Jet Propulsion - How does a space rocket work?

Ever worndered how spacecrafts move in space? If space is vacuum, where does the propulsion come from? These are questions that I'm sure most of us have asked ourselves at some point, and the answer ir not that complicated. First of all, I believe we should start by explaining what is the difference between an aircarft's jet engine and a spacecraft's rocket engine.

Jet Engine

A jet engine feeds off of air. The higher the density of air, the better performance it will have. There are tipically two types of jet engines used in aviation, turbojet and turbofan. They have the same working principles but significant differences in design.

Turbojet Engine

Turbojets work by compressing air, then mixing it with fuel, burning the mixture in the combustor and passing the hot, high pressure air through a turbine and a nozzle. In this type of engines all the air that is absorbed passes through the compressor. The compressor is made up of fans with many blades attached to a shaft. Some move and others are stationary. This way the air flows in the same direction while it is compressed. The compressor is powered by the turbine. After the combustion, a jet of gas is shot backwards providing thrust.

This thrust comes from Newton's Third law of Physics: for every action, there is an equal and opposite reaction, and from conservation of momentum. The conservation of momentum is a fundamental concept of physics such as the conservation of mass and energy. It states that in a closed system momentum is constant, neither created or destroyed, but changed through forces. The force exerted by the gas jets pushes the aircraft forwards.

Turbofan Engine

Turbofan engines follow the same principles, converting internal energy in the fuel to kinetic energy. The main difference is that they use a fan, also powered by the turbine, at the inlet, which bypasses the compressor and the combustor, and part of the flow is directly exhausted through the nozzle. The fact that this air flows at a lower velocity, but with a higher mass, makes the thrust more efficient in subsonic speeds. At higher speeds, the drag produced by the fan makes it less efficient. 


Now that we understand how these engines work, let's compare with the ones in spacecrafts.

Rocket Engine
The main difference between jet engines and rocket engines is that the latter carry on with them all the propellant, so they don't even need to be moving in a mass of air to work.

The principle of operation is the same, a high-speed jet of gases expelled by the exhaust produces thrust. There are two types of rocket engines according to their fuel system: solid and liquid. If we take a look at a section of both of them, we'll be able to understand better how they work.

As you can see, they are basically the same. The difference is the type of fuel they use. Solid rockets have been around for quite a bit. The Chinese invented gun powder in the 9th century to make weapons and fireworks, and it was slowly introduced in Europe until around the 13th century. Early rockets used to work with this or any other sort of powdered propellant, until liquid and hybrid rockets started replacing them in the 20th century, due to their efficiency and controllability. A simple solid rocket can't be shut off or throttled, as they contain all the mixture needed in the same chamber where they are burnt.

Modern rockets are 100% controllable, that's why we can use the for space travel. With liquid propellant it is a lot simpler, as you can increase or decrease the amount of fuel and oxidizer in the mix, or shut it of completely.

Next, you can see a skech by NASA, where they explain how to calculate a rocket's thrust.

I hope you have learned a little bit more about these wonders of human ingenuity. Please, comment if there are any question regarding this or any other subject.