Just before Starship SN5's 5 August 2020 flight, there appears to be a leak:
Was this planned or accidental?
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Sign up to join this communityDo you mean the trail of white vapor coming from the lower side as shown in the image below?
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.