Help with rendezvousing in high orbits

[4747]

I have a contract to put a part on a satellite in a high Kerbin orbit (>GEO) and am looking for advice on how to rendezvous. Last time I did something like this, I couldn't get within less than 40km of the target, so I ended up just burning toward it, which worked because it was just a crew rescue contract and I could eva the kerbal over to the rescue ship, but this definitely won't work for this. I have docked a few times in low orbits, but is there any different techniques for higher ones?

Also: is there a way to increase the orbit line resolution?

[Streetwind]

Rendezvous are a multi-step process, and it's hard to figure out how to help you if your description of what went wrong isn't a bit more detailed.

Generally, it involves five steps: (1) get into a similar orbit, (2) match inclination, (3) match the orbit, (4) phase the orbit, (5) match the target. However, the order you perform these steps in isn't set in stone. Even "get into a similar orbit" doesn't necessarily have to come first, despite what it might sound (but to be fair, in most cases it does come first). You make the call what you want to do next. It is not uncommon to do some steps more than once, edging a little closer each time.

As an example, let's take a rescue contract which is in an 80km by 90km low Kerbin orbit with zero inclination. If I were to do this rendezvous, I would...

(1) ...start by launching into a similar, but intentionally not too similar orbit. Something like 75km by 75km. The more the semimajor axis (think average altitude) of my orbit is different from that of the target, the quicker it is to phase the orbit, but the lower the precision will be. I will typically time this launch so that the target spacecraft is directly overhead the launch site, so that I insert into orbit slightly behind it, but it's not a necessity - it just makes the process faster.

(2) I'll select the marooned spacecraft as my target, so I get an AN/DN marker. I'll make a maneuver node there with a plane change to cancel out any minute deviations from 0° inclination that I may have picked up during launch. Even a tiny inclination mismatch can screw with some of the later steps.

(3) Then I will look at where the target spacecraft is along its orbital path, relative to where I am. Because I am in a lower orbit, I want to be behind the target (hence my launch timing in step 1). Lower orbits are faster, so I will be catching up to my target from behind. Now a judgement call must be made: am I close enough behind? Not close enough? Too close? The answer depends not just on the current position, but also on the difference in orbits. The greater the difference in SMA, the faster I will catch up with the target on each orbit around Kerbin, so if I am really close to the target but the orbits are too dissimilar, I will overshoot. It is best to err on the side of not being close enough, both in your launch timing, and in selecting how close you make your approach orbit to that of the target.

(3a) To help this judgement call, I will make a maneuver node somewhere ahead of me, and give it enough dV to cross the target orbit. This will display a close approach marker, and also fill in numbers in the rendezvous tab of the flight data display in the lower left. I will then select the maneuver node and toggle it one orbit ahead into the future. The close approach marker and data will change. If the marker jumped ahead of the intersect, I am too close, and must either force a rendezvous immediately, or pass the target and wait a week or two to come up behind it again, or pass the target and then assume an orbit higher than the target so that it will begin catching up to me instead and I can do the rendezvous from the other side. If the marker is still behind the intersect, I will push the node another orbit into the future.

(3b) The goal is to get myself into a position where the maneuver node shows a close approach marker more or less on top of the point where the orbits intersect at some point in the future. There will typically be two such points, and having a close approach at either one is fine. This whole process of one spacecraft catching up with the other is called "orbit phasing", and its purpose is to get both spacecraft to arrive at the same point in space at the same time. It doesn't matter how well I match the target orbit if I am not phased correctly and thus will never be in the same place as the target at the same time.

(4) Most often, I will not get a close approach marker on top of an intersect, no matter what I do. I just blindly picked my initial orbit, after all, so there's no guarantee that it is suitable for rendezvous. If this happens, I must match orbits better. I will typically phase the orbit until I am quite close behind and in danger of overshooting soon. Then, I will make a maneuver node next to the target orbit's periapsis, and make a burn to roughly match apoapsis. My orbit will turn from a 75 by 75 km orbit into something like a 75 by 89 km orbit (the target being in an 80 by 90 km one as you recall). Because I am doing this burn next to my target's periapsis, this means I am not just matching SMA more closely, but my eccentricity will also align with the target's eccentricity (the so-called argument of periapsis will match).

(4a) After this burn completes, I will once again make a maneuver node, have it cross the target orbit, and see if I get a close encounter near the intersect. As my orbit still differs slightly, I will still phase forward ever so slowly with each revolution around the planet, but the steps will be really small now. I will find the point in the future where the close approach jumps from behind the intersect to in front, and then I will click and drag the maneuver node around the orbit and observe how the close approach changes. Typically I can find a close approach of under 300 meters like this. I may also occasionally find that my intersect disappears completely while I do this, because dragging the node to a different part of the orbit means it no longer has enough dV to cross orbits with the target from that position. In which case I add more dV and then repeat the process. This is also the part where inclination mismatch really interferes, so hopefully I did my step 2 properly.

