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?
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.
Why do methane rocket engines require these igniters too?
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.