How should I place RCS thrusters?

[DaElite101]

I heard something about placing RCS thrusters and CoM. Ideally, where should I place them so that they're most efficient?

[DD_bwest]

i believe you want the center of thrust for the rcs to be as close to CoM as you can or you will get some rotational forces when you want to translate up, down, left n right.

i could be wrong. i remember hearing it should be on the CoM not long after i started, and never really looked into why lol i just accepted that it works lol 

[foamyesque]

The end goal, as @DD_bwest says, is to get your RCS thrust to be aligned through your CoM.

How you do that depends on how much torque you need to get out of them. The Apollo spacecraft, for example, put them close to the longitudinal CoM; the lever arm of the space-craft's radius was enough to provide the needed torque. KSP rockets tend to be much bulkier, so sometimes you do paired nose & tail setups for pitch and yaw control. Doing it that way makes it trickier to balance to the CoM, though.

 

Edited September 10, 2016 by foamyesque

[DaElite101]

How do I view the CoT for the RCS thrusters only? 

[foamyesque]

Just now, DaElite101 said:

How do I view the CoT for the RCS thrusters only? 

 

With mods. I believe RCS Build Aid allows it, but I can't say for sure since I don't use it. I usually place mine by eye and fiddle.

[DaElite101]

1 minute ago, foamyesque said:

 

With mods. I believe RCS Build Aid allows it, but I can't say for sure since I don't use it. I usually place mine by eye and fiddle.

Okay. Wow, that was a fast reply. Also, I'm going to put large reaction wheels on my mothership. Should I place them at the ends so that they don't bend the craft?

[foamyesque]

Just now, DaElite101 said:

Okay. Wow, that was a fast reply. Also, I'm going to put large reaction wheels on my mothership. Should I place them at the ends so that they don't bend the craft?

 

It depends. Reaction wheels don't care about their placement, but if the stack is floppy they can cause the ends to swing around a lot. Can't really advise you specifically; it depends a lot on your mothership.

[DaElite101]

So, just as long as I add struts/beams and stuff to make it more stable, the stack won't bend the ship?

[foamyesque]

6 minutes ago, DaElite101 said:

So, just as long as I add struts/beams and stuff to make it more stable, the stack won't bend the ship?

Generally the weak points are decouplers, struts, and the ends of radially mounted stacks. Some struts running across the gaps or tying the ends together generally work. A screenshot would help here

[DaElite101]

3 minutes ago, foamyesque said:

Generally the weak points are decouplers, struts, and the ends of radially mounted stacks. Some struts running across the gaps or tying the ends together generally work. A screenshot would help here

Oh, okay. Sorry to bother with all the questions, I really appreciate your help. Here is the ship in question:

Here's the ship and the cargo bay view. There are struts connecting the reaction wheel stack to the cargo bay walls, and on the underside, mirror struts connecting the stacks to the cargo bay floor. 

[foamyesque]

That's an awful lot of reaction wheels. What are you trying to do with them?

[DaElite101]

17 minutes ago, foamyesque said:

That's an awful lot of reaction wheels. What are you trying to do with them?

I'm going to dock this craft with others to build the mothership. RCS of course adds weight and I only use it for docking. However, with most of my projects being large and with my limited knowledge of placement, I've had no choice.

EDIT: The mothership has to be able to go places, which means it has to be able to turn to actually get going, obviously. This will consist of a lot of docked modules, so it's going to need a lot of reaction wheels and docking connections.

Edited September 10, 2016 by DaElite101

[foamyesque]

Hm. I think you're carrying more wheels than you need; the piece in the screenshot should only need one, perhaps two. For orbital work, turn off SAS and just be patient, maybe with higher timewarp. The piece in your screenshot should be fine, but large amounts of torque could cause problems with the docking port connections.

 

As far as the RCS thrusters go, have you thought about using the LFO vernor thrusters? They have more thrust, better TWR, better Isp, and don't require a separate kind of tank (and LFO tanks have better mass ratios than monoprop ones). For purely translation purposes, a set of four spaced around your CoM will cover dorsal, ventral, and lateral motion, and fore/aft can be done by putting them at the front and back of the sidemounted Mk2 stacks. If you also need pitch and yaw, instead of the CoM-mounted translation thrusters, do two sets of four, fore and aft. That gives you pitch and yaw as well as the same translations, it's just a little trickier to balance them out to avoid rotation.

