Tips for landing

[Hello moto]

I have landed on the mun about five different times but I wasted a lot of fuel and I'm sure there is some more fuel saving way.

Does anyone have any tips on how to land on the mun easier?

[OtherDalfite]

Try to time out your burn so your velocity reaches around 0 m/s before you hit the ground. That, and try to start your burn low, going down at 10 m/s from 40,000 meters is very very wasteful of fuel. Also, try going from a low orbit (around 5000 meters or so). Also in your initial burn, burn at max throttle and do not go down unless you are at your last moments of touchdown, as using the engine at it's strongest setting increases the bang for your buck, so the fuel isn't wasted.

[Mr Shifty]

• The most efficient is a constant altitude burn, particularly for low TWR vehicles.
  

1. Lower your periapsis to near the surface -- safe altitude where you won't hit things, but as low as possible for the terrain you'll be flying over.
   
2. When you get near the surface, turn retrograde and put the engines on full burn, but you want to adjust your ship's attitude to keep your vertical velocity at 0. (Your altitude should not change.)
   
3. When your surface speed gets down to controllable speeds, start throttling down your engines. Adjust your ship's attitude to keep your slowing down horizontally and use throttle to control your rate of descent.
   

• Kosmo-not made a great video showing this a month ago or so.
  

[KerbMav]

Also in your initial burn, burn at max throttle and do not go down unless you are at your last moments of touchdown, as using the engine at it's strongest setting increases the bang for your buck, so the fuel isn't wasted.

That true? Never heard that before - or didnt listen/read attentively enough.

How would I calculate the time needed at full thrust to negate a certain velocity to zero?

Does velocity equal dV?

[blubbermonkeys]

If you happen to use mechjeb, try pointing retrograde and doing a suicide burn. you need to time it right but its my personal preference.

[JiWint]

It's simple.Burn at your retrograde ( circle with rotated +).Wait until the Surface speed (a little above nav-ball) is around 10m/s,then click ctrl to make your burn a little weaker,and when you're closer to the ground,burn at your retrograde to keep ~1 or 5m/s (depending on your landing legs (if you have some) )

[Sean Mirrsen]

The thing with saving fuel on landing is the same thing as with saving fuel on takeoff - the less time you spend fighting gravity, the better. Ideally, you must time your one full-thrust burn exactly so that your velocity hits zero a meter or two from the ground. That's ideally. In practice, you are unlikely to time it that well, so do a quick experiment and see how much acceleration you are capable of. 1G is around 10 m/s per second. If you can pull 1G, you will cancel 100m/s of velocity in 10 seconds, and travel around 600m down - meaning around 650m is a good time to start slowing down if your speed is 100m/s. So the exact point of where you start that burn is dependent on the exact rocket and trajectory, but it's usually best to start earlier than later. Just remember how fast you can slow down, and how soon you'll be hitting the ground, and try to not make the latter be sooner than the former.

[Mr Shifty]

Suicide (i.e. retrograde all the way down) burns are NOT the most efficient burn. There was an [thread=39390]extensive forum thread[/thread] debating this a few weeks back and it's confirmed. It's most apparent in low thrust vehicles, but true for all vehicles. The most efficient way to land is to use as little thrust as possible to fight gravity, i.e. keep your altitude constant during the deceleration.

[capi3101]

@KerbMav - you could determine the time with a third-order kinematic equation, assuming full thrust on your engines and accounting for the target's gravity, accounting for the rate of change of mass of the ship (i.e. the rate of jerk), and knowing both the starting and ending elevations. Oh, and your initial velocity; final (desired) velocity is zero, of course.

Yeah, there's a hell of a lot of math there. It's vector math too, I should add. At least it reduces to two dimensions...

[MrPopcup]

Just keep it easy and simple, no one masters landing for the first time, so:

Start by lowering periapses as low as you can then before reaching it start burning retrograde until your speed is controllable, then start killing your horizontal velocity. Then burn for a bit of time when you are almost close to the surface and keep your velocity below 30m/s Then throttle up and keep your velocity 1 m/s.

[SnappingTurtle]

My best way to land is to get a very shallow trajectory to the surface. Then, when you're almost over a good landing spot at about 15 to 20 km burn toward the horizon so you lose most of your lateral speed. I'm not an expert but I think that would remove the problem of losing efficiency by losing your descent speed too early. Remember to be in surface mode on the navball.

The method shown in the video might be okay for a pre 0.21 Mun but I wouldn't recommend it these days. Probably better to get a more accurate landing. I'm not convinced by that video anyway, the guy spends a long long time burning at greater than 45 degrees off the horizon, spending more fuel resisting gravity than slowing down for several minutes.

Edited August 30, 2013 by SnappingTurtle

[Mr Shifty]

The method shown in the video might be okay for a pre 0.21 Mun but I wouldn't recommend it these days. Probably better to get a more accurate landing. I'm not convinced by that video anyway, the guy spends a long long time burning at greater than 45 degrees off the horizon, spending more fuel resisting gravity than slowing down for several minutes.

The lander starts with a TWR of 1.09. You have to burn for a long long time to reduce your speed at that thrust no matter how you do it. If you use a suicide burn, you have to start burning at something like 29 km of altitude, otherwise you're lithobraking. The constant altitude method showcased in the video allows you to burn near the surface, minimizing the amount of thrust you're going to be using descending. He burns at that 45 degree angle specifically to counteract gravity and no more. If your altitude doesn't change, you're using just enough thrust to resist gravity. Likewise when you're taking off from a non-atmospheric body, you want to first gain enough altitude to keep from hitting mountains, then thrust horizontal as much as possible while maintaining your altitude constant: you'd rather gain speed then waste energy fighting gravity to gain altitude. It is far more efficient to gain altitude by burning prograde when you're at orbital speeds than by burning radially during a sub-orbital ascent. The constant altitude landing method is exactly the same thing in reverse.

[blizzy78]

You have to burn for a long long time to reduce your speed at that thrust no matter how you do it.

I find it difficult to estimate when to start burning. I'm pretty sure it must be possible to calculate that using the lander's TWR and the current velocity - how would I go about that?

[SnappingTurtle]

You're always going to have enough fuel or be able to add more. Unless you're aiming for really tight margins just estimate and feel free to err on the side of caution. As for calculating when to start burning I guess you could take the thrust of the engine and subtract the weight at the Mun's surface, divide by the mass to find your upward acceleration and divide by your speed to find how long it will take to stop. You could use that information in a suvat equation to get an altitude to start burning at.

@Mr. Shifty

I see, good explanation.

Edited August 30, 2013 by SnappingTurtle

[Kosmo-not]

It was a nice video; I liked the music. However, I think he should have explained more on how starting the burn at a low altitude minimizes gravity losses. That is obviously the key to the whole thing.

[Dunbaratu]

And of course the most important thing - keep your eyes on the navball and use IT to aim your craft. Looking up at the screen is sure to confuse the heck out of you as usually you're looking at the craft with controls reversed and watching the rotation on screen will mess with your head and make you fail. Just remember to point at the retrograde marker and as it drifts off aim toward to keep it centered. It will keep wanting to shift off center, as you are in an unstable equilibrium situation (like balancing a long stick on its end in your hand and constantly moving your hand around to keep it from falling over - landing on retro rockets is like that.). but don't look at the lander while you do it - look at the ball.