need to align rcs between docked ships/space stations

[Vist]

Hi everybody

Just want to ask if it's necessary to also align the RCS parts between docking ships or with a space station? Does it matter if they're aligned or not when docked?

The red dots are the location of RCS

I've seen videos and tutorials for docking but they don't seem to show the RCS are aligned as well. :">

Thanks in advance for your reply.

Edited July 29, 2015 by Vist

[Ripper2900]

That's no problem. As soon as the ships are docked, they're seen as one single ship. And rcs functions as such.

[Alshain]

It does in fact matter for some translation. It will still work but some of the jets may fire at angles to the direction your are going and that isn't as effective.

Edited July 29, 2015 by Alshain

[Grumman]

The closer they're aligned, the more efficient they would be, I think. It doesn't matter for rolling or forwards/backwards movement, but pitch, yaw and other translations could be up to 30% less efficient if the RCS nozzles are at the 45 degree marks instead of the 90 degree marks.

[Yasmy]

I used to believe (based on "common sense") that RCS block orientation mattered, but after some Kasuha-style extensive testing, it appears that orientation is meaningless for RCS.

Having RCS blocks on the diagonals neither uses twice as much fuel per second, nor produces net thrust reduced by 1/sqrt(2) per firing block, relative to placing rcs blocks on the cardinal directions, when firing in a cardinal direction (ijkl).

Edited July 29, 2015 by Yasmy

[FancyMouse]

It does in fact matter for some translation. It will still work but some of the jets may fire at angles to the direction your are going and that isn't as effective.

http://i.imgur.com/LWscYDQ.png

Not the whole picture - the "less effective" one has thrust from the other side of RCS as well, so it's doubling the RCS mono consumption, delivering 1.4x thrust. It is less efficient than the below one but gives more thrust. Design choice, of course.

EDIT: RCS consumption what is that :-(

[Yasmy]

Below there are two shots of the same craft. The RCS blocks on the diagonals are in a different action group from the blocks in the cardinal directions, for easy testing.

Define the thrust of one RCS block as T, and the monopropellant consumption rate as R. You may expect the net thrust and fuel consumption of the two craft to be:

Cardinal: 2T, 2R

Diagonal: 4T/sqrt(2), 4R

Thus you may expect the thrust per unit fuel to be T/R and T/R/sqrt(2), respectively. I did. I did a large number of tests with different test rigs in space and on the pad.

For all my tests, the net thrust and fuel consumption rates were essentially identical. I timed the fuel drain in the tests below, and I measured the accelerations, and they were the same. To me this says that as far as RCS placement goes, don't try to use logic/physics/math. Slap em on, and they will work the same in either configuration. Someone else may wish to verify my tests. I haven't done them since 0.90.

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Edited July 29, 2015 by Yasmy

[Sharpy]

It doesn't matter for Precise Mode (capslock) which throttles the RCS. It does matter for maneuvers with full-power RCS.

Aslain's picture is optimistic in the ports being symmetric. If your "RCS direction" (relative to the root part) is the red arrow, full thrust will actually propel you in the dark cyan direction when they operate at full power. In precision mode, one will be throttled enough that the direction will match.

[Yasmy]

That is just wacky behavior, Sharpy. Who ordered that silliness? I'm going to have to do some more testing to see that for myself.

My tests were done without Precise Mode on, but I did not test at arbitrary angles like you are depicting. Just in ijkl and in diagonals (ij, il, jk, kl).

They definitely conflict with Alshain's image. (Which I expected to be correct, but found to be wrong.)

- - - Updated - - -

Vist's question is very easy to test. Build a core with RCS, duplicate it, rotate by 45 degrees, or any arbitrary number of degrees, attach the two and launch. If orientation of the RCS blocks matters, translations will induce rotations. Easy-peasy. I did a quick test, and it worked just fine. I didn't quantify monoprop usage as a function of angle mismatch, but there were no induced rotations.

Edited July 29, 2015 by Yasmy

[FancyMouse]

Just did a test. My test setup is regular 4-port RCS using 4x symmetry. Here's my result table. Actual number doesn't matter - look at the ratio

Mono consumption rate | acceleration

H/N 0.42 | 1.26

ijkl(x setup) 0.30 | 0.64

ijkl(+ setup) 0.21 | 0.65

It sounds to me that it is what we imagine geometrically, except that KSP pretends being smart enough to decrease the thrust for the x setup case in order to have a consistent RCS experience - but mono consumption is adjusted correspondingly. This means you can't improve thrust by placing it differently - it has been compensated by KSP internal control system.

Interesting - this means we should never place RCS in a x fashion. It will be just 40% waste of mono.

EDIT: wipe out my earlier edits. nothing to worry about except clarifying that I don't have ModuleRCSFX

Edited July 29, 2015 by FancyMouse

[Yasmy]

Thanks for the test, FancyMouse. Definitely different from my 0.90 test, which showed identical consumption in x vs + setup. I still have 0.90, so I could retest that and current. Maybe later.

[Vist]

Thanks everybody for your comments/insights/inputs

Will stick to placing RCS' in a cross pattern and dock with them aligned as close as possible.

[Sharpy]

Vist's question is very easy to test. Build a core with RCS, duplicate it, rotate by 45 degrees,

It won't work with 45 degrees as the RCS will balance out. Anything other than multiple of 45 (0, 45, 90, 135, 180...) merely possibly wasting more fuel (choice between + and x pattern). It's at arbitrary angles where the trouble begins. Try 5 degrees - the one on the side won't fire at all, only the one on the opposite side, pushing the craft at an angle.

I had a whole batch of fuel trucks with botched RCS, where the angled RCS would require correcting with "J" after each "I" burn (while "K" worked okay) because the bottom RCS ports were misaligned.

[Vist]

Hi guys, just found out about mods in assisting in docking ="> most notably this one that just got released awhile ago called "Docking Target" made by TG626

[Alshain]

Hi guys, just found out about mods in assisting in docking ="> most notably this one that just got released awhile ago called "Docking Target" made by TG626

Yes, there are several mods to help. That one I've never seen before but if you like realistic, that is very good looking as it's very similar to the real deal (even the clamp sound effects!)

Docking Port Alignment Indicator is a popular choice if you want to see the craft your flying, it's just an instrumentation view rather than a camera like that one.

NavHud has a target port indicator, and for a more simplistic version of that, NavBall Docking Alignment.

Edited July 31, 2015 by Alshain

[Vist]

Yes, there are several mods to help. That one I've never seen before but if you like realistic, that is very good looking as it's very similar to the real deal (even the clamp sound effects!)

Docking Port Alignment Indicator is a popular choice if you want to see the craft your flying, it's just an instrumentation view rather than a camera like that one.

NavHud has a target port indicator, and for a more simplistic version of that, NavBall Docking Alignment.

Just had a spin with the mods you mentioned they're all good and quiet enjoyable, especially NavHud

Had to add the other docking ports stuff into the Docking Target's config file it to work/show up on them. That made me want to have the stock RCS thrusters' thrust I was using be adjustable also. Am suddenly finding myself want to be precise with alignment all for fuel efficiency.