Problems with orbital rendezvous

[Mitchz95]

I'm having some weird problem with making an orbital rendezvous.

1 - I get into orbit. The ship I want to dock with is in a low ~80km orbit around Kerbin, so I find myself with a close encounter right from getting into orbit.

2 - I refine my orbit so that we will come within 18km of each other on our next encounter, and 0.6km (600m) for the one after that. I quicksave and wait for the second encounter.

3 - Our ships are getting close now, about 5km apart. Our relative velocity is about 30m/s; should be easy enough to counter.

4 - At around 2km apart, our relative velocity suddenly skyrockets, shooting upwards of 600m/s within a few seconds! My attempt to counter it only succeeds in lowering my periapsis below the surface of Kerbin.

5 - I reload my quicksave and wait for the first encounter (18km apart). This time I'm able to counter the relative velocity fine, but it keeps going up by 0.1m/s as soon as I stop my engines. My repeated attempts to keep it at 0.0 eventually push the retrograde marker to the side of the navball, at which point my engines start pushing my periapsis down.

What am I doing wrong? Is this a bug? How do I fix it?

Edited November 18, 2013 by Mitchz95

[phunk]

5) Are you at the same altitude when you cancel out the relative velocity?

[Soda Popinski]

18km is too far. If you adjust yourself to 0.0 m/s, then you're essentially in the same shaped orbit but with your apo and peri 18 km apart / off phase (I think). You'll slowly drift further apart. Wait until the 2nd encounter, and you should be close enough. Target the rendezvous craft, and keep the target within your prograde mark using the RCS thrusters to translate left/right up/down. That way you don't increase your relative velocity too much by using your main thruster to aim yourself at it. Use H & N to adjust relative velocity. I usually go 1/100 of my distance or so.

I also really recommend the Docking Port Alignment Tool. It's unrealistic to try to dock with a spacecraft solely by eyeball from the outside. This just gives you an instrument window to help you line things up.

[Mitchz95]

Yeah, I see now our orbits were too different and that's what was causing the problems. I was able to kill the relative velocity before the weird speed boost at the second rendezvous, and was able to approach and dock from there.

Thanks for the help, everybody!

[Jason Patterson]

The first situation is bizarre. I can't think of anything other than a change in the navball setting that would cause that large a different between velocities, and none that would give those particular values. I guess that if the target were switched between two objects you would wind up with the same type of thing happening; in any case, it's weird.

The second situation is just what happens when you have two objects in non-identical orbits. Even though you were reasonably close, 18km is more than enough difference to give large relative velocities. You would want to burn toward your target until you were closer to try to zero out that relative velocity (my rule of thumb is no more than 10 m/s per 1km separation.)

[Kasuha]

I just had a strange experience in Eeloo orbit. I established orbit with a ship made of three parts docked together. While in orbit, I undocked two parts from each other trying to redock them in a different way. It was much harder than normal docking, the part kept running away, every time I matched speeds and position (within a few meters of distance) it started accelerating away in random direction. Then I undocked the third part and left it hanging there - immediately it started accelerating away and when I finished docking of the two initial parts it way already 200 m away with relative speed 7 m/s to the rest.

Normally docking ports do not give any significant impulse on undocking so I was really wondering what's going on.

[Kerbart]

4 - At around 2km apart, our relative velocity suddenly skyrockets, shooting upwards of 600m/s within a few seconds! My attempt to counter it only succeeds in lowering my periapsis below the surface of Kerbin.

5 - I reload my quicksave and wait for the first encounter (18km apart). This time I'm able to counter the relative velocity fine, but it keeps going up by 0.1m/s as soon as I stop my engines. My repeated attempts to keep it at 0.0 eventually push the retrograde marker to the side of the navball, at which point my engines start pushing my periapsis down.

What am I doing wrong? Is this a bug? How do I fix it?

Don't watch the movie "Gravity," it will poison your mind. Instead, switch to map view.

When you burn to get closer to your target, you're not just "traveling in space" and bridging the distance between you and you're target, you're also adjusting your orbit. [4] Sounds like you've converted your orbit in something highly elliptical, hence the sudden velocity change. The same with [5]; you've changed your orbit into something that temporarily gets you closer.

Thought experiment: you're behind your target. So you burn aiming your ship towards it, right? Well, if that is a short distance (two, three minutes max) it will work. But if the distance is longer... You've just added delta-V to your orbit. Which means you've increased your semi-major axis. Which means that your orbit period increased. Which means that each orbit you will actually fall behind on your target. What you really need to do in such a case is actually burn aiming away from the target. Your velocity drops, so does your orbit, and now your orbital period is less than the target and you'll gain ground each orbit.

Here's what I do for rendezvous:

1. Fix plane inclinations. Once you designate a target you should see the difference in inclination, as well the ascending and descending nodes. That's the point where you need to change inclination to match it with the target. Around Kerbin, anything less than 1° will be fine, but expect trouble if it's more than, say, 5°.
   
2. Get close. 18 km is not close, 5km is as far as I usually venture, with an exceptional 8-12km if I really #### my orbits.
   
3. There's a time to be patient and save fuel. Rendezvous is not that time. Move too slow and you'll hit the part where your orbits diverge. I usually aim to bridge the gap (whatever it is) in about a minute. Is the gap 5000m? Then I burn for 85m/s. Don't overdo it though; every m/s spent will be doubled as you'll have to slow down too.
   
