Rocket propulsion refers to an object being accelerated (it’s velocity changed) by the expulsion of stored “propellant” material from that body. This ejected matter has momentum, meaning that due to momentum conservation the body’s momentum must then change; causing a thrust effect in the rocket.

The physics behind rockets was put to use as early as 1232 A.D., unsurprisingly motivated by warfare. The Chinese used fire arrows to defend against Mongol invaders. These were solid-propellant rockets containing gunpowder in a tube, which once lit produced gases which escaped through a hole in the back of the arrow, causing thrust. Fire arrows are the basis of the legend of Wan-Hu, a Chinese general who attached forty-seven rockets to a chair and disappeared after ignition, never to be seen again. These rockets also laid early foundations for the bazooka, used by the U.S. in WWII, which launches rockets out of a tube to better control their direction.

The idea of staging, discovered by a German fireworks maker, is fundamental to modern rocketry. This is where a primary rocket is ignited and carries smaller rockets which ignite once it burns out. The first stage rocket may then detach to reduce the weight of the overall rocket and allow for greater heights and speeds to be reached, as the smaller rockets have reached a certain height ‘for free’, i.e. they haven’t had to use up their own propellant yet. This can also be done with several small booster rockets attached in parallel to a big rocket, for example the NASA Space Shuttle.

Rockets may be differentiated by the type of energy source they use. These include chemical combustion, nuclear reaction and radiation, although radiation methods are not limited to the sun – they can also involve microwaves and laser beams sent from Earth to a flying receiver, known as a light sail. These three methods define three categories which rockets can be divided into: chemical propulsion, nuclear propulsion and electric propulsion.

Chemical rockets are the most widely used type of rocket, with two common types being solid propellant and liquid propellant. These rockets have a combustion chamber in which chemical reactions between fuel and oxidizer (which together make up the propellant) produce exhaust gases, ejected to provide thrust. Solid propellant designs are often simpler, however liquid propellant rockets allow for greater efficiency and control of fligh

Nuclear thermal rocket (NTR) designs use nuclear fission to heat propellant and are efficient in generating extremely high thrust. These rockets have undergone ground tests, the earliest taking place in the 1950s, although they have never been flown. The Pentagon plans to launch an NTR in 2025, which would allow for more agile and rapid transit through space than current rockets.

Electrical rockets come in three main types – electrothermal, electrostatic and electromagnetic, each having its own pros and cons. Electrothermal rockets have relatively high thrust but low efficiency when compared to electrostatic and electromagnetic. Electrostatic are very low thrust but very high efficiency. Electromagnetic thrusters can in theory combine the high thrust and high efficiency of the other two types but they pose many difficulties in both the theoretical and engineering aspect.

Electrothermal rockets work by electrically heating a propellant and then accelerating it out of a nozzle. The main limitations on these types of thrusters are:

  1. How hot you can heat the propellant before melting the rocket.
  2. How much propellant you can heat per unit time.

Due to these limitations, electrothermal rockets only produce a few Newtons of thrust. However, they are much more efficient than chemical rockets and so are useful once in space.

Electrostatic thrusters ionise their propellant and use the Coulomb force to accelerate the ionise propellant out of the back of the rocket to produce thrust. This produces extremely high exhaust velocities. However, electrostatic thrusters only produce a few hundredths of a Newton of thrust. Despite this, electrostatic thruster are the most common type of electric rocket thruster in use today because of their extremely high efficiency. They are often called ion engines.

Electromagnetic thrusters also ionise their propellant However, instead of using the Coulomb force to accelerate the ionised propellant, they instead use the Lorentz force. This allows for much greater thrusts. This, in theory, allows electromagnetic thrusters to combine the high efficiency of electrostatic thrusters with the relatively high thrust of electrothermal thrusters. However, the power requirements of electromagnetic thrusters is very large and to control the magnetic fields used, superconducting magnets are needed. This makes for a difficult and expensive engineering endeavour. Because of this, research into electromagnetic thrusters lags behind that of electrothermal and electrostatic thrusters.


By Adam Bourke, Liam McManus, William Cosgrave and Jason Basquill.

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