How Rockets Work
To understand rockets, you need to know Newton's laws of motion:
- An object will remain still or keep moving the way it's already moving until some force changes its movement.
- Force equals mass times acceleration (f = ma).
- For every action there is an equal and opposite reaction.
The first law makes it obvious that a rocket won't move until some force starts the movement. To make the movement happen, we use the second and third laws.
Newton's second law is a calculation that shows how much force you need to move (accelerate) an object. This and other calculations are important to figure out how much force you'll need to move the rocket.
To get the required force, we use Newton's third law. This says that if you apply a force in one direction, an equal amount of force will be applied in the opposite direction. To accelerate a rocket upwards, we need to apply a downward force from the rocket.
We do this using a "reaction engine" which is a remarkably simple and familiar concept. Even if you've never heard the term "reaction engine", you've seen one. Imagine a balloon filled with air. If you open the lip and let the air out, the air will accelerate out of the balloon, in whichever direction the lip is pointed. That's the action force. The reaction force accelerates the balloon in the opposite direction.
This principle forms the basis of all chemical reaction rocket engines. You accelerate some mass out of the rocket in one direction to move the rocket in the opposite direction. We call the expelled mass "rocket fuel" or "propellant". The force is called "propulsive force" or "thrust".
You can use any kind of mass for propellant. The balloon uses air but that's not very efficient. Water and steam can be used for amateur rockets. Higher-performing rockets need more efficient chemical propellants, and they need to accelerate it faster.
This is where Newton’s second law of motion comes in: Force equals mass times acceleration (f = ma). This means that if you want more thrust you need to shoot out more propellant, or you need to accelerate that propellant more. Rockets accelerate the propellant by burning it, so it shoots out at very high speed. The biggest, fastest rockets are the ones that can push out the most propellant with the greatest acceleration.
Accelerating a rocket is a delicate balance between weight/mass and thrust. If you make a rocket bigger, it adds mass, requiring more propellant, which adds more mass. Rocket engineers strive to design systems with low mass, high-energy propellants and efficient engines.
Note that the medium through which the rocket is traveling will have some impact. Air resistance creates a resisting force so it has to be accounted for. A rocket could be designed to work underwater but the resistance will be much higher. A vacuum is the most efficient medium because there's no resisting matter at all.
A common misconception is that rockets have to "push against" something else to move. For example, some people think rockets wouldn't work in space because there's no air to push against. That's not how they work, though. They move by pushing propellant away from themselves—nothing else is required.
There's one more important application of Newton's laws to rockets: What happens when the fuel runs out. The first law states that a moving object will keep moving the same way unless a force changes that movement. If a rocket's fuel runs out before it reaches orbit or some more distant destination, the force of gravity will change its movement and it will fall back down. However, if the rocket is able to reach orbit or interplanetary space, it will keep moving without requiring any more fuel. It's common for rockets to use most of their fuel to begin their voyage but keep some for future "burns" to adjust course or land somewhere.
Note: Most rockets need four main things to be useful:
- The payload is the thing you want to get into space. It is usually a spacecraft of some kind, containing people, scientific instruments and/or supplies. Note that the payload is usually only a small part of the whole rocket system.
- The control system guides the rocket and keeps it on course.
- Propellant/fuel is the bulkiest, heaviest part of almost all rockets.
- The engine is where the fuel gets burned and accelerated out of the nozzle at the bottom of the rocket. It is common to have multiple engines in a single rocket.