RCS/Reaction Wheel Issues

[Amraam0313]

Had a question about small craft in orbit. Trying to design a craft to land on Moho so I am trying to keep it considerably small and compact. Using a Mk1 lander can and putting most of the fuel tanks, batteries, etc. inside the can to limit physical size. The problem arises when I try to use RCS thrusters for any type of docking or maneuvering the craft literally goes crazy, and all the RCS thrusters are firing to keep it steady. I've basically done everything in the game unmodded, never had an issue with larger craft and docking, maneuvering etc. but have issues when I go small. Also have issues with it when I build HECS2 probes that are compact.

I try to mess with the reaction wheels, turning them off and what not, but then lose the ability to easily dock and what not, should I just put a separate reaction wheel on the craft and turn the reaction wheels off of the lander can?

[bewing]

If you will please post a craft file, that would be helpful here.

 

[Spricigo]

Probably caused by misalignment of RCS thruster, worsened because the vessel its so compact and light. A translation Input causes a rotation and  SAS try to compensate for it with the opposite torque and just go crazy. The simple solution its to set RCS to only respond to translation inputs and let the rotation be handled by Reaction Wheels.

In settings turn on "advanced tweakables"

Right click on RCS thrusters, [show activation toggles]

turn it off for Pitch, Yaw and Roll

 

Edited April 18, 2017 by Spricigo

[Freshmeat]

I do not recall whom to give credit, but a very important piece of information is: When you engage precision mode (caps lock on std keyboard layout), your reaction wheel and RCS thrusters will try to balance out any imbalanced translational thrust. Also, the reduced output makes maneuvering a lot easier.

[Spricigo]

13 hours ago, Amraam0313 said:

Using a Mk1 lander can and putting most of the fuel tanks, batteries, etc. inside the can to limit physical size. 

I just realised how big a factor it may be.  As I said misaligned RCS thrusters may exert torque (induce rotation). 

For translation we have F=m*a. For rotation we  have {torque} ={moment of inertia} *{angular acceleration}. 

The moment of inertia of a body is the sum of the moment of inertia of all its parts. The moment of inertia of a mass concentrated in a point is I=m*d² where d is the distance between the mass and the axis/point of rotation. 

You said most parts get moved onto the lander can. Since most of the vessel's mass is near the CoM the moment of inertia is low.  Your vessel is very easy to rotate. 

 

[blakemw]

You might try to master the art of docking without RCS. It's quite easy as long as the docking port is aligned with the engines, especially if the docking port is at the nose. All you need is a good reaction wheel so the vessel is reasonably nimble. Docking without RCS involves getting close to the target vessel, killing relative velocity, getting the docking ports to aim at each other ("control from here", and setting the other docking port as target is very useful here, as is "target lock" on an advanced probe core), and then flying forward so the vessels dock. I find it's usually quicker than docking with RCS. 

Eliminating the RCS has two benefits: First it eliminates unbalanced RCS. When you try to do translation with RCS it's easy for the thrusters to jitter the vessel around, sending it slightly off-course. It can be worked around in variuos ways, but it's a pain. Pure reaction wheel rotation is perfectly clean without side effects and you can rotate the ship and direct thrust to "pull" the prograde marker over the target marker (the trick is that a burn will always pull the prograde marker towards the reticule, hence this technique is sometimes called "prograde pulling").

The second benefit is it saves a lot of mass: the mass of monoprop and all the RCS thrusters to achieve balanced thrust adds up, it's not a big deal for low orbit, but when you're sending stuff a long way, like to Moho, you don't really want to bring any unnecessary mass. A final thing to note in relation to this is that RCS thrusters are generally grossly overpowered for small vessels, there is a similar problem with aircraft control surfaces, the AV-R8 winglet is perfectly sized for a "Twin-Boar" rocket and there's nothing small enough to be actually suitable for smaller rockets. While you can dial down the authority limiter/thruster power this doesn't stop them being excessively heavy/costly for the small vessel. The flipside is small vessels are incredibly nimble with reaction wheels alone, making it a great solution for small vessels.

 

When I do use RCS it's usually only for forward/reverse thrust. Even when doing "aim, fly forward, and dock" docking it's pretty useful to have reverse thrust, not essential, but useful. When I do this I limit the actuation toggles to fore/aft so that the reaction wheels take sole responsibility for roll/yaw/pitch.

Edited April 19, 2017 by blakemw

[Spricigo]

@blakemw granted it's a nice skill to learn.  But RCS is convenient,  It's much better to turn it on and touch N,  than rotate 180°, limit the thrust to 1%,  touch shift and quickly hit control.

Laziness may hinder you in the long run but is assured to be gratifying immediately. 

[blakemw]

2 minutes ago, Spricigo said:

@blakemw granted it's a nice skill to learn.  But RCS is convenient,  It's much better to turn it on and touch N,  than rotate 180°, limit the thrust to 1%,  touch shift and quickly hit control.

Well that's funny, you just said exactly what I said in my last paragraph ;). Reverse thrust is convenient to have. Though I normally don't bother on vessels less than 20t.

Edited April 19, 2017 by blakemw

[Spricigo]

Well, if is available for reverse thrust it's just +60kg for lateral thrust.  Also the main point is not reverse thrust but very low thrust compared to the main engine.