Precision Landing Technique

[Wallygator]

UPDATE: Made my first attempt at this tonight. Re-read all the suggestions below. Selected Minmus as my beginner zone due to low gravity and close proximity to Kerbin (!)

1) put my ship in a low 5k orbit.

2) adjusted inclination to assure I was passing directly over target vessel (which was previously landed for this purpose)

3) I waited until the target was well past the horizon during approach (no sense in burning such that I would pass through the ground)

4) I force switched to target mode

5) During the descent I burned in various degrees around my retrograde indicator to keep it centred on the target indicator

6) Simultaneously, I monitored my descent rate to assure I was within my lander deceleration envelope

7) By gently pulsing my burns as needed, and keeping the vectors aligned, I was able to get within 100m surface distance on my first try.

I'm pretty sure that with just a little more practice I can crash my landers on top of one another (not that I would try that). Thanks a lot to all for the really great help!

Cheers,

W

-----------------------------------

Thanks Guys - I appreciate all the input so far. I consider this answered for now. I'll repost my experience and what techniques which you have all recommend seem to work for me.

More later...

--------------------------------

Hey Guys,

Not taken the time to try this, as I have not yet had the need to "precision land" anything (that is, other than precisely on the "proper end" softly on the target planet). So... the question/scenario...

You have selected a piece of equipment you have previously landed on the surface as you're target. While in descent toward your target (let this be a base in this example, not that it matters) and with your navball in surface mode, is it correct to assume that if you keep your retrograde marker positioned on the target marker you will land on/very near the target? Obviously paying attention to your descent rate as you want to arrive in one piece ;-)

I'll give this a go tonight - just checking up front. Also, if there are any other techniques I'm unaware of, then that would be great.

Thanks in advance!

W.

Edited May 22, 2014 by Wallygator
UPDATED: It Worked!

[Hodo]

It depends on the planet I am landing on. If it has an atmosphere I can land pretty much where ever I want because most of my craft are space plane designs with VTOL abilities. On airless moons it is a bit more challenging but only because my craft have a much larger footprint than a typical lander.

[Wallygator]

Ah... So to be more specific I'm inquiring about powered non-space-plane landings sans parachutes.

Sorry, thought my OP was not specific I guess.

Hopefully I shall also one day get to the concept of wings...

[KerbMav]

This is basically it, the easiest way to use this method would be to put your orbit above your target, null all your relative (mind the rotation of the planet) horizontal velocity and just drop directly from above. The shallower your coming in the more you will have to take care of not crashing into hills or even the ground if your flight vector is really flat!

Combining this I tend to aim a bit behind my target and cancel my horizontal velocity when I am above my target but already quite low above the ground, depending on the gravity of the planet/how quickly my vessel will accelerate vertically.

[Kasuha]

is it correct to assume that if you keep your retrograde marker positioned on the target marker you will land on/very near the target?

I was trying it and found the approach very ineffective because if your retrograde marker is on your target, you're going to undershoot so you have to "hover" for extended period of time to get there.

Similarly I tried Target mode and keeping the prograde marker on/near my target, it also works somewhat but not very comfortable and you can crash rather easily.

The most comfortable way of doing it I found is using map view and keeping the terrain intersect on/near the target.

Enough TWR and fuel is recommended.

Scott Manley has a

on the subject.

[AlexinTokyo]

Scott Manley (unsurprisingly) has an excellent tutorial video for precision landing (rescue) ships on non-atmospheric bodies.

I can't post the link, because I'm on my work network, so no YouTube, but it shouldn't be hard to find.

Basically it involves using one of the motion equations to get the horizontal distance to target, and then using Pythagoras to calculate the straight-line distance, so the distance-to-target reading can tell you when to start you burn. It's not the most efficient method, but it works well.

Regarding your method, if you keep your retrograde marker on the retrograde-target marker, you will hit the target. If your target is stationary on the surface, then I think this should be true in both surface and target navball modes (but not orbit mode).