 

How much does your CoM move, tanks empty v. tanks full, and what are you planning to put on those docking ports?

[DaElite101]

16 minutes ago, foamyesque said:

Hm. I think you're carrying more wheels than you need; the piece in the screenshot should only need one, perhaps two. For orbital work, turn off SAS and just be patient, maybe with higher timewarp. The piece in your screenshot should be fine, but large amounts of torque could cause problems with the docking port connections.

 

As far as the RCS thrusters go, have you thought about using the LFO vernor thrusters? They have more thrust, better TWR, better Isp, and don't require a separate kind of tank (and LFO tanks have better mass ratios than monoprop ones). For purely translation purposes, a set of four spaced around your CoM will cover dorsal, ventral, and lateral motion, and fore/aft can be done by putting them at the front and back of the sidemounted Mk2 stacks. If you also need pitch and yaw, instead of the CoM-mounted translation thrusters, do two sets of four, fore and aft. That gives you pitch and yaw as well as the same translations, it's just a little trickier to balance them out to avoid rotation.

 

How much does your CoM move, tanks empty v. tanks full, and what are you planning to put on those docking ports?

Yeah, I have considered them, but for a craft this large I figured it'd be tricky. Right now I'm going to try launching and testing it. I'm planning to put a vehicle with several docking ports for crafts to dock, and a orbital VAB-esque structure surrounding a central docking port. On the side ports I plan to put docking areas for 2 SSTOs along with extra fuel tanks and power supply. I also plan to add ISRU onto the craft and one of the SSTOs will deliver ore to it. I have OPM and I'm going to mess around and explore in the Sarnus system first. I know that an SSTO can haul it's own ISRU around but I prefer to have it on the main vehicle. The ISRU/Ore storage will go around the structural core (Not the ship I have shown you) on both sides to resolve any balance issues. The plans I have laid out are vague, but once I start prototyping it should be more clear.

EDIT: Actually, I'll go to sleep now. Right now it's really late where I am.

Edited September 10, 2016 by DaElite101

[juanml82]

I monitor their placement by using the RCS Build Aid mod.

I also use 5 way rcs blisters from the MRS mod or MK2 expansion.

I usually place two "rings" of blisters, in 4x symmetry, (total of 8 blisters, at two different heights) checking with RCS Build Aid that they produce as little undesired movement around the average CoM as possible. By using two rings when rotating, the top set of blisters pushes in one direction while the bottom set of blisters pushes in the other direction, which increases the rotation speed.

[dire]

One subject that's only tangentially related to your RCS problem: you're using an awful lot of those mk0 LF cans. You could easily swap all 4 of them for a mk2 LF tank just before your adapter and it would make that part of your craft significantly less wobbly. Reducing part count also reduces lag, which will be an issue with the sort of large station you're planning to build.

Another suggestion, you could probably cut 2 or 3 SAS modules out of your payload; since you're going to want your other station parts to also be able to rotate in order to dock, they're going to need to bring SAS modules of their own. SAS torque doesn't care where it is on the ship. RCS torque, on the other hand, -does- care where it is. The further out from your CoM each individual RCS port is, the more leverage they'll have to help rotate your ship for those really tight turns. In terms of translation, people have already mentioned that they should be centered about the CoM.

I also noticed that you are strutting your solar panel/radiators. I have to wonder how you plan to get them into orbit like that. Are you going to use KAS/KIS and assemble them in situ?

[Diche Bach]

I probably do not do it quite right, but here is my scheme in synthesis:

1. Always 4-point radial symmetry. I've used 3 and that is functional, but 4 seems to work better.

2. Try to place thrusters so that all thrusters on the vessel line up relative to the cross-section of the vessel (difficult to explain this, and I recently uninstalled some mods, so now several of the craft that would exemplify it best cannot be loaded in VAB . . . but: imagine a rocket that starts out pointy on top, spreads out to 0.625 at the beginning of the command module, then expands to 1.25, then eventually to 2.5. In this case, I'd place the RCS groups that attach to the smaller diameter portions on the end of girders so that those RCS packages lined up with the ones attached directly to the surface of the 2.5m diameter. WASD mod, Editor Extensions Redux, and a bit of clipping are integral to this).

