If space is a vacuum, then how do space capsule's retrorockets steer?

Asked by: Mr. Wood

Answer

The fact that space is a vacuum nicely isolates this problem as a demonstration of Newton's third Law, commonly phrased as 'every action has an equal and opposite reaction.'

It is not necessary for the rocket exhaust to push against anything EXCEPT THE SHIP ITSELF. You see, when the combustion of fuel takes place inside the rocket (think of a long vertical cylinder with the 'bottom' open) the exhaust gasses produced expand quickly in all directions. The molecules slam into anything in their path exerting a small force each. Thus some molecules push against the 'right' side and some push against the 'left' side, and all these sideways forces cancel each other out. Some molecules slam against the 'top' of the cylinder, but since there is no bottom of the cylinder, there is no force to cancel this out! Therefore the net force will be in the 'up' direction.

Another way to think about the situation is as a conservation of momentum problem. Any isolated system will conserve total momentum. Thus if you imagine a stationary ship, plus the fuel and oxygen molecules on the ship, p=0. If a few trillion atoms of exhaust are shot in one direction into the vacuum of space, the rest of the ship must move in the OPPOSITE direction with the same momentum (mass x velocity)

You can simulate this situation by throwing some bean bags (or bowling balls, or any other objects you have lying around) in one direction as you sit in a chair with wheels or stand on roller blades or ice skates. Whatever direction you push on the object, the object will push back on your hand with the same force, but in the opposite direction. Therefore you and the object you throw will move in opposite directions.
Answered by: Rob Landolfi, Science Teacher, Washington, DC

Retro-rockets are usually used to decelerate, not steer, a spacecraft. They operate like any rocket engine in the vacuum of space, by propelling gases in one direction to create an opposite and equal force on the craft. There is no need for air for those gases to 'push against' thanks to Newton's third law of motion.

'Steering' a spacecraft means changing its existing orbit or trajectory. That is usually done by using small attitude control rockets to point the vehicles main, larger rocket engine opposite to the direction you want to head. The main rocket engine is then fired to alter the vehicles original path. That same main rocket engine can be used as a retro-rocket by using the small control rockets to point it in the direction of motion before firing it.
Answered by: Paul Walorski, B.A., Part-time Physics Instructor