Does it matter where you place SAS?

[Talonsin]

Is the placement of the SAS module important like the position of the RCS jets? Or can I just slap the SAS anywhere as long as I have one?

Thanks

[jebster]

I did a quick test of this. I placed a single SAS module mounted on an OSCAR radial attachment port. Excepting the weight unbalance, the navball seemed to move as regularly as if I had placed it linearly to the thrusters.

In short, no, it doesn't matter where you place SAS.

[SRV Ron]

They seem to be best near the top of the rocket. Keep in mind that if you launch command module capsules, you won't need them. The built in SAS of the command modules is strong enough to do the job.

If the launch vehicle is extremely large and is becoming difficult to control, use an SAS reaction wheel on top of the second stage.

[Captain Sierra]

Yes and no. It does not matter where you put it, but keep staging in mind. If you only need it on the boosters for an asparagus launcher (like me) slap them on top of those and they drop with the boosters so you don't over torque. Just be sure you don't accidentally drop them in staging before you mean to. That will ruin your day to make your rocket suddenly a pig.

[eurybaric]

So basically the SAS torque will always be applied from the center of mass and not where the actual part is?

[sal_vager]

Don't forget guys, SAS is just the computer, reaction wheels are the bit that does the work, it's going to take ages for the distinction to settle in with everybody but it is kinda important that we don't confuse them.

[Motokid600]

An SAS part does not exist anymore. Your talking about reaction wheels and YES it does matter where you place them. Reaction wheels near the center of mass are a total waste unless its a small craft. For large craft I place reaction wheels as far from the center of mass as possible. It's a leverage issue. I put a wheel at the very bottom in between an engine and fuel tank and then at the top on the command pod. Now these pods have their own torque, but its too minuscule for large craft. So I disable it to save electricity.

Unfortunately the stock reaction wheels do not come with enough variety ( there's what..two? ) So I use NovaPunch wheels. There low profile and there's one for every size up to 5m. On REALLY big craft I double up reaction wheels and pack on a lot of batteries.

I love these wheels. Rcs is damn near obsolete. I only use Rcs for docking and to fine tune orbits now. So all in all treat reaction wheel placement just as you would rcs placement. ( balance wise )

I will add to what I said. The center of mass is not always in the center. If for instance you have a large, full fuel tank at the bottom. The CoM will be lower. Therefore placing reaction wheels at the bottom will be a waste and might actually hurt you. So instead place the RW's at the top and the craft will be more responsive. Again. As far away from the CoM as possible. That CoM is your fulcrum. So let's say you have to flip over a large rock. You have a five foot pole and a ten foot pole. Which one will flip the rock more easily?

Edited August 11, 2013 by Motokid600

[AbuMaia]

I put a wheel at the very bottom in between an engine and fuel tank and then at the top on the command pod.

How do you do that? I tried it once, my engines never got any fuel from the tanks. Do you route fuel lines around the reaction wheels?

[xub313]

An SAS part does not exist anymore. Your talking about reaction wheels and YES it does matter where you place them. Reaction wheels near the center of mass are a total waste unless its a small craft. For large craft I place reaction wheels as far from the center of mass as possible. It's a leverage issue. I put a wheel at the very bottom in between an engine and fuel tank and then at the top on the command pod. Now these pods have their own torque, but its too minuscule for large craft. So I disable it to save electricity.

Unfortunately the stock reaction wheels do not come with enough variety ( there's what..two? ) So I use NovaPunch wheels. There low profile and there's one for every size up to 5m. On REALLY big craft I double up reaction wheels and pack on a lot of batteries.

I love these wheels. Rcs is damn near obsolete. I only use Rcs for docking and to fine tune orbits now. So all in all treat reaction wheel placement just as you would rcs placement. ( balance wise )

I will add to what I said. The center of mass is not always in the center. If for instance you have a large, full fuel tank at the bottom. The CoM will be lower. Therefore placing reaction wheels at the bottom will be a waste and might actually hurt you. So instead place the RW's at the top and the craft will be more responsive. Again. As far away from the CoM as possible. That CoM is your fulcrum. So let's say you have to flip over a large rock. You have a five foot pole and a ten foot pole. Which one will flip the rock more easily?

Actually the reaction wheels should be placed close to the center of mass, not away from it. I see your logic but reaction wheels don't work like levers. To move an object with a lever the other end must be "pushed" like when using RCS. Reaction wheels, however, rotate around the part's center, not the ship's. So to get a rotation about the center of mass, the part must be at or near the ship's center of mass.

To visualize this, imagine grabbing a long piece of rebar and trying to rotate it with one hand. Grabbing one end and rotating it is hard and is offcenter, but rotating the rebar at its center will preserve the position of the center of mass and make it easier to rotate.

[Bean]

I had a big moonbase module with rocket lander stages at the edges, with RW, they almost tore the ship apart, like twisting a straw from both ends in different directions. Definitely need them close to the CoM as xub313 says.