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# How do I know the suicide altitude?

## Question

I really want to know how to decelerate efficient for landing.

example, landing to mum, If I decelerate too early, my vessel will drop down, and I have to decelerate again, this will waste some fuel. If I decelerate too late, It's dangerous to crash.

so, How to know, what altitude is best to decelerate can save fuel?

I see MechJeb didn't have this Item that can display this information. But I use its auto landing function, I see its operation, it's good, very efficient. no more redundant action. But, I don't like auto-operation,  I like to control by myself hand

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Moving to Gameplay Questions.

Here's a fairly simple manual technique that works reasonably well: use a little trick with maneuver nodes and let KSP do the math for you.

Here's how it goes:

Put yourself into a suborbital trajectory,  so you're on a collision course with the ground.

Switch to map mode, and drop a maneuver mode right exactly at the point where your projected course intersects terrain,  i.e. right at the impact point.

Drag the handle of the node until the projected post-maneuver trajectory collapses to a point (i.e. Ap right at ground level where the node is).

Switch back to flight view, make sure the navball is in SURFACE relative mode (not "orbit"), and set SSS to "hold ".

Now you're set, and the maneuver node's burn time indicator tells you what you need:

The "time until burn" is how long until you go splat. The "estimated burn time" tells you how long your burn will be.

So all you do is wait until the time-until-burn is about 60-70% of the estimated burn time, then max the throttle.  For example,  if the estimated burn time is 10 seconds, you'd wait until it's about 6 or 7 seconds until the node before goosing it.

It's not *exact*, so you'll need to keep an eye on the terrain and make small adjustments as you approach touchdown,  but overall it works pretty well.

Incidentally,  if you don't mind *some* mod assistance,  you may like to consider BetterBurnTime. It doesn't automate anything;  you're still doing all the flying yourself.  It just gives more accurate burn info on the navball, and automatically provides the above-described info (time until impact, estimated burn needed) without your needing to tinker with a maneuver node.

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Actually, Mechjeb does give a suicide burn readout, if you add it to a window.

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3 hours ago, Kryxal said:

Actually, Mechjeb does give a suicide burn readout, if you add it to a window.

Yep! I typically add that readout (found under the Misc group when customizing your MJ windows) to the bottom of my Surface window. It seems to do a much better job of calculating burns for shallow entries than the one in KER.  And if youre using SmartASS for holding surface retrograde, it's really easy to dial in a few degrees of pitch or yaw to fine tune your landing approach.

I'll also add the current biome so I know where I'm at for science gathering purposes.

Edited by Norcalplanner
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23 hours ago, Snark said:

Moving to Gameplay Questions.

Here's a fairly simple manual technique that works reasonably well: use a little trick with maneuver nodes and let KSP do the math for you.

Here's how it goes:

Put yourself into a suborbital trajectory,  so you're on a collision course with the ground.

Switch to map mode, and drop a maneuver mode right exactly at the point where your projected course intersects terrain,  i.e. right at the impact point.

Drag the handle of the node until the projected post-maneuver trajectory collapses to a point (i.e. Ap right at ground level where the node is).

Switch back to flight view, make sure the navball is in SURFACE relative mode (not "orbit"), and set SSS to "hold ".

Now you're set, and the maneuver node's burn time indicator tells you what you need:

The "time until burn" is how long until you go splat. The "estimated burn time" tells you how long your burn will be.

So all you do is wait until the time-until-burn is about 60-70% of the estimated burn time, then max the throttle.  For example,  if the estimated burn time is 10 seconds, you'd wait until it's about 6 or 7 seconds until the node before goosing it.

It's not *exact*, so you'll need to keep an eye on the terrain and make small adjustments as you approach touchdown,  but overall it works pretty well.

Incidentally,  if you don't mind *some* mod assistance,  you may like to consider BetterBurnTime. It doesn't automate anything;  you're still doing all the flying yourself.  It just gives more accurate burn info on the navball, and automatically provides the above-described info (time until impact, estimated burn needed) without your needing to tinker with a maneuver node.

whaoo! It sounds great, I will try this method. Thank you !

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On 7/30/2018 at 2:46 PM, krosshuang said:

whaoo! It sounds great, I will try this method. Thank you !

I will suggest instead of buring at 60-70%, burn at 100%. Because reaction time.

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On 7/29/2018 at 9:06 AM, krosshuang said:

I really want to know how to decelerate efficient for landing.

example, landing to mum, If I decelerate too early, my vessel will drop down, and I have to decelerate again, this will waste some fuel. If I decelerate too late, It's dangerous to crash.

so, How to know, what altitude is best to decelerate can save fuel?

I see MechJeb didn't have this Item that can display this information. But I use its auto landing function, I see its operation, it's good, very efficient. no more redundant action. But, I don't like auto-operation,  I like to control by myself hand

I made on excell a nice formula for it (to do it automatically).

you need several data:

1) VehicleAcc = your ship aceleration (and should be the minimal with the fuel you just have, in any way it will accelerate a bit eventually) but you cannot switch between them. (the next may be lower or you may even take too much time at changing the steps).

2) PlanetAcc=the landing gravity on surface (yes its not really that much above but, its better to be on the safe side). (in m/s^2 not in g)

3) StartHeight= your altitude (and with my equation you are supposed to not have horizontal velocity, too much of that and you will crash).

4) startVelocity=your vertical speed. (I take it positive when going down).

5) LandHeight= the desired altitude (because except on some Minmus zones normally is not 0)

you can skip 2 and 5, if you have the distance to the surface, even so it should be exact or you will crash.

because a lot of math, the speed at which you have to start the desaceleration can be found by.

MaxSpeed = sqrt((startHeight + 0.5* startVelocity^2/PlanetAcc - LandHeight) / (0.5/PlanetAcc + 0.5/(VehicleAcc-PlanetAcc))

but because reasons I do not understand you cannot rely on the speed you see so you better rely on the altitude, for that.

Time to free fall (before starting it).

FallTime= (MaxSpeed -StartVelocity)/PlanetAcc

or directly

FallTime=[sqrt((startHeight + 0.5* startVelocity^2/PlanetAcc - LandHeight) / (0.5/PlanetAcc + 0.5/(VehicleAcc-PlanetAcc))-StartVelocity) ]/PlanetAcc

now having the fall time. the altitude you have to start your stop is

Altitude to start your rocket = StartHeight-(startVelocity*FallTime+0.5*PlanetAcc*FallTime^2)

if you do correctly you will always have some small amount to fall, because the rocket aceleration is just a bit more in the end (unless your rocket does not change much).

Important notice, your rocket needs to have quite a LOT more acceleration than the PlanetAcc. If its the same or lower you cannot land.

and the closer it is to the same aceleration the more fuel it will expend. I recomend at least 10 times more. (not hard on mon and less hard on Minmus, may be really hard on other planets).

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