Ship veers around when I thrust up with an asteroid.

[Toastedbuns]

Hi, I've been having a problem while trying to complete and asteroid contract. My ship will not stay on retrograde or any one spot when I'm trying to make a burn while I'm carrying an asteroid. I have to resort to spinning 360 degrees to the retrograde node to make incremental burns instead of a steady burn. It's not gamebreaking, but just frustrating. Thanks in advance. 

[Snark]

A screenshot would help diagnose the problem.

Most likely, you don't have your thrust axis perfectly aligned with the asteroid's center of mass, so your thrust is inducing torque that's causing a spin.  To get properly centered, unlock the joint of your Klaw, choose "target center of mass" on the asteroid, then rotate your ship until the target marker is perfectly centered on the navball.  Then re-lock the joint.

Aside from that:  depending on how your ship is put together, it's possible you could have a design that's especially prone to this sort of problem.  There are various possible solutions, but it would depend on your ship design, thus the need for a screenshot.

[Dfthu]

Did you target the asteroid's center of mass?  Go to 7 on this 

pic are good too.

Edited January 17, 2016 by Dfthu

[Toastedbuns]

Thanks both of you, I didn't even think of right clicking the asteroid. 

[Geschosskopf]

1 hour ago, Toastedbuns said:

Thanks both of you, I didn't even think of right clicking the asteroid. 

Well, if you've never done that before, there's something else you should know...  The asteroid right-click menu lets you rename the asteroid.  This only shows up when you don't have any ships docked to it---as long as you have a ship in contact, it will have the ship's name in the tracking center.  So mostly it's for creating new moons by a "catch-and-release" strategy.

But back to your thrust alignment problem....

You can target the asteroid's CoM during your final approach to it, before the Klaw makes contact.  However, the Klaw only grabs when it hits more or less perpendicular to the surface and asteroids are lumpy.  If the spot on the asteroid surface is at too much of an angle to the incoming Klaw, you'll slide along the surface a lot before the Klaw bites.  This carries the ship well off the CoM line, meaning you have to make a bigger adjustment later.  In extreme cases, it might be that the correction required will exceed the angular play in the Klaw and you'll have to release and find a different spot to grab.

To avoid this problem (most of the time), it's a good idea to 1) target CoM before contact and 2) unlock the Klaw's pivot before contact.  That way, when you hit an angled lump on the asteroid, the Klaw will pivot and hopefully bite while the ship remains lined up with the CoM.  There will probably still be a little sliding, however, so before locking the pivot, lean the ship as required to get back facing the CoM.

[Kryxal]

A further possibility, more related to ship design: make your pusher with widely-spaced engines, at least 3, and fueled from a central stack.  This means you can adjust thrust levels to better put the CoT behind the CoM.

[Snark]

Other ship-design issues:

• You want your center of thrust as close behind the grab point as possible.  i.e. you don't want a long, skinny rocket poking way way out from the asteroid.
• Beware of engine gimbal, especially if you have a long skinny rocket; it can actually make the situation worse, when SAS tries to gimbal the engine to correct for orientation, and it makes the rocket bend at the Klaw joint in precisely the wrong direction and you get a runaway error.
• Trying to solve the problem by putting lots of reaction wheels on the rocket that's doing the pushing can have the same effect.

One approach I've used that worked pretty well was to completely disable all torque sources on the pusher rocket (engine gimbal locked, all reaction wheels disabled), and then dock some other ships to the asteroid that have very powerful reaction wheels, but disabled engines.  That way, the source of torque isn't tied to the source of thrust, and it doesn't cause that problem.

And then there's the kookiest contraption I ever built, which was a setup to pull an asteroid.  Had a Klaw on the back of the tug, and the engines were mounted out on long curved booms so that they were on the sides of the asteroid.  It only handled up to class C, but it was fun to watch.    Pulling has the advantage over pushing in that it's inherently stable; errors in centering tend to correct themselves rather than grow.

If you have KAS installed, you could probably work out a winch-and-cable design to make pulling easier.

[Wemb]

One final comment, even if you can't get the CoM aligned perfectly, depending on bad the torque you have is, it's possible that you might be able to overcome it with reaction wheels, etc. but at a lower thrust - try reducing the thrust and seeing if your reaction wheels, etc. can compensate when your thrust isn't as large.

 

Wemb