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Comment by autonomousErwin | Hacker News Reader

That was crazy, 50% of me thought as it was coming in, especially as it pitched towards the tower, "they've overcompensated and are going to bring the whole tower down" but they absolutely nailed it.

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Even without the catch step, I always feel like their boosters are coming down way too fast way too late, with engines reigniting startlingly close to surface. Never ceases to surprise me.

Every moment your engines are burning at anything less than maximum thrust, you are wasting fuel. You want to relight as late as possible with as high thrust as possible for maximum efficiency. But that it actually works is nuts.

I remember playing KSP, I had used some mod to program a moon lander to do a suicide burn, a maximum thrust burn to land. I did the maths myself and when it actually came to the landing I was 100% certain I had either a bug in my code or did the math wrong because I was coming in way too fast, or so it seamed, but to my surprise the thing slowed down in time! Mind you ksp is way simpler than the real world and the spaceship could handle some… error, the landing was hard but successful

Point is it always seems like you’re coming in too fast

PEDANTIC - max efficiency isn't necessarily tuned to max thrust.

The math is pretty intuitive; every second you're burning is another 9.8 m/s of delta V you have to spend counteracting gravity instead of slowing the craft down (minus air friction) so minimizing deceleration time cuts your fuel expenditure massively. It also means you have to carry less fuel so you weigh less and you spend less fuel for the same acceleration. It's the inverse of the rocket equation really, less mass is easier to slow down and you waste less energy (ie fuel mass) decelerating faster.

The higher the thrust, the lower the gravity losses. Gravity losses go down as you approach orbital speed but they aren't zero until you're in orbit. And when boosting out of orbit, the higher you are the less benefit you get from the Oberth Effect, so you want to do the burn before you rise too much, which implies sufficient thrust.

Exactly. In addition, by delaying the relight as long as possible you're giving drag as much time as possible to do the work for you (assuming you're not at terminal velocity, so not applicable for the ship.)

Prior to today the only landings you've seen are Falcon 9, which has to do a "hoverslam" because it can't throttle low enough to actually hover.

Even just one engine in minimum thrust would make the rocket go up when empty.. so the computer lights the engine at the precise right moment so it will have 0 velocity at 0 altitude, then it cuts off the engine. "Hoverslam".

The Starship booster is different, it can actually hover.

I have the same feeling watching Starship when the telemetry on the bottom right of the screen showing “0km” but landing burn hasn’t started. Later did I know that it was meant to start the burn only a few hundreds _meters_ from the ground

It may also be that the video feed is delayed (e.g. due to compression) relative to the telemetry.

Yeah, it also always catches me off guard how late they reignite, while at the same time I'm always surprised by how slow the liftoff is even though all the boosters are on full throttle. Of course both things make sense given the respective mass at both points in the process, but given that it looks like the same rocket from the outside, a bit unintuitive.

Yeah but they did pass some flame down much of the tower. Pretty quick though, probably just cleaned it with fire.

Saving up those refurbishment costs, cleaning the tower ahead of time!

Yeah it definitely looked sketchy for a sec but was probably fully as intended

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