Translating without rotating

[kiwiak]

How can i achieve that?

How do i place mu rcs thrusters?

[Captain Sierra]

What's happening, if you haven't already figured out, is that your RCS is unbalanced and is creating torque forces during translation. To fix this, you'll need to balance them. This can be done by first displaying the center of mass. If you don't already know, this is done by pressing the weight icon in the lower left in the editor.

Now, you want to place either one or two sets of thrusters in 4x radial symmetry. 2 is good for both rotation and translation, while 1 is better if you have sufficient torque from reaction wheels to rotate.

For 1, place them exactly level with the center of mass. It's advisable that you double check this and adjust with your fuel tanks tweaked to be half full (or whatever they'll be at roughly while you are translating) to best balance.

For 2, you want to place them a distance off the center of mass, BUT you want them to both be the same distance away. This setup also enables rotation and is harder get out of line by center of mass shifting, but is also more difficult to balance from the start.

Hope this helps!

-Sierra

[kiwiak]

Thanks.

Maybe i shoudl use some fuel balancing plugin to prevent com shifting?

[Anton P. Nym]

Placement of the RCS blocks is the biggest part; as Cap says above, symmetrical about the centre of mass (either on it, or pairs of blocks on either side at equal distances from it) will do the trick.

Another helpful tip is to leave SAS on; this will let the system make corrections for you if you're not exactly on-spot with your blocks. Since the CoM tends to move as you use propellant, it's hard to get it perfect for all cases.

When you're up close to your target, hitting the Caps Lock key (if you're using the standard controls) will switch your RCS to "fine" mode to give you more precise control... this also automatically balances the RCS thrusters to compensate for shifts in the Centre of Mass and will help kill unwanted rotation.

-- Steve

[lajoswinkler]

With SAS function on, there isn't any rotating due to slight center of force offset relative to center of mass. Always leave it on.

You can go with 3 RCS thrusters, you don't need 4 of them because they will compensate, but I mostly use 4 to get more force.

Or you can use thrusters distally (if you want more torque for rotation), but their total net center of force needs to be in CoM.

[Claw]

As an example, here are two basic options you can go with.

Option A:

Place two sets of thrusters around the body of your craft, about equal distance away from your Center of Mass.

- This gives you the benefit of double the RCS thrust

- You can translate and rotate independently using this.

- This allows the SAS to use differential thrust to account for CoM movement.

Option B:

Place one set of thrusters around the CoM of your craft

- This keeps your weight down.

- You can translate and rotate independently using this.

- Doesn't adjust well to big CoM movement.

If you happen to only place one set of thrusters at only the top or bottom (as labeled for Option A), then you will end up rotating while trying to translate. The closer your thrusters are to the CoM, the less you will rotate.

Hope that helps.

[m4rt14n]

how about for space plane? Where should you place them ?

[Captain Sierra]

how about for space plane? Where should you place them ?

spaceplanes are tricky, as there are limited spots. You may want to consider using a combination of RCS blocks and linear RCS ports. The same methods apply, so use your better judgement and experiment around. A quick trick for testing is to use hack gravity and test it on kerbin without the need to go all the way to space. You can tweak the fuel levels to simulate as if you had just burned up to orbit.

[Taki117]

I recommend getting RCS build aid, it shows you how various translations with RCS will affect your ship. Its very useful.

[Claw]

how about for space plane? Where should you place them ?

For space planes I typically worry more about placing RCS blocks for translation and leave the rotation to torque (if we're talking about small ships). So the "Place-anywhere" linear ports (Like Capt Sierra said) are usually easier to deal with. You can put a few by the nose and some tucked up on the sides of the tail behind the wings. I find the linear ports are easier to deal with because the RCS thruster blocks tend to get blocked by wings, tails, engines, etc. You can certainly use them, but they get a bit tricker to keep from blocking. EDIT: That being said, placement considerations like the picture I posted still apply.

And yeah, gravity hack makes space plane RCS designing a lot easier. Launch your ship and press Alt-F12. That will bring up the debug tool bar. From there, you can just select "Hack Gravity" and play with the thrusters.

[arq]

If you use fine control (default key: CAPS LOCK), it will autobalance your RCS by modulating thrust. Even the most carefully constructed and balanced craft will not stay that way when you begin burning off fuel, so this (or a mod) is the best way to do it.

[kiwiak]

I didnt know that fine controll ballances thrust of rcs!

I thought that it just reduces torque power.

[Claw]

Fine control reduces the amount that you thrust. This means the SAS has extra authority to keep the thrust balanced.

I don't think that fine control, by itself, auto balances, but I would have to try.

[Anton P. Nym]

Fine control reduces the amount that you thrust. This means the SAS has extra authority to keep the thrust balanced.

I don't think that fine control, by itself, auto balances, but I would have to try.

I think fine mode's RCS thrust modulation was added in v0.21; I could be wrong on the version, but I do remember the devs saying it'd been introduced.

-- Steve

[Kasuha]

Isn't this simply controls issue? With RCS, you rotate using WASD and translate using HNIJKL keys. If you have SAS on, it usually does not matter much if your RCS thrusters are balanced or not.

[Anton P. Nym]

Isn't this simply controls issue? With RCS, you rotate using WASD and translate using HNIJKL keys. If you have SAS on, it usually does not matter much if your RCS thrusters are balanced or not.

SAS helps damp that out, but only to a degree; I find if RCS positioning is off by enough (or you don't have enough torque to compensate) then there will still be some unwanted motion to skew the maneuver. SAS will eventually damp that out but the orientation will be off at the end and require manual correction.

-- Steve

[Kasuha]

SAS helps damp that out, but only to a degree; I find if RCS positioning is off by enough (or you don't have enough torque to compensate) then there will still be some unwanted motion to skew the maneuver. SAS will eventually damp that out but the orientation will be off at the end and require manual correction.

-- Steve

My point was that if you use WASD (and don't switch to docking mode), you'll always get rotation, regardless how well your RCS thrusters are balanced. OP was not very specific about what his problem is.

[Kerba Fett]

Another way to keep your RCS balanced is to put a ring of them in line with the COM as already suggested. (I use 4 way symmetry) Then using the tweakable controls empty your fuel tanks. Place another ring of RCS blocks in line with the new COM. Finally you place a third ring of RCS blocks halfway between the others. Now refill the fuel tanks and assign each ring of RCS thrusters to an action group.

The idea is that when your ship is full of fuel you use the first ring of RCS blocks you placed. When it's empty you use the second ring of RCS and when it's half full you use the middle ring of RCS. You can also use the first AND the middle ring for 1/4 fuel or the second AND the middle ring for 3/4 fuel.

The three sets of RCS blocks allow you to move your center of thrust as your center of mass changes.

[codepoet]

Also bear in mind that the CoM will move around as fuel is consumed from your tanks. You can use a fuel balancer mod to move it back to where it needs to be for RCS to be balanced. I recommend the PWB Fuel Balancer for doing this, (but then I would as I wrote it!) Others are available (TAC fuel balancer).

Edited February 6, 2014 by codepoet

[Jouni]

Another option is to learn to live with unbalanced RCS. It helps if you don't have too much thrust. If a short tap on translate controls changes the velocity only by 0.01 m/s, the lack of balance is pretty much a non-issue.

Things are a bit harder with lighter craft, because the RCS thrusters are so powerful. If you need RCS only for docking, the Docking Port Alignment Indicator mod can be very helpful. When the ship starts rotating after using translate controls, you just compensate with rotate controls to keep it pointed at the orange marker.