# Thrusters and Spinning Wheels

## How do you Control Attitude in Space?

Because the control surfaces that an airplane uses cannot work in airless space, a spacecraft relies on a different form of attitude control. To change orientation, a spacecraft applies torque (a twisting force) by firing small rockets or by spinning internal wheels.

To rotate a spacecraft, a pair of thruster rockets on opposite sides of the vehicle are fired in opposite directions. To stop the rotation, a second pair is fired to produce an opposing force. The Space Shuttle’s Reaction Control System provides the thrust for attitude maneuvers (roll, pitch, and yaw), and for small velocity changes along the Shuttle’s longitudinal axis (front to back). See the Space Shuttle Discovery at the National Air and Space Museum's Steven F. Udvar-Hazy Center.
Credit: National Air and Space Museum, Smithsonian Institution

Some satellites use a system of reaction wheels—basically gyroscopes—to change their attitude in space. The rapidly spinning wheels carry a lot of rotational momentum that the spacecraft can tap to change its own orientation. To do this in all three dimensions of space, the spacecraft must use three rotating wheels oriented at right angles to one another.

## How Can Thrust Help Change Attitude?

Most spacecraft use small thruster rockets to control their attitude. To rotate a spacecraft, a pair of thruster rockets on opposite sides of the vehicle are fired in opposite directions. To stop the rotation, a second pair is fired to produce an opposing force.

Q:

## What is a gyroscope?

A:

A gyroscope is an object designed to turn very quickly around one axis. A fundamental property of gyroscopes is gyroscopic inertia. Gyroscopic inertia is the tendency of all objects rotating around an axis to maintain a fixed orientation, or direction of spin and position—if there are no outside forces. In other words, a rotating object tends to stay in the same position.

## Did You Know?

The space shuttle had 30 different attitude control thrusters in its Reaction Control System, and the International Space Station uses four 600 pound Control Moment Gyroscopes (CMGs) to keep the station oriented.

## Pop Quiz

Which of the following objects benefits from gyroscopic motion?

A) Frisbee
B) Yo-Yo
C) Football
D) All of the above

All of the Above: The nature of gyroscopic motion assists in the stability of footballs, yo-yos, Frisbees, space satellites, even the Hubble Space Telescope.  Footballs and Frisbees fly farther because of their tight spins.  And yo-yos move vertically up and down the string easier, because the spinning motion keeps the yo-yo’s axis perpendicular to the string.