When/How to preform suicide burn?

[ToukieToucan]

Using Kerbal Engineer but the readings dont make sense to me:

 

SuicideAltitude (or SuicideDistance) was at 0m so I started to burn but crashed into the surface at 600 m/s.

[Streetwind]

Sounds like something you might want to ask in the Kerbal Engineer mod thread. Perhaps the math for that indicator makes certain assumptions about your ship's TWR profile that just aren't true because you're using a super weird design or something? *shrugs* Best to ask someone who knows how it works under the hood.

[Trann]

Something sounds off: the suicide burn altitude should never be 0 if have a suborbital trajectory.  On descent, it should display the minimum altitude to perform a 100% burn and survive (for safety, I tend to burn 100m above this value until I slow to about 10m/s then come in with more control).  Haven't seen it do this yet on any ships I've build (though none are "super weird" ) Hopefully, not a serious bug.

[Snark]

Here's a quick mod-free technique to come up with a pretty good estimate for deciding when to start a suicide burn.  Even if you don't use it as your primary go-to technique, you can still use it as a backup "sanity check" for the numbers you get out of some other mod, such as KER:

1. Get yourself on a suborbital trajectory.
2. Zoom way in on the map view, and carefully place a maneuver node right on the spot where your trajectory intersects the surface (i.e. your impact point).
3. Grab the "retrograde" handle on the maneuver node and drag it until your projected post-maneuver trajectory collapses to a point and the prograde/retrograde handles start flipping back and forth.

There, you're done.

The "estimated burn time" will tell you how long your suicide burn is.

The "time to maneuver" tells you time until impact.

So just start your burn when the "time to maneuver is around 60-70% of the estimated burn time.  (The exact percentage depends on variables such as your angle of descent and your local TWR, but "60-70%" is a rule of thumb that I've found works pretty well with a reasonable safety margin, and is a lot less hassle than trying to actually calculate it.)

My own personal technique basically boils down to the above, except that I'm running my BetterBurnTime mod, which is doing essentially the same thing and presenting essentially the same information, except that it's an automatic display on the navball and saves you the trouble of mucking around with maneuver nodes, like this:

...with the added advantage over the "stock" technique that it takes into account your increasing TWR as you burn fuel, so it'll present more-accurate numbers.  You can think of it as "KER lite" (very, very lite... it's a much smaller, simpler mod than KER), if you like. 

[Sharpy]

KER gives you pretty much the vertical component of the burn only - or the timing for a vertical drop.

In normal deorbiting, if you burn "skywards" at 0 distance, you'll die from gently scratching the surface at insane speed. If you burn retrograde at 0, most of thrust will go towards killing your horizontal speed, while your craft plunges towards the surface hardly slowed down, and by the time the horizontal component is killed off and your engines point downwards, it's far too late.

Also, the mod can't adequately predict the landing point altitude (say, suicide burn at the edge of crater, plus minus 1km depending on where you land!), sometimes messes up active engine thrust (e.g. counting towards suicide burn thrust of engines that just don't have enough fuel), and generally means suicide if you follow it to the dot.

I'm using it as 'advisory'. First, it adjusts the number continuously, so watch if the suicide burn altitude drops or grows. If it grows, by the time distance reaches 0 it will be way too low.

Also, I try to keep the Suicide Burn Distance at about 10-30% of Suicide Burn Altitude - say, my Suicide Burn Altitude is 10km, and I'm 1km above it - I start the burn, until I'm some 20-30% away (say, Altitude 4km, I'm at 5km - next burn when I'm at 4.5km.) That way I'm not wasting much.

I almost never use it for SpaceX style 'running touchdown' - only in direst situations. I add some 100m to the final suicide burn, and throttle down when my speed is at 20m/s (at altitude of some 100m), move throttle until my vertical speed starts very slowly dropping - and I try to move the throttle such that it remains 20% of the altitude. 50m - 10m/s. 10m - 2m/s. 2m - 0.4m/s and letting the craft touch down at that.

I don't do this only when I'm very, very tight on fuel. Even then I'll add some 3% to the suicide burn, throttle down a little if I see "suicide burn distance" grows, and perform a controlled, constant-speed descent over the last 2-3m.