4. Once I get close (hint: when your target is passing your ship as you look at it from a 90° angle. You need to brake before that) reduce velocity difference to 0 (syncing the orbits in effect).
   
5. Repeat as needed. This is where big burns pay off: I usually get within docking distance in two burns. You can take slower steps but then you need more of them, spending the same amount of delta-v
   

I'm sure there are more fuel efficient ways of doing this but it's an effective way to rendezvous.

[Soda Popinski]

I just had a strange experience in Eeloo orbit. I established orbit with a ship made of three parts docked together. While in orbit, I undocked two parts from each other trying to redock them in a different way. It was much harder than normal docking, the part kept running away, every time I matched speeds and position (within a few meters of distance) it started accelerating away in random direction. Then I undocked the third part and left it hanging there - immediately it started accelerating away and when I finished docking of the two initial parts it way already 200 m away with relative speed 7 m/s to the rest.

Normally docking ports do not give any significant impulse on undocking so I was really wondering what's going on.

My guess is you were using time acceleration while approaching. The game uses approximations in time accelerated mode, which seems to result in "teleportations" when you go in and out of time warping. I even see spacecraft I'm trying to rendezvous with pulling in one direction or another when using Physical Time Warp.

[spearsp]

I'm getting the same thing, in a similar environment, and I'm no novice when it comes to rendezvous and docking. Likewise, I have a 3 part vehicle - SSTO launch vehicle, orbital transfer vehicle and lander. I decouple the OTV from the launch vehicle, and transfer to Mun, leaving the launch vehicle in orbit around Kerbin. Once in orbit around Mun, I decouple the lander from the OTV, and land. Then take off and re-establish orbit, and rendezvous with the OTV.

First time round, I had a remote guidance system on the OTV and Mechjeb on the lander. As soon as I decoupled, the OTV accelerated away from me. I aborted the flight and restarted without the remote guidance system. This time, when I decoupled, it didn't do anything strange. However, once I got to within 2km of the OTV after landing on Mun, the relative speed jumped (instantaneously) from 1.0 to 280, and the relative speed on the navball seems to be out of whack. If I click REL- on Mechjeb, however, it does point me in the right direction. I managed to get to within a few meters of the OTV, but it keeps accelerating away from me at about 1m/s/s. Docking is impossible. I considered the possibility that it might be trying to target something else, so I unselected and reselected the target. When I unselected the target, there was no target selected, so that does not appear to be the problem. When I reselected the target, it went into the same state.

I have repeated this several times, with the same results. Any ideas?

Mods installed: B9, Quantum struts, FAR, Proc. Fairings, Kerbal Joint Reinforcement, Mechjeb, Laser guidance system, Stretchy tanks, Tac Fuel balancer.

OTV and Lander use parts from B9, Mechjeb and stretchy tanks.

EDIT: I have removed Exsurgent engineering, Firespitter, proc. Fairings, Kerbal Joint Reinforcement, KineTech Animation, ResGen and the laser guidance system from the game, and the symptoms have gone. I'll post here again and on the offending plugin's forum once I've established who the culprit is.

Edited March 16, 2014 by spearsp

[Kryxal]

Somebody has a Kraken Jr., then?

[FREEFALL1984]

Here's what I normally do, I'll get into a low orbit and make sure I'm about 20 degrees behind my target, then I'll spend a little time perfectim my inclination, then I'll time warp to "catch up" with my target, now here's the cool but, I'll wait until I'm more or less half the distance behind my target as they are higher than me. so for instance if I'm at 80km and my target is at 100km, I'll make sure I'm about 10km behind them. which means I'll be roughly 22.3km away from the target as the crow flies, and then I'll perform a transfer to their orbit, now I completely ignore the fact that there is another ships and then circularize my orbit, assuming I performed an accurate transfer and got my altitude to within about 200m of the target and I circularized accurately to within about 200m, I can should in theory be less than 1000m away from my target, on almost perfectly matching orbits with almost perfectly matching speed and now you can gently follow the velocity vector to catch up/fall behind. Basically if you spend as much time perfecting your vessels orbit as you did when you first set up your space stations orbit, you should only have to worry about timing your transfer correctly, which after a bit of practice and experience should be a doddle...

Please note... I'm not an astrophysicist and the whole "half the distance behind as the distance below" thing is not mathematically perfect as it doesn't take into account orbital radius or initial velocity difference, and any errors in it would be amplified exponentially over larger distances. it's just a figure I've found and seems to work quite well for the most part

[ctbram]

This is real and it is some kind of physics bug in the game and it got me screaming at my mic trying to make a Twitch video demonstrating orbital rendezvous.

Yet the navball goes 0.0 0.1 0.2 0.3... all over again.

Allow me to quote Neuton - A body in motion will stay in motion unless acted on by an EXTERNAL SOURCE.  So at a relative velocity of 0.0 and a thrust of 0.0 the two ships should stay 10m apart until hell freezes over and not magically start accelerating towards each other!

So what the love is causing this?  I was trying to demonstrate orbital rendezvous on Twitch and ended up SCREAMING and cussing and spitting and generally having a live meltdown on air because I could not get two ships together and STAY THAT WAY!!!!!!!!!!!

WHAT IS CAUSING THIS?  I have a save and can reproduce it to prove it is happening!!!!  SO there is some kind of loveing bug in ksp!!!  I am running 1.12.3

 

[Gargamel]

Any issue that might be software related was most likely worked out in the decade since this thread was started.

Please do not crosspost the same question in multiple locations, it only leads to confusion and fractured discussions.