Suicide Burns

[flightmaster]

So I was getting quite curious about trying a suicide burn to land a ship on a body without an atmosphere (the Mun in this case, it's the easiest to get to) and after a lot of inefficient approaches I decided to try and figure out how to do one properly. I heard about a method of creating a manoeuvre node at ground level and retrograding it all the way to zero, but that doesn't work very well as I never knew when to start the burn (right at and/or half way to the node doesn't work) so that was also quite inefficient. In the end I gave up on this method and decided to use the formula given in a thread I found to create an Excel document that gave me the altitude to start burning at. It's pretty close, not 100% accurate but not too bad!

What do you guys think?

Here is the link http://www.2shared.com/file/D05vVYaR/Suicide_Burn.html

[Simon Ross]

Have tried variations of the suicide burn, but it really is hit and miss a lot of the time. What does actually work is the staircase burn which Apollo used in real life. Certainly not the most fuel efficient way to land but does give you a lot of control

[chuangatronic]

My suicide burns usually end up stopping 100m above the ground because I chicken out. I think I've only managed two successful suicide burns

[Kasuha]

Suicide burns are dangerous and often inefficient. The problem with them is that when you're moving at 500 m/s at the beginning, you simply cannot start burning at exactly the right time so that it ends 1 m above the ground, you'd need to start burning at the right 1/500th of a second even if you knew when.

Using maneuver nodes helps, you need to start burning 100% - 70% of the burn time before the node. But the map view is often not very accurate and if you put your node below the real surface, you're screwed. I recommend using this method only when you need pinpoint landing.

For normal landing where you don't care too much where you land, use the "horizontal approach" - at apoapsis, bring your periapsis down, just high enough to not hit surface (you might want to check kerbalmaps for heights). Then at periapsis start braking and when your altitude starts decreasing, gradually raise your pitch to keep your vertical speed small (~10 m/s) while still killing your horizontal speed. When you kill enough of your horizontal speed, let the ship descend at reasonable rate.

This method is usually much more effective than your average suicide burn and only slightly less effective than absolutely perfect suicide burn. And unlike suicide burn, you can make it this way every time unless you are really careless about your vertical speed or height of hills you might run into.

[AmpsterMan]

Also, if you can get your ship to hover with no horizontal velocity, you can then use RCS to really pinpoint your landing site. It's quite fun to do actually

[maltesh]

One of the things you can get Mechjeb to show you is a Suicide Burn Timer, which gives you a numeric countdown to the last instant you can save your craft from impact by slamming full throttle.

[ZedNova]

I usually overengineer my rockets simply and have plenty of fuel for them so i can hover at 100m on the mun for a long time.

[angeldust]

One of the things you can get Mechjeb to show you is a Suicide Burn Timer, which gives you a numeric countdown to the last instant you can save your craft from impact by slamming full throttle.

Really? never seen it!

[toric5]

i usulay start my maneuver scuicide burn about 5-10 seconds before i need to. (exempt in tylo, then more than that.)

[Epthelyn]

Really? never seen it!

In landing guidance there'll be a "Time until Impact" display somewhere, unless I'm mixing up other mods or just imagining things.

[maltesh]

It's one of the Custom Window Menu Items.

And is included in the Landing Info Custom Window Package.

[flightmaster]

One of the things you can get Mechjeb to show you is a Suicide Burn Timer, which gives you a numeric countdown to the last instant you can save your craft from impact by slamming full throttle.

This doesn't tell you when to start burning though, only when you're about to hit the ground!

[zekes]

Hahaha what?

[togfox]

This doesn't tell you when to start burning though, only when you're about to hit the ground!

Im pretty sure it shows you the last possible point you need 100% thrust to avoid hitting the ground. If it shows a negative value youd better have springs for landing legs

Edited December 22, 2013 by togfox

[flightmaster]

Hmmm, I'll have to give this a go. I was basing my manoeuvre nodes on the time until impact given by Mechjeb, assuming that it was when I was going to hit the ground. Maybe that's why it never worked for me.

[MGCJerry]

Suicide burn? Oh, you mean just park your rocket nose first into the targeted celestial body and hit as hard as you can? Yup, done that a few times...

