How Does a Rocket Work?
A rocket doesn't push against the air. There's no air in space! Instead it throws hot gas out the back — and Newton's Third Law pushes the rocket forward. Let's find out how.
Newton's Third Law
Over 300 years ago, the scientist Isaac Newton wrote down three Laws of Motion. The Third Law is the one that explains rockets:
What does that mean? When you push something in one direction, it pushes you in the opposite direction with exactly the same force. Every single time.
- You push off a swimming pool wall → the wall pushes you away.
- You throw a ball forward → you get a tiny push backward.
- A rocket throws hot gas backward → the gas pushes the rocket forward.
A rocket engine burns fuel to create incredibly hot, pressurised gas. That gas rushes out of a nozzle at the bottom of the rocket — and Newton's Third Law slingshotts the rocket upward.
The Balloon Analogy
Blow up a balloon and let it go without tying the end. What happens? The air rushes out the back, and the balloon flies forward! That's exactly how a rocket works — just with hot gas instead of air, and at much higher pressure.
The balloon doesn't push against the air around it. The reaction is between the balloon and the escaping gas. That's why rockets work perfectly in the vacuum of space.
Thread a string through a straw. Blow up a balloon, pinch the end (don't tie), and tape it to the straw. Let go — your balloon rocket will zoom along the string!
Why So Much Fuel?
To escape Earth's gravity, a rocket needs to reach a speed of about 11.2 km/s — that's 40,000 km/h, or 25 times faster than a bullet!
The trouble is: the heavier the rocket, the more fuel you need. But more fuel makes the rocket heavier, which means you need even more fuel… This is the tyranny of the rocket equation, discovered by the Russian engineer Konstantin Tsiolkovsky in 1903.
The result? A typical rocket is about 90% fuel by mass when it launches.
Rocket Staging
Here's a clever trick engineers use: staging. Instead of one big rocket, build several rockets stacked on top of each other.
- The first stage fires, lifting the whole rocket. When its fuel is used up, it drops away (jettisoned) — the rocket is now lighter!
- The second stage fires. Because it doesn't have to carry the heavy, empty first stage, it's much more efficient.
- A third stage (if there is one) might fire to push the spacecraft into orbit or beyond.
Dropping empty stages is like finishing a bottle of water and throwing away the bottle before running a race — you don't carry useless weight.
SpaceX Falcon 9 does something even cleverer: it lands the first stage back on a drone ship in the ocean so it can be used again. Reusability dramatically cuts the cost of getting to space.
Real Rockets
- Saturn V (NASA, 1967–1973): Still the most powerful rocket ever flown. It sent 12 people to the Moon. Thrust at launch: 35,100 kN (about 7.9 million pounds of force).
- Falcon 9 (SpaceX): First rocket to regularly land and reuse its first stage. Workhorse of modern satellite launch.
- Starship (SpaceX): The largest and most powerful rocket ever built — designed to take humans to Mars. Thrust: ~74,000 kN.
- Ariane 6 (ESA): Europe's main heavy launch vehicle. Puts science satellites and commercial payloads into orbit.