[Music throughout] While thrusters are great for moving a spacecraft around, they aren’t as good, or efficient, at controlling where the spacecraft points.
Instead, rotating wheels, and conservation of angular momentum make it possible to turn in any direction without any outside influence.
Most space telescopes, from NASA’s Hubble to Webb, and everything in between, use components called reaction wheels to change where they are pointing. The reaction wheels run on electricity, unlike thrusters which are less precise and can run out of fuel.
The reaction wheels are basically flywheels driven by electric motors. They spin to store angular momentum. By slowing or speeding the rotation of a given wheel, changing the amount of momentum, a computer can precisely adjust how the spacecraft points around its center of mass in one plane. With three wheels set at specific angles, a satellite can control its pitch, roll and yaw to point in any direction and then hold that position without any change.
NASA’s Nancy Grace Roman Space Telescope has added its set of reaction wheels to the main spacecraft body, or bus. Roman has six reaction wheels, rather than the necessary three, to give it more angular momentum for faster pointing, as well as complete redundancy should any one wheel fail.
Each of Roman’s wheels is 18 inches across – about the size of an extra-large pizza – weighs roughly 45 pounds and spins up to 4,000 rpm. As long as the solar cells provide them with electricity, the reaction wheels could repoint Roman indefinitely except that they have one function requiring thrusters.
The reaction wheels counteract the pressure from sunlight, which exerts a tiny, but steady, force that, over time, would push Roman out of alignment with whatever it is observing. The wheels can exactly offset that pressure by slowly spinning faster.
When a wheel finally reaches its maximum rotation speed, it needs to “unload,” or slow down, so that it can begin to freely vary its speed again. Roman engineers gently fire specific thrusters, causing the maxed-out reaction wheel to compensate the rotational force by slowing down.
Thrusters are also critical for any action that physically moves Roman. Reaction wheels can only rotate the spacecraft in place, they can’t cause a change in location. Fortunately, Roman’s extensive set of thrusters will be well fueled and fully up to the task, positioning the spacecraft and unloading momentum so that the reaction wheels can keep it perfectly pointed at the stars. [NASA]