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Comment by java-man | Hacker News Reader

I noticed a delay between video and audio - the announcer on the NASA official live broadcast said splashdown before the the capsule splashed down on video. Was it intentional (in case something happened)?

Also, what were these puffs on thermal camera after the main chutes were deployed?

https://www.youtube.com/live/m3kR2KK8TEs

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My suspicion was they were burning excess propellant, rather than attitude adjustment while under the parachutes. Though who knows how much propellant remained. It could be quite a bit more than it appears was used.

Not just excess - excess and toxic. Hydrazine derivatives and nitrogen tetroxide, IIRC. They are hypergolic, too, so the easiest way to vent them is just to run the engines until empty. However, to prevent moving the craft too much, you do short bursts.

On the press conference they mentioned the RCS was used to orient the craft with the most sturdy part facing down for the ocean impact.

Otherwise I would also just bet on RCS venting like in Apollo.

There should be an opposite thruster for each axis. I wonder if the short bursts were due to heating limits.

There are opposed thrusters, but I assume that in atmosphere and under parachute canopy it’s harder to make sure they are perfectly opposed.

Heating likely plays a role as well.

I am not a rocket engineer, but I have read How Apollo Flew to the Moon and Ignition!: an informal history of liquid rocket propellants, both of which cover these issues. Highly recommended.

The short bursts are just the period of the control cycles. Control cycle starts, loop sees error, commands thrust; next control cycle starts, loop sees error is nulled (or in deadband), commands no thrust.

In the post splashdown conference, they mentioned that these were indeed attitude control bursts to orient for favorable orientation for water impact.

It was for attitude adjustment.

RCS (Reaction Control System) which you can see on Artemis I internal video as it falls down https://www.youtube.com/watch?v=-QbYrs5SZ5M

I was wondering about that too, I assume maybe there was some additional adjustments needed to land in the right spot, but they didn't mention it on the stream.

Yeah, they looked intentional - there are no reaction wheels on the capsule.

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