Center of Mass?

[Brotoro]

How does KSP calculate the center of mass of ships with partially empty tanks? I mean, does it calculate the center of mass of a partially-full tank assuming the remaining fuel is at one end of the tank...or does it just use the decreased mass of the partially-full tank and assume the CoM of that tank is at the center of the tank?

I could do some experiments, of course, but I thought I'd ask if anybody knew the answer first.

And I want to know because I'm pondering making a VTOL aircraft, and I expect keeping it balanced as fuel is used to be a major difficulty.

[BubbaWilkins]

The CoM of the fuel tanks doesn't change, just the value. So the best way to build a balanced VTOL is to build the fuselage so that the CoM of the craft is as near as possible to the CoM of the tanks. Same with Helicopters.

[rryy]

I believe each fuel tank's center of mass remains in the same place as fuel is burned, but a group of fuel tanks would normally have a shift in the CoM as the fuel in the front tanks is used.

[SilentWindOfDoom]

I would like to know this too. I struggle to create balanced RCS systems to make docking something other than a screen punching, teeth grinding affair and i seem to be doing okay, but i'd hate to find out someday in Duna orbit or whatever that i cant move worth a damn because my cog has shifted.

[ThePyrateCaptain]

Well... In the 3d modelling program, mine is blender, there is a way to "fix" the center of mass and lock it in. Hehehhe

[bsalis]

And I want to know because I'm pondering making a VTOL aircraft, and I expect keeping it balanced as fuel is used to be a major difficulty.

IMO an easy way is to make the tank arrangement 'Plus' shaped. So symmetric on both axis. A couple of examples below. I have stripped away wings to show the tanks.

1 seater

3 seater

Edited June 21, 2013 by bsalis

[OrbitusII]

Yes, but that's not how the CoM of ships is calculated in KSP (perhaps individual parts, but not whole vessels). The center of mass can (sort of) be explained as the average of all the masses of the objects and their positions, so bigger objects farther out will shift the CoM farther towards the new big object, versus a lighter object at the same location which won't move the CoM quite as much. Now this explanation is just a sort of visual aid and not necessarily used in real science, but if you get the concept then you should be good.

(Somewhat) Helpful equation: M_n*(x_n,y_n,z_n)+M_n+1*(x_n+1,y_n+1,z_n+1)+ ... ad infinitum

Where M is mass of each object n and (x,y,z) is the position of the object.

Again, note that this is a visual aid with a simple way for the CoM to be calculated, don't go into a college physics class and say "this is the way!" because while it may work for KSP, real life and KSP physics are significantly different (I came up with it while eating breakfast, so there's that too).

If I did do it right, then terrific! Physicists, feel free to correct me if something isn't right; it'd be good to have someone who does this professionally in here to help.

[Fried]

Yes, but that's not how the CoM of ships is calculated in KSP (perhaps individual parts, but not whole vessels). The center of mass can (sort of) be explained as the average of all the masses of the objects and their positions, so bigger objects farther out will shift the CoM farther towards the new big object, versus a lighter object at the same location which won't move the CoM quite as much. Now this explanation is just a sort of visual aid and not necessarily used in real science, but if you get the concept then you should be good.

(Somewhat) Helpful equation: M_n*(x_n,y_n,z_n)+M_n+1*(x_n+1,y_n+1,z_n+1)+ ... ad infinitum

Where M is mass of each object n and (x,y,z) is the position of the object.

Again, note that this is a visual aid with a simple way for the CoM to be calculated, don't go into a college physics class and say "this is the way!" because while it may work for KSP, real life and KSP physics are significantly different (I came up with it while eating breakfast, so there's that too).

If I did do it right, then terrific! Physicists, feel free to correct me if something isn't right; it'd be good to have someone who does this professionally in here to help.

or if you use fuel lines to equal it all out at the same time you don't need the math, but if you have three tanks and burn only from the front one you will shift your com back.

[Zephram Kerman]

The CoM of the fuel tanks doesn't change, just the value. So the best way to build a balanced VTOL is to build the fuselage so that the CoM of the craft is as near as possible to the CoM of the tanks. Same with Helicopters.

I agree with this. So all you have to do is build the vessel so that CoM is exactly at the center of the fuel tank(s), and put the VTOL engine at that same point. Here's an example:

(By the way, this one features your wonderful BirdDog landing gear system.) Basically what I did here was to build the core and fuel tanks, position the VTOL so that it matches CoM, then position the outer wings for proper CoL. It would have been easier with a single fuel tank, but I was feeling inspired at the time to do twin-hull stuff.

[Brotoro]

OK guys...thanks. If the CoM of the individual tanks do not change as fuel is used, this will make things much easier.