Need help with CoM

[Chel]

Hi all. I've built many craft over the years, but have only now actually been paying attention to the CoL, CoM, and CoT. The CoM should be in the middle, for a craft to be stable. But the problem is, I'm having trouble seeing if it is in the middle or not. So, are there any tips on how to find out? Any mods that can help? 

[bewing]

The CorrectCoL mod may well be what you are looking for.

 

[Foxster]

3 hours ago, The_Cat_In_Space said:

The CoM should be in the middle, for a craft to be stable.

Not necessarily. 

[Guest]

The CoM doesn't need to be in the middle. It just needs to line up with the CoT (for spacecraft) and be in front of the CoL (for aircraft). You can see this pretty easily with the indicators in the SPH/VAB.

[funk]

RCS Build Aid is what you're looking for, shows torque at COM/ACOM/DCOM and many more things which are useful for building.

[MaximumThrust]

First of all, it dependents of what you are building.

If it's a lander or rover, its good to have the CoM very low.

If it's a rocket, it's good to have the CoM low, but not very far from the middle, to provide lever for the gimbals to pitch and yaw. I guess this is the info you saw somewhere.

If it's a reentry vehicle, the CoM needs to be near to the side you want to point prograde.

If it's an plane/spaceplace, it needs to be in-front of the center of lift. In KSP having the two at the exactly same point also works for me. 

The CoT needs to be in-line with the CoM in almost every situation. Small misalignments can pass using reaction wheels, surface controls, RCS or gimbal (although the last one isn't reliable with the in-game/mod stabilization tools).

If you are dealing with RCS, the CoT they produce should be inline with the CoM for you be able to translate (move to the sides, and front-back) without rotating, and away from the CoM to have a better lever to pitch, yaw and roll.

I think the only exception for CoT (from main engines) and CoM to be aligned is for planes, you can have the CoT pointing a little below the CoM, which will make the craft pitch up. If you have straight wings, pitching up will be necessary to keep altitude. Also helps with design, you can put the engines bellow the wings. A VTOL plane I did using this, it flies very well:

Planes like this (with engines above the wings) have to tilt them up, to avoid making the engines force the nose down:





Like @funk said, the RCS Aid mod is fantastic. Have so many useful options that I would need to keeping writing a lot to describe them all.

An orbital shuttle I did using it:



The yellow marker is the wet CoM (all the tanks filled), and the red one is the dry CoM (all tanks empty). Since they are in the same spot, my RCS will behave the same way all the time. In the picture it's simulating an lateral move to the right. Since the CoT from the RCS and the CoM are aligned, I get almost no rotation torque (0.01 kNm) when doing this. If they weren't aligned, I would get an red arrow showing the direction and force of this rotation, and the torque value. You can also do this for other directions, main engines and parachutes. It's amazing. 

Edited March 9, 2018 by MaximumThrust

[Zhetaan]

3 hours ago, MaximumThrust said:

[Large-size advert for RCS Build Aid]

Have a link:

 

On 3/8/2018 at 12:53 AM, bewing said:

[Small-size advert for CorrectCOL]

And this:

 

[Chel]

On 3/8/2018 at 6:56 PM, Brikoleur said:

The CoM doesn't need to be in the middle. It just needs to line up with the CoT (for spacecraft) and be in front of the CoL (for aircraft). You can see this pretty easily with the indicators in the SPH/VAB.

Right. So as long as it's in front of the CoL it will take off, and not start flipping in the air before crashing.

[Guest]

1 minute ago, The_Cat_In_Space said:

Right. So as long as it's in front of the CoL it will take off, and not start flipping in the air before crashing.

More or less. It's not a completely ironclad rule though: highly draggy parts near the nose can cause the craft to flip as you build up speed, and canards near the nose can become such highly draggy parts if they stall before your wing does, also causing your craft to flip. Note also that Mk 2 parts are highly draggy and function as lifting surfaces ---> a 2.5 m or Mk 3 craft with a Mk 2 nose could bite you even if the CoM and CoL line up nicely in the SPH.