3. ESSENTIAL: the sets of 4 MUST be lined up with respect to the two planes the four thrusters form through the center of the vessel. You should be able to start at one of the thrusters in a set of four at the top, and draw a straight line from it down the whole length of the vessel, and every subsequent thruster below it must line up with it as close to perfect as possible.

4. Always try to have at least one set of 4 at the superior ("top" prograde) end of a vessel, one set of 4 at the inferior ("bottom"), include more in the middles depending on mass of the vessel and space them roughly equally. So the final "payload" section will have at least two sets (one "top" one "bottom") then the stage below it will have at least one toward its "bottom," and the next stage same, all the way to the lowest stage where I will use RCS; I often do not use it until in space, but sometimes really clumsy craft need it right from the launch pad to maintain maneuverability.

5. For each section of the vessel that might need to be involved in docking manuevers, make at least one of the sets of four thursters comprise five-way thruster modules (either one of the integral modules or five of the "put anywheres" arranged on a girder end) to allow for prograde/retrograde translation and more effective roll.

6. Place most or all monopropellant fuel tanks toward the final stage so it doesn't get inadvertently jettisoned with an otherwise spent stage. The game seems to do a good job of using up stuff from bottom to top, so including a fraction of the mono fuel in lower stages is okay.

This seems to work pretty well for me. Can dig up some screen caps if anyone is curious to see what I'm talking about. Be curious to hear if anyone who knows more than me has any helpful suggestions for my particular approach.

ADDIT: here is one that shows pretty much what I'm talking about. The larger diameter lower stage(s) has already been jettisoned, but just imagine there was a larger cylinder attached below the engine that had RCS attached directly to the tanks, thus those RCS modules on the girders would "line up" with them.

This module is a 4 Kerbal capacity passenger vessel that I'll be using for tourists contracts (1 pilot and up to three tourists) and later in the game I'll often keep one on my backup launchpad for rescue missions and also use it for personnel transfers to bases and stations and the like: thus a bit of extra battery power, some supplies (the food carton looking things with green markings) and solar panels to allow for up to about a week in space when at max Kerbal load.

I've tended to use the linear "place anywhere" ports more than these integral 5-point modules (which come from a mod I think). The place anywhere smaller version has like twice the thrust power of these. But I recently noted that five of those place anywheres are about 4 times more massive than one of these integral 5-point modules so I'm checking out using them instead.

Edited September 10, 2016 by Diche Bach
pic more detail

[DaElite101]

15 hours ago, dire said:

One subject that's only tangentially related to your RCS problem: you're using an awful lot of those mk0 LF cans. You could easily swap all 4 of them for a mk2 LF tank just before your adapter and it would make that part of your craft significantly less wobbly. Reducing part count also reduces lag, which will be an issue with the sort of large station you're planning to build.

Another suggestion, you could probably cut 2 or 3 SAS modules out of your payload; since you're going to want your other station parts to also be able to rotate in order to dock, they're going to need to bring SAS modules of their own. SAS torque doesn't care where it is on the ship. RCS torque, on the other hand, -does- care where it is. The further out from your CoM each individual RCS port is, the more leverage they'll have to help rotate your ship for those really tight turns. In terms of translation, people have already mentioned that they should be centered about the CoM.

I also noticed that you are strutting your solar panel/radiators. I have to wonder how you plan to get them into orbit like that. Are you going to use KAS/KIS and assemble them in situ?

I'm going to put it into the VAB and launch it that way with a couple of 3.25m rockets.

[dire]

Well, you don't need to worry about the monstrosity of drag that they would be because they're going to break sometime early on your ascent to orbit. Solar panels can't handle dynamic forces; they shatter.

[DaElite101]

7 minutes ago, dire said:

Well, you don't need to worry about the monstrosity of drag that they would be because they're going to break sometime early on your ascent to orbit. Solar panels can't handle dynamic forces; they shatter.

Actually, they were just fine. They survived to orbit but I was too lazy to do anything with it. Too much lag. The mothership idea was too much. Guess I'll pick it up again when I get a better computer.

[foamyesque]

3 hours ago, dire said:

Well, you don't need to worry about the monstrosity of drag that they would be because they're going to break sometime early on your ascent to orbit. Solar panels can't handle dynamic forces; they shatter.

 

The fixed panels that he was using don't.