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When I was watching the flight readiness test firing of the Vulcan’s BE-4 engines, I noticed that there were igniters similar to the ROFI igniters seen on liquid hydrogen rockets. Why do methane rocket engines require these igniters too?

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    $\begingroup$ Uninformed speculation - because their engine startup sequence runs fuel rich and they want to avoid unburned methane building up around the area? In fact it might be an additional safety precaution just for engine tests to reduce fire/explosion risk to the test stand due leaks or uncontrolled shutdown. Only true source for this is probably the risk assessments for the test which may not be public. $\endgroup$ Aug 27, 2023 at 8:45
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    $\begingroup$ Methan does not ignite with air or oxygen spontaneously. $\endgroup$
    – Uwe
    Aug 27, 2023 at 11:09
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    $\begingroup$ @GremlinWranger - I don't think the ROFIs are used to burn off the normal flow of fuel that occurs during the startup sequence, but rather fuel that may have inadvertently leaked out prior to the startup sequence. At least that's what they were used for during Shuttle, and I assume for Delta IV and SLS also. I have never heard what exact problem they are trying to solve, my guess is possible instability in the exhaust area. I seem to remember that engineers thought it was not really necessary for Shuttle but it was done anyway just to be safe. I would like to know more about the exact reasons. $\endgroup$ Aug 27, 2023 at 14:13

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Whilst methane is indeed less flammable than hydrogen there is still a very real risk of unburnt methane accumulating under the vehicle during engine chill down. Space X thought this wouldn't be a significant issue and that the methane would dissipate before accumulating to dangerous levels, they discovered this was an issue the hard way.

Space X have solved the issue using water sprays but the more conventional ROFIs are a well understood solution to the problem.

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    $\begingroup$ It might be too much conjecture to use the July 11, 2022 fireball incident as proof of anything, as we really don't know what happened since it was related to a spin start test that went bad. Elon said at the time they will have sparklers, but "This particular issue, however, was specific to the engine spin start test (Raptor has a complex start sequence). Going forward, we won’t do a spin start test with all 33 engines at once." He seemed to imply that sparklers would not have helped in that situation. I seem to remember that even with hydrogen it was debated whether it's really needed. $\endgroup$ Aug 27, 2023 at 12:54
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    $\begingroup$ @StevePemberton they did add a fire suppression system between this test and the next one though which does suggest something is needed (or at least, if not really a necessity, worth the effort, just in case) $\endgroup$ Aug 27, 2023 at 14:39
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    $\begingroup$ The issue ULA is solving with ROFIs and the issue SpaceX solved with a nitrogen/water spray system are two totally different issues. In ULA's case, they want to ignite any stray methane before it could get anywhere and damage the rocket. That is why they use ROFIs, which start fires. In SpaceX's case, they want to avoid igniting the very combustible mixture of CH4 and LOX that the engines expel during spin prime tests (a type of test that ULA does not conduct) - hence the inert gas spray system. Using ROFIs would basically guarantee there is an explosion during every spin prime. $\endgroup$
    – quinnkenri
    Aug 27, 2023 at 18:53
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    $\begingroup$ @675longtail - I think they are the same issue: Unwanted flammable propellants. The methods for dealing with the issue, and the reason for the presence of the flammable gasses, are very different though; as you explained. However, I would note that ROFIs should not cause an explosion if used for SS spin primes, instead it would be a rather controlled, but large, fire. $\endgroup$ Aug 27, 2023 at 21:29
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    $\begingroup$ It should also be noted that the ROFIs are expendable pyrotechnic devices that require special training to handle and must be replaced after each use. Picking an alternative solution that can be used as often as they want as long as they have nitrogen and water to feed it seems very much SpaceX's style. $\endgroup$ Aug 27, 2023 at 21:47
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Why do methane rocket engines require these igniters too?

They don't.

SpaceX has launched Starhopper three times, Starship 7 times, and Super Heavy once without them. SpaceX has conducted at least 35 static fires of Starhopper, Starship, and Super Heavy, and many pre burner and spin prime tests, as well as over 1200 test firings of Raptor 1, 1.5, 2, and 3 at McGregor. All without burn-off igniters.

Relativity Space has launched Terran 1 and performed at least two static fires. I did not see any burn-off igniters on the live stream, although it is somewhat hard to see.

LandSpace has launched 朱雀二号 twice, and on the second attempt, it has become the first methalox-powered launch system to achieve orbit. I don't think there is a live stream of the first (failed) attempt, but on the successful attempt, it does not appear that there are any burn-off igniters.

SpaceX had two incidents in which a mix of methane and atmospheric oxygen caught fire, but in both cases, they were able to fix the issue without requiring burn-off igniters:

In July 2022, during a spin prime test of all 33 engines of Super Heavy, methane accumulated under the orbital launch mount, found an ignition source, and exploded. This was fixed by installing the so-called FireX system, a fire suppression system which uses high-pressure nitrogen and water to fill the space below the engines with a fine mist of water and nitrogen, displacing the oxygen and preventing sparks.

In April 2023, during the first integrated flight test of the Starship system, a fire developed inside of the aft skirt of Super Heavy. Note that this issue cannot be addressed with burn-off igniters: the whole point is to not have a fire there. SpaceX has addressed this issue by installing a system which purges the engine bay using an inert gas. It could be seen operating during the recent static fire test, but the real tests will come on the next test flights.

So, while there can be issues with gaseous methane pooling in places it isn't supposed to, it does not look like these issues require burn-off igniters to fix them. It seems to be a decision by ULA to use them, not a requirement of methalox engines in general.

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  • $\begingroup$ "This was fixed by installing a system which purges the engine bay using an inert gas." Maybe, we'll see if it was fixed. $\endgroup$ Aug 31, 2023 at 10:29
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    $\begingroup$ You are, as always, correct. I changed the paragraph. $\endgroup$ Sep 1, 2023 at 20:23

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