ISS Attitude Control - Torque Equilibrium Attitude and Control Moment Gyroscopes

Summary

This script explains how the International Space Station (ISS) maintains its orientation in space. It introduces the concept of reference frames, contrasting Earth's fixed frame with the ISS's LVLH frame. The script discusses challenges like atmospheric drag and gravity gradient torque, which affect the ISS's spin. It explains torque equilibrium attitude (TEA) and how control moment gyroscopes (CMGs) and reaction wheels are used for attitude control, conserving fuel by reducing reliance on thrusters.

Takeaways

• 🌌 A reference frame is an imaginary box that defines directions, like up, down, left, right, forward, and backward.
• 🛰️ In space, satellites maintain a fixed orientation relative to space in an inertial reference frame, unlike on Earth's surface where the frame is relative to the ground.
• 🔄 The International Space Station (ISS) spins once every orbit to keep one side facing Earth and the other facing the direction of travel, creating a local vertical/local horizontal (LVLH) reference frame.
• 💨 Even in space, the ISS experiences atmospheric resistance, which affects its spin and orientation.
• 🌐 The orientation of the ISS's solar arrays impacts the amount of air resistance it encounters, leading to changes in spin rate throughout its orbit.
• 🔄 The ISS is subject to gravity gradient torque, where different parts of the station experience varying gravitational pulls due to their distance from Earth.
• 🔄 To counteract external forces, the ISS is placed in a torque equilibrium attitude (TEA) to balance air resistance and gravity gradient torques.
• 🛠️ Control moment gyroscopes (CMGs) and reaction wheels are used to control the ISS's orientation without using thrusters, conserving fuel.
• 🔄 Reaction wheels can become saturated, or reach their maximum speed, limiting their ability to counteract external forces.
• 🚀 The ISS uses a combination of CMGs and reaction control system thrusters to maintain its orientation and desaturate the CMGs when needed.

Q & A

What is a reference frame?

A reference frame is an imaginary box that defines directions such as up, down, left, right, forward, and backward, or in technical terms, the XYZ axes.

How does an inertial reference frame differ from a non-inertial one?

An inertial reference frame has its orientation fixed relative to space, whereas a non-inertial reference frame, like the one on Earth's surface, is fixed relative to Earth.

What is the LVLH reference frame?

LVLH stands for local vertical/local horizontal and is a reference frame where the imaginary box is defined relative to Earth's surface below the spacecraft.

Why does the International Space Station (ISS) need to be reboosted periodically?

The ISS needs to be reboosted periodically due to the tiny amount of atmospheric resistance it experiences even at its altitude, which causes it to lose altitude.

How do the orientation of the solar arrays affect the ISS's air resistance?

The orientation of the solar arrays affects the ISS's air resistance because they change the surface area facing the oncoming air, which in turn changes the amount of air resistance.

What is torque equilibrium attitude (TEA)?

TEA is an orientation of the ISS in the LVLH reference frame where the air resistance torque and the gravity gradient torque oppose each other to balance out.

Why is it necessary to actively control the attitude of the ISS?

The ISS requires active attitude control to maintain its orientation and to perform tasks such as docking with supply vehicles.

What are control moment gyroscopes (CMGs) and how do they work?

Control moment gyroscopes work on the principle of conservation of angular momentum. They use spinning wheels to create angular momentum, and by adjusting their orientation, they can control the spacecraft's spin.

What is the limitation of reaction wheels?

The limitation of reaction wheels is saturation, which occurs when the wheel reaches its maximum speed and can no longer counteract external forces effectively.

How do CMGs avoid the saturation issue?

CMGs avoid saturation by rotating independently to adjust their angular momentum vectors, allowing the ISS to counteract external forces without reaching a maximum speed limit.

Why is it advantageous for the ISS to use a torque equilibrium attitude?

Using a torque equilibrium attitude is advantageous because it minimizes the need for thrusters, which require fuel and can disrupt delicate experiments in zero-g conditions.