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Precise landings


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I have no problems with orbital maneuvers, but I'm now looking to put a fueling base on Minmus. In order to do this I'm going to need to be able to land at an exact spot over and over.

How does one get an exact landing where desired? Maneuver nodes are inadequate because they don't take into account the deceleration that has to be done while descending, changing the flight path; they also don't take into account rotation of the body. I suppose part of this could be mitigated by doing all of the deceleration/landing burn in one go just before landing, but this requires a higher thrust engine than you'd otherwise need and still risks you smacking into the object at high velocity.

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but this requires a higher thrust engine than you'd otherwise need and still risks you smacking into the object at high velocity.

Maybe. This could be a problem, but the way I solve this problem is I don't do the final "lineup" burn at the target, but some altitude above the target. This is slightly wasteful, but it mitigates this problem.

You also have a couple advantages given your problem setting: your fuel shuttle, whatever it is, is presumably landing mostly empty, which means your turn and thrust capability is as good as it's ever going to be. If you can get to Minmus orbit if your tanker is full (an obvious design requirement), you must have reasonably sizable Minmus TWR when empty, right? At least this would be my expectation. (So for example, even though when full my VTOL rover has a meagre ~1 m/s acceleration, when almost empty it's about 8 m/s, which is pretty good for a Minmus landing even when I'm being careless.)

Here's how I solve the same problem, for my fleet of Minmus fuel shuttles:

I first obviously designed something really easy to land, and dock. So for example, my fuel shuffle is basically a giant VTOL rover, with well balanced downward pointing engines, and a probe core in the exact center pointing up, a port on one end, and a klaw on the other. It looks downright stupid doing transfer burns (see here), but it's also very easy to land.

If I start my descent 10 minutes before impact, that means that the point of impact is going to be slipping about 5.5 kms on me (if at equator), which is about five-ish degrees. I eyeball it, naturally erring towards overshooting by a bit.

The navball makes very clear to you when you're about to be right over the target -- at this point to be clear I am still high over the target (perhaps as much as 500 meters or so, hopefully a bit less), but once I get lined up vertically pretty much all I have to do is drop, which makes things much, much easier -- this is wasteful of a few dozen m/s deltaV or so, but then again, I'm about to refuel with the "free fuel" anyway, and my safety margins aren't that tight. Again, because the fuel shuttle is empty in your problem setting, killing horizontal velocity should require rather a relatively short burn, so you should have plenty of warning so you can "time" it right. (Versus the lift back into orbit when full. :) But there timing doesn't really matter at all.)

Then of course the vertical landing is very easy. I usually land within about 100m or so. (I obviously try to "miss" a bit so I don't destroy everything.) Then I line up and dock. See here.

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The good news is that Minmus is very forgiving, since the gravity is so low and velocities correspondingly slow.

I generally handle precise landings thus:

1. Do the retro-burn so that I'm descending at a reasonable angle towards the target

2. Make it so that my projected point of impact is a bit beyond the target (i.e., I'm going to overshoot)

3. Put the navball in surface-relative mode and orient myself so that I'm pointing pretty much surface-retrograde

4. Watch the "anti-target" marker slide towards the zenith (that's the point at which I'm directly above it)

5. Rotate to point "horizontally retrograde" and do a burn to kill my horizontal velocity (i.e. so that my surface-relative retrograde marker is at the zenith)

6. Now I'm just dropping vertically towards the target, so it's just a matter of landing as per usual.

I realize it's not completely ideal... but dV around Minmus is so small that any minor inefficiencies don't matter much, and it's really simple and easy to do. And even if it's not perfect, it's not flagrantly wasteful, either-- works pretty well even when the velocities are higher, such as on the Mun.

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Nav ball and nodes all you need.

1. Establish low orbit (15km Mün)

2. Burn retro until path ends on target (about 1/8th of orbit away)

3. At lower 1/3 of trajectory, burn retro and antil-radial to establish parabolic arc ending just past target.

4. Over target kill all horizontal v (use Nav ball surface mode)

5. Use Nav ball to kill most of vertical v, say to 30m/sec or so

6. Use Nav ball target mode to fine tune

7. Set down 200m away with nice soft landing

That's the way I do it at least. Obviously adjust trajectory when doing node burns with normal/anti-normal vectors so path is crossing through map target icon. Don't be so aggressive the first time, use a few parabolic burns.

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This is how I learned... Scott Manley's how to make a precision landing. It's technically for a surface rescue, but the technique works for any precision landing.

Once you get the hang of it, you can land anywhere you want.

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Edited by Just Jim
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How does one get an exact landing where desired?

Put rover wheels on your lander, that way even if you're off by a km or so, you can easily drive up to your base.

Minmus is so lo-g that most engines will have enough impulse to make due, and some will probably manage on RCS alone.

As to easy soft landings, put your navball reference to "surface" and have SAS point to retrograde, then you can easily and efficiently throttle your way down (Kerbal Engineer has a "suicide burn" counter that tells you exactly when to burn (as in meters left to the point where you need to start your burn), I often use that as an indicator).

Edit: or use one of the other excellent suggestions mentioned above.

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