Large ships veer off course... because CoM off-axis from CoT?

[reaction_wheel]

I've been playing KSP for years but never actually mounted a crewed interplanetary mission, because, well, you know... this game is hard.

So I've assembled a great big crewed interplanetary exploration vessel in orbit around Kerbin, but every time I try to put it through an orbital maneuver with any meaningful amount of thrust, it veers off course. In fact, this has happened to me in a couple of different games with radically different ad-hoc ship "designs". Of course I have reaction wheels, SAS, etc. etc... I've even trying putting KAS struts everywhere to keep the ships from wobbling, but no dice.

This veering off course happens whether I'm performing the maneuver manually with SAS, or whether I try to have MechJeb manage it.

I'm guessing that this is because my chimera of a spaceship has been cobbled together in such a way that the center of mass is off-axis from the center of thrust.

My question is: Does anyone know of any mods or techniques for determining how far off-axis the CoM is from the CoT, so that either the mass or the engine thrust can be trimmed to center it?

I'm guessing that there may be some way to do this with the MechJeb "Attitude Adjustment" window, but I haven't been able to figure out how.

Or alternately: Is there some other reason I haven't thought of why my larger ships won't maneuver straight in orbit?

Thanks!

[Gargamel]

Since you are using MJ, there is a setting for showing your COM as a red dot on the vehicle.  It's under Attitude adjustment.  You can also display your COt and COL with this.   I have never used the CoT element before, but I would assume it only shows up under thrust.   You might have to quicksave and then play with it, get it tuned, write down the settings, and revert, then reset your engines. 

The bigger issue though, is that If your ship is off balance with full tanks, the COM will only shift on you as you burn fuel.   You will have to adjust on the fly as you burn, or do some serious fuel transfer dancing as you burn. 

[GrouchyDevotee]

12 minutes ago, reaction_wheel said:

Or alternately: Is there some other reason I haven't thought of why my larger ships won't maneuver straight in orbit?

Are you sure its a Centre of Mass/Centre of Thrust issue? You can check CoM and CoT in the Vehicle assembly building, but if you've docked several modules in orbit that won't help.

If you've mixed different size engines for aesthetic reasons, or engines with different gimbal ranges you might be able to adjust the output without needing to load a MOD.

If you've added some modules in orbit make sure you have set "control from here" in your control pod.

Make sure you didn't set the trim.

A screen shot would help.

 

 

[Starwaster]

Install the RCS Balancer mod and then load/build the ship in its entirety in the VAB. The balancer can show you the results of applying engine thrust as well as from RCS.

Also try enabling Differential Throttle in the MechJeb Utilities menu

[Plusck]

I don't know about MechJeb, but KER gives you "thrust offset angle" information in flight, which I use all the time to balance odd loads (and especially asteroids).
If you can't find the equivalent in MechJeb, I don't think there's any reason not to just add KER to your mods. I don't think the overhead is very large.

[Guest]

You can eyeball it pretty effectively with the CoM/CoT indicators in VAB/SPH. Just get them aligned, add a reaction wheel or two, and you'll be fine most of the time.

As @Plusck says, KER will display thrust torque for each stage too. You can fine-tune it away with that -- just watch the number as you drag the engine around and get it as close to zero as you can.

As with most things, thrust torque is best solved by designing it away -- keeping the engines and tanks on the same axis. Sometimes this isn't possible or feasible (e.g. spaceplanes), so you need to design in ways to compensate for it. Reaction wheels will cancel out small amounts; RCS ports placed near the tips of the craft will correct larger amounts with a small fuel expenditure, and gimballed engines at the tail will handle even larger amounts. The latter two will have a fuel cost but unless your design is completely out there, it will amount to a rounding error in your burns.

Finally, if you have engine clusters, you can correct for it manually by adjusting the thrust limiter on the engines to the side that's pushing harder. It's fiddly and annoying but sometimes the only way.

 

[Zhetaan]

There are a couple of possibilities here.

I'll refrain from repeating what others have said as much as I can.

