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A typical reaction wheel
used in NASA spacecraft |
Why do the reaction wheels have to continuously spin? If a reaction wheel were to ever stop spinning, it would create an abrupt change in torque that would cause the spacecraft to jitter. This sudden jitter would result in undesirable attitude errors; therefore, reaction wheels are kept continuously spinning. The energy required to speed up and slow down the spinning reaction wheels on the Core Spacecraft will be collected from the Sun via the spacecraft’s solar panels.
As you might imagine, if the reaction wheels are constantly kept spinning, there will be a limit where they can be accelerated no more, due to physical or material constraints (the reaction wheel mechanism is built to handle spin only up to certain speeds). It will be necessary to “dump” the momentum forces built up in the reaction wheels. The spacecraft’s momentum management software continuously makes adjustments to control momentum buildup, with the help of a set of electromagnets, called torquer bars, onboard the satellite. These torquer bars consist of iron bars wrapped with copper wire. When an electrical current is run through the wire (the energy to produce this current also comes from the Sun) a magnetic force is created. These electromagnets attempt to align themselves with Earth’s magnetic field, and as they move to do so, create a torque onboard the spacecraft.
The momentum management software autonomously and continuously employs the torquer bars to compensate for secular (non-periodic) momentum increases, allowing it to maintain reaction wheel spin rate within the designed operational range. But if torquer bars can produce forces capable of changing spacecraft orientation, why don’t we just use torquer bars instead of reaction wheels to control the attitude of satellites? Reaction wheels are necessary because they can produce forces enabling us to point the spacecraft in the desired direction quickly and smoothly—it would take too long for torquer bars to move in the desired manner.
GPM engineers have recently performed a trade study to determine the size of reaction wheels and torquer bars necessary for the GPM Core Spacecraft. They first considered all of the requirements that the Core Spacecraft must fulfill to be able to accomplish its mission. For example, it must be able to slew (change position in space) by 180 degrees every 36 days to keep the proper side of the spacecraft facing away from the Sun. It must accomplish this move very quickly so that little time to collect science data is lost. In addition, all parts of the Core Spacecraft must be designed such that they will disintegrate in Earth’s atmosphere when, at the end of its lifetime, the Core Spacecraft falls to Earth in an uncontrolled reentry process called “Design For Demise” (for more information on the Design For Demise, click here to view a previous GPM Monitor article). Typically, reaction wheels are made of stainless steel or titanium, very dense elements that would not meet GPM’s Design For Demise requirements. Therefore, GPM Core Spacecraft’s reaction wheels will be made of aluminum, and require a larger dimension than typical wheels.
Given these requirements, the size and shape of the Core Spacecraft, and other data, GPM engineers ran a series of computer programs to simulate different sized reaction wheels and torquer bars on the GPM Core Spacecraft. Their goal was to determine the best combination of torquer bars and reaction wheels that would work in together to control the attitude of the GPM Core Spacecraft in a manner allowing it to meet mission objectives. After trying various combinations and sizes, here are the study results:
To meet its attitude control requirements, the GPM Core Spacecraft will employ a set of four 50 Nms (Newton meter second) reaction wheels in combination with three 300 Amp-m2 torquer bars.
By Eric Holmes/GPM Guidance, Navigation and Control Engineering
Lead
and Lena Braatz/Booz Allen Hamilton