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©2018 Reaching Space Science. 

Rocket Science: Propulsion

June 5, 2019

One of the biggest technical challenges in the rocketry is the strength needed to get the rockets off the ground and in to space. To do this, rockets operate on Newton’s 3rd Law of Motion: “To every action there is an equal and opposite reaction.” In this way, rocket engines are dispensing mass in one direction and benefiting from the reaction that occurs in the opposite direction. The same principle applies when letting free a balloon that has been inflated with air or when a gun recoils after being fired. To do this, rockets rely on what is called thrust to get to space, which is the force made by the rocket’s engines in a propulsion system.




To achieve this power, a rocket’s propulsion system is tasked with two main jobs. Firstly, these systems must achieve enough thrust to push against the force of gravity that is working against the rocket it in order to get to space. Rockets must reach a certain speed to escape gravity’s pull. This speed is known as the “escape velocity”. As a rocket climbs through the atmosphere, air resistance will try to pull it back to Earth as well. This force also fights against the engines’ thrust. Each planet has a different escape velocity. On Earth, this speed is approximately 11.2 kilometres per second, which is around thirty times as fast as a bullet leaving a handgun. In comparison, the escape velocity of the Moon is 2.38 kilometres per second. It is lower on the moon because the force of gravity here is less than it is on Earth. The second job of the propulsion system is to make sure that the payload that the rocket is carrying reaches an orbit around the Earth. Once it space, a rocket can make manoeuvres to place the payload in the right location depending on its destination.


The propulsion system of a rocket includes all components that work to make the needed thrust, including the propellants and engines. Larger and heavier rockets require a greater number of engines. Of the many components of these engines, two are of particular importance: the rocket nozzle and the combustion chamber. In typical chemical engines, propellants are combined in the combustion chamber where they chemically react by burning solid or liquid fuel in to gas at very high pressures. Then, these gases are pushed quickly through the engine's nozzle. This nozzle acts as a spout where these hot gases are pushed through to create thrust for the rocket with enough momentum to lift the vehicle off the ground. Because of the very high temperatures and heat transfer felt by these parts of the engine, they are typically cooled. The amount of thrust that is produced by these powerful rockets depends on multiple factors. These include how fast the gases leave the nozzle and the flow rate for the fuel that passes through the engine. When this force is high enough, the thrust will be greater than the counteracting force of gravity pushing against it and the rocket will lift-off en route for outer space.


 Space Shuttle Challenger launch in 1983. Credit: NASA


There are many different forms of rocket engines, but to date, only chemical rocket propulsion has been used to bring rockets to space. These engines produce hot exhaust gases by burning a propellant. These propellants are made up of a fuel and an oxidizer and can be in either solid or liquid form. Oxidizers are a type of chemical that fuel needs in order to burn. On Earth the most common oxidizer is oxygen because it is present in the air around us (the atmosphere contains around 21% oxygen). However in space, the lack of atmosphere also means a lack of oxygen so rockets must carry their own oxidizer with them. Another type of propulsion system that has seen various research and development is nuclear propulsion. In this engine, a nuclear energy source is used to heat the propellant. Nuclear energy is created from reactions that involve the cores, or nuclei, of atoms. No nuclear engine system has yet been used to launch rockets into space. Electric propulsion systems also exist but these systems cannot produce big amounts of thrust, so they are not usually used to launch rockets but instead for powering spacecraft once they have been launched in to space by a rocket.



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