Is RCS worth it for larger ships?

[Fenris]

Well, for me it'll be my largest ship, I know it's tiny compared to what I've seen others do.

I don't know if this will help or not:

Rough ship design:

Height: 90m on launch pad

Fuel: Liquid fuel 10,080

Mono Propellant: 3,000

Where I believe most of the weight will come from:

3 Rockman x200 tanks in center, 1 Rockman x200 tank on 4 nacelle points (for a total of 7 tanks, a top down view makes the ship look somewhat like an "X")

Ship's purpose:

Transport of smaller craft to planets/moons orbit.

Scientific scanning

Transporting of Kerbals

The ship will not enter atmospheres (at least not on purpose). The ship will also not dock, other ships will dock with it.

I'm not asking anyone to do the math for the exact figures, I'm just trying to get a general idea of if it's worth it or not.

Also with positioning RCS thrusters am I better off with an equal amount on the front and back of the ship creating a "pivot point" near the center of the ships mass or could I just cram a bunch on the back of the ship and have the same effect? Also, is this an item were more have a greater effect or do people just put a bunch on for appearance?

Thanks

Edited October 27, 2013 by Fenris

[Skyrunner27]

RCS is always worth it.For big ships and small. Not only can it help stabalise your ship(especially with .21 and .22 SAS), but it can also be used as emergency Delta-V. I have had probes that didn't need RCS to stabalize, but the RCS I brought along allowed them to establish minmus orbit or burn to the Mun on my manned mission when the injection stage fails.

[Jordanpumah]

like he said, but if you have, say, a refueling station in orbit, and it's crewed, all you need is an escape pod to get them out,

but if you can squeeze it in, fire extinguishers are always good

[ScottyDoesKnow]

It's worth it just to save the frustration of slowly maneuvering large ships around. Toss on 4 of the bigger nozzles at the top and at the bottom of the center stack, throw 4 of those little RCS tanks on as well. If you're doing interplanetary stuff, maybe 8? You can try it and find out.

If you're docking, it's important to place the nozzles equidistant from the center of mass. Even if you're not, place them at the top and bottom for the most leverage. More thrusters is more power, but uses more RCS.

[Fenris]

Awesome, thanks for the info!

[Vanamonde]

RCS is only *necessary* for translation on axes other than that of the main engines. However, it's often quite useful. For example, if you are making fine-adjustments to your planetary approaches from a long way out in space (such as the half-way point of the interplanetary transit), RCS can be enough to adjust even the trajectory of very large ships, on the order of kilometers per key-stroke. Also, I like to use the mother ship's RCS tank as a reservoir from which to refuel carried vessels.

[Sirine]

RSC is a must for ship AND probe with docking port to do axis refinement docking.

RSC is a must for interplanetary travel, like above "Vanamonde said, to fine tune your trajectory.

RSC is a secondary, don't bring too much along. Ideally, 100~120 units, cause they are heavy.

If you need to use RSC for lifting a rocket from KSC to orbit, redo that rocket. There are no need RSC for lifting purpose. To balance your lifting, use small rocket engine to compensate instead.

[Thogapotomus]

It certainly comes in handy. I prefer to use reaction wheels instead, though. Weighs less and there's no limit to how much you use it. Well, you need electricity but one 2.5 meter battery is probably more than you'd ever need. Even if you only have one or two reaction wheels, you can physical time warp and make the turn within seconds (real time).

You can't be quite as accurate (less than .1 m/s) during interplanetary maneuvers, but I just make a tiny adjustment burn closer to the target. Probably still more delta V efficient than carrying a few tons of monopropellant for the whole trip.

If you need to dock however, make sure to bring the rcs along.

[allmhuran]

It's not *always* worth it, but at the size you've listed it is still effective. Position the thrusters as far as possible from the centre of mass while still having balanced thrust to get the most lever effect.

