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How to do precision landing?

Question

I wish to land near a supply depot that I landed before on the Mun. The problem is, no matter how I try, I land some 3 kilometers away from the target. How do I land within a 100m of the target?

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With an atmosphere everyting gets more complicated. But I assume you want to land near a mun/minmus base. A good simple method is to do this:

1. Go into a low orbit. The less excentric,  the better.

2. Use normal or antinormal burns to get your orbit to pass directly above your target.

3. Plan a retrograde maneuver node. Watch as its predicted path hits the ground.

4. Move the node, foward or backwards so that it lands directly above your target.

5. Execute? Well,  not yet. The tricky business is to counter the movement of your target,  since it is in the surface of a moving planet or moon. So place the end of the orbit beyond your target (how much is a matter of experience). Then execute the maneuver.

6. Watch as you descend, and monitor your path and your target's position, as both you move and the target moves foward on the ground. Then tweak it little by little with more short retro burns. Your target moves a little,  you burn a little and so on. This will work if you left room to maneuver ahead of you,  viz,  your end path lying further from your target (because burning retrograde will always shorten the distance of your orbit's end).

7. When you pass directly above your target,  point retro in relation to the SURFACE (and not 'orbit') and kill your speed completelly. You will then hover slowly towards your destination,  just control your descent speed. But only do this when close enough: do it prematurelly,  and the target will keep moving foward as you slowly fall towards the surface. 

Now, also consider this: 

1. You under-shot and now your orbit ends on the ground short of the target?  Burn RADIAL. It will throw your orbit end further,  without changing speeds.

2. Is your target a little to the side of your path?  Tweak with small  normal or antinormal burns.

3. I always build surface outposts as close to the equator as I can. It will make me worry with only one axis of displacement,  viz,  I will start from an equatorial orbit and only worry about how far/ near the target is. But  the farthest it is from the equator,  the more you have to worry about it displacing in two different axes in relation to your aproaching ship: further/closer, and to the sides,  forcing you to compensate for more variables.

4. Building outpost on the exact poles will eliminate all issues mentioned here. However those sites are impractical for other reasons,  above all,  the huge delta-v requirements to go into polar orbits in the first place. Also,  usually the higher the ground,  the easier to land on it, however the advantage quickly disapears as the site moves away from the equator. Yup,  equatorial bases are usually the best choice.

I'll do some pictures,  re-visit this post later on and they will be there.

Edited by Daniel Prates

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I could write a long, detailed explanation on how to land literally on top of your depot...

But the truth is, I learned from the master, Scott Manley... and he explains it much better than I can.

This is how I learned to land almost anywhere I want:

Oh, quick note, it's technically a rescue mission, but the same rules apply for what you need.

 

Edited by Just Jim

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For sheer laziness, mods like mechjeb and trajectories are pretty good at predicting your landing site.

Or I suppose you can kill all horizontal velocoty relative to the surface before suicide running your way into stuff.

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After a bit of experimentation, I settled on a method, that I have briefly described in this imgur album: 
https://imgur.com/a/zsMOp

I later discovered this method is very similar to the "reverse gravity turn landing" method described by GoSlash27 in this post (unfortunately, all the images are missing):

 

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^ what Psychogre said. The 'reverse gravity turn' is a great method for pinpoint landings while keeping a low DV budget. It works because you're under positive guidance and control during most of the approach, rather than in unguided free- fall. It makes it easy to set your lander precisely where you want it, even in an unrehearsed landing on a body you've never approached before.

I usually set 2x the landing DV when I plan on this method, but it usually turns out within 15% of the theoretical minimum. The additional reserve is necessary because you might want to alter the landing zone, which costs a lot of DV.

 

Best,
-Slashy

 

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15 hours ago, Xd the great said:

Or I suppose you can kill all horizontal velocoty relative to the surface before suicide running your way into stuff.

This is how the Apollo missions did precision landings. you lose some DV over a suicide burn, but especially on minmus it's a very small loss. I'd suggest practicing on Minmus. Come to a hover a kilometer up and then nudge your ship til you've over your target and drop down.

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2 hours ago, Tyko said:

This is how the Apollo missions did precision landings. you lose some DV over a suicide burn, but especially on minmus it's a very small loss. I'd suggest practicing on Minmus. Come to a hover a kilometer up and then nudge your ship til you've over your target and drop down.

I prefer to do it in orbit, but your method seems to save more delta v then mine.

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1 hour ago, Xd the great said:

I prefer to do it in orbit, but your method seems to save more delta v then mine.

They would both work, but yea...you want to slow down as close to the ground as possible because otherwise you're going to start accelerating due to gravity and have to spend more fuel to slow down a second time. This is the idea of a suicide burn. coming to a hover closer to the ground means you have less distance to accelerate again.

