Newton's Third Law in Action: How Rockets Really Fly

On April 20, 2023, SpaceX's Starship — the most powerful rocket ever built — lifted off with 33 Raptor engines producing about 7.9 million pounds of thrust, more than twice the thrust of NASA's Saturn V. Every one of those engines works on the exact same principle a deflating balloon uses to zip around a room: push mass one way, and you get pushed the other way.

- How Newton's Third Law explains rocket propulsion - The difference between chemical, ion, and nuclear thermal propulsion - How specific impulse measures engine efficiency - Why rockets need to carry their own oxidizer in space

Sir Isaac Newton's Third Law states that for every action, there is an equal and opposite reaction. A rocket engine burns fuel and blasts hot gas out of its nozzle at extremely high speed — that is the action. The reaction is the rocket itself being pushed in the opposite direction. This is why rockets can work in the vacuum of space, unlike propellers or jet engines: a rocket does not need air to push against, because it carries and expels its own mass.

Most rockets, including the Saturn V and SpaceX Falcon 9, use chemical propulsion: a fuel (like refined kerosene or liquid hydrogen) combines with an oxidizer (usually liquid oxygen) in a combustion chamber, igniting a controlled explosion that shoots hot gas out the nozzle at speeds up to 4.5 kilometers per second. Chemical rockets produce huge thrust quickly, which is why they are used to escape Earth's gravity, but they burn through propellant fast and are relatively inefficient over long distances.

Ion engines, used on NASA's Dawn spacecraft and Deep Space 1, work completely differently. They use electricity (often from solar panels) to strip electrons from xenon gas atoms, creating charged ions, then use an electric field to accelerate those ions out of the engine at speeds up to 90,000 miles per hour. The thrust is tiny — about as much force as holding a single sheet of paper in your hand — but ion engines can run continuously for years, making them far more fuel-efficient for long deep-space missions.

NASA and DARPA's DRACO program is developing nuclear thermal rockets, which use a nuclear reactor to heat hydrogen propellant directly, rather than burning it. This could roughly double the efficiency of chemical rockets, potentially cutting a crewed trip to Mars from about nine months down to just a few months, reducing astronauts' exposure to dangerous cosmic radiation.