There is a limit beyond which RCS is basically useless. At that point you can start adding little engines instead and assigning them to action groups, or position your main engines such that you can also use them to turn. The one on the right in the image below, for instance, has nuclear engines out on extensions, and pointing both forward and back. Turning left means turning on the left retrograde engine and the right prograde engine.

[Kasuha]

To turn a large ship around, RCS is better than number of torque pieces you would need. Unless you start simulating RCS with engines which is also possible.

[Vanamonde]

To clarify, I NEVER use RCS for attitude control, except during docking, when time is critical. Otherwise, it's too wasteful of finite fuel. Small ships don't need it, big ships can take their time rotating on feeble pod torque while in space, and you're better off using control surfaces in atmo.

[Kasuha]

big ships can take their time rotating on feeble pod torque while in space

You either didn't try it on big ships, or you have a lot of patience. Or use mods which do the work for you. Last time I tried it on my ship which I consider big, after 10 seconds of holding the control the navball moved by its first pixel.

[allmhuran]

You either didn't try it on big ships, or you have a lot of patience. Or use mods which do the work for you. Last time I tried it on my ship which I consider big, after 10 seconds of holding the control the navball moved by its first pixel.

Turn on 4x physics warp. Much faster, since physics warp doesn't just increase the speed of time, it also (mistakenly) seems to increase the magnitude of forces for some reason.

[Fenris]

Thanks again for all the answers, I didn't expect so many answers so fast. This forum has great members

And I did add RCS to the ship based on the information here.

[Psycix]

Personally I think RCS is pretty redundant in most situations:

-Rotation, small ships: The pod provides enough reaction wheels. Otherwise, add one.

-Rotation, large ships: Patience, even a small pod can rotate a jumbotank if you give it the time it needs. You will need to think ahead though, make maneuvers far ahead and hold that button for a minute on physical timewarp. If you are the kind of person that realises he has to flip his bunch of jumbo's 180 degrees in 20 seconds, this will not work for you.

If it gets too big and slow, add one or a few big reaction wheels, they are very powerful. Again, just be patient and rotate carefully, you should not expect a big ship to rotate as fast and agile as small ones.

Also make use of the thrust vectoring. Finish a maneuver node with a spin towards your next one or use a small low throttle burn to create angular movement. Turn off SAS and let it spin.

Another trick is to asymmetrically disable some engines.

-'Extra' delta-V: You mean that delta-V you lost in the first place by adding mass? Remove it or replace the RCS tanks by fuel tanks and you'll go a lot further.

-Fine tuning trajectory: This is indeed useful, but I do not think this is a must. If your TWR is low enough (disable engines?) you can fine tune interplanetary encounters just fine.

Docking:

-Translation, small ships: Thinking ahead and quickly rotating + doing small burns with the main engine can make RCS redundant.

-Translation, medium ships: This is THE perfect situation to use RCS.

-Translation, gigantic ships: Tons of RCS can be used, but I prefer swapping it out for control-grouped bipropellant engines facing forward and/or sideways.

Edited October 27, 2013 by Psycix

[Kulebron]

I managed to save mass and get better TWR & Delta-V with RCS instead of vectored engines.

2 LV-T45 would weigh 500 kg more, have less thrust than 2 LV-T30. Instead I put 4 RCS blocks (200 kg) and 1 tiny RCS tank (250 kg)

[Kulebron]

-Translation, medium ships: This is THE perfect situation to use RCS.

-Translation, gigantic ships: Tons of RCS can be used, but I prefer swapping it out for control-grouped bipropellant engines facing forward and/or sideways.

Do you have calculations how much they weight? Great idea!

[allmhuran]

-Translation, gigantic ships: Tons of RCS can be used, but I prefer swapping it out for control-grouped bipropellant engines facing forward and/or sideways.

Yes to everything, particularly this part. Plus you don't have to carry up a lot of engine mass. In theory just one engine is fine, since even big ships tend to be pretty easy to roll. One engine at the CoM could be rolled to face any direction required for translation. Of course, two is going to be the way to go in practise in order to keep the ship mass balanced. Point being, you don't need to add engines pointing in every direction. One on each side for translate left right/up down (using roll to change between them), one on the front for translate back, and use the main engines on tiny throttle for translate forward.

