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Comment by WalterBright | Hacker News Reader

> early indications suggest that this week’s ispace failure could have been caused by the lander running out of propellant just before it touched down

I practiced a lot with the Lunar Lander game in the 1970s. The trick is to let it fall until the last second, then hit the thrusters full blast to hit 0V as it hits the surface.

Burn too soon, and you run out of propellant and crash.

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https://en.wiktionary.org/wiki/suicide_burn

You can still apt install that game, I think the package name might be something like moon-lander. I play this regularly and keep a file for games that don't have highscore storage (like Dino "no Internet" game as well, also so you can compare across different computers). Can confirm this is the best strategy and also the hardest option.

There are many, many incarnations of it. The one I played was written in BASIC and ran on a dot matrix tty.

Interesting that the SpaceX rockets have the same behaviour when landing. I read somewhere that was because they have no throttle control and are on/off, but I'm no authority.

The key reason is that an extra second of time spent hovering or close it is another second of resisting gravity, which takes fuel, so the most efficient hypothetical ‘burn’ is one big bang ‘hoverslam’ right at the end and 0 seconds spent fighting gravity

The SpaceX Falcon 9 rocket does have throttle control, but at minimum thrust the thrust-to-weight ratio is still greater than 1, which means they have to hit 0m/s at exactly 0 feet of altitude or they start going up again. I’d imagine they time their ‘hoverslam’ to take 2x minimum thrust (or some other multiplier) and then adjust up or down through the manoeuvre.

They have throttle, but not enough throttle range. Merlin is a 845 kN engine that can throttle down to 338 kN. When the rocket is mostly empty, that's too much thrust to hover. So they have a very short window to start the landing burn, and also not enough fuel to abort and try again.

Given the constraints, it's surprising they don't lose more rockets during the landing phase.

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