RCS - less is more?

[mellojoe]

For docking with other vessels in space, is it better to have fewer RCS thrusters? or more?

I've gone with the more is more philosophy, but I struggle so much docking. I've even tried MechJeb docking assist, and it just seems that it dances all over the place.

I'm going to attemt to build a docking vessel with as few RCS thrusters as possible and see if that helps or hinders.

What do you guys think? What have you found?

[deadshot462]

For smaller ships/landers, fewer RCS thrusters is definetely better.

For bigger ships/rockets/boosters, just a few extra well placed RCS thrusters can make a big difference. On bigger ships, more RCS thrusters just help rotate faster.

I would keep the number of thrusters low on any ship that has to do fine adjustments for docking/maneuvering

[Camelotking524]

You can dock without RCS if you want a challenge, but placement matters more then number IMO.

[Atanar]

For docking placement is key. See that you place the 4-thruster block at the height of the centre of mass in 4-symetry and align them with the cardinal directions, this makes it most controllable with the least amount of RCS.

And yes, you can dock over 100 tons with those 4 thrusters:

(the rock is 600 tons from a challenge and the craft itself about 130, the docking was the 2 big docking ports, docking was done manually using less than a 1m RCS contrainer, the thrusters are hidden on the middle tank)

Edited June 8, 2013 by Atanar

[NeoMorph]

Use the gravity hack to balance your RCS above the launch pad.

http://forum.kerbalspaceprogram.com/showthread.php/28983-How-To-Balance-RCS-On-Your-Payload?highlight=balance

[Conarr]

I've found that on smaller vessels putting the RCS blocks at the center of mass makes it pretty responsive, but on larger vessels the farther from the center of mass the better.

[Atanar]

I've found that on smaller vessels putting the RCS blocks at the center of mass makes it pretty responsive, but on larger vessels the farther from the center of mass the better.

He's talking about docking. Having thrusters off center for translation is a waste of fuel. Turning is a diffrent thing of course.

[Eric S]

For docking placement is key. See that you place the 4-thruster block at the height of the centre of mass in 4-symetry and align them with the cardinal directions, this makes it most controllable with the least amount of RCS.

And yes, you can dock over 100 tons with those 4 thrusters:

With patience yes. For those that can't manage the patience do NOT use 8-symmetry, you'll waste a lot of RCS fuel that way. You'll want the same 4 symmetry above and below the center of mass by equal distances, and you'll want them at least 2.63 times as far above or below the center of mass as they are out from the center of mass. So if you're doing this on a 2.5m central stalk, you'll want the RCS quads at least 3.27 meters above/below the CoM. If you don't, then the quads can do odd things trying to compensate for the fact that the thrust they want isn't exactly on a cardinal direction. NOTE: I haven't done testing to see at what point the RCS quads start firing two at a time, so you may not actually need them that far, that's just the value I use to be sure.

He's talking about docking. Having thrusters off center for translation is a waste of fuel. Turning is a diffrent thing of course.

I think he's talking about balanced above and below the CoM, which doesn't change translation, only rotation. Not having your center of RCS thrust in the same place as your CoM is just plain bad in every way, if that's not what he's saying.

Edited June 8, 2013 by Eric S

[Pontiac]

@Atanar

Wouldn't that still depend on how you're docking and what you're docking with? If you're presumably lined up along the Z, with a momentum going towards the docking point, you've got left, right, up and down to play with still. If you have RCS in the center, just as a pair or quad, and if you're positioning of the RCS is off by a little bit, you could introduce rotation. If you put a quad at the ends of your vessel equi-distance from the COM, you'll be able to evenly move the 4 directions with less chance of rotation.

[Conarr]

He's talking about docking. Having thrusters off center for translation is a waste of fuel. Turning is a diffrent thing of course.

Doh! I sit corrected. Sorry guys.

[steveman0]

I usually do a set of 4 equally above and below the center of mass. Even with the largest of vessels this has proven to be enough for docking. I use the docking camera to get my rotation right first and then lock it in with ASAS and just the smallest burst of RCS will get the translation right given enough time. I've docked >100 tons with just those 8 blocks and ~30 RCS fuel.

[Atanar]

@Atanar

Wouldn't that still depend on how you're docking and what you're docking with? If you're presumably lined up along the Z, with a momentum going towards the docking point, you've got left, right, up and down to play with still. If you have RCS in the center, just as a pair or quad, and if you're positioning of the RCS is off by a little bit, you could introduce rotation. If you put a quad at the ends of your vessel equi-distance from the COM, you'll be able to evenly move the 4 directions with less chance of rotation.

Good point. But what if you don't get the 2 distances exact? would that not rotate the craft as well? Is it really less chance? It all boils down how close you get the "centre of thrust" of the RCS to the COM. Too bad we can't display that.

[steveman0]

Good point. But what if you don't get the 2 distances exact? would that not rotate the craft as well? Is it really less chance? It all boils down how close you get the "centre of thrust" of the RCS to the COM. Too bad we can't display that.

You can still end up with rotation but if they are far enough out it is easy to get it pretty close and then ASAS will keep you from turning with small corrections. When making a translation velocity adjustment the RCS remains on for another second or two after I let off to counter act the rotation and then the whole rocket just goes into a silent drift until the next manual correction is needed.