Help with grabbing unit and center of mass?

[Genesis6496943]

Hi everyone!

I posted over on Reddit about this a couple of days ago and got some answers but thought I'd post here and see if anyone had anything else to add or clarify the situation.

My issue is as follows: When using the AGU, you're supposed to be able to target center of mass of whatever you're attaching to. This presumably still works with asteroids, as I went through the asteroid redirect tutorial again to make sure it worked there. That doesn't appear to be an issue.

What is an issue is that I can apparently no longer target center of mass for debris. I know all about how the AGU is supposed to work - attach, use free pivot to target centre of mass, lock, and you're good to manipulate the object's path using engines/RCS/whatever.

Why can I no longer do this with debris? Reddit user nguyenco replied with the following statement to that question: "As of the latest update, I don't think that you can target the center of mass of debris."

This makes no sense to me for a number of reasons. It renders the AGU relatively useless compared to the way it was before, while still retaining (To my knowledge) the nonsensical flaws that it has always had in the past. Being able to rescue stranded Kerbals, or being able to transfer fuel, or being able to control the engines/SAS/RCS of the debris come to mind as examples of this.

Despite all of that, apparently it's now impossible to know the CoM for a spent stage, but you can approach an asteroid and somehow magically know where it's CoM is without doing science of some sort? Where is the logic - even the Kerbal logic - in something like that? There is none.

Last but not least, I can't find mention of them changing the AGU in any of the changelogs, nor on the KSP Wiki.

What's going on with this? Is this a bug? Did Squad change it and not put it in the changelog? If they did, why would they make a change like that, which makes absolutely no sense?

Thanks for your time and any replies you might be able to give me!

[Pecan]

No changes that I know of.

When you target anything, it's the CoM that is targetted, isn't it? What/where are you getting as target?

Secondly, if you pull with any lateral thrust the CoM will aline on that axis, then you can lock the claw.

[Genesis6496943]

No changes that I know of.

When you target anything, it's the CoM that is targetted, isn't it? What/where are you getting as target?

Secondly, if you pull with any lateral thrust the CoM will aline on that axis, then you can lock the claw.

Most of the things I want to attach to are an almost identical piece of staging: a stack seperator at the "top" followed by a tricoupler, followed by the FL-T800 fuel tanks and their engines. The only thing I'm allowed to target when approaching the debris is the stack seperator, which is of course nowhere near the center of mass. I've tried right clicking all over the debris, and I get nothing. Once the AGU attaches, it stops treating the debris as a target as it becomes part of the ship, and I therefore lose the ability to try and target it's CoM - not that I can figure out how anyways.

As for your suggestion about lateral movement, I presume you mean using RCS to try and translate in some direction other than forward and backwards? I'll try that out but I don't see how that would actually align the CoM... Doesn't make sense to me just thinking about it, but I'll give it a shot in a couple of hours and report back when I've tried it. Even if it does work, it still doesn't explain why I can't target CoM to begin with.

Thanks for the reply!

[Genesis6496943]

I`ve had no luck with aligning with the CoM by pulling or moving laterally along any axis, or by combining the two. Anyone have any other ideas?

[Pecan]

For 'lateral' I should have said 'linear', non-rotational. If you pull a thing the CoM will align behind the point at which you pull.

I had assumed you were selecting the target from the map view, but I'm surprised it makes any difference in flight mode - it never has for any other sort of target selection (ie; docking).

[Genesis6496943]

Longitudinal force then I've tried that but it didn't work particularly well. Perhaps more RCS thrusters would help? I only have 4 on a small tug.

As for targeting, I've tried targeting directly and from map view. I'm still only able to target the stack seperator, and not the CoM.

Thanks for answering!

Edited January 27, 2015 by Genesis6496943
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[Pecan]

Upload your save game (persistent.sfs) to dropbox or similar if it's stock and I'll take a look at it.

[Genesis6496943]

I uninstalled MechJeb and TAC Fuel Balancer (Neither of which were being used, but just in case), started a new game in sandbox mode, copied over the .craft files from my other save, and launched everything exactly as it is in career mode.

https://www.dropbox.com/s/03mpfnxpr09lfd4/persistent.sfs?dl=0

The debris and deorbiter should be a few meters apart, ready to attach, in a ~105km orbit and on the light side of the planet. Action group 1 will put away the solar panels if you need to, and I've already disabled all the SAS and engines on the debris (it still has a tiny bit of fuel left in it).

Thanks so much for all your help, even if we can't figure it out. You've been great!

[Pecan]

Well you're absolutely right about the targetting; it's always the root (or highest part in the tree after decoupling) that gets targetted. Never noticed that before, since I'm targetting to dock and using the Navball Docking Alignment mod.

Attaching sideways at one end still lets the load align under thrust though. Make sure you 'free pivot' after attaching, then apply thrust to pull the load.

I would suggest you try a 'pull' design with radial engines rather than relying on RCS for that. A push engine like yours will have the opposite effect (pull string, it straightens out, push it and ... it doesn't).

ETA: Excellently prepared and presented save-game, by the way. It really helps us help you if you can do something like that.

[Genesis6496943]

It's the least I can do - you're going out of your way and taking time out of your own day to help me. Anything I can do to facilitate your help I'm happy to take the time to do!

I'll try a pull design when I can later on. It makes sense, perhaps the extra power and where I attach the AGU will help. I tried pulling using just the RCS and the debris never really aligned itself at all, it just kept swinging around.

I'm still confused about the whole thing though. I looked on my laptop (where I originally played KSP, but not for many months now) and I have a number of saved ships designed specifically for deorbiting debris, and all of which use a very similar design to what I have in the save I uploaded. I'm now certain that you used to be able to somehow target CoM of debris, or else I wouldn't have a load of designs saved for doing exactly that. Oh well, time to adapt and move on I suppose.

Thanks for all your help!