Stop vessel rotating when using RCS translation?

[dryer_lint]

Yes, as usual, I know the title is confusing... anyways, I've been told that if you want to be able to use RCS to translate a ship, you have to have the RCS thrusters right on the ship's center of mass. I've tried it and it works, but the problem is, now I'm building larger ships and I need more RCS ports. So putting a big group right on the center of mass won't work anymore. Does anyone have any ideas? Probably a mod or something?

Edited June 20, 2014 by Nikola7007
answered

[Kryxal]

Having groups at each end tends to work if they're fairly well centered on the CoM ... and if they become not-so-centered, capslock toggles fine control which balances things better instead of just blasting the ports facing the right direction.

[Jean Deaux]

*Hoping for future releases of bigger RCS thrusters now that we have bigger ship pieces parts!

[Taki117]

THis is the exact thing you are looking for! It gives you a readout of the direction of translation and how much torque is being applied so that you can balance your RCS ports. Also, if you are looking for more powerful RCS ports I suggest KW Rocketry because it has some really powerful ones, it also has some placed at a 45 degree angle so that you can put them off center and still have centered thrust.

[dryer_lint]

Ah, thanks! I knew there must be a mod for it!

[Claw]

Yep, all of the above are good suggestions. I haven't used RCS build aid, but I hear it's good.

If you don't want to use mods, then when you sprinkle RCS ports around your ship, think about the CoM acting like the balance point on a long board. If you don't want (or can't) put matching mirrored RCS ports at both ends, then place them so they balance. If you have 1 port way out on side A that is 10m from the CoM, you can place 2 ports on side B of the CoM 5m away. Or 3 ports at 3.3m away. Or 2 separate ports on side B, 1 at 2.5m and the other at 7.5m.

However you split it up, the number of meters on each side of the CoM (when you add it up) should be equal. Or you can sort of eyeball it and let the auto-balancing SAS help out.

[DeMatt]

If you press the Caps Lock key, you toggle Fine mode on your controls (the arrows on the meters in the lower-left change from orange to blue). This automatically adjusts all your RCS thrusters's output to have the same effective torque.

So if you have your RCS thrusters reasonably balanced, e.g. one ring somewhere forwards and one ring somewhere aft, they'll thrust at a balanced rate and not turn your ship when you translate.

It will, however, reduce their output.

[TMS]

I always wonder why people give the advice about having one ring of RCS at the CoM. That advice was roughly correct prior to 0.21's SAS tweak, but it's been largely rendered moot after successive SAS recoding.

As has already been mentioned, engaging SAS will adjust each RCS port's thrust proportionately to their distance from the CoM, giving a 'levelled' amount of directional force. It can't fix absurd leverage in huge ships, but it will handle most situations, and particularly movements in CoM due to fuel usage.

Best advice is to place RCS rings at the furthest points of your vessel, engage SAS, and let it do it's work. Stick a reaction wheel on there if the vessel doesn't have enough control authority.

For larger and more unbalanced crafts, it's also worth tapping the RCS controls, giving the vessel's SAS the necessary time to torque back into position. As also mentioned, fine control can also help.

[Claw]

I always wonder why people give the advice about having one ring of RCS at the CoM. That advice was roughly correct prior to 0.21's SAS tweak, but it's been largely rendered moot after successive SAS recoding.

Because some people might only want 1 ring of RCS. Some people make great attempts at controlling part counts and/or mass. (Though massless parts are a whole 'nother issue...)

Not having it near the CoM creates a problem, even with SAS control refinements.

[Red Iron Crown]

It was more beneficial to have just one ring of RCS thrusters back when they had mass (pre-0.23.5), especially on smaller craft. Now I use two rings on just about everything.

[SRV Ron]

There will always be a rotation issue when using thrusters for translation. This is because while you can balance them to the ship COM during construction, that COM will change as fuel gets used during flight. Therefore, the best combo is a pair of thrusters on each end of the ship but have SAS enables so it can cancel out the unintended rotation caused by the changing COM of the ship.

[cantab]

IMHO it's still worth trying to balance things well. SAS can be pretty wasteful of monopropellant when it's handling really imbalanced things.

Normally what I do is balance for half-full fuel and monoprop tanks, though if I expect to do most docking with a full or empty fuel load I'll balance for that instead. I don't have RCS build aid so I'm just eyeballing the distances. I'm also mindful of how much CoM shift there is as fuel is burnt off. If a ship can have just the one fuel tank per stack then by balancing the components above and below it you can reduce shift from LFO depletion to next-to-nothing, and monoprop tanks can be placed at or balanced around the CoM like RCS thrusters themselves.