Understanding KER's Suicide Burn Figures

[HebaruSan]

I was intrigued when these numbers appeared in my KER window a few versions ago. Landing still takes me multiple tries on occasion, so any hints could be very valuable.

However, I have yet to figure out how to use these numbers effectively. (Thanks to BetterBurnTime, I now have numbers I do understand, so the below is less pressing, but I might still learn something.) Just in case my problem is conceptual: My understanding of a suicide burn is that you get into a trajectory that intersects the surface of a body with no atmosphere, and then you wait as long as possible to start burning, with the goal that at the exact moment you finish, you have 0 velocity and 0 terrain altitude. It's supposed to be difficult but also the most efficient way to land.

I would think the thing we're trying to determine is, "When should I start my suicide burn?" Or at least, "How long should that burn be?" But I can't figure how to decide that based on these numbers. For starters, they change; above is early in a suborbital hop on Minmus, below is later at apoapsis:

No thrust was applied in between these screenshots; it's the same trajectory, just with the lander at a different point along it.

I would think the (Alt.) value would tell me at which altitude I should start my suicide burn. But it increased by about 300 meters while I was just coasting, so it must mean something else.

(Dist.) sounds like it might be how far the craft currently is from the point where the burn should start, but that can't be right, because we should be closer to that point in the second screenshot, not farther away.

(dV) I would expect to be the magnitude of the needed burn. But again, this increased while the burn needed to stop should have remained constant.

I've attempted to use these interpretations to land and crashed every time. I also looked for wiki pages or previous discussions about this and didn't find any. Can anyone please explain what these values mean and how to use them?

[sarbian]

From what I see I would say that to land you start your burn at "Suicide Burn (Alt.)" and you will travel for "Suicide Burn (Dist)" while you lower your vertical and lateral speed. At you will use "Suicide Burn (dV)" delta V.

[Evanitis]

That -sounds- logical, but either this explanation, or the numbers can't be correct, otherwise the values wouldn't change while coasting. Though I can't be much of a help in figuring this out, as I never used KER. Yet, I'm pretty interested.

[Padishar]

Those readouts are not currently very accurate except for near vertical descents.  This is the code that calculates them and it can be seen that this only uses the vertical speed, the terrain height immediately below the vessel, the current altitude and it assumes surface gravity and that the full thrust of the engines acts vertically.

Altitude is the altitude (above the ground) at which you should start the burn.  Distance is your vertical distance from this altitude.

[HebaruSan]

1 hour ago, Padishar said:

Those readouts are not currently very accurate except for near vertical descents.  This is the code that calculates them and it can be seen that this only uses the vertical speed, the terrain height immediately below the vessel, the current altitude and it assumes surface gravity and that the full thrust of the engines acts vertically.

Altitude is the altitude (above the ground) at which you should start the burn.  Distance is your vertical distance from this altitude.

Thanks! I'll try zeroing out my horizontal speed to see if that makes a difference. It sounds like (Dist.) is the one to watch---hit 'Z' when it gets close to 0.

Kind of funny since KER also has an Impact Time stat that seems to take into account both the full velocity vector, differences in gravity, and the distant terrain.

[Padishar]

Yes, though the impact time and position readouts take no account of the burn.  For a non-vertical trajectory, the landing burn will cause the impact point to move which could result in a significant change of terrain altitude.  To get a really accurate result would need a simulation of the landing burn's effect on the trajectory but it should be possible to improve it significantly from what it is now with only a few tweaks to the calculations.  Instead of the potential/kinetic energy calculation it does to determine vertical impact speed (the DeltaV = line), it should instead calculate the velocity vector at that point of the orbit (using the result of the impact calculation for the precise location) relative to the surface and use the magnitude of that as the deltaV.  The burn time for that deltaV can then be calculated and a countdown to the start of the burn displayed.  Giving the radar altitude would be inaccurate over hilly terrain though the altitude above sea level could be given.