How to time suicide burns?

[imaspacestation]

So supposedly the most fuel-efficient way of landing on a void world is to circularize as low as possible and then burn "at the last second" to kill all horizontal velocity, so that you are very low to the land at that point and are not wasting fuel fighting the Mun's gravity.

But how do you time that "last second" when you're supposed to start burning?

[Trann]

Kerbal Engineer Redux has a read-out for suicide burns. Bob's yer uncle.

- Suicide Burn Altitude (height above terrain to start burn)

- Suicide Burn Distance (distance to suicide burn altitude)

- Suicide Burn DeltaV (velocity change required to zero vertical speed)

http://forum.kerbalspaceprogram.com/threads/18230-1-0-2-Kerbal-Engineer-Redux-v1-0-16-6/

[imaspacestation]

Ok, and without mods?

[georgTF]

Quicksave (F5) before you de-orbit, then try a few practise runs. Reload with F9. Without a clear readout you need first hand experience if you want try seat-of-the-pants suicide burns. I do them often enough when i've used the same craft several times.

Bear in mind that craft mass and engine thrust change the exact way to do the maneuver. Gravity too, but that one is obvious.

Personally, i waste more fuel due to picking an uneven landing site and then correcting, than i do to less than efficient, semi-suicide burns.

[Mahnarch]

Hmmm... I usually come in steep (vertically(ish), with low horizontal speed at around 5-8km) and it seems to work pretty good for me.

My "first" landers are usually a 400 with a 909 at the business end, surrounded by four 400s w/ 909s mounted on radial decouplers.

I use the four radials to land. I usually have enough fuel left over to use them to get to a few thousand feet back into orbit and kick them off - using the single center to return to Kerbin.

I also, sometimes, use the F5 simulation to "practice". Especially with targeted landings.

[Red Iron Crown]

In stock: Set a maneuver node at the point of impact and drag retrograde until node Ap hits the surface. Start the burn when time to node equals node burn time.

[Grumman]

Without testing it yet, I'm going to make a guesstimate...

Find the surface gravity of the moon you want to visit: g(Minmus) = 0.491

Find the radius of the moon you want to visit: r(Minmus) = 60,000

Calculate g^2*r / 2: k(Minmus) = 7,232

Find the maximum acceleration of your ship, and divide k by the square of that number. That should be approximately how high you need to be when you start your burn. That's assuming you are almost in a circular orbit.

Edited June 8, 2015 by Grumman

[Snark]

In stock: Set a maneuver node at the point of impact and drag retrograde until node Ap hits the surface. Start the burn when time to node equals node burn time.

^^This. It's a really handy technique, is quick and easy, and doesn't require any mods.

Also, your actual time to start the suicide burn will be slightly after time-to-node = burn-time. More to the point, your actual ideal suicide burn time will be when time-to-node is some fraction of the burn time. Just how big a fraction it is will depend on two things:

1. Your angle of approach. Shallower angle (i.e. you're coming in almost horizontal) = you need to start burn earlier.

2. Your TWR relative to the body you're landing on (e.g. if you can do 10 m/s2, and you're landing on the Mun where g = 1.63 m/s2, then your effective TWR is about 6). Higher TWR = you can start the burn later.

The absolute shortest burn you'll need will be if you're coming straight down, and your effective local TWR is very high. In that case, the fraction is 0.5; i.e. if your node burn time is 20 seconds, you'd start when you have 10 seconds until the node.

Since that's the ideal and you never reach it, you want to allow a little bit of safety margin. In practice, I find that a factor of 60-70% strikes a nice compromise; I'm usually not coming in super close to horizontal, and for any vacuum world except Tylo, my local TWR tends to be pretty high. So (for example) if the node burn time is 20 seconds, I'll wait until it says 12 seconds or so before I start the burn. (And of course I do a quicksave early enough that if I'm wrong, I can try it again.)

The technique doesn't work if there's atmosphere, but then, in that case you're probably using chutes instead of a suicide burn anyway.

