method for rcs tourque / sas torque translation

[TheReaper]

is there a way for me to calculate rcs strengh to sas torque or the other way arround? rcs number their position and angle would matter in the equation but just assume 4 rcs angle snapped right on Center of mass, would there be any equation that could describe torque as rcs strengh or otherwise? it would help me reduce parts if i go straigh sas torque and no rcs monopropollent weight, or no sas full rcs weight.

[MarkyJ279]

Simply mechanics calculation. Torque is essentially a moment so if you just want a snapshot we can use the moment formula which is simple.

Moment/torque = Force x Perpendicular distance

RCS has a strength of 1KN. Assuming you have a simple setup and mount 4 radially on a 1.25m diamter part and make it spin in a circle. Take the distance from centre (radius so that's 0.625m here) x Force of 1KN x 4 (because there are 4 thrusters). You would end up with 2.5 KNm of torque if you tried to roll.

Reaction wheels are easier. It lists torque in the description and assuming your craft doesn't bend like a banana you can directly compare your RCS torque with the value for reaction wheels regardless of placement. The cheapest reaction wheel for example has 8KNm of torque so for your 1.25m part the reaction wheel would be stronger.

Note that reaction wheels don't scale with size or placement. If we bump the core part up to 2.5m then reaction wheels stay the same but RCS gives (1KN x 1.25m x 4) 5KNm but RCS is very similar that cheap reaction wheel on the 3.75m parts (1KN x 1.875m x 4 = 7.5KNm). Stick your RCS on arms out from the main body and roll-torque goes through the roof. Have sets of thrusters at each end of a long craft and you'll flip end over end much quicker with RCS than reaction wheels (same formula applies but now you're measuring length of craft between thrusters and CoM) for. On the flip side reaction wheels are simpler both in placement and logistics of operation.

Edited July 30, 2014 by MarkyJ279