Need tips for RCS placement for docking

[Invader Jim]

I found some old threads and some people say put RCS on CoM and some people say put them equidistant from CoM. Have things changed for 1.0+? I never pulled off docking in .90. Which thrusters and where?

Any simple tips and pics would be appreciated. Thanks!

[capi3101]

If you're not adverse to mods, I'd suggest you try out RCS Build Aid; it was designed for this very purpose. Also comes in real handy if/when you want to start puttering around with spaceplanes. It is mostly working in 1.0.2 though for best results (for the moment anyway) you'll want blizzy78's toolbar as well.

I've generally found that the Apollo-style, 4 multi-directional thrusters on the CoM works best. Now, if you've got a massive vessel that doesn't turn particularly easily, you might want to consider sets on either end to assist with rotation (and RCS Build aid would help you position them with a minimal amount of unintentional torque). By and large though, for docking purposes you'll want RCS mainly for translation, and that works best with sets near the CoM.

I might also suggest NavyFish's Docking Port Alignment Indicator as another vastly useful mod when it comes to simplifying the docking process. That one is definitely supported in 1.0.2, and it makes docking much easier; when he featured that mod, Scott Manley put the interface over the navball because he didn't need it with DPAI in action - that's how useful it is.

[Invader Jim]

I docked without any mods. I am going to try the RCS Build Aid. I used four RV-105 block thrusters positioned on Center of Mass. However, it was not quite precise enough. I would tend to move in unintended directions slowly and lose my north-south orientation. I fixed it by switching to SAS, correcting my orientation and then proceeding with RCS. I wish RCS would work just like Kerbalnaut RCS jet packs. Then I would have no problem docking.

[Gaarst]

It depends on what you use RCS for.

If you want to move your ship around then you have to put your RCS on the CoM.

If you want to orientate your ship more easily then you have to put your RCS as far from the CoM as possible because torque depends on the distance to the CoM.

When translating, if your RCS is only off your CoM by a few centimeters it will induce torque in the ship, if your SAS is not active or not strong enough to counter it.

Or you can decide that RCS is for the weak and dock without it. I did it once because I forgot to put RCS on my ship; the ship was small so it was easier than I expected

[Invader Jim]

It depends on what you use RCS for.

If you want to move your ship around then you have to put your RCS on the CoM.

If you want to orientate your ship more easily then you have to put your RCS as far from the CoM as possible because torque depends on the distance to the CoM.

When translating, if your RCS is only off your CoM by a few centimeters it will induce torque in the ship, if your SAS is not active or not strong enough to counter it.

Or you can decide that RCS is for the weak and dock without it. I did it once because I forgot to put RCS on my ship; the ship was small so it was easier than I expected

Do you use both SAS and RCS at the same time?

[Gaarst]

Do you use both SAS and RCS at the same time?

Yes I do when docking: as I said if you're off your CoM, your ship will gain torque, and as you cannot predict what exact amount of fuel you consume, you don't exactly know where your CoM will be when approaching the ship to dock; so leaving SAS on, set on target (most of the time), will keep your ship to the right orientation when you translate.

Then I usually deactivate it when about to touch the other ship to let them dock correctly.

Edited May 6, 2015 by Gaarst

[Supernovy]

Note that when you're in precision control mode (caps lock) the game does a dumb auto-balancing with RCS, scaling the thrust they put out with the inverse of their distance. If you have the same amount of thrusters on each side of the CoM at the same radial size (e.g. both sets on 2.5m parts), you can translate without rotating and rotate without translating.

[Laie]

Gaarst's advice is valid: if you need RCS to rotate, the ports should be way out; if you want to translate (move sideways without turning), the combined force of all applicable RCS thrusters should be near your CoM.

Your CoM will probably shift a fair deal over the course of the mission, though, but don't sweat it. SAS and reaction wheels can compensate for a lot of RCS imbalance. Placing the pods so that "they won't be too far off, on average" will do nicely.

[Rickenbacker]

Symmetrically placed, equidistant from the CoM, and as far out as you can get them. That's always worked for me.

Oh yeah, and use the CoM you figure you'll have when docking, not the CoM you have when the tanks are full .

[Invader Jim]

Symmetrically placed, equidistant from the CoM, and as far out as you can get them. That's always worked for me.

Oh yeah, and use the CoM you figure you'll have when docking, not the CoM you have when the tanks are full .

How do you know where is the CoM in this case? Now that I have successfully docked my impression is that docking is not nearly as challenging as precisely placing your RSC thrusters in the correct places.

[Laie]

How do you know where is the CoM in this case?