I usually just give a little throttle to slowly bring my speed down and land wherever as I'm doing my 'landing'. But I've only ever landed on Kerbin & Mun that way so I may have to resort to other techniques elsewhere...

[Superfluous J]

I come in fairly horizontal and aim to crash beyond my target a bit, and then eyeball it coming in when deciding where to burn.

Generally I come to a horizontal stop within a km or so of my target, maybe 1-2km above the ground, once I've gotten used to coming in to that location.

Once you've done it a couple times you get a feel for when to burn and if you're not doing it more than once or twice, getting it exact is probably not a big deal. And learning how to be exact in that situation is assuredly not important.

[John FX]

This is a good way to do it using not too much fuel. Video is pre-0.22 so make your Pe 6-7km instead of 2.4 (Highest mountain on Mun is 7km.) Most of your fuel use is to reduce your orbital velocity. It`s fairly efficient.

[Hotblack Desiato]

john fx, your video gave me an idea: how about using the landing wheels instead of landing legs. with those wheels it should be easily possible to land at 150m/s horizontal speed and then use the brakes to get the lander to 0m/s.

the only possible difficulty is: get that lander to descend at a few m/s (less is better), but it should be doable with actiongroups.

[Frostiken]

How do you 'create a maneuver node on the ground'?

[Kasuha]

john fx, your video gave me an idea

It's not his video, unless Kosmo-not is now posting under a pseudonym.

how about using the landing wheels instead of landing legs. with those wheels it should be easily possible to land at 150m/s horizontal speed and then use the brakes to get the lander to 0m/s

Your wheels will immediately break at 150 m/s and your ship will crash. Even when landing on wheels you need to bring your horizontal speed below 50 m/s and vertical speed below ~2 m/s. The difference to vertical landing is very small and landing legs provide better damping.

With normal landing (and current landscapes) you'll kill your horizontal speed at sufficient height to have enough control to land vertically.

How do you 'create a maneuver node on the ground'?

Create it anywhere on the trajectory and move it with mouse.

Edited December 22, 2013 by Kasuha

[MonkeyFuzz]

John FX, if I made that kind of landing, zipping along at 200m above the Mun's surface, it'd be the biggest nailbiter landing for me since my first Mun landing in 0.12. I usually burn off my orbital velocity between 4-10km.

I know it turns into a huge amount of gravity drag and a long drop to burn off fuel, but I don't trust the terrain to not have a mountain come up over the Munar horizon and smack me at 200m/s.

[Kasuha]

John FX, if I made that kind of landing, zipping along at 200m above the Mun's surface, it'd be the biggest nailbiter landing for me since my first Mun landing in 0.12. I usually burn off my orbital velocity between 4-10km.

I know it turns into a huge amount of gravity drag and a long drop to burn off fuel, but I don't trust the terrain to not have a mountain come up over the Munar horizon and smack me at 200m/s.

All it takes is to try it. It's actually rather easy landing and avoiding things that "jump up" from behind the horizon is not hard if you watch for them. And it only means long slide along the surface if your rocket has very low TWR.

What you see in the video is kinda extreme and it can be only used on old Mun surface. Now you need to start higher and you only need to keep your vertical speed relatively low, 10-30 m/s is usually ok unless you're running into something.

Very similar approach was used on real Moon landing, too.

[CobraA1]

Suicide burns need to be properly timed, which is very hard to do. I consider that something that you'd really need an addon like MechJeb for.

A "stairstep" approach where you bleed of speed every so often works. It's easy to build ships with enough fuel for this.

Scott Manley has an interesting approach - he approaches the body at a very shallow angle, so that most of the speed is horizontal rather than vertical. Then you don't need as many stairsteps and waste less fuel than a steep angle approach.

[maccollo]

Don't do suicide burns myself. They are merely the least inefficient way of doing an inherently inefficient maneuver.

The more you burn vertically the higher the delta V penalty from gravity losses.

*edit

Well... sometimes I do if the body I'm landing on has low very low gravity like minmus, however for mun gravity or higher surface scraping is always the preferred landing approach.

Edited December 22, 2013 by maccollo