RCS Thruster Placement Tips?

[IriathZhul]

I've been having trouble getting my RCS thrusters to behave properly. I know I'm supposed to place them as far as possible from the center of mass, which I've done. The ship will turn fine enough, but when it comes to translation maneuvers, anything other than forward/backward makes the ship behave very, very strangely, moving in directions that have nothing to do with what I'm telling it. It's basically uncontrollable and it's becoming impossible to perform docking maneuvers.

So what's the best way to place the thrusters? I've been using the 4-way block thrusters, usually with four of them in 4-way radial symmetry on one side of the ship, far away from the center of mass. Any help at all is appreciated.

[SalmonellaDingDong]

You're supposed to place them equidistant from center of mass, not necessarily the farthest away you can. For small ships you can just stick the rcs blocks right on the CoM and get away with it.

[boa3532]

Another way to look at it... Build your portion of your ship (command module if you will) you expect to be in orbit with. Turn on Center of Mass and place the RCS right in the center. Then build the rest of the ship that you'll use to launch into space.

Or build your entire rocket, take off the parts you'll use to launch the command module, turn on CoM, align RCS in the center of CoM, then reattach the rest of the ship.

[GoldenShadowGS]

All you need is 4 RCS centered on the center of mass of the final stage. The more mass your ship has, the more sluggish maneuvers become. Adding more RCS just helps you waste monopropellant faster, especially if it has to fight itself, correcting your heading(using ASAS) when translating.

[SmallChange]

Salmonella is right. The trick is placing them equidistant from the center of mass. Once you've done that, I suggest RCS balance testing on the pad by turning off the gravity in the debug menu. One you turn off the gravity, you can translate all around the center.

BTW, would you guys consider that cheating if only used for research?

[UNSC]

I don't understand how you could cheat in a single player game.

[GoldenShadowGS]

We need a gizmo that shows center of RCS thrust that we can use to help line up the RCS ports to the center of mass.

[chainedlupine]

I am assuming that is what is happening to you is that your ship is skewing as you apply translation thrust.

In other words, there is an imbalanced load and applying translation thrust causes your ship to start to rotate. Possibly even skew rather violently (if the imbalance is great).

Four 4-way thruster blocks placed at the dead center of mass will always work for small ships, because their CoM doesn't shift much. For large ships, your CoM shifting due to fuel burning will make such a simple arrangement stop working.

The lever-arm approach works, which is what has already been suggested to you -- Two sets of four 4-way thrusters, equidistant from your CoM. However, even this system will start to fail if your CoM shifts a lot.

One method that works really well (from what I hear, haven't tested myself) is to use radial thrusters only. Instead of placing four 4-way blocks, you place two linear thrusts along the exact long axis of your ship. Repeat this for the two major axis of your ship. This is supposed to make it easier for you to correct against CoM shifting, particularly if you place multiple sets of linear thrusters and then activate/deactivate them as your CoM changes. ie: Change the length of the "lever" you are using.

[Vanamonde]

Are you losing control because of thruster placement, or because ASAS goes berserk with the RCS and gets into a continual spasm of over-corrections? Those are two separate issues. If the latter, try putting SAS on the ship instead of ASAS, which will steady the ship but leave RCS to your manual control.

[SciMan]

@Vanamonde: So very true, as a matter of fact I can't remember a single ship I've built since 0.18 that had ASAS on it, mostly due to the "continuous over-correction" you mentioned, but also because I use MechJeb.

However, while MechJeb is much more well-behaved than ASAS with respect to over-correction, it's still not perfect, so I have developed a few habits that may or may not make a difference, but they seem to work well enough:

1. I always use at least one SAS module per MechJeb on any ship I build, so that there is at least a little damping of rotation near MechJeb's desired attitude set-point.
   
2. I ONLY let MechJeb use the RCS when the ship is extremely hard to turn with pod torque, and even then I usually add another SAS unit first. This is because MechJeb is always making tiny attitude corrections, and if the RCS was on, it would be using monopropellant when it doesn't need to.
   
3. When I use MechJeb to align a ship to a docking port, I usually manually line it up by eye first, it seems to take less time that way. Additionally, I can use the RCS when eyeballing the alignment, which further speeds up my docking.
   
4. If I have a ship that is tumbling in space and time warp will not engage due to "under acceleration" or something like that, I usually try to kill the rotation with manual controls first, then let MechJeb null out whatever rotation is left over.
   

Basically, each of these habits/strategies/methods I use is designed to allow me to take full advantage of MechJeb's superior pointing precision, while at the same time limiting its ability to quickly turn the craft in case it either does something I did not intend it to do, or to save monopropellant for when it is actually needed (docking translation maneuvers, for example).

Due to MechJeb being better behaved than ASAS, these techniques should be useful to people who use ASAS in their designs, as well.

[bsalis]

The CoM has moved as the fuel is consumed. That is why translation is causing the craft to turn.

