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Just before Starship SN5's 5 August 2020 flight, there appears to be a leak:

white plume on left side of rocket

Was this planned or accidental?

Nate Barbettini
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Joe Jobs
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1 Answers1

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Do you mean the trail of white vapor coming from the lower side as shown in the image below? enter image description here

That is pre planned and you can actually see something similar when a Falcon 9 (or most other rockets) is preparing for a launch.

As I understand it, its from the LOX (liquid oxygen) boiling off as it warms up inside the tanks. The gas is pulled from the tanks through bleeder valves to help reduce stress from excess pressure (as I understand). Since the raptor engine also uses liquid methane, which has a temperature similar to that of the LOX, gaseous methane may also be expelled, although that is highly unlikely due to the risk of explosions from the mix of methane and oxygen and the contamination caused by unburned methane. You can see the cloud because the gas is still quite cold and water in the air condenses.

note- the image shown is from a screenshot of the official Spacex recording of the launch.

edit- changed part regarding methane.

Lonely Fox
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    Yup, you always see such plumes with rockets carrying cryogenic propellants. – Loren Pechtel Aug 06 '20 at 00:24
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    Is it oxygen boil-off or is it the helium that is bubbled through the cryogenic fuels to keep them cold in the tank? – Moo Aug 06 '20 at 01:16
  • Are you sure liquid methane leak is not dangerous? Isn't there a risk of explosion? – Joe Jobs Aug 06 '20 at 10:44
  • @Moo Well, in that case it would be a mix of helium and oxygen, as the sole purpose of the helium is to make some oxygen boil off and cool the rest. That said, I'm not sure if SpaceX uses subcooling at all for these tests. Feels wasteful, they're not after maximum performance, the plumbing would complicate things, and helium is much more expensive than LOX. – TooTea Aug 06 '20 at 10:54
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    The methane tank is at the top, so this is clearly oxygen. I can't remember whether it is also vented into the atmosphere, or always vented into the flare stack which is slightly out of the frame in this still. – Jörg W Mittag Aug 06 '20 at 12:28
  • You probably do not want to vent both oxygen and methane at the same time. If you need to vent methane you probably want to make sure you burn it as you do so. –  Aug 06 '20 at 12:49
  • @TooTea, I thought that the helium was only used to backfill fuel and oxygen tanks as they emptied in flight. Also, according to this article the sn-5 did use liquid fuel – Lonely Fox Aug 07 '20 at 01:10
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    @LonelyFox Yes, using helium for pressurization is very common, and yes, with the exception of SRBs, rockets do use liquid propellants. But the "bubbling" that Moo suggested refers to a special pre-launch subcooling procedure used on the Falcons. – TooTea Aug 07 '20 at 06:19
  • Always @LorenPechtel ? I don't see them with Ariane 5 –  Aug 08 '20 at 09:10
  • @JCRM: I had a look at some launch videos of Ariane 5 and it does vent, just not very much. The plume is fairly high up: my guess is from the second stage. I suspect the first stage must vent somewhere that isn't visible. –  Aug 08 '20 at 10:42
  • What happens if the rocket has to leak methane too, in order to prevent explosion? – Joe Jobs Oct 02 '20 at 16:49
  • @JoeJobs well if the rocket does vent methane then it will probably end in an explosion if they don’t control it in time. Apon investigation though I’ve discovered that the rockets do have methane that evaporates, but it is collected inside the tanks and piped to the far edge of the pad and burned in a “flare stack” – Lonely Fox Oct 05 '20 at 02:46