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A slightly simpler method might be to find the speed at which your craft "hovers" above the Mun; that is, the throttle amount at which its velocity relative to the surface does not change. Come in close as possible, and slowly "hop" your way over to the depot by increasing your throttle slightly and manually tilting your navball in its direction.

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21 hours ago, Daniel Prates said:

4. Building outpost on the exact poles will eliminate all issues mentioned here. However those sites are impractical for other reasons,  above all,  the huge delta-v requirements to go into polar orbits in the first place. Also,  usually the higher the ground,  the easier to land on it, however the advantage quickly disapears as the site moves away from the equator. Yup,  equatorial bases are usually the best choice.

Polar orbit are not Dv expensive, save for Kerbin polar orbit (by 100-200m/s), or more generally polar orbits achieved from the ground (and even then, it's much easier and somewhat cheaper than matching any orbit which does not go above site).

 

If you achieve polar orbit, by achieving equatorial orbit then doing a plan change, you are wasting a lot of fuel indeed (much worse if you achieve a low equatorial orbit). But the right way of achieveing polar orbit is by refining the encounter with the body you want to orbit, usually with a midcourse manoueuver (<10m/s than what you'd need otherwise) so that when you enter the SOI, you are already in a polar hyperbola. Then the cirularization burn cost pretty much the same as one for an equatorial orbit. And you can land anywhere on the body if you're patient enough.

 

Polar orbit are impractical for station though, as it is easy to achieve polar orbit, but it requires timing and patience to match one in particular (whereas there is only one equatorial plan).

Edited by Kesa

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6 hours ago, Kesa said:

Polar orbit are not Dv expensive, save for Kerbin polar orbit (by 100-200m/s), or more generally polar orbits achieved from the ground (and even then, it's much easier and somewhat cheaper than matching any orbit which does not go above site).

Interesting, I agree in some respects.

Expensive is what I consider entering the mun's SoI and then having to spend delta-v correcting it to a polar stance.  In my perception, a system with a mun station in equatorial orbit, and a mun outpost in an equatorial position, undoubtedly is cheaper to transit to/from then a station in polar orbit to a polar outpost, not to mention you also have to go between kerbin and said station, or perhaps even, directly from kerbin to the outpost. I think it would be mostly a matter of opinion/style what is the best approach, since 'colonyzing' the mun can be done in a variety of ways, be it with a supporting station in orbit or not. In my playstyle I try to do the 'budgeted' type of craft, to come to/from an orbital station, so saving delta-v anywhere I can is crucial, hence, my 'expensive' comment. I will agree though that KSP is somewhat permissive in that area and its not hard to design craft with a lot of surplus delta-v, as compared to what would probably be a delta-v budget in our real-life moon, for instance. Doing a polar-oriented station+outpost system is not impossible, neither particularly difficult. Just, not how I would do it. I fully agree with your data, btw.

Edited by Daniel Prates

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There's a lot of good advice here so all I can really add to this is encouragement and a bit of praise.  Really to hit within 3km or so is pretty damn good when you're starting out.  You're doing fine!.  I quite often land that far away too.

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1) Landing within 3 km  over a distance of 11400 km is actually superb.

2)  Landing with 100 meters consistently is probably not possible. I've managed to get my CEP down to about a quarter kilometer which is usually plenty good enough with the lunar tractors I've built. 

3) The technique I've employed is to first get myself into an orbit which takes me directly over my target. An orbit on the order of 100 km is ideal. You don't want to be too low as this makes it hard to burn off lateral velocity. Too high makes it expensive to burn off vertical velocity. Wait until you are approaching your target installation and burn retrograde as hard as you can. You want to come to a near complete stop relative to your target installation with the installation just very slightly ahead of you. Burn until your orbital predictor shows you will impact the mun just about a km or so short of your installation. Now remember that you cannot control lateral and vertical speed. You can only trade one for the other. This is why you needed to come as close as you could to a dead stop. As you burn  to kill your horizontal speed you will gain lateral speed relative to your installation and will therefore tend to begin to overshoot it. This is why you needed to apply some lead at the beginning. Now, depending on the performance characteristics of your lander, this next bit can vary but the principle is to fall as far as you dare until you get to the point where you have just enough thrust to burn continuously  until your vertical speed drops to about 5 m/sec within a couple of tens of meters of the surface. this will minimize the amount of time you spend picking up horizontal speed. If you've done all of this right you should land within a couple of hundred meters of your target.

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5 hours ago, Ironcladsix said:

2)  Landing with 100 meters consistently is probably not possible. I've managed to get my CEP down to about a quarter kilometer which is usually plenty good enough with the lunar tractors I've built.

Assuming you can target something, during your final approch, chase your retrograde vector to stay on top of the target away vector, you can land litterally land on top of your target, as shown in the video linked earlier in this thread.

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