Depending on the initial conditions, though, I can imagine that you might have too much horizontal velocity to land at the right time. As long as you keep an eye on the other flight parameters (esp. altitude AGL), though, you should be fine.

EDIT: Ninjad. Twice.

[Wallygator]

Thanks kerbmav! Perfect. Answered.

[allmhuran]

Yes, it does mean you will impact the surface very close to your target, as long as there are no mountains or anything in the way.

However, this doesn't necessarily imply that you will perform a safe landing near the target, because just keeping your retrograde marker on top of target marker might mean you are coming in with a very high horizontal velocity at relatively low altitude, which is almost certainly going to result in explosions.

Generally if I am trying to make a point landing, I begin a deorbit burn approximately opposite my target landing position to bring my periapsis down until it is positioned directly above my target at about 5000m altitude. Then as I swing around further I burn again to put my trajectory into the ground "just past" the target. How far past is "just past"? Well, this is something best learned from experience, and which depends very heavily upon the thrust to weight ratio of your vessel. For a ship with very high thrust to weight, it's close to the target. For a ship with low thrust to weight, it's much further past the target.

At that point I coast in for as long as possible. The most efficient way to land is to fire your engines at 100% thrust at exactly the right time to ensure that you hit exactly 0m/s at exactly the same time as you reach the surface. Obviously nobody is actually going to be able to do this, but that's the goal you're shooting for.

When I am "approaching" the target position I burn hard retrograde, which will decrease my horizontal velocity, pulling my intersection point with the ground back towards the target, as well as decreasing my vertical velocity. At this point it is always better to be too high than too low: if you're too high you can pitch your nose down towards the horizon and burn to kill off horizontal velocity, then coast along and drop vertically onto the target position (which is inefficient, but extremely accurate). If you're too low, you'll either overshoot (because you'll have to stay aloft longer than you want to in order to be able to kill off all of your horizontal velocity in time) or you'll crash into the terrain at a high horizontal speed (because you tried to kill off all your horizontal velocity but didn't have time to do so).

I normally aim to come in vertically for about the last 100m or so of my descent. Less is more efficient, more is safer.

Edit: Ninja'd! But this is a bit more detailed.

[Wallygator]

KerbMav, also thanks!

[Laie]

The best way I figured out myself (which is far from perfect): plant one, two, three manoever nodes (as few or many as needed) to get a nice path to the desired spot. The ideal path looks like a launch in reverse. Leave a little wiggle room: you will eventually land a little short of the spot you're aiming for, while the Mun (or whatever) keeps turning below you. I do not know how to calculate that offset, but after two or three tries you get a good feeling for it.

[michaelhester07]

What i've been doing recently is to lower my orbit when near the target to "fairly low" (as low as it gets before I start crashing into mountains) and aim it so I pass over the intended target. Then when the target is almost directly below me I hit the de-orbit burn and line up the retrograde marker to the target.

That technique you described works well assuming you're directly over your target. It doesn't work if you're angled off because the planet/moon's gravity will pull you down. You'll fall short then.

The target retrograde marker wants to drift away from your retrograde marker so if you put your marker on the opposite side of zenith and keep it there till the target retro is on the zenith then you can land within 100 meters of your target.

I have yet to prefect an atmospheric targeted landing like that though.

[KerbMav]

I have yet to prefect an atmospheric targeted landing like that though.

I only ever managed these with correction burns inside the atmosphere.

[Rickenbacker]

I usually eyeball it, but when I do use maneuver nodes, I try to set up my orbit to pass very low over my target. I then set up a node directly over the target, that zeroes out all my horizontal velocity. Then I just start my burn juuuust after the estimated burn time (I.e. I don't wait until I'm halfway there like I normally do for orbital changes and what not). That way I end up stationary over the target and only have to drop a short distance and land.

[Tank Buddy]

Scott Manley has your answer:

[Wallygator]

Hello All, Posted an update in the OP - Thanks again for the help!

Cheers,

W