Spaceship drags to the side

[Cabadam]

I was all proud, I built this monstrosity of a ship in LKO over several launches, docking together the pieces until I finally had my "Interplanetary Mothership".  Powered largely by LF and Nuke engines, it's goal was to be capable of traveling the solar system without need for coming home to refuel or anything.

Did some tests in place to see how well the reaction wheels could turn it, not great, not horrible.  But then, the combined weight of this thing is... I either 243tons or 540tons, I'm not sure exactly how to read the mass in KER (it lists the vessel mass as 243,143 / 539,472kg).  I'm guessing the 540 is correct.  But hey, I've got just shy of 5,000 deltaV available.

Alright, what should my first act be?  Well, my orbit isn't quite circularized anymore, let's do that.  A nice easy exercise.  Get the maneuver node setup.  Full thrust from all 20 nuke engines.  And it starts moving... and then it starts turning, and turning, SAS can't keep it pointed to the maneuver node nor can I manually do so.  Forced to cut engines before my periapsis decided to drop below the 70K mark.

Well that was depressing.

The engines are balanced - 5 sets of 4 (each set attached through the quad adapter) organized in a cross: +  Left, right, top, bottom, and center on the rear of the vessel.  I verified all 20 engines actually lit up too.

That leaves me to assume my CoM isn't right.  Which is fair and I definitely did not consider that during planning.  I had the core of the ship with 6 docking ports and then basically started "plugging in" things I wanted.  Lander with a rover, passenger modules, and (my suspicion of the problem) a refueling lander.  Idea was to carry up fuel that had been mined on a moon somewhere back to the mothership so I could continue the voyage.  That lander is much larger than the rover lander on the opposite docking port.

Alright, story time aside, questions.  Where do I go from here?  Is there a good way to see the center of mass in flight?  I know it can be viewed in the VAB, but the entire ship never existed in one piece down on the ground.

If not... is the only solution to try to get all the pieces of something like this together in the VAB before launching up the pieces into orbit for assembly?  I'm not sure I can even FIT this thing in the VAB...

Am I missing another possible cause of the ship spinning when the engines fire up that might be easier to resolve?

[Grogs]

In my experience this is common with big ships with lots of things docked/attached to the sides. Just a couple of thoughts:

- On big ships, I try to avoid just hitting 'Z' and instead use shift to gradually increase thrust. This helps avoid snapping things off of docking ports, etc.

- Try and increase thrust gradually and see how high you can go before the ship pulls off-axis. Reaction wheels and engine gimbals can compensate to some extent, and maybe 1/2 or 1/3 thrust is good enough for what you need.

- Look at what direction the ship rotates off target. That should give you a clue as to which side is too heavy. If you have some fuel tanks with empty space, you may be able to shift fuel from one side to the other to make things more balanced. Or you could try redocking things in different positions to see if that helps.

- You also try lowering  the thrust on the engines opposite the side it turns. For example, if the ship rotates to the left, lower the thrust of the engines on the right. This will effectively move your center of thrust to the left, closer to the CG.

 

Good luck!

[Vanamonde]

The game's camera points at the center of mass by default. Rotate the camera around the ship and the COM will become evident as the non-moving point that stays at the center of your screen. 

[Snark]

16 hours ago, Cabadam said:

Where do I go from here?  Is there a good way to see the center of mass in flight?

The problem is that your center of mass doesn't line up with your center of thrust.  So there's two ways you could fix it:

1. Move the center of mass
2. Move the center of thrust

The easiest way to move the center of mass is to pump fuel around... but the degree to which you can do that will be limited by things such as "where do you have available empty tanks that you could pump fuel to" and so forth.

As to moving the center of thrust... you might actually be well set up for that, because of this:

16 hours ago, Cabadam said:

5 sets of 4 (each set attached through the quad adapter) organized in a cross: +  Left, right, top, bottom, and center on the rear of the vessel.

What you can do is to tinker with the thrust limiters.  If the craft tends to curve left when you thrust, try lowering the thrust limiter of the engine on the right, and so forth. 

How effective that will be (and whether it will be enough) will depend on how far off-center your CoM is, and also how far apart your engines are.

 

16 hours ago, Cabadam said:

Is there a good way to see the center of mass in flight?

The way that @Vanamonde describes is basically the only way to do it in flight, unless you use a mod.  Rotate the camera view, the CoM is the center of the camera rotation.

[Cabadam]

Thanks everyone for your responses today.  I'll be heading in game in a little bit to try out some things.

11 hours ago, Grogs said:

- On big ships, I try to avoid just hitting 'Z' and instead use shift to gradually increase thrust. This helps avoid snapping things off of docking ports, etc.

I'll keep this in mind.  I definitely hit Z    Fortunately, all of my docking clamps are facing directly forward and directly backwards, nothing to snap off sidewise.

11 hours ago, Grogs said:

- Try and increase thrust gradually and see how high you can go before the ship pulls off-axis. Reaction wheels and engine gimbals can compensate to some extent, and maybe 1/2 or 1/3 thrust is good enough for what you need.

I'll give this a shot.  I have a large number of reaction wheels on this thing to try to make it vaguely maneuverable, I'll see if I can go at a lower thrust.

 

11 hours ago, Grogs said:

- Look at what direction the ship rotates off target. That should give you a clue as to which side is too heavy. If you have some fuel tanks with empty space, you may be able to shift fuel from one side to the other to make things more balanced. Or you could try redocking things in different positions to see if that helps.

It rotates to the left - and front left is the docking port where I have the "heavy" fuel lander attached.  Right now, most of the fuel tanks are full, but yeah I may be able to burn some off and then shift things around.  Just have to remember that moving forward.  Unfortunately, I don't have a whole lot of space to rearrange what I have docked.  I mean, the two landers could probably flip flop, but they are on the front left and right, so my guess is that I would just begin turning to the right after that.  Might be an interesting test though to verify cause.

 

11 hours ago, Grogs said:

- You also try lowering  the thrust on the engines opposite the side it turns. For example, if the ship rotates to the left, lower the thrust of the engines on the right. This will effectively move your center of thrust to the left, closer to the CG.

 

5 hours ago, Snark said:

What you can do is to tinker with the thrust limiters.  If the craft tends to curve left when you thrust, try lowering the thrust limiter of the engine on the right, and so forth. 

How effective that will be (and whether it will be enough) will depend on how far off-center your CoM is, and also how far apart your engines are.

I am not in game at the moment - I forget, can you cap an engine's max thrust while already in flight?  I know you can in the VAB.  Unless I am completely mis-remembering, the engines are well spread out.  I'm almost considering posting a picture of the craft, but if I'm being honest with myself it is pretty hideous.  If Frankenstein built a spaceship

6 hours ago, Vanamonde said:

The game's camera points at the center of mass by default. Rotate the camera around the ship and the COM will become evident as the non-moving point that stays at the center of your screen. 

Didn't know that - will give this a try.  Surprisingly, I feel like I didn't notice the camera noticeably off center last night.

[Snark]

3 minutes ago, Cabadam said:

can you cap an engine's max thrust while already in flight?

Yup

[Cabadam]

Playing with the camera a bit, I can confirm that it does indeed center slightly over to the side where it pulls, so it seems like that is indeed the cause.  As for resolution, I had to drop the four engines on that side down to 50% thrust, and then SAS is just BARELY able to hold on the maneuver node (yaw is almost but not quite completely to the left) - but it does do it.  I may drop it down to 40% just in case I ever want to turn the other way sometime

Thanks for all the suggestions.  She ain't perfect, but it looks like she's going to get underway afterall!