Rendezvous in a "Strange" Orbit

[Enigma179]

So I sent a probe out to Duna, and messed up a bit because it ended up on a polar orbit (pretty much 90 degrees to the equator). This meant that when it came time to come home, despite having around 500 delta v to spare for the Hohhman transfer I couldn't get the right exit trajectory, and had to reach escape velocity and improvise from there instead. With the last rockets and empty fuel tanks ejected I had just enough monoprop to adjust the orbit to get home, and then set up a steep aerobrake before fixing into a stable orbit around Kerbin.

Now here's the problem: the original plan was to score the aerobrake, pull into equatorial orbit around Kerbin and from there dock with my space station, (equatorial eastward orbit at 105 km) to "deliver atmospheric samples" or whatever the probe was supposed to be doing there. I'm out of monoprop now though, so instead of that I'm sending up a recovery team to dock with the probe, then de-orbit instead. Thing is, the orbit around kerbin is hardly ideal;

It's travelling westward, with a descending node of -136.2 degrees relative to the space station (which itself is more or less equatorial). It has a periapsis of 428 kilometres and an apoapsis of 924.3 kilometres. The probe is simple enough, a stayputnik with a few antennae for decoration, some batteries and solar panels (running out of power isn't a problem, obviously), a single (now empty) 100 unit monoprop tank with 4 RCS thrusters placed around the centre of mass (well, the centre of mass when the tank was full) and a clampotron docking port (standard size).

I really really would like to collect it safely but my current attempts have been less than successful; I wait for the space centre to pass under a point of its orbit, then launch, but the difficulties lie in compensating for Kerbin's movement while trying to work out the right trajectory, while having a rocket with enough delta-v to carry up a team that can safely return and land home afterwards.

How would you guys go about it? I'm no stranger to orbital rendezvous, having erected that station in 5 launches (including the hellish placement of a full jumbo 64 tank last) prior to the Duna mission, but it's getting into the right orbital inclination that's my issue. I don't know how useful it would be due to the orbit being in an opposite direction, but the station has a full Jumbo 64 Tank attached, so one could go up and dock with it to refuel and pull out a lot of extra delta-v if that's something that could be considered.

[stupid_chris]

Hmm, you could send a craft from the launchpad to a westward orbit. Then you could simply plan a burn at the AN or the DN and put yourself on the same inclination as the probe. Then since you simply rendez-vous with it as you normally would on an equatorial orbit. Docking is gonna be touchy as you're gonna try docking something small with something rather large, which isn't what usually happens, so you'll want to be extra careful on your approach speed as you could easily bump it off. Then you have too choices: either you return with it on Kerbin, either you dock to your station. Returning to Kerbin is simple, just deorbit. For your station it will be harder. The "easiest" way to do it would be to burn at the DN or AN to change your inclination, it might take two burns since you have a 136 degree difference. Then you could rendez-vous with your station and dock as usual. Shooting straight from the launchpad to the same orbit as your probe is doable, but very hard, so I do not reccommend if you go with stock. MechJeb could do it though.

Anyway, good luck!

[b0b349]

You sound like you know what you are doing more then I do. The only thing I can think of is to make an orbit at roughly the same angle, so that you are parallel to the station, then aim your rocket westward near kerbin and eastward on the other side. So that you rotate your orbit around the earth until it is lined up. Other then that I got nothing.

[Reavermyst]

This would be tricky to accomplish but doable! The main trick is to have plenty of Delta-v available for both your launch stage(s) and your orbital stage(s) to put your craft on a similar orbital direction and to match the inclination. This means turn Westward and fight gravity on your way into orbit which will take a lot more Delta-v than your typical eastward launch. If you're up for the challenge, you could try to match inclination during your launch, but in my honest opinion, it'll be simpler to pop a maneuver node down and do it in orbit. It takes a lot more Delta-v to do so, but it's easier to accomplish on a mental standpoint. You don't need to worry about the eccentric orbit of your target, in fact, that makes it even easier! All you'd have to do is circularize your orbit on the same inclination(preferably in a much lower orbit), then pop up a maneuver node to figure out when the object will be at or near it's periapsis in order to conserve fuel for your transfer. From there, it should be the same old rendezvous scenario!

