Buzz
How does the ISS maintain its orientation?Robert Frost:
Typically, attitude control is managed by four control moment gyroscopes (CMGs). Each CMG features a 220-pound (100 kg) wheel that spins at 6600 rpm, producing an angular momentum of 3500 ft-lb-s (4742.5 N-m-s). The principle is that when torque causes the ISS to rotate, these wheels can adjust their position to alter the ISS's angular momentum, generating a counter torque. CMGs are a more delicate approach compared to thrusters, ensuring that microgravity experiments remain unaffected. However, if needed, thrusters can provide assistance when the torques become too large.
To reduce reliance on thrusters, during stable operations, a technique called momentum management (MM) is employed. This involves positioning the ISS at a torque equilibrium attitude (TEA), which has been carefully calculated by the ground team a year or more prior. This TEA allows gravity and atmospheric forces, within a 15-degree variance, to nearly cancel each other out over an orbit. The CMGs then fine-tune this balance to achieve zero torque.
During critical operations, we often cannot maintain a TEA, and thus, we switch to an attitude hold (AH) mode. Examples of such operations include docking or berthing. Maintaining an attitude hold is more demanding as it requires additional effort, often exceeding the capacity of the CMGs alone. Furthermore, firing thrusters during these sensitive operations can create problems.
For these situations, we develop a flight rules matrix to ensure safety. For instance, thrusters are prohibited from firing when the robotic arm's end is within 2 feet (0.6 m) of the vehicle. The last thing we want is for a thruster to shake the arm, causing it to collide with a module and potentially puncture it. If the timeline shows the arm will be so close, ADCO (attitude control flight controller) will disable thruster assistance.
Docking and berthing events can trigger abrupt changes in momentum. To prevent damage to the docking or berthing mechanisms, we temporarily disable the entire attitude control system during these activities. You may notice, when watching NASA TV, that the vehicle's attitude can significantly deviate during these moments.
The attitude control computer (GNC MDM) houses the software responsible for performing all necessary calculations related to attitude control. It compares the actual attitude with the commanded attitude to calculate the error that needs to be corrected. The system is highly sensitive, so much so that we can detect when the crew wakes up by observing the changes in the CMGs as they start moving inside the vehicle. Additionally, the software requires user-provided parameters such as vehicle mass properties and inertia tensors, which are stored in data slots known as CCDBs (controller configuration databases). We maintain a variety of these CCDBs for different vehicle configurations. For example, when a Progress cargo vehicle arrives and docks with the Russian Segment, a CCDB specific to that configuration is used. Once it departs, we switch to another one.
This post originally appeared on Quora. Click here to view.