First, use Alt-X to cancel any trim in case you have accidentally set it.  However, trim doesn't work when you use SAS, so if you're veering while using SAS (I don't think MechJeb counts as using SAS for this) then it isn't your trim.

Thus the main reason that you veer off course is most probably due to a thrust offset that exceeds your gimbal capability and a thrust value that exceeds your reaction wheel or RCS torque.  Because torque depends on both the force and the length of the lever arm, you can choose to modify one or both to get the result you want.  If it's not a spaceplane, it's usually silly to use RCS just to fly in a straight line on a long-term mission (and temporary--you're out of luck when you run out of monopropellant), so adding more is not the best option.  I will also assume that a complete redesign is not an option, seeing that you already have this in orbit.  Symmetry is your friend, but since that ship has sailed rocket has been launched, let's try to fix the rocket you have rather than build the rocket you need.

You can reduce engine thrust either differentially or unilaterally, but the drawback to this is that your burns will take longer.

You can reduce the torque angle by adding a counterweight.  If you want an adjustable counterweight, see which direction the vessel wants to veer and add a locked fuel tank to the other side.  Putting this somewhere such that the line between it and the centre of mass is at right angles to the thrust vector is best but I assume that you don't have an abundance of docking ports, so use what you can.  If the rocket starts to veer towards the side with the added tank, then transfer fuel out of the tank and into the core stack until it does fly in a straight line.  Fuel isn't especially massive, though, so it may take a lot.  An ore tank would work better, but you likely have fewer places to put ore.  The drawback to this is that you add dead weight, so you lose capability (less delta-V, less acceleration, longer burn times, etc.)

Of course, you can also shorten the lever arm; if you assembled this craft in orbit, then unless you have an engine right in the middle of the engine module (which you probably do--and I don't know how many engines you're using here--but it's worth mentioning), there's no reason why you can't move the engines closer to the middle of the vessel, or even the front (I use puller designs to move asteroids about).  Just be sure you don't turn the rearward parts of your rocket into a barbecue with impinging engine exhaust.  This means minding the gimbal angle, too.  The drawback to this is that you lose some manoeuvrability:  that should make sense because the problem in the first part is that your engines can spin your rocket round too easily.  You also end up with a bit of 'clunkiness' in your design.  On the other hand, the pretty rocket that doesn't fly is certainly not superior to the ugly rocket that does.  Tweak it until it works, then ship it!  ...Refinements can wait for Version 2.0.

Edited March 2, 2018 by Zhetaan

[GuyWithGlasses]

12 hours ago, reaction_wheel said:

 

Or alternately: Is there some other reason I haven't thought of why my larger ships won't maneuver straight in orbit?

Thanks!

Yes, there is another issue to consider. 

If the ship consists in multiple ships glued toghether, try doing the maneouver after clicking "control from here" in one docking port alignned with the main ship (ive experimented similar issues with this problem with mj)

 

 

Edited March 2, 2018 by GuyWithGlasses

[dreadanaught]

I was wondering, I built a mid-sized manned mobile base that seems to work well. However, while testing it on Kerbin near the KSC, I noticed the steering was slightly off, and I kept having to adjust its direction because it was veering to the side. I know that this thread is originally from spacecraft, but can someone please help me with this? Thanks!

[reaction_wheel]

On 3/1/2018 at 9:37 PM, GrouchyDevotee said:

Are you sure its a Centre of Mass/Centre of Thrust issue? You can check CoM and CoT in the Vehicle assembly building, but if you've docked several modules in orbit that won't help.

If you've mixed different size engines for aesthetic reasons, or engines with different gimbal ranges you might be able to adjust the output without needing to load a MOD.

If you've added some modules in orbit make sure you have set "control from here" in your control pod.

Make sure you didn't set the trim.

A screen shot would help.

 

 

For the record, I built a similar ship in the VAB, but with appropriate symmetry around the CoT, then sent it up in pieces, reassembled it, and it steers just fine.

So I'm going to hypothesize that my earlier steering issues were entirely due to off-center mass distribution. Lesson learned!