As Chris mentioned, don't be afraid to get mechjeb to make these maneuver nodes for you. It'll save you lots of time and even help you determine where the Ascending and Descending nodes are really located in your orbit which is extremely important in determining the correct time and direction of your inclination burn. Shoot, if you just want the issue resolved regardless of HOW it gets done, you can let the auto-pilot do all the hard parts! But that's entirely your call.

(Note: my sense of direction may be terrible, as I can't tell if the red line on the navball means north or south. If The red line is north, then flip my directions around! I got it right the first time, then ignore this!)

[purpletarget]

This is pretty straightforward if all you're going to do is go up, dock and de-orbit.

Just wait till KSC is about 1-2 minutes from passing under the An or Dn before launching, and use a a north-westerly heading if it's the An, and South-westerly if it's the Dn. The math for the heading is pretty simple, if you take a look here: http://forum.kerbalspaceprogram.com/showthread.php/24623-Tutorial-Video-Krash-Test-Kerbals-How-to-science%21?p=387855&viewfull=1#post387855

Just in head cheese math, I'm thinking an azimuth heading around 320 for northerly, and 220 for southerly. You'll only need about 300-400m/s additional delta-V compared to a regular easterly launch, and probably another 2-300 after your RV to push the probe Pe down into atmo...aerobraking can do the rest.

[Reavermyst]

This is pretty straightforward if all you're going to do is go up, dock and de-orbit.

Just wait till KSC is about 1-2 minutes from passing under the An or Dn before launching, and use a a north-westerly heading if it's the An, and South-westerly if it's the Dn. The math for the heading is pretty simple, if you take a look here: http://forum.kerbalspaceprogram.com/showthread.php/24623-Tutorial-Video-Krash-Test-Kerbals-How-to-science%21?p=387855&viewfull=1#post387855

Just in head cheese math, I'm thinking an azimuth heading around 320 for northerly, and 220 for southerly. You'll only need about 300-400m/s additional delta-V compared to a regular easterly launch, and probably another 2-300 after your RV to push the probe Pe down into atmo...aerobraking can do the rest.

Definitely a ways of approaching it if you understand how to apply your mathematics in KSP, but I suppose your videos would assist in that if he really wants to go the no-mod route! And if he does so, I'll be pretty impressed! Well, not that I'm easily impressed enough by the very notion of folks being able to do these things with math. I can barely opperate a calculator to convert the distance between Alpha Centari and Sol down to Kerbal standards of measurement!

Edited June 3, 2013 by Reavermyst

[Uberick]

This isn't too bad. If you already know how to rendezvous and dock, you've got the toughest part down. Side note for changing inclination. . . it costs less DV the farther out you are, for extreme inclination changes adjusting the eccentricity of your orbit can make it cost less fuel.

Now then, to rendezvous! Firstly, you want to wait until the target orbit is overhead. In this case I've decided to burn south, southwest to catch up with it. (I used 5300 d/v total for this, including the launch(which is about 4800 of it) make sure to bring excess if you want to deorbit or meet up with your station)

Once you hit 10km, do your gravity turn. Try to aim along the compass to where you the orbit looks like it sits. For this one, I've guesstimated that it's south, southwest(obv). Since 270 is west, and 180 is south, I split the difference and go with 225 as my heading.

Set up a phasing orbit just on the inside or outside(depending on if you are ahead or behind the target) of the target's orbit. A phasing orbit is just a slightly different orbit that will intersect the target's orbit at one or two points(or get close enough for target mode). Since this orbit is "inside" the target orbit, we will be circling the planet faster, and will gain a little bit of distance with every orbit. Watch the closest intersect nodes. As they get closer(you should get a feel for how much you are gaining every orbit) slowly expand the size of your phasing orbit so the increment of distance that you "catch up" to the target orbit gets smaller and smaller. Once you get a node that's reasonably close(within 50km ish), create a maneuver node at your periapsis and add delta v until the intersects get close enough for a rendevous.(In this case, about 2 km) Since we have made our periapsis close to the target's periapsis, we can make adjustments there without throwing ourselves into a wild orbit and risk missing the target.

I used probes for this so they don't have docking ports, but that's close enough:) Now you have your probe samples!

Edited June 3, 2013 by Uberick

[Enigma179]

Thanks for all the help guys! Fired up KSP (I don't use Mechjeb, personally, HAL demonstrated we can't trust dem machines) and tried again following some of the advice, and managed to rendezvous, dock and de-orbit without a hitch! I'm about a quarter of the world away from kerbal space centre but precision landing can be another lesson for another day.