[Cannon_Fodder]

RCS used to increase their thrust output to self balance the rocket. There were exploitable bugs with the calculations. So now they have a fixed amount of thrust. I am hoping they sort of change it again. Give them a MAX thrust then let the calculations dial back the thrust to keep things balanced.

[engraverwilliam]

use the linear thrusters. put four at center of mass. place two on the nose and two facing aft. you will thank me later. dont use the four way RCS they waste fuel and are harder to control.

[Tw1]

I read somewhere you might be able to see how the CoM moves as fuel depletes when they make the tweakables system.

Also, I made this thread suggesting extra tools to guide precise placement of things like RCS. http://forum.kerbalspaceprogram.com/showthread.php/25485-An-idea-to-make-the-centre-of-X-system-more-useful-in-craft-design

[RoboRay]

Because of the ASAS (and even MechJeb) oscillations, it's much more propellant efficient to to put a single set of RCS around the craft's center of mass, Apollo CSM style, than having multiple sets spaced out from the CoM.

You can do this with quads, like Apollo, or with linear thrusters pointed outward. Just remember to add forward/aft thrusters if you go with linear ports. The linear ports are the most efficient way to do it, because they provide no attitude control and so waste no monoprop on the oscillations.

You can make it easier to keep the craft balanced by putting your fuel tank in middle instead of down by the engine.

Edited April 25, 2013 by RoboRay

[IriathZhul]

Are you losing control because of thruster placement, or because ASAS goes berserk with the RCS and gets into a continual spasm of over-corrections? Those are two separate issues. If the latter, try putting SAS on the ship instead of ASAS, which will steady the ship but leave RCS to your manual control.

I haven't been using ASAS, since combined with RCS it just burns monopro as fast as if I'd been rotating constantly. I use ASAS once I've manually used RCS to stabilize myself, but then I turn it off and let the default torque do the work, and it's been fine for most maneuvers. The RCS problem is when I'm trying to translate in pre-docking maneuvers. It's madness.

I dunno. My ship isn't that big (just a small tugboat with 1440 fuel and some nuclear engines), so maybe it's handling fine and I'm just terrible at docking.

[NeoMorph]

I actually stumbled onto a really good setup. It also uses the single ring of RCS pods like apollo but offset slightly to give better torque. You then add a set of four "place anywhere" RCS nozzles around the CoM.

The secret to having it work great for translation is setting the quad pods on an action group and you toggle them off when you want to use translation. Gives awesome control, even on big wieldy rockets. When you need to realign you use the gyros for fine control and turn on the quads for gross control. Saves MonoProp too as you don't get the firing all over the place when just staying in one position.

I used to HATE balancing my RCS setup and it used to drive me nuts. Now I seem to have made docking about 100x easier.

[ComradeGoat]

I'm having great success with the unidirectional thrusters only: a ring of four at the top, another ring of four at the bottom (or positioned equidistant from the COM if it's not central) and a few on each end. This has no ability to control roll, but that doesn't matter all that much for docking and if you have a crew module, the reaction wheel will handle that.

This arrangement seems to be very tolerant to the COM changing and to carrying off centre loads. Just approach with SAS turned on and small rotational forced will be held at bay. It almost never gets into annoying monoprop-wasting oscillations, like the ones that can happen with the 4-ways, which I've pretty much given up using now.

[Malaugrym]

Salmonella is right. The trick is placing them equidistant from the center of mass. Once you've done that, I suggest RCS balance testing on the pad by turning off the gravity in the debug menu. One you turn off the gravity, you can translate all around the center.

BTW, would you guys consider that cheating if only used for research?

Computer simulations exist. Reference: Scott Manley.

[Merinsan]

I've been having problems too, primarily because of the mission objectives. I am docking fuel tanks into a space station, using a small probe. The fuel tank needs RCS blocks, because if not, it'd be unbalanced. The probe needs them, because after docking it flies off to deorbit. Lastly, I want no more than 4 blocks, because I need to keep part counts low.

It never occurred to me before that I should switch some off. I'm going to give it a try tonight.

[foamyesque]

The thing about putting them far from CoM mostly applies if you're using them to assist in attitude control on really big (multiple hundreds of tons) rockets and/or stuff with no pod magic torque. For translation purposes it's irrelevant and putting them further away from the CoM will result in exaggerating any skew issues going on.

Also note: radial distance from the CoM still counts as distance from the CoM and will provide attitude control lever arms, just as it did with the Apollo CSM pictured above.

[iDan122]

You place them symmetrically on an equal distance from the center of mass of your orbiter. The law of the Lever here is used: the momentum of a side from the point of pressure is distance to the force by te force pointed perpendicularly from the lever. Since the impulse of the thrusters is the same, everything depends on the distance.

[nebfer]

We need a gizmo that shows center of RCS thrust that we can use to help line up the RCS ports to the center of mass.

My Brother wants this.