider a volume-limited and hazardous chemical igniter like TEA-TEB?

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    $\begingroup$ The other engines restarted probably only in vacuum. F9 has to restart when going supersonic, engines first. Maybe the spark is not powerful enough to be reliable in these conditions? The chemicals will ignite even when injected into subcooled LOX (and possibly even on contact with solid frozen one. $\endgroup$
    – jkavalik
    Commented May 25, 2016 at 4:53
  • $\begingroup$ Good point. I wonder if the amount of TEA-TEB used to ensure a good start contributes to the green flame seen after landing. space.stackexchange.com/questions/15400/… $\endgroup$
    – Russell Borogove
    Commented May 25, 2016 at 5:27
  • $\begingroup$ Restarting an engine several times requires not only ignition of the fuel, it also requires starting the turbo pumps. Using a solid gas generator for this purpose also limits the number of possible restarts. $\endgroup$
    – Uwe
    Commented May 25, 2016 at 9:07
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    $\begingroup$ The spark ignitor needs to withstand the conditions of the center of the combustion chamber if it's to be reused. Not an easy feat. They used spark ignitors in the Shuttle main engine... several hundreds of them per engine. $\endgroup$
    – SF.
    Commented May 25, 2016 at 11:06
  • $\begingroup$ There were 3 "torch" ignitor chambers in each SSME. One in each preburner, and one in the MCC. Each chamber had an O2 and H2 line to it, and 2 electrical ignition devices. So 3 ignition systems, each with 2 sparkers. $\endgroup$
    – Organic Marble
    Commented May 25, 2016 at 13:17

2 Answers 2

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The way you ignite a rocket engine depends a lot on the fuel/oxidiser combination. Some combinations, like Liquid Hydrogen/Liquid Oxygen (HydroLox) ignite very easily and are ignited using a glorified spark plug (this is the case of the RL10, used on the Centaur Upper Stage). The RP1 (Kerosene)/Liquid Oxygen mixture is harder to ignite, and requires more energy. You also can't afford to have fuel pool up in the combustion chamber and then ignite all at once (it could be too brutal for the engine to withstand). In this case, you inject TEA-TEB with the oxidiser, which causes it to ignite, and only then is the fuel injected.

NASA article on ignition systems: http://blogs.nasa.gov/J2X/tag/ignition/
Centaur/RL10: http://spaceflight101.com/spacerockets/atlas-v-551/

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    $\begingroup$ From what I've read, spark ignited hydrogen engines divert a small amount of propellant to an ignition chamber to bootstrap ignition with a small spark. Is this not feasible with kerosene engines? $\endgroup$
    – Russell Borogove
    Commented May 26, 2016 at 16:20
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    $\begingroup$ Ah, I didn't know that. It might be, but kerolox is still harder to ignite (I think, but my chemistry is rusty, that it is due to the complexity of the kerosene molecule, compared to H2), so you might still not have enough energy with a spark. $\endgroup$
    – amyinorbit
    Commented May 26, 2016 at 16:29
  • $\begingroup$ Aha -- some googling on the topic of using kerosene in internal combustion engines supports that. Thanks! $\endgroup$
    – Russell Borogove
    Commented May 26, 2016 at 21:46
  • $\begingroup$ How about using a heating system to vaporise kerosene and then use it in a flame torch? Wouldn’t that be technically feasible, let alone optimal? $\endgroup$
    – karthikeyan
    Commented Jun 17, 2018 at 8:09
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    $\begingroup$ @amyinorbit: That doesn't mean you need anything more than a sparkplug; it just means you need more spark in the plug. $\endgroup$
    – Vikki
    Commented Feb 24, 2019 at 4:32
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A spark won't do anything until the engines are already pumping fuel/oxygen

Rocket engines use a LOT of fuel/oxygen. Each Merlin 1D engine has a 12,000 horsepower propellant pump (~100,000 HP total, just in the pumps). But the pump itself is like a jet engine, driven by the expanding gasses - so you need fuel+oxidizer already in place to generate the gasses to run the pumps to generate the gasses. It's simpler to use phosphoric than to have a secondary pump.

Similarly, large jet engines often use a "huffer" to get going if completely off, they too need a lot of input gas just to run them enough to start. The huffers themselves are jet engines, but much smaller, and use electric motors to start them. Most commercial jets also have an APU, which is a mini huffer onboard (and electric generator)

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