Precision landing

[G'th]

So lets say hypothetically, I have a command pod I want to land a specific point back on Kerbin. And I want to use a ~30km periapsis to produce a safe reentry.

Now, the question is, at what point in my orbit do I lower my periapsis (Let's presume the orbit is more or less equatorial for simplicities sake) so that when I reenter the atmosphere I will land within a particular zone I designate beforehand?

I would imagine the answer requires some math or something, but that is beyond my ability. My general "throwing things and seeing what sticks" method has thus far proven ineffective as I have been unable to pinpoint where to do my burn so that my eventual landing is where I want it.

My first thought was to create a glider out of my command pod, but this did not prove wise. (though in the name of safety I of course still kept chutes so Jeb did not die in that experiment)

[RSwordsman]

I've had quite the bit of luck with using maneuver nodes and tracking the spot where the planned trajectory (orange dotted line) intersects the planet. It's wise to overshoot your landing zone a bit to account for drag on atmospheric planets. When you get closer and air resistance is introduced, then you adjust your path basically by the seat of your pants (short burns between straight up and retrograde while watching the map view). I use this method to plunk capsules down pretty much right off the beach at KSC.

Happy landings.

Edited January 5, 2014 by RSwordsman

[Superfluous J]

You COULD calculate it, probably, but there are at least 4 factors and I wouldn't be surprised if calculus was involved. Best to hit F5, re-enter, and then hit F9 to adjust your trajectory to shoot longer or shorter.

(drag of craft, angle of entry, speed of entry, and height of the ground you eventually land at)

[Duke23]

About to start working on this myself. I pretty much have to just pick a general area "over there a little before the blue line ends". I think if you play around with it enough it'll become easier just like everything else we used to botch every time in this game. lol

[nhnifong]

NASA put wings on their ship so they could weasel out of the problem. So do I.

[TechnicalK3rbal]

Why technique (DR and FAR) put the peri apsis about 10 degrees ahead of ksc. Usually works. put it 30 degrees for pods.

[I_Killed_Jeb]

There's a table and graph floating around here for this purpose

[katateochi]

Maths! no, no you don't need to invoke that ritual. Personally I'd go with what 5thHorseman said and do some trial and error, after a while you'll develop a mystic sixth sense for when to de-orbit to hit your target. Also what nhnifong says about adding some wings (if the craft design allows it) so you have some control once in the atmo can help quite a bit.

edit - almost didn't write this, cos well, haters gonna hate; but MechJeb is a good teacher, just don't rely on it.

Edited January 5, 2014 by katateochi

[Plur303]

As other's have said, you just need to practice and get a feel for it. It all depends on what kind of orbit your are de-orbiting from. Saying "make a PE of 30,000 meters here" doesn't make sense because it depends on your velocities and orbit characteristics that you are coming from. Deorbiting from an orbit with a AP of 100,000 meters and an orbit of 250,000 meters will have different outcomes even if they both have a PE of 30,000 meters in the same spot.

[Captain Sierra]

Well, after talking to friends with aerospace degrees and getting them to explain the maths to me, it's a lot of calculus. I know calculus, and that sh--, 'stuff' is over my head. Avoid the math, trial and error. Assuming you are using DRE (your scenario sounds that way) that's the best bet. No DRE, do a full retro burn killing all your orbital velocity right over the target. You'll fall like a rock to your destination, for me, accurate to within a kilometer.

[FlamedSteak]

I'm still struggling with this. I can usually nail a target within a few kilometers on Duna, but on Kerbin I'm always over or undershoot by a good 300-1000 km.

[Mmmmyum]

If you get the maths from somewhere do please tell me, FAR provides all the necessary information but calculus is involved. Which I cannot do yet

[SpaceSphereOfDeath]

If you get the maths from somewhere do please tell me, FAR provides all the necessary information but calculus is involved. Which I cannot do yet

If you want I could caculate it for you.

On topic: I usally go with scott manley's technique, 20km periapisis over the large peninsula to the east of the KSC. For pods I find it works very well

[rkman]

I'm still struggling with this.

Even NASA struggles with it.

Satellite GOCE Crashes Back To Earth

November 10 2013

http://www.ibtimes.com/update-satellite-goce-crashes-back-earth-sunday-better-chance-winning-lotto-then-getting-hit-debris

"Update 4 p.m. EST: CNN reports the satellite is expected to make its re-entry around 7 p.m. EST, but the exact crash location is still unknown."

[cypher_00]

Hah there is a Kerbal reference in that article:

The satellite created a "high-resolution map of the boundary between Earth’s crust and mantle – called the Moho,"

[zarniw00p]

Mechjeb displays the predicted landing zone on planets with atmosphere pretty well. You can still work with maneuver-nodes to plan your reentry and you don't have to use autoland to use this feature. Just click "Show landing predictions" in the "Landing Guidance" tab. It will display your predicted landing zone as a blue marker on the map view. It also can be used to tell your predicted apoapsis after aerobraking.

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-23-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-1-1

[RickRastardly]

Hah there is a Kerbal reference in that article:

The satellite created a "high-resolution map of the boundary between Earth’s crust and mantle – called the Moho,"

Noticed that to, bit of a coincidence.

[Scarecrow88]

If you want to work with a 30km periapsis as the basis of your de-orbit, try a few tests, starting with it over your desired landing point. If you land long, move the periapsis west of your target; if you land short move it east. The amount you land long or short by should give you an indication of how far you need to move it. Shouldn't take too many attempts to get it close, and if you use a quicksaved position before you start, it shouldn't take to long to do.

Bear in mind that the position your periapsis ends up in relation to your target will only work for subsequent landings if the height of your orbit is the same.

Edited January 5, 2014 by Scarecrow88

[magnemoe]

Mechjeb displays the predicted landing zone on planets with atmosphere pretty well. You can still work with maneuver-nodes to plan your reentry and you don't have to use autoland to use this feature. Just click "Show landing predictions" in the "Landing Guidance" tab. It will display your predicted landing zone as a blue marker on the map view. It also can be used to tell your predicted apoapsis after aerobraking.

http://forum.kerbalspaceprogram.com/threads/12384-PART-0-23-Anatid-Robotics-MuMech-MechJeb-Autopilot-v2-1-1

Yes it works pretty well however it work better if you come in from an higher orbit as drag works over shorter time.

Manual landing with mechjeb indicators uses far less fuel but is less accurate if you just do an 20 m/s burn to get an aerobrake.