Reaction Wheels are Great Fun
How do you keep a spacecraft large or small stable in space? Reaction Wheels are one way. Writing software to control one reaction wheel of four is tough, but this toy/tool lets you test them. at least if they are small.
I love playing with this beast. it is to simulate the operation of reaction wheels that are used to stabilise small spacecraft. I was at Sydney Uni discussing our StratoDrone and how to build a test flight involving a zero pressure balloon. More on that later, but we now have a plan. This reaction wheel test jig is hugely expensive and it sits on a partial ball that then sits on a vertical stand in a half hemispherical hollow fed by air. Basically it floats on air and the reaction wheel results are clearly seen. You can test your electronics and your programming. Although these are bigger than those used for a cubesat, they still work the same and produce this visual action. It of course does not go around fully as in space. It hits limits in the ability to roll and pitch, but the actions can be seen clearly. I love this tool so much.
You can also see that the various axis are written on the perspex above the reaction wheels. There are only three needed in a basic unit, but 4 are often provided in case one fails. It is placed at such and angle that it influences all three axis. If one fails, then the 4th replaces it and the working two have to counteract the influence it has on them. It means it also has to spin 3 times more to achieve the same result in replacing the faulty unit.
There are other platforms and ways to test reaction wheels. even hanging a test object on a long cord from the ceiling will give a good indication of what is happening at almost zero cost, but this tool is way more elegant and will give a more accurate result.
I love space toys! – erh – I mean space tools.
When to use a Reaction Wheel
The ISS uses Gyros and not reaction wheels. They are not the same, but I will not go into that in detail here, but I will say that Gyros don’t always work. They reach a stage where they get saturated and require a sort of reset and thrusters are needed to get things stable again.
A control moment gyroscope (CMG) is an attitude control device generally used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. They are big and heavy. They are used on board the ISS and spin constantly. No manned craft or craft visiting the ISS has ever used gyros. A gyro can exert torque along any axis by turning the gimbals. It is also very big and heavy. The whole assembly is roughly spherical in shape.
As for reaction wheels, once the reaction wheel reaches its maximum angular velocity, it must be slowed down and the resulting torque must be counteracted with thrusters. Usually there always are some reaction control thrusters and if precise attitude control is needed, there are reaction wheels in addition to thrusters. The reaction wheels maintain precise stability until they reach their limits. They are however very useful for cubesats. Cubesats may also be further stabilised along the earth’s magnetic field by the use of a powerful magnet on board the spacecraft. care must be taken in the design for the reaction wheels not to fight the magnetic orientation.