Please explain my Craft's stability

[Jestersage]

I am away from my home, so I cannot upload the craft.

Anyway, I am making a XS-1 clone, so it have wings, with 1 orange tank, 1 rockomax 32 at bottom of the orange and a rockomax 16 above the orange. Attached to the side is the payload. Between the 32 and the orange is a SAS wheel. Entire thing is propelled by a RS25 and 2 aerospike.

What I found is that, if I have the SAS on, the entire thing is hard to control, while with the wheel off it's smooth flying.

Why is it harder to fly with the wheel?

[Gaarst]

The reaction wheel is probably too powerful. Reaction wheels and control surfaces do not respond to input the same way: reaction wheels pretty much apply full torque instantly while control surfaces take some time to deploy and react depending on the airflow. By forcing your craft to move using the reaction wheel, you "throw" your surfaces out of the airflow by the difference in response or simply because the torque of the wheel exceeds the torque the surfaces can apply/withstand.

Reaction wheels and control surfaces usually don't work that well together, but it can be amplified depending on the craft itself (ie: shape, CoM and CoL positions...).

It's just an hypothesis but SAS is known to destabilise crafts when too much torque is available. Kind of hard to say for sure without a picture of the craft or a craft file.

Edited June 23, 2017 by Gaarst

[bewing]

Probably due to the timing of the feedback loop. If it flies OK without the wheel, then any oscillations must damp themselves out by averaging.

 

 

Edited June 23, 2017 by bewing