[Warzouz]

20 hours ago, Snark said:

Here's a quick mod-free technique to come up with a pretty good estimate for deciding when to start a suicide burn.  Even if you don't use it as your primary go-to technique, you can still use it as a backup "sanity check" for the numbers you get out of some other mod, such as KER:

1. Get yourself on a suborbital trajectory.
2. Zoom way in on the map view, and carefully place a maneuver node right on the spot where your trajectory intersects the surface (i.e. your impact point).
3. Grab the "retrograde" handle on the maneuver node and drag it until your projected post-maneuver trajectory collapses to a point and the prograde/retrograde handles start flipping back and forth.

There, you're done.

The "estimated burn time" will tell you how long your suicide burn is.

The "time to maneuver" tells you time until impact.

So just start your burn when the "time to maneuver is around 60-70% of the estimated burn time.  (The exact percentage depends on variables such as your angle of descent and your local TWR, but "60-70%" is a rule of thumb that I've found works pretty well with a reasonable safety margin, and is a lot less hassle than trying to actually calculate it.)

That's a very good technique I always use. Beware that if your TWR is low (1 to 1.5), you may start burning sooner.

[Superfluous J]

In my experience KER ALWAYS underestimates the suicide burn. I tend to burn first when it's at 1000m even though that's egregious. 500's probably better. Lower than that and you're asking to become a splat mark.

[Sharpy]

@5thHorseman In my Moho landing, KER would overestmate the suicide burn by a margin significant enough that I'd run out of fuel before touchdown. I managed to stick the landing at last, as I switched Vectors off, KER calculated suicide burn for the remaining nukes, I began burning good 500m below the recommended altitude, then switched Vectors on for a second when I was less than 500m above the surface, bringing the Nuke estimate distance near zero (then throttling them down enough that it ran to zero exactly, before throttling back up to max.) That's yet another technique - perform the suicide burn with your weaker engines way under the estimate, and correct with stronger ones.

[magnemoe]

1 hour ago, 5thHorseman said:

In my experience KER ALWAYS underestimates the suicide burn. I tend to burn first when it's at 1000m even though that's egregious. 500's probably better. Lower than that and you're asking to become a splat mark.

Two issues with burns and nodes, fist mostly affect low twr crafts, you have to brake 200 m/s with an TWR of 10 m/s so the burn takes 20 seconds, You start burning 20 seconds before node or 200*20=4 km up. after 15 seconds you are more than 1000 meter up as you are braking. 
As Warzouz says you can wait until you have 60-70% of time left. 

Second affect high fuel ration crafts, as you brake you use fuel and your TWR goes up and you brakes faster. 

[eonmoon]

I use the time on KER as a guideline, usualy I start a burn at the 2-5 k mark, drop off a bit of speed. then again as I get closer, so far I have yet to crash a ship... tho mechjeb hits ALL the time, stupid lil buggers brain was not scanned right. that or its a pre ksp scan they used

[Superfluous J]

3 hours ago, magnemoe said:

Two issues with burns and nodes, fist mostly affect low twr crafts, you have to brake 200 m/s with an TWR of 10 m/s so the burn takes 20 seconds, You start burning 20 seconds before node or 200*20=4 km up. after 15 seconds you are more than 1000 meter up as you are braking. 
As Warzouz says you can wait until you have 60-70% of time left. 

Second affect high fuel ration crafts, as you brake you use fuel and your TWR goes up and you brakes faster. 

I know all this.  I just expected KER to know it too

[FyunchClick]

On ‎23‎-‎8‎-‎2016 at 1:59 PM, 5thHorseman said:

In my experience KER ALWAYS underestimates the suicide burn. I tend to burn first when it's at 1000m even though that's egregious. 500's probably better. Lower than that and you're asking to become a splat mark.

For extremes like Tylo, you may even need to keep a margin of a few tens of kms (I think last time I went I got a KER suicide burn distance of 0 about 50 km up, and ended up needing to start around 70 or 75 to actually not splatter; lots of reloads on that one).