The only real drawback I've found to this technique is when I've got a base on the surface, and I'm trying to land right next to it. When that happens, I can't drop a maneuver node on the point of impact, because the game thinks I want to select my base instead. The workaround is to go to map settings and temporarily turn off display of whatever icon I'm using for my base, at least long enough to drop a maneuver node before I turn the display back on again.

[Red Iron Crown]

It's not so much your absolute TWR but how much your TWR changes during the burn that affects how much later you can start. I just go with time to node = node burn time because it's simple and leaves some safety margin.

[Chaia]

I can't recommend KER's suicide burn function. Went to mun with like a 30° angle, started to burn at the stated time and crashed right into the mun with like 150 m/s left

[pandaman]

From my experience KERs suicide burn function works only on vertical velocity. So it gives a very false reading if coming in at shallow angles. It works ok when coming down close to vertical, but I still burn a bit early just in case.

[match]

I just noticed a stat in the ship info window a few days ago. Something like "delta v to 0 vertical al speed". Kind of forgot about it until reading this thread. It might be worth checking out.

[MarvinKitFox]

Take your current Altitude, Speed, Angle, Local gravity and rocket TWR.

Write them down on a piece of paper.

.

Look back at the screen, notice how much less that Altitude reading is now.

Write down the new reading.

reach for your calculator, and look back at your screen as you hear an earth-shattering Boom.

.

.

Now press F9, and use the advice from Red Iron Crown, a couple posts back.

[Guest]

There is mod called like Diazo's Vertical Velocity Control, or something like that exactly for some purpose.

[imaspacestation]

In stock: Set a maneuver node at the point of impact and drag retrograde until node Ap hits the surface. Start the burn when time to node equals node burn time.

Oh, I didn't even think of trying something like that! Thanks for the tip!

[ajburges]

KER suicide burn only considers vertical velocity. You need to reckon the added time for horizontal velocity. (or use wheels to make horizontal velocity safer)

I try and chase the burn altitude to correspond to my fall rate, but I'm frequently off and quick saves are a necessary safety feature.

Another trick is that you can use the map trajectory to eyeball most of the decent. Eliminate half orbital velocity at low orbit to select site. Then burn near retrograde with throttle such that your craft is half way between collision and Ap. You shouldn't need too much dV for final approach then, so inefficiencies are more forgiveable.

[Snark]

It's not so much your absolute TWR but how much your TWR changes during the burn that affects how much later you can start.

That too.

I just go with time to node = node burn time because it's simple and leaves some safety margin.

The nice thing about the technique is that you can use it more than once on a single descent. i.e. if you find that you slow way down and still have a significant ways to fall, you can cancel the node, place another one, and do a 2nd, shorter suicide burn.

[Sovnheim]

While it is not per se the most efficient, after a few tries I highly recommend Red Iron Crown's approach.

I tend to arrive at a medium orbit and perform successive burns at retrograde to reduce horizontal velocity. A well placed maneuver node will help time the second-to-last burn, the one that will kill the remaining horizontal velocity and the most of the vertical.

For reference, that was more or less what was done in the Apollo missions.

[Vectura]

Just to note, I had one particularly hair-raising suicide burn. I activated the Vertical Velocity Control mod just as I hit KER's suicide burn time, and I ended up stopping a couple of meters above the surface. Was kinda scary!

[Warzouz]

MJ also do suicide burns but it has a hard time with high horizontal velocity (maybe he can't get the landing point precise terrain altitude). Sometimes it fails... Further more, with low TWR, suicid burns aren't possible. you have to "hover"** to kill horizontal while keeping vertical under control. That's not efficient but safer (especially for Tylo...)

** not aiming pur retrograde but a little higher

Edited June 9, 2015 by Warzouz

[gmiddlemass]

If you don't mind using mods then you might also want to have a look at the "Landertron" mod by XanderTek.

http://forum.kerbalspaceprogram.com/threads/61294-1-0-XT-Landertron-Smart-Retrorockets-for-Landers-and-Spaceplanes-v0-08-Oct-10

It adds a few retro rocket parts which will calculate the correct time for a suicide burn and fire at the correct point automatically. Kind of scary to watch but worked quite well when I tested it last (before v1.0)