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Comment by modeless | Hacker News Reader

I don't believe they throttle the engines up or down much during the second stage burn. Fuel decreases ~linearly and thrust is relatively constant. Acceleration increases as fuel mass decreases.

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Don’t they throttle back at MaxQ?

Yes, on the first stage.

Oh! Second stage. Misread.

No, second stage has 3 vacuum engines and 3 atmospheric engines, so they'll have to be able to throttle for the cutover.

The engines are able to throttle but they run all of them at 100% throttle for most of the second stage's first burn, I expect. For efficiency. The telemetry showed all of them firing at the same time. The vacuum engines will be used by themselves for on orbit maneuvers and the sea level engines will be used by themselves for landing, but for the first burn to reach orbit they need all the thrust they can get.

I would be surprised if that was the case, my reasoning to that is that computing where a thing is going, when it's under going with changing acceleration AND changing mass, is pretty complicated. Especially if you already have the capability to throttle the engines and keep 'a' constant.

They might, I'm not saying your wrong, I'm just saying that I cannot imagine how you would justify the added complexity of doing it that way.

The computations are complicated but not that complicated relative to everything else SpaceX is doing. It's much more important to optimize the propellant mass by using it most efficiently than to simplify some computations. And it's probably most efficient to burn the propellant as fast as possible.

Any extra time spent during a burn is wasted fuel. Intuitively, any time before the rocket is in orbit, some part of the rocket thrust is resisting the force of gravity or else it would fall back down to earth. The longer that time is, the more thrust (and thus fuel) was spent negating that force. It's the main reason why the Falcon 9 boosters do a 'hoverslam' on return and land at close to full throttle - any extra time during that burn is less fuel efficient.

Better fuel efficiency = more payload to orbit = plenty of justification for the extra complexity.

Admittedly gravity losses are more significant at the beginning when the booster/ship are ascending purely vertically than later in second stage flight which is mostly horizontal, but definitely still a factor.

Designing a rocket that's strong enough to survive 100% thrust at maxQ also wastes fuel, because you've overbuilt.

The rocket is well beyond max Q by the time the second stage engines ignite. There’s no reason not to run them at full throttle at that point.

I thought the hoverslam was because the Merlin can't throttle down sufficiently to actually hover?

Yup, apparently Raptors have more dynamic range in their throttling.

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