Cannot control asteroid when pulling

[jbdenney]

Having an issue with trying to move an asteroid.

It is a class E about 1300t. I used a grabber attached to the end of an extended cylinder piston on the back of the central core so that I could pull it instead of pulling.

No matter how slowly I apply thrust the rock starts rotating badly. Not just a wobble but a definite roll. I did target the center of mass so that is pretty straight.

I tried to get a picture in from i.imgur but it is not working for me tonight so I will have to describe it.

Just so you know I use only stock parts.

Central Core is Rockomax Jumbo-64 fuel tank topped with an Advanced reaction Wheel (Large) and a Mk1-3 command pod. There are also a bunch of RCS trusters, monopropellant tanks, Solar Cells, Ore Storage containers. 

On the back side is a 3PT Telescoping Hydraulic Cylinder that extends with an Advanced Grabber unit. The extension is needed to get the grabber beyond the outrigger tanks. During launch the extension is retracted so it will fit on the booster.

There are four outrigger tanks.

Two outriggers are  Two FL-TX1800s and a FL-TX900 with a Thud mounted on the front side as far forward as possible pointing straight back.

The other two are also Two FL-TX1800s and a FL-TX900 with a Thud mounted on the front side as far forward as possible pointing straight back but they also have Convert-O-Tron 250 with an attached Drill-O-Matic Excavation Miner mated with a large radiator on hte back end.  The resource drill and converter is to refuel once I am on the asteroid.

When I am ready to start pulling I reset the engines to 20% max thrust. When I start the burn the rock starts rolling badly. It is not wobbling but rolling.

As far as I can tell I have the Center of Mass pretty well centered. Even if I use the RCS system it will not hold keep the rock stable.

I do not know if I need to add one or more Reaction wheels. Even turning the rock is very slow and sluggish.

Also one other point. I used this tug to grab a Class A rock and after that mission the 3PT Telescoping Hydraulic Cylinder is bent. It is mounted flat on the rear end of the tank but the shaft has a distinct curve. It is bad enough that when I extend the piston and control from the grabber it lines me up off center of the center of the rock. I had to do the last few meters of the capture by eye.

Could the bent piston be causing the issue?

The Thuds are a far forward as possible but do I need to extend them further from the central core to get a wider thrust base? If so - how do I do that. On the end of struts extending sideways? In that case how to I set them up so I can get the tug into orbit and then extend them into position? BTW - I also use the Thuds to get the last bit of DetaV to get into orbit.

Is there something else I could be missing?

 

Thanks

Bruce

 

 

Edited May 24, 2020 by jbdenney
speeliong

[OHara]

Are you using SAS ? 
If I understand your craft, the Thuds are pointing opposite their usual direction, relative to the control point.  If a tiny amount of roll starts, then the SAS would gimbal the Thuds in a direction that makes it worse. (Pitch and yaw controls don't flip so don't have this problem.)  You can turn off the Roll response of the Thuds (enabling the option in in-game Settings: Advanced Tweakables).  Or you can change the 'control from here; part or select 'Control Point: Reversed'.  Or you can leave SAS turned off. Or some better solution you'll probably think of.

If it is not SAS, maybe one of the Thuds' exhausts is being blocked by the asteroid, which would cause a pitch or yaw.  I can imagine the gimbal to correct that pitch/yaw with partially blocked Thuds could induce some roll.

[jbdenney]

Yes - I am using SAS. When I am capturing the rock I am flying backwards and the Thuds are pointing at the rock and the Tug is controlled from the grabber in the back pointing backwards. It took a while to get used to having to reverse Retro and Pro for the Thuds while the RCS worked as expected with the backwards orientation.

When I start to pull the rock I have moved control to the forward MK1-3 capsule and the Thuds are still pointing back to the rock.

Either way the Thuds are pointing in line with the central axis of the tug. When I am catching the rock the engine thrust points in the direction of movement and when I am trying to pull the rock the engines thrust point opposite the direction of movement and that is when the roll happens. I have not tried to "push" against the rock  since there is no way to have the Thuds push toward the attached rock.

I will have to try turning off the gimble controls and see what happens.

Edited May 24, 2020 by jbdenney
left out a word

[AHHans]

Well, this is a case where a picture would say a lot more than some hundreds of words. I don't really have a good idea how your craft looks like.

The issue that @OHara mentions occurs when the control direction is not pointed in the direction into which the engines apply their thrust. But it could also be that your thrust vector isn't really pointed 100% through the center of mass. (I never got that perfect when pushing class-Es, so I always had to apply some torque to keep the craft straight. So I limited my engine thrust to what the RWs could compensate.)

I understand that you are pulling the asteroid. That only gives you any benefits if the asteroid is free rotate behind your craft (see pendulum rocket fallacy). Do you have the pivot of the claw locked or free?

[jbdenney]

Finally figured out how to get IMGUR to work.

My Grabber picot is locked - I will try setting it free.

Below is my Tug. the Grabber is retracted at this point. When extended the grabber extends past the Convert-o-Trons so It can grab the rock.

Here it is attached to  Class E 1,300t rock...

H

Here is a pick of my Bent Cylinder...

DO I need to extend the Thuds away from the core to give a wider thrust base?

 

Thanks

Bruce

Edited May 24, 2020 by jbdenney
Added sentence

[AHHans]

19 minutes ago, jbdenney said:

Here it is attached to  Class E 1,300t rock...

Thanks.

So when you choose to control from the command-pod (with the control direction as default) then you shouldn't have any SAS issues. When you control from the claw, then the gimbal of the rocket engines will be in the wrong direction. (I.e. their torque will fight with the torque from the reaction wheels and RCS (if active).)

It would also help to see the target marker from targeting the CoM of the asteroid. Does that move when you are accelerating?

19 minutes ago, jbdenney said:

DO I need to extend the Thuds away from the core to give a wider thrust base?

Probably not. If you want to give them more leverage to generate more torque, then you would need to move them farther away from the asteroid, i.e. make the rocket longer. What may be a problem is if the asteroid is too close to one (or more) of the engines and screens the exhaust (thrust occlusion). In that case that engine would not generate thrust and the thrust from the engine on the other side would not be balanced.

[jbdenney]

the target center of mas does not move when I apply thrust.

[jbdenney]

Ok - think  I found the issue. I had to reposition where I grabbed the rock. I moved it a few meters in the direction of where I was rolling. There was a bulge that I am now on top of and it seems stable. now for a 26 minute burn at 20% engine thrust. 

[jbdenney]

Yep - it was my position on the rock. 26 min burn at 20% thrust (96kN) and it was steady as a rock (no pun intended). I got some back and forth rocking but it was perpendicular to the direction of thrust so it was not bad.

Now all I have to so is get it into a firm Kerbin Orbit so I have the chance to bully it into a Mun or MInmus orbit.

Thanks every body.

Bruce