No S3 Kerbodyne sized reaction wheels / stabilizers?

[Northstar1989]

Torque is fixed, but angular acceleration = torque / moment of inertia, and any distance between the torque source and the center of mass increases the applicable moment of inertia.

Moment of inertia is a property of the shape and size of the body. Except for any effects of reaction wheel placement on the overall shape/size of the rocket, it is unaffected by where you place the reaction wheels...

I was mistaken in saying that SAS is more effective at the ends of the rocket- I was thinking of RCS, which *IS* more effective the further it is from the Center of Mass- I'd like to see somebody dispute THAT!

Regards,

Northstar

[Claw]

RCS thrusters generate FORCE, not TORQUE.

The difference being, torque depends on location of the rocket.

The further from the Center of Mass RCS thrusters are located, the more torque they generate with the same force...

SAS, on the other hand, generates torque, not force, which I forgot when I made my last comment...

Therefore, RCS DOES benefit from being placed as far from the Center of Mass as possible.

SAS DOES NOT benefit from being placed away from the Center of Mass...

The following diagram, stolen from an earlier post on this subject by "mhoram", illustrates the forces generated by each type of control system.

http://i.imgur.com/JUUe9bU.png

RCS thrusters *DO* benefit from leverage, unlike SAS.

Regards,

Northstar

Thanks for this Northstar. In fact, RCS MUST be placed away from the CoM to be effective. If you could somehow place it exactly on the CoM, it would result in nothing but translation (and no rotation). Of course, it's hard in practice to place it on the infinitely small CoM point. Reaction wheels, on the other hand, could be placed exactly on the CoM or at the very ends. If the object is completely rigid and the CoM position unchanged, rotation due to a reaction wheel is the same regardless of reaction wheel position.

Unfortunately, it's easy to throw around terms like Torque and SAS, but it's a bit trickier to intuitively understand what's actually happening. Since torque also results from forces (like RCS) being applied over a distance (as you pointed out).

Nice work. For practical purposes, it seems that quantity reigns over location. I've tended to space out reaction wheels to give some kind of balance between the command pod and the starting primary engine, so sometimes thats near the CoM and at other times not.

Thanks. And yes, quantity reigns over location because location has no bearing at all (when talking about reaction wheel's effectiveness). Spacing them out is generally the best option though because it reduces bending and reduces the likelihood of a craft tearing itself apart.

Edited May 22, 2014 by Claw

[Northstar1989]

I'm glad everybody could come to some sort of a consensus...

Regards,

Northstar