Edited October 27, 2013 by allmhuran

[Vanamonde]

You either didn't try it on big ships, or you have a lot of patience.

I routinely use it on big ships, and I do have a lot of patience. If a ship is going to take, say, 45 days to get to Duna, what does it matter if it takes 5 minutes to rotate 180 degrees?

[Superfluous J]

I routinely use it on big ships, and I do have a lot of patience. If a ship is going to take, say, 45 days to get to Duna, what does it matter if it takes 5 minutes to rotate 180 degrees?

Other than the fact that those 45 days to Duna are actually a short sprint at max acceleration which can be ended instantly without interaction by a very common mod (Kerbal Alarm Clock) while those 5 minutes take 5 minutes of holding down a key, and even if you don't hold the key down the whole time you can't easily set a timer for it as it's hard to predict exactly when you'll need to stop rotating?

Nothing, really

[Thogapotomus]

I routinely use it on big ships, and I do have a lot of patience. If a ship is going to take, say, 45 days to get to Duna, what does it matter if it takes 5 minutes to rotate 180 degrees?

Although I agree with most of what you've said completely, if my ship was going to take 5 minutes to rotate (or over 1 minute at x4 warp) I'd rebuild it lol. A reaction wheel here and there isn't gonna break your dV budget

Also, that's a good picture for explaining RCS locations for new players, Kulebron.

[Vanamonde]

I call it "pod torque" because it used to be that only capsules had it, but I was meaning to include all the reaction wheel types within that term. It's just RCS that I don't like to use for attitude control because it's not necessary, and the finite fuel supply is better saved for other purposes.

[maccollo]

Since I don't build space stations (It just ends up being tedious for me) almost I never ever use RCS. I find it to be mostly useless extra mass and parts.

taking of into orbit engine gimbal and pod torque is enough to keep me pointed in the right direction.

In orbit the rocket can be rotated quickly enough with pod torque + physical time acceleration if it's sluggish.

For interplanetary fine tuning the transfer stages usually have low enough TWR to be able to make those fine adjustments.

[Fenris]

Personally I think RCS is pretty redundant in most situations:

-Rotation, small ships: The pod provides enough reaction wheels. Otherwise, add one.

-Rotation, large ships: Patience, even a small pod can rotate a jumbotank if you give it the time it needs. You will need to think ahead though, make maneuvers far ahead and hold that button for a minute on physical timewarp. If you are the kind of person that realises he has to flip his bunch of jumbo's 180 degrees in 20 seconds, this will not work for you.

If it gets too big and slow, add one or a few big reaction wheels, they are very powerful. Again, just be patient and rotate carefully, you should not expect a big ship to rotate as fast and agile as small ones.

Also make use of the thrust vectoring. Finish a maneuver node with a spin towards your next one or use a small low throttle burn to create angular movement. Turn off SAS and let it spin.

Another trick is to asymmetrically disable some engines.

-'Extra' delta-V: You mean that delta-V you lost in the first place by adding mass? Remove it or replace the RCS tanks by fuel tanks and you'll go a lot further.

-Fine tuning trajectory: This is indeed useful, but I do not think this is a must. If your TWR is low enough (disable engines?) you can fine tune interplanetary encounters just fine.

Docking:

-Translation, small ships: Thinking ahead and quickly rotating + doing small burns with the main engine can make RCS redundant.

-Translation, medium ships: This is THE perfect situation to use RCS.

-Translation, gigantic ships: Tons of RCS can be used, but I prefer swapping it out for control-grouped bipropellant engines facing forward and/or sideways.

I'm still a newb ... but a lot of what you said is along the same lines as what I was thinking. I just figured after reading some of the post that even though I don't plan on using them much if I can spare the extra weight I might as well put them on ... just in case.