Uneven angular torque compensation while translating using RCS

[Wampa842]

While building space stations, I like using RCS-powered "tugboats" to move parts around, but recently I haven't been able to build a decent one.

When I translate using RCS with SAS on, it also rotates the boat. This only happens when I'm translating to one direction, usually left, but it has also happened with all six directions. In all of those cases, none of the other directions had this problem.

It happens because, as I noticed, SAS doesn't counter the angular torque of RCS thrust when translating on the bugged direction. The markers at the bottom left corner show that the SAS' reaction wheel applies some torque when translating on all but the bugged direction. I've yet to find what really causes this. My crafts are evenly balanced, and it also happens on a vanilla install.

Edited November 26, 2013 by Wampa842

[larsmaehlum]

I have found this to happen when I don't have exactly 4 or 8 thruster blocks around the center of mass. Seems it calculates something wrong when you try to thrust left, but no thruster block is able to apply force in exactly that direction and it has to do a partial thrust on multiple blocks. Not sure if this is your problem. Do you have a screenshot of the craft in question?

[Dante80]

Use this, your COM is probably not aligned with the RCS blocks, thus producing rotation when translating. Have in mind that the COM of your ship changes dramatically during a flight, both due to fuel consumption and docking other parts for tugging.

[Wampa842]

Eight RCS blocks perfectly balanced around the CoM, everything but the antennae is stock.

I decoupled the docking port just below the 2-1m adapter, tried it in space, same problem. I can send you a .craft file if you'd try it.

[Rosco P. Coltrane]

I noticed the same as larsmaehlum. My tug, that worked perfectly in 0.20, has "rows" of three thrusters and it's drifting off axis all the time.

[Dante80]

. I can send you a .craft file if you'd try it.

Sure, send it over. I like space tugs very much..C:

[Wampa842]

https://docs.google.com/file/d/0B20HEDvnk-sycWVHVWY3V0RVbFk

Should be all-stock. I didn't include the boosters.

[Dante80]

No worries, I'll send it up with style...going to test now..XD

[Dante80]

Okies, here are some suggestions.

1. As mentioned, the RCS blocks are a little off for your COM, and a lot off when you spend some of the RCS fuel in your tank. Here are a couple of screenshots:

As you can see, the position of the RCS fuel tank unbalances the craft when you spend some of the fuel for rendezvous, tugging and docking. A nifty way to counteract this is to change your part order a little and use the heavy RTGs as weights to balance your crafts COM and DCOM. Here is the result:

2. Even if you have a perfectly balanced tug though, you might still encounter problems. The reason is that your COM changes as soon as you dock with a payload to ferry. If we are talking about station parts, a good suggestion would be to use your tug as a subassembly, adding it to the payload when designing in the vab and adjusting the payload RCS blocks to compensate for the new COM. If you don't want to add RCS thrusters to your payloads though, you will still have problems. Adding more reaction wheels to the tug might help, as long as you limit your thrusting to small intervals.

3. And even if you have a balanced payload/tug assembly, you might still encounter problems. I have also sometimes encountered the bug you speak of, and it may well stay that way in some situations. Having a balanced ship though will help a lot in limiting it.

Hope that helps, cheers. Here is a craft file for the modded tug if you want to test it...C:

http://www./download/iy5row5vnyye97e/

Edited November 26, 2013 by Dante80

[Wampa842]

I have that RCS helper, it showed no rotation, and no change in DCoM at all. Besides, the problem is not with the balance, it's with SAS. Like I said in the first post, it compensates angular torque on all but one axis.

I might try redownloading the whole game and pray for resolution.

[Dante80]

I have that RCS helper, it showed no rotation, and no change in DCoM at all.

Then you might have a mod conflict or an old version, because the tug you gave me is definitely not balanced in DCoM.

Try out the modded tug I posted and see if you can replicate the problem you have.

[wasmic]

While building space stations, I like using RCS-powered "tugboats" to move parts around, but recently I haven't been able to build a decent one.

When I translate using RCS with SAS on, it also rotates the boat. This only happens when I'm translating to one direction, usually left, but it has also happened with all six directions. In all of those cases, none of the other directions had this problem.

It happens because, as I noticed, SAS doesn't counter the angular torque of RCS thrust when translating on the bugged direction. The markers at the bottom left corner show that the SAS' reaction wheel applies some torque when translating on all but the bugged direction. I've yet to find what really causes this. My crafts are evenly balanced, and it also happens on a vanilla install.

I think it's a known bug - C7 (I think it was him, anyway) said that a fix has been made and is waiting on 0.23. You're using docking control, right? Using staging and hnjikl will allow you to circumvent that bug for now.

[Wampa842]

I think it's a known bug - C7 (I think it was him, anyway) said that a fix has been made and is waiting on 0.23. You're using docking control, right? Using staging and hnjikl will allow you to circumvent that bug for now.

The kind of answer I was looking for, thank you. I guess I'll just have to wait until the next update.

One thing still remains, though. I built a tug back in .21 and I copied the .craft to my .22 career save - it worked perfectly. That one either had a flawless design, or it's a bug with VAB.

[DMagic]

Two things; are you using fine control mode, activated with CAPS lock? And are you using docking mode for translation controls?

Fine control mode should help at least a little by reducing thrust from some of the RCS blocks to compensate for any offset in the COM.

And docking mode doesn't work right for RCS translation controls as wasmic said. When you switch to docking mode and use the WASD keys it will make SAS momentarily switch to damping mode (the orange arrows on the SAS indicator), which can really screw up your heading in some cases. The behavior of this isn't entirely predictable and sometimes thrusting in one direction will trigger damping mode while others won't.

Only use the translation controls in staging mode and you should avoid this problem.

[arq]

Two things; are you using fine control mode, activated with CAPS lock? And are you using docking mode for translation controls?

Agreed, I find that fine controls reduce the necessity of meticulously trying to balance RCS. It also helps if while in this mode, you just repeatedly tap the translation keys rather than holding them down.