visualize CoM and thrust on a ship in flight?

[king of nowhere]

while in the VAB, we all know to visualize the center of mass and the thrust vector. but i know no way to do so in flight.

i just assembled in orbit a complicated monstruosity; i had checked that all its parts would be balanced before the flight. but that was with full tanks. the thing on the top was balanced with the rest of the ship, provided its fuel tanks were full. now they are half full, and the ship is tilting to the side. and i have no idea how to check the new CoM.

I can fix the problem. I have 24 engines radially, i can turn off a few of them until they compensate the imbalance. but trying to estimate the deviation by eye is a nightmare. that's why i'd really need to see the CoM. and i can't really do that in the VAB.

 

P.S. imgur is not letting me upload stuff at the moment, so no picture. not that it would be particularly relevant anyway, i'm just asking ways to see my CoM

[Okhin]

You can empty the tanks in the VAB, it will be reflected on the CoM display there. Don't forget to refill them before launch though. I had my share of craft with empty tanks on the pad because I wanted to be sure I maintained alignment with some empty tanks.

[HansAcker]

28 minutes ago, king of nowhere said:

but i know no way to do so in flight

MechJeb can visualize CoM and velocity vectors in flight. You'll find those options in the "Attitude Adjustment" menu:

Edited November 8, 2020 by HansAcker

[Snark]

On 11/8/2020 at 6:17 AM, king of nowhere said:

i have no idea how to check the new CoM

Well, at least at a very rough level, just look where the camera is pointing (i.e. at the middle of the screen).

Unless you manually set the camera to focus on sometihng else, by default the game always points the camera at the CoM of the current vessel.  So you can just look at the center of your screen and that's it.  (It's easier to see where the spot is if you rotate the camera back and forth a bit-- the CoM is the part in the middle that doesn't move when you rotate the camera, because it's at the center of rotation.)

Dunno if that's good enough for your purposes, but it can give you at least a rough idea.

[king of nowhere]

2 hours ago, Snark said:

Well, at least at a very rough level, just look where the camera is pointing (i.e. at the middle of the screen).

Unless you manually set the camera to focus on sometihng else, by default the game always points the camera at the CoM of the current vessel.  So you can just look at the center of your screen and that's it.  (It's easier to see where the spot is if you rotate the camera back and forth a bit-- the CoM is the part in the middle that doesn't move when you rotate the camera, because it's at the center of rotation.)

Dunno if that's good enough for your purposes, but it can give you at least a rough idea.

it's not good enough for my purposes, as the push can be off-center by just a little bit and it will already set the ship rotating. even with a dozen large reaction wheels.

and while this game has a mod for everything, i am reluctant to install them.

ultimately, i solved this by trial and error. deactivate sas, turn on rockets at full speed, see which way the ship deviates. shut down one of the engines on the opposite side, and repeat the process. i was able to find a good enough balance that sas could keep me stable by shutting off two of my 24 engines. with that i was able to land on minmus and refuel, and with full tanks the ship is balanced.