Catching asteroids

[Warzouz]

I'm thinking on a mission to catch and orbit an asteroid (C, D or E) around Kerbin to dig fuel out of it.

There are quite few tutorials but I'm not sure how to do it. Here is wnat I under stood

Asteroid

- Go to tracking station and choose one with the best peripasis and orientation with Kerbin (I failed to see the Kerbin SOI, how ?)

Ship

- Add a Claw

- Add lots of reaction wheels or RCS

- Pack lot of fuel depending on what to do with the asteroid.

- Pulling won't work (it'll push the asteroid back so no acceleration)

Get there

- Asteroid is on a hyperbolic trajectory inside Kerbin SOI, but on an elipitc in Kerbol SOI

- escape Kerbin SOI on the reverse course of the asteroid. How do you do that exactly ?

- Slow down to match its velocity. This seem a waist of fuel

Catch it

- Use the grapple grossly in the middle of it.

- When grappled, use the pivot to align the CoM

Change orbit

- Avoid Kerbin collision is easy, the sooner the deviation is, the less costly it is.

- Is it possible to aerobrake, which altitude do you recommend ?

- Is it reasonnable to change inclination of a C, D or E ?

Thx for the tips.

[Blu_C]

I'll try to answer as best I can, though it is really easier to understand after you have tried it a couple of times and watched a couple videos.

Now first big question is have you gotten to the point where you have no trouble with docking? IF not then STOP and learn how to dock first. This will teach you a bunch of skills like how to set up a rendezvous and move your ship around in a way that won't get you killed when in close proximity to the asteroid.

It sounds like your main problem is with getting there:

Get There

There are 2 ways to do this. The easier way is to probably leave Kerbin SOI and set up a rendezvous outside. This will both ultimately require less fuel (especially for the larger asteroids) and gives you more time to set up the encounter.

Do you already know how to get to another planet from Kerbin? If not then STOP and learn to do that. The process is much the same here. I'll be assuming you know how to reach another planet.

Set the asteroid as your target and then create a maneuver node inside Kerbin SOI, then begin playing with the values and node position to get the closest encounter you can manage. The closer the better.

Once you leave Kerbin SOI change your velocity to Target mode. You will notice that the prograde/retrograde indicators will change. They now show you the realitive motion between you and your target. The retrograde one is the most important when you are trying to slow down as burning in this direction will bring you to a stop in relation to the target (though this will likely begin to change again slowly as time goes by.

Your goal is to get the prograde direction to point directly at the target. The sooner you do this the less fuel you'll need to burn later as early course corrections result in larger changes over time than later corrections. To get the prograde marker pointed at the target point your nose at the target and take note of the direction the prograde marker is in. Now turn in the opposite direction about 45 degrees and make a burn. You should notice the marker slowly closes in on the target node, but it may be slow.

I like to keep an eye on the map view from here and watch the closest approach indicator. As you make this burn the closest approach should be getting smaller. If it starts getting larger than STOP and try doing small burns in different directions until you find one that begins making it smaller again (a good trick is to point at the target then do small burns at 90 degrees off left/right and up/down, and away). When you find a direction that starts narrowing the encounter distance keep burning.

Once you get it as low as you can time warp for a bit. You will notice that the prograde drifts away from the target most likely, this is because you aren't in the same orbit and it is ok.

After you have closed some distance (say, covered half the distance) repeat the above process. If you can get the encounter distance to under 1,000km (or less!) then you are close enough that you can warp to the closest approach and simply get the velocity vector pointed at the target and burn toward it.

Now, once you start getting close it is time to worry. I like to set my encounter to be within 1km, but not exactly on. This makes it so that if I bungle my breaking burn I don't slam into the target. Point your rocket retrograde and take note of how quickly you are closing with the target and how far away you are. This will give you an idea for how soon you need to start your breaking burn. If you keep pointed retrograde realitive to the target velocity then when the difference reaches near 0m/s you will be on approximately the same orbit as the asteroid.

Change Orbit:

Yes it is possible to aerobreak, I suggest not going too deep in the atmosphere. Certainly not below 40km, though I avoid going below 50km. If I think I need to slow down more I'll fire my engines near periapsis as well to take full advantage of of the increased efficiency. I like to be pretty conservative with my captures if possible (simply getting the orbit to within the mun's orbit) if possible and worry about getting it into the exact orbit I want until later. This makes building the rocket to capture it less costly.

[Warzouz]

Thanks a lot. I won't try very soon (I'm still building some reliant and cheap but heavy launchers).

Yes I know how to dock, even I'm not too good at rendez-vous (final approach is quite hard). A for transfert, there seems to be quite a difference with regular planetary transfert. Usually you change orbit to get you periaps (outer planets) or apoapsis (inner planets) at Kerbin location and get you apoapsis or perapsis at the target body on the other side of Kerbol. With asteroids, it's not the same, you usually can't wait half on aorbit to meet it.

As for aero capture so a "around 50km" should get a reasonable hyperbolic trajectory on an eliptic orbit under the moon SOI ?