Cillian Thomas - 24/01/2020

Various types of propulsion for rockets, planes, and other flying objects rely on the same principle to propel themselves forward: Newtons’ third law of motion, i.e If body A exerts a force on body B, body B exerts a force equal in magnitude and in opposite direction onto body A. In other words, every action has an equal and opposite reaction. That means propelling mass with a force out of a rocket nozzle will propel the rocket in the opposite direction of the propelled exhaust gas with an equal force.

Image Via Tom Benson, Source: NASA

This applies to the movement of airplanes, jets, rockets, and even boats and cars. The difference with rocket propulsion is that it must work in the vacuum of space, so the method of creating the propulsion force cannot rely on the fluid surrounding itself, such as a boat where someone can use a row to propel the boat forward.

One of the most common ways to create this propulsion force is via combustion, in other words, controlled explosions. Generally, this involves two substances called a ‘fuel’ and an ‘oxidizer’, where the fuel is a chemical compound that really wants to give away its electrons, and the oxidizer really wants to accept electrons. Once heat is supplied to the fuel and oxidizer, a chemical reaction occurs (i.e combustion occurs) giving off exhaust gases and heat. This exhaust is propelled out of the rocket and thus the rocket is propelled in the opposite direction.

Image Via Nancy Hall, Source: NASA

In rockets propulsion, there are three main methods of using combustion to propel a rocket and they are generally characterized by the state of matter of the propellents (the propellents are the combination of fuel and oxidizer). These are solid propellant, liquid propellent, and hybrid propellant rockets (propellent is the combination of fuel and oxidizer).

Solid propellant rockets consist of a ‘grain’ that contains the fuel and oxidizer in solid form. When ignited continues to combust indefinitely. The simplest form of a solid rocket is a firework for example. Solid rockets offer a simple method of propelling a rocket without the need for complex mechanical components. A disadvantage to solid rockets is the fact that it is virtually impossible to stop and/or reignite combustion which can be quite dangerous.

Image Via Tom Benson, Source: NASA

Liquid propelled rockets store the propellants in liquid form in high-pressure tanks. These liquids are often required to be kept at extremely low temperatures in order to stop them from turning into a gas again (i.e boiling). Also quite often these rockets require quite a lot of complex turbomachinery to feed the liquids into the combustion chamber. Nonetheless, they are capable of propelling the exhaust gas at extremely fast velocities.

Image Via Tom Benson, Source: NASA

Lastly, a hybrid rocket contains the oxidizer in liquid form and the fuel in solid form. These rockets tend to be very safe, but come at the cost that the combustion process is more complex than solid or liquid rockets. A reverse hybrid may also be used where the fuel is in liquid form and the oxidizer in solid form.

Image credit Jack Rockley, source link

There are many other types of rocket propulsion that I have not discussed such as electric propulsion, nuclear fission and fusion propulsion, gaseous propulsion, and much more, but for the purposes of this blog post, I intend to discuss the most common types of propulsion. These other methods of propulsion if developed may have merit at some point in the future for space travel.