Difficulty Docking Around Duna

[eddiew]

Hey guys, wondering if maybe you can help with this one; I can't for the life of me dock two ships around Duna. I've been playing KSP for a couple of months, I can dock vessels around Kerbin, Minmus, and Mun without issue.

When it comes to Duna however... it's like my direction indicator for Hohmann transfer nodes is a moving target. If I'm piloting manually, I'll bail out and there'll be a couple of m/s left on the node and I won't get the intercept. If I let MechJeb do it, he'll flip the ship end over end with 0.3m/s left - possibly forever. Sometimes I get to ~5km, with prograde marker directly over the target marker, and yet somehow I'm getting further away from it!

This happens in a variety of ships; I've been Hyperediting them out there just to see, and it's a very consistent problem. It can even be seen when putting the reticule over the node marker, it will slide away from underneath me over the course of a few minutes.

Can't tell if I'm being blindingly stupid and not accounting for something, or whether I've got a Kraken lurking beneath the red sands... any thoughts appreciated!

*edit* It's mod related. I'll binary search it until I figure out the culprit.

Edited January 11, 2015 by eddiew

[moogoob]

Are your orbits too circular? Sometimes when that happens "floating point errors" cause the trajectory to jump around a lot. Happens to me around Minmus all the time.

[eddiew]

I did have an issue with that, where the intercept markers were flickering, but even if I nudge the target into a 100x110 orbit, I still can't seem to lock onto it.

I'm worried about the way the (edit: manoeuvre) marker is sliding out from underneath me; if I put the reticule over it and turn SAS to stability and timewarp, it really shouldn't go anywhere - and yet it does. Something somewhere is messing with my angles, but I don't know what

Edited January 11, 2015 by eddiew

[merendel]

some pictures of the craft might help but spitballing a few possible issues. Its always possible your geting phantom forces from something on your craft but that would likely be noticable in your orbit shifting significantly over time for no reason.

1 I'm asumeing your well clear of atmosphere but best to confirm

2 Is your nav ball in target mode? Your orbital prograde lining up with your target will not nessicarly get you to the target. you want your target relative prograde to stay lined up as you approach

3 are you sure you were looking at the target indicator and not the anti target indicator (cant remember if the extra marker is stock or part of enhanced navball off the top of my head) I've had a few incidents where I thought I was burning to a target but was actualy burning away due to following the wrong marker.

I did have an issue with that, where the intercept markers were flickering, but even if I nudge the target into a 100x110 orbit, I still can't seem to lock onto it.

I'm worried about the way the target marker is sliding out from underneath me; if I put the reticule over it and turn SAS to stability and timewarp, it really shouldn't go anywhere - and yet it does. Something somewhere is messing with my angles, but I don't know what

This is sounding to me like your reading the navball wrong or have it in orbit mode instead of target mode (click on the display above the navball with your velocity till it says target) Nothing in orbit is static, if you point your nose at an object and then stop all rotation it will drift out of alignment as it cruises along its orbit around the planet. Basicly if its just ahead of you in orbit and you point strait forward with the planet being "Down" and turn on stability its going to slowly fall away as it moves down around the planet. Its not that something is messing with the angles its just that your craft isnt being rotated to follow your orbital path, if you waited half an orbit you'd be faceing directly away.

[eddiew]

1. I've tried it with several craft, and they all have this behaviour - even a simple pod/tank/engine/mechjeb that I hyperedited out to Duna did the same thing; node marker drifts off at the end of the manoeuvre. I let MechJeb follow it for 3 minutes, and it ended up radially outwards from Duna at 0.3 m/s with no sign of changing its mind.

2/3. I should have said "node marker" not "target marker". Apologies for being confusing about the problem! I'll edit the previous post.

And as said; I've pulled off docking around Kerbin, Mun, Minmus, and back in 0.25, Eve. I have no idea what's going on around Duna today, but I'm increasingly sure it's not user error

- - - Updated - - -

Ok, I hoofed out half my mods, started a new game, cheated two probes into Duna orbit, and had absolutely no problems rendezvousing and docking them.

Right, well, time to work out what's mucking my game up then

Thanks for trying to help guys!

[merendel]

well the manuver node marker will tend to drift as you get closer to completing it. The nodes represent instantaneous velocity changes, as an instantaneous change in velocity is impossible and we most do them over time they have some measure of inaccuracy. The game tries to add up your vector changes over the burn and subtracts that from the remaining change on the node. Slight inaccuracies throughout the burn will have no immediate visible effect but they show up towards the end.

Say for example you want a 100m/s prograde burn. You point prograde but your .1 degrees off in the normal direction. It looks like your right on the node all through the burn but your actually adding a small amount of normal vector. the game accounts for that and adds a comparable amount of anti normal to the node to correct the error. This isn't visible at first because it may only be .5m/s vs 50 m/s of prograde left in a node at the halfway point. However when you get down to .5m/s of prograde left that antinormal correction component is half of the remaining node and so the manuver node will drop down to halfway between prograde and antinormal.

Mechjeb is normaly prety good at following nodes although its possible a mode is screwing things up. However mech jeb tries to follow the node as precisely as it can including the correction the node will produce for overburn and steering errors. If for whatever reason mechjeb cant hit the node to withen its error margian (.1m/s default as I recall) it will keep trying untill it does sometimes resulting in it spining around and thrusting back and forth trying to follow a node, getting less and less effective as you move further from the time of the node.

Honestly I'd recommend you fly the nodes manually to learn how to do it. By all means use mechjeb to plan out the manuver if your not confident in doing that yourself (although you should learn that too) but try to fly the node yourself. a human mind is better at deciding when your close enough for the job than a script that just keeps trying even if its makeing things worse. Try watching 5thHorsman's fundamentals serries, particularly the 3 episodes on rendezvous and docking. you can find the first episode here

[eddiew]

I appreciate the help, but I'm really not new to rendezvous and docking manoeuvres. Around Kerbin/Mun/Minmus I've launched from the ground and met a station without problems, both by hand and by MechJeb. I only use the latter because, frankly, I'm not interested in doing docking manoeuvres or plotting Hohmann transfers anymore. Did a dozen or so, that was enough. I mostly like building ships and seeing if they fly, moreso when they're also atmospheric planes, but dragging the lines around and executing things to the nearest 0.1 m/s doesn't do it for me - so I let Jeb do it instead

The Duna issue however, is something new. Circularising at ~70km is weird; node markers will veer off towards a vertical angle away from planet, and no amount of thrust in this direction will complete the node. If I do get to within, say, 3km of my rendezvous target, there's a good chance that my navball will show 0.1 m/s, yet somehow the distance is increasing by 100m every couple of seconds. Within a few seconds, the reading is several m/s and the retrograde-vs-target is pointing straight upwards. I'm pretty darn sure this is not just me being dumb