Rcs

[KvickFlygarn87]

Is there some sort of trick for placing RCS? I try getting it as close a possible to the CoM, but it's always slightly off making translation (especially side-to-side, i.e. j and l) spin the craft. If there is no trick is there a mod I can use?

[Vector]

Place them symmetrically in sets of 2 or more, so they are symmetrical about the center of mass. Also remember that as fuel is burned, the center of mass moves, so you want to predict where it will be at the time you will be using RCS.

If they are near the center of mass then they can't apply much rotation due to short moment arm. Place them farther from CoM (but symmetrical about CoM) and they will stabilize their own torque.

[KvickFlygarn87]

So, for translation, I should put them far from CoM?

[Vector]

If they are symmetrical about CoM then for translation it doesn't matter how far from CoM they are. For rotation, farther is better, and rotation will be used to counteract any accidental spin you may get from translation.

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Edited October 4, 2013 by Vector

[Kasuha]

That first image above is not just "bad" RCS placement. It is just plain wrong.

If I know I'll need a lot of translation, I usually put one set of RCS thrusters at each end. If I'll need just a little, then placing one set around center of mass is usually okay.

Always add some torque too and when translating have SAS turned on. It will deal with most stray forces.

[radonek]

There is a great mod rcs build aid for this. It can show torque forces imparted by rcs for both dry and ladden CoM.

[SofusRud]

I just place them at CoM and have reaction wheels do the rotation part. reaction wheels also compensate for drift when translating

[SRV Ron]

That first image above is not just "bad" RCS placement. It is just plain wrong.

If I know I'll need a lot of translation, I usually put one set of RCS thrusters at each end. If I'll need just a little, then placing one set around center of mass is usually okay.

Always add some torque too and when translating have SAS turned on. It will deal with most stray forces.

For steering, that is the best location. For translation, better to use a set at the top and a second set at the bottom since the center of mass will change as fuel gets used up.

[capi3101]

I usually build my designs with a minimum of twelve blocks, and only when I know my craft is either going to be too heavy to steer on pod torque alone or the torque isn't all that much to write home about (like on a smaller probe core such as the OKTO2). 4 as close to the top of my craft as I can manage, 4 at the bottom, and 4 at the center of mass, more at each location if I know I'm moving a heck of a lot of mass around. Usually the ones on top and bottom help with rotation, and the ones near the center of mass help with translation.

[Dkmdlb]

When translating, turn on SAS before translating. This will correct for any small imbalance in the RCS placement. Then, after the translation, turn SAS off or else it will keep using RCS fuel.