Context-Menu on the tanks and drain them as much as you think it appropriate. You may even go the extra mile and design a vessel where the CoM barely moves while the fuel is being drained. That's what I used to do. But as I said, I found that SAS & reaction wheels can compensate for a lot. You may not slap on the thrusters any old how, of course, but you don't need to be extremely precise either.

[SlinkyBlue]

Symmetrically placed, equidistant from the CoM, and as far out as you can get them. That's always worked for me.

Oh yeah, and use the CoM you figure you'll have when docking, not the CoM you have when the tanks are full .

I identify with this. I have had very good luck using the small structural pylons (the tiny, thin scaffolding in the structure tab) as a way to extend RCS thrusters out farther from the ship. This is very effective.

Give it a shot, OP. If you have 4 RCS thrusters, directly over the Center of Mass, extended out from the hull by a few meters with scaffolding, you gain a massive amount of maneuverability that tends to be very predictable and easy to control. I find doing anything different ends up confusing me quite a bit, resulting in wasted RCS and sometimes multiple attempts to dock.

[LordFerret]

I do RCS placement in stages, balanced and based on CoM. For example: placement on the lander, then placement on the lander + transfer stage, and then placement on the lander + transfer stage + lifter (when required, and when so - using Vernors). Also, I find keeping the RCS blocks on the same plane (part diameters) helpful in balance as well, especially with respect to docking.

[arkie87]

I usually place one set on either side of the CoM for orientation control and one set as close as possible to the CoM for translation, thus, three sets total.

[SkyeRangerDelta]

Based off of what I understand, RCS thrusters for docking should be placed on the center of mass for the payload.

I always place a symmetrical 4 thrusters on the dead center of mass. You can see where it is by clicking the "center of mass" button in the VAB.

[capi3101]

If you're using RCS Build Aid at this point, you want to design your craft such that A) your CoM does not shift during flight (i.e. the "DCoM Offset" should say 0.00 m) and with the mod in Translation mode, the torque is as close to zero as you can get it for the left, right, up and down directions.

You can go ahead and use a twelve-port configuration (four on the CoM to translate, eight to turn) to get the best of both worlds, provided you can stick to those general guidelines; in fact if you're working with a craft that's both beefy and difficult to turn (say if you've got something with no inherent torque controlling the ship like an OKTO2) you'll probably want the twelve-point configuration. Just bear in mind that when you put that the mod will tell you you've got torque after you set the first quad of thrusters on the end of the craft, and then the challenge becomes finding a spot on the other end that cancels out the torque. If you run out of craft, put the second set of thrusters as far down as you can get them and start adjusting the position of the first set. It's an interative process.

Something else you might want to check is torque levels based on the various reference points - there's a button marked Reference that'll let you switch between CoM and DCoM. If you have no torque on both reference points, you can rely on your craft to be completely controllable under all circumstances. If not - and this happens usually when you can't get the CoM to not shift - you'll need to base your thruster placement on which Reference point best matches the intended condition of the craft when you go to dock it. If your goal is to dock a full fuel module, it'll probably still have most of its mass, so you use CoM. If your goal is to dock a nearly empty space capsule, you're probably going to want to use DCoM. SAS becomes necessary if and only if your craft is in a state between the two.

Now, for SAS planning - use your two reference points and find where and in what direction the greatest amount of torque RCS Build aid is telling you will occur with the design. Take the amount of torque and multiply it by 1.5 - that's how much counter-torque you'll need to have available with SAS units. Reaction wheels provide 5 kNm of counter-torque, Stabilizers provide 15, and Big ASAS provides 30. Add what you need and then re-balance your ship's RCS.

Hopefully that's all helpful; just like anything else in KSP, there's a lot of factors to consider when it comes to RCS placement, but it gets easier to know about how to set things up after you've gained some experience. Best of luck.

[cicatrix]

My experience says that placing RCS thrusters equidistant from CoM is more reliable than placing them on CoM. The reason is that CoM shifts as you burn your fuel/oxidizer/monopropellant. When time comes to translation the thrusters that had been perfectly placed on 'wet' CoM suddenly apply parasite torque to your craft since the 'dry' CoM has shifted. The farther your thrusters from the CoM (provided they are symmetrically placed) the easier the translation would be. Of course, you will spend more monopropellant this way but in nearly all situations you don't spend all your monopropellant anyway, personally I bring more than half back home every time.

[KerbMav]

I would like a button to stop RCS from working with rotation on demand.

Edit: And more on topic: Where - if really - is the difference in putting the thrusters on top and bottom, left and right instead of diagonally? (This could for sure be explained better ... )

Edited May 8, 2015 by KerbMav