Propulsion moves things like spacecraft or jet planes forward by pushing something out of the back. Think of a balloon that you blow up and then release. The air rushing out of the back pushes the balloon forward. This happens because of a phenomenon described by Sir Issac Newton: "every action has an equal and opposite reaction."
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How are rockets designed?
How does DS1's engine work?
What is a propellant?
How does solar electric propulsion (ion propulsion) work?
How do conventional rockets work?
What is thrust?
Every action has an equal and opposite reaction?
What are the types of rocket propulsion?
How is rocket propulsion different from jet propulsion?