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After reviewing the split second ignition of the 3 Starship SN 9 Raptor engines on their successful liftoff and ascent on video, thoughts began to focus on their relight issues during the "seven seconds of terror" Hoverslam landing technique.

Specifically, during a launch, the engines are static and upright. Gaseous methane, lighter than air, will accumulate near the top of the combustion chamber.

However, during the landing maneuver, the Raptor nozzle is in the equivalent of a Category 5 hurricane airstream. This would seem to potentially cause a great deal of unstable, turbulent airflow around the nozzle, and act as a vacuum aspirator, pulling the fuel air mixture out before sustained ignition can be achieved.

Jets solve this issue by protecting the flame with a "can". Could it help the Raptor to "keep a pilot light on" using can(s) to ensure reliable ignition for their powered landings?

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  • $\begingroup$ I think the ignition methods are radically different in rocket engines. $\endgroup$
    – Carl Witthoft
    Feb 11, 2021 at 13:31
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    $\begingroup$ The Raptor's restart problems are not due to ignition difficulties, but to plumbing. The incorrect amounts of methane and oxygen arriving at the combustion chamber. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Feb 12, 2021 at 15:50
  • $\begingroup$ @PcMan which extreme turbulence and unpredictable air flow would only exacerbate. $\endgroup$
    – Robert DiGiovanni
    Feb 12, 2021 at 16:15
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    $\begingroup$ @RobertDiGiovanni um, NO. If your turbulence and airflow is affecting your mass flow rate through your turbopumps, you have WAY bigger problems than just ignition!!! $\endgroup$
    – CuteKItty_pleaseStopBArking
    Feb 12, 2021 at 18:08
  • $\begingroup$ @PcMan um, it would not affect the mass flow rate, but could affect their ratio in the ignition area. A relight in a 200 mph crosswind should be tested. Head on will be better because the fuel and oxygen are being rammed back into the nozzle. But, yes, certainly, the flow rates must be good. $\endgroup$
    – Robert DiGiovanni
    Feb 12, 2021 at 19:15

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The problem wasn't the ignition mechanism - that's done with a sparking system. The problem (appeared) to be the fuel supply to the engine. You could see it was attempting to relight (and successfully lit a few times), but appeared to not be getting the stable fuel supply to maintain the flame. A can can't help with that.

The biggest problem with spinning around to land is it's a massive dynamic change which means liquids move all over the place when you do it, in both the tanks and the pipes. Getting that stable enough to light every time is... problematic, hard and awesome that SpaceX is trying.

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    $\begingroup$ Do you have any evidence that liquids slosh in the pipes? I would be surprised if there are voids in the feedlines. A reference would be great. $\endgroup$
    – Organic Marble
    Feb 12, 2021 at 3:30
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    $\begingroup$ No, that's not what pogo is at all. Slosh occurs at a liquid/vapor boundary. BTW Raptor doesn't use chemical igniters. You might want to give your answer a tune up and stick to what can be proven or at least clearly state where you're guessing. $\endgroup$
    – Organic Marble
    Feb 12, 2021 at 3:43
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    $\begingroup$ Fair call. I was pretty loose on "slosh" - was more meaning to talk about chaotic movement than the actual physics of "Slosh Dynamics". Wild changes in local "gravity" causes liquids to move all over the place is more what I was getting at. I didn't feel the OP wanted a full physical treatment, so I kept it pretty loose. $\endgroup$
    – throx
    Feb 12, 2021 at 3:54
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    $\begingroup$ @throx the header tanks (really a second stage tankage feeding the same engines to land (instead of continuing on into space)) is designed to handle "slosh" (maybe not entirely suceessfully). The air turbulence around the Raptor nozzles may lower the odds even further, which is why it has been proposed to get it vertical and stable with a drouge 'chute, which would also give the nozzles a much more predictable and reliable ram air flow (if only to keep the fuel/lox in its chamber to relight). $\endgroup$
    – Robert DiGiovanni
    Feb 12, 2021 at 7:58
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    $\begingroup$ @RobertDiGiovanni If you have another question like "Why doesn't Starship use parachutes", then I suggest you ask it so other people can find it (assuming it's not already there). Comments aren't for followup questions. $\endgroup$
    – throx
    Feb 14, 2021 at 22:42
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Flame holder will probably not withstand temperatures inside engine. What maybe is the reignition problem is that LOX header tank is 50m far from engines. Due to that lenght there is probably pressure and mass flow drop and therefore unproper feed of oxidiser. If they try ignite all 3 on landing i think it's gona be even worse case then SN 8&9.

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  • $\begingroup$ They ignited two engines on SN8's flight with no oxidizer pressure issues, and despite low pressures from the fuel tank which was much closer to the engines. I figure we've seen the last of the fuel pressure issues until they go back to fully autogenous pressurization, which I expect they'll do eventually (it's likely way down on their list of priorities, since they can easily just include some helium tanks and still do almost all of their satellite launch business). $\endgroup$
    – Christopher James Huff
    Feb 14, 2021 at 2:57

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