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Comment by lutorm | Hacker News Reader

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.

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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 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.)

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.

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