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Landing on airless bodies


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There is one case where you need to de-orbit to land from a higher orbit and that's if your thrust capacity isn't very good. For example, if you currently have 500 m/s speed, and it's going to take you 2 minutes of constant burn to get rid of all of it, but in those two minutes you hit the surface because you started very low, then that's not very good. You need to be able to kill your horizontal speed in the time it takes to fall the vertical distance to the ground as you are no longer going fast enough to orbit but still not going slow enough to land.
The solution to this is to angle your craft so some of the thrust counteracts gravity. Indeed I believe the best way is to just always burn retrograde (measured in surface mode not orbit mode!). If done precisely and neglecting finite turning speed, you'll reach the point where your lateral velocity is killed and you're pointed at the zenith, which is then time to throttle down and descend for landing.
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the best way to do it is to drop your PA as low as possible around where u want to land then try and kill all your horizontal velocity around the PA that way u dont have to kill as much of the gravity velocity for eg i try to burn only 3 times from kerbin to the mun to get max benefit so burn from 70km around kerbin trying to get your orbit to come in from behind the muns so it will boost your craft towards it meaning you will 1 get a boost of speed as it pulls u closer to its orbit and 2 u will be slower as u hit PA on the mun and in that 1 burn from kerbin try and get the mun PA as low as u can eg if ur coming into the big crater about 4km and the second burn would be at PA kill all horizontal speed and u will find u will be really low with very little vertical velocity ... the last burn would be a suicide burn if u dare but id say u would be falling fairly slow once u need to do that

take note of how high the land is from sea level and any hills u will be coming over on your way in ... iv had some outrageous scrapes on the side of craters etc on the way down the lower u can come in the more dV u will save overall

have fun them more u practice the easier it gets and showing off to you friends is the fun part

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I looked again, and it seems you are right : my bad

but I still think de-orbitate from a lower orbit is better, especially because you will have to cancel your vertical velocity which will come when you will "fall" from your altitude

Well people are agreeing with that. Me too. I was just disagreeing with the claim that it's due to the speed being slower when you're low, which is the opposite of true.

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The solution to this is to angle your craft so some of the thrust counteracts gravity. Indeed I believe the best way is to just always burn retrograde (measured in surface mode not orbit mode!). If done precisely and neglecting finite turning speed, you'll reach the point where your lateral velocity is killed and you're pointed at the zenith, which is then time to throttle down and descend for landing.

It is correct that the most efficient way is to keep burning retrograde.

It is false that this can always be conducted from any low altitude. Depending on your thrust you do need a minimum distance to stop, and you need to make sure that distance doesn't involve you intersecting the surface of the planet before getting to the stopping point. A proper suicide burn involves knowing how low you can risk starting the burn from. Too low and it becomes a more literal suicide.

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It is correct that the most efficient way is to keep burning retrograde.

No, it isn't. There's a bunch of math on this here: http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

It's counterintuitive, but it's more efficient to come in as low as possible and burn slightly off-retrograde to keep your altitude constant as you slow down, rather than burning retrograde and allowing yourself to fall. The difference is more pronounced at lower thrust-to-weight ratio.

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No, it isn't. There's a bunch of math on this here: http://forum.kerbalspaceprogram.com/threads/39812-Landing-and-Takeoff-Delta-V-vs-TWR-and-specific-impulse

It's counterintuitive, but it's more efficient to come in as low as possible and burn slightly off-retrograde to keep your altitude constant as you slow down, rather than burning retrograde and allowing yourself to fall. The difference is more pronounced at lower thrust-to-weight ratio.

That seems to assume you're landing on a smooth sphere with no terrain features, where it's safe to travel at something like 500 m/s sideways a few meters off the ground, such that when you finish killing your horizontal speed, you've got no distance left to fall at that point. If you're landing in a more real situation, where your orbit has to start above the terrain features, then wouldn't you end up coming to a stop not AT the ground, but several kilometers above it, and then you have to add in the fuel spent on that vertical descent straight down against gravity?

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Perform the initial part of your landing burn at the minimum possible safe altitude. Then transition from burning off-retrograde (constant altitude trajectory) to something more like a standard retrograde suicide burn as your speed drops. The very end of the landing trajectory can look the same, you just start (and do most of the near-orbital-speed braking) at a lower altitude. The important part for efficiency is to prevent yourself from losing altitude while you're moving relatively quickly, but slower than orbital speed. You can control vertical speed during landing (by adjusting pitch) to gradually descend however you like, so you'll be close to the ground by the time you counteract the last bit of your horizontal speed.

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It's counterintuitive, but it's more efficient to come in as low as possible and burn slightly off-retrograde to keep your altitude constant as you slow down, rather than burning retrograde and allowing yourself to fall. The difference is more pronounced at lower thrust-to-weight ratio.

Wouldn't it be more efficient to start at an altitude high enough to be able to perform a pure retrograde burn the whole way down?

Starting "as low as possible" is nice but you have to get there first, and that takes fuel.

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Wouldn't it be more efficient to start at an altitude high enough to be able to perform a pure retrograde burn the whole way down?

Starting "as low as possible" is nice but you have to get there first, and that takes fuel.

Actually, targetting a low periapsis can be done for very little DV if you're able to make a maneuver in advance. When I launch for Mun or Minmus, I try to get as low a target periapsis as I can from the transfer burn, then a small mid course (or earlier) burn to really nail down the target periapsis. Of course, it's always a trade off, killing a huge amount of orbital velocity only a few KM from the surface gets.... hairy.

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Wouldn't it be more efficient to start at an altitude high enough to be able to perform a pure retrograde burn the whole way down?

Starting "as low as possible" is nice but you have to get there first, and that takes fuel.

Nope. High orbits are bad for efficiency, you never want to be any higher than you absolutely have to. Target a low periapsis on all encounters, perform all capture, landing, ejection, etc burns at low altitudes. Oberth effect means the higher orbital velocity (kinetic energy) of a low orbit is actually useful, whereas the potential energy of a high orbit isn't good for anything. The only thing that's good to do at high altitudes is inclination changes, but you can easily do those from the apoapsis of a highly eccentric orbit where the periapsis is still at a low altitude.

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