(4b) Once I have my close approach of under a few hundred meters, it's just a matter of time to wait for the node to come up and execute the burn. I will typically turn my engine power down really low for this (by rightclicking on the part), and carefully use the throttle so that the burn takes three to five seconds despite expending only 1-2 m/s worth of dV. This greatly helps with precision. I will look at the rendezvous data display in the lower left to see my close approach change in real time as I burn. Often I can get even closer than the maneuver node suggested. I've managed to get an impact trajectory more than once!

(5) And then I just wait for half an orbit until I come up right next to the target, and light my engines one last time to match it and remain stationary.

 

Edited January 23, 2023 by Streetwind
Typos, typos everywhere!

[king of nowhere]

15 hours ago, 4747 said:

I have a contract to put a part on a satellite in a high Kerbin orbit (>GEO) and am looking for advice on how to rendezvous. Last time I did something like this, I couldn't get within less than 40km of the target, so I ended up just burning toward it, which worked because it was just a crew rescue contract and I could eva the kerbal over to the rescue ship, but this definitely won't work for this. I have docked a few times in low orbits, but is there any different techniques for higher ones?

Also: is there a way to increase the orbit line resolution?

why couldn't you get closer than 40 km? what is your problem exactly? how do you try to perform rendez-vous?

rendez-vous on high orbit is no more difficult than on low orbit. I find it even easier, because it's cheap to maneuver and this gives you additional options.

anyway, if you get to 40 km distance and then you burn towards the target, it works. you can get a rendez-vous that way. why do you think it won't work this time?

[Kryxal]

My usual method is to match inclinations, change to an orbit that will intersect the other orbit at a near tangent, then fine-tune from there.  When I go from trailing to leading on the next orbit (or vice versa), that's the time to adjust this orbit to get an intercept.  This can generally be done with just prograde or retrograde inputs near the intercept point, and maybe an extra, null maneuver node to make sure the next displayed intercept is the following orbit.

[TheOrbitalMechanic]

If you've docked before, you should be good to go. There's no real difference in rendezvous procedure based on orbital altitude.

Just in case, though, here are the steps I would follow to rendezvous as simply as possible:

Spoiler

1. Launch into an orbit either significantly lower or higher than your target's orbit.

2. Click on your destination spacecraft and set it as your target. This adds some new navball markers, showing you where the target is relative to you.

3. Make sure your inclination doesn't differ significantly from your target's; if it does, fire normal/antinormal at the ascending/descending node. An inclination difference of 0 degrees is ideal, but you could probably make it work with up to 0.5 degrees of difference.

4. Make a maneuver node raising or lowering your apoapsis to your target's orbital altitude. Some new markers should pop up showing your closest approach to the target. Fiddle with the positioning of the maneuver until your closest approach is as close as possible--ideally, within 5 kilometers. Once you're satisfied, execute the maneuver.

5. As you approach your target, your navball should automatically switch to Target mode, which will show your velocity relative to the target itself. If it doesn't for some reason, click on the velocity indicator to switch the mode yourself. As you reach your closest approach, fire retrograde to cancel out all of your relative velocity. Note again that this is your velocity relative to the target, and not your orbit--you won't knock yourself out of orbit doing this (unless your navball isn't in Target mode).

6. Now that you aren't moving relative to the target, and are reasonably close to it (hopefully within 5-10 kilometers), point yourself directly at the target and fire your engines. Don't go too fast--maybe 20 m/s max? You don't want to miss your target and have to approach again, or worse, crash into it. Make sure your prograde vector is directly in line with the target marker on your navball. It will drift to one side as you approach, so use RCS or angled engine burns to keep it in line. Do this until you reach your target. Be sure to keep your speed at a manageable level, especially as you get closer.

7. Once you're within 100 meters (or really, as close as you like), cancel out all your velocity again, switch to RCS, and dock. The actual act of docking is far less complicated than the rendezvous, so I won't go into detail--you mentioned you've docked before anyway.

This isn't the *most* efficient way to rendezvous; there are certain things you can do to minimize time spent approaching and fuel used, but these are the basics.

[4747]

Thanks! I think I just need practice and not to worry about if the orbits are precisely matched as long as I'm getting an encounter with relatively low relative velocity.

[king of nowhere]

9 hours ago, 4747 said:

Thanks! I think I just need practice and not to worry about if the orbits are precisely matched as long as I'm getting an encounter with relatively low relative velocity.

you don't have to try and match orbit, in general. you have to get a close encounter (possibly with relatively low velocity), and then you zero velocity when you are close