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Comment by tpmx | Hacker News Reader

Two of the three engines flaming out in the last few seconds, was that intentional? Something similar happened last time too, but at least this time it landed.

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Yes. They need 1 to land - more than one means too much thrust.

Last time they only lit one, it failed to light, and boom. This time they lit all three hoping at least one would work, then turned off the ones they didn’t need.

I assume they had the idea independently of the guy who asked musk why they didn’t light more than one and shut down — musk replied “because we’re dumb”, but I guess that tweeter could claim to be a rocket scientist now.

Quick edit to this - they need two for the rotation to happen. Last time they lit two and one failed, so they did not have enough thrust to rotate it. This time they lit three and turned off one for the rotation, then turned off a second once it was rotated.

It was three engines through the whole rotation. They turned off two within about a second of one another after transitioning out of rotation.

Yep, you’re right, just saw the video from SpaceX. Good call. This actually makes me wonder if they significantly changed the landing profile since they’re scrubbing velocity so much faster with three thrusters, it may explain why the single thruster landing portion seemed slow compared to previous attempts.

Link to the tweet: https://twitter.com/elonmusk/status/1357256507847561217

To start both main and backup engine was standard procedure planned for Soviet lunar landing LK, at liftoff. If everything's fine, backup engine shuts down.

"The rocket company", https://en.wikipedia.org/wiki/The_Rocket_Company , a great and quite realistic book, reminds about importance of knowing history; there, a company explicitly maintains a library of previous technical solutions to problems encountered.

This is very interesting. Blending fiction and non-fiction, it sounds like a very finely detailed position paper.

This wasn't done for SN8 and SN9, where they only lit one engine. Musk said that this time around, they were going to light all three and then shut two off, thus providing some engine redundancy during the landing.

Two are necessary for the rotation and one for the landing. SN9 tried to light two and one failed, now they're lighting all three for the rotation. Here's a video on SN9 with one burning and one flameout: https://youtu.be/z8PzW8imgAE?t=401

I wonder about raptor re-ignition. They have it down with Merlin but it’s not torch ignited nor full flow. I wonder how well that torch handles being in a lit combustion chamber and what the insides of the preburners are like after shutdown...

Ps so so easy to armchair quarterback. I don’t want to sound like I, even one tiny bit, am not constantly dumbfounded by SpaceX’s accomplishments!

They were landing on one iirc but this one did look very fiery and there was a fire started that eventually caused it to explode a while after landing.

Yes, they can't throttle the engines down that much, so it turns them off once their thrust is no longer needed.

I believe so.

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