We’re all familiar with the image of an enormous rocket taking off from Earth, thrusters spewing out flame and smoke, powering the craft through the atmosphere and out into space.

But once the rocket has reached its destination, whether that’s in Earth’s orbit or circling another planet, other thrusters come into play, and they’re just as important as those that blast off from Earth.

Rockets, satellites, and any other object humans send into space must regularly complete orbital manoeuvres; this might be to avoid colliding with another satellite or object, to get into a better orbit, or to even allow passengers to transfer between spacecrafts, such as at the International Space Station.


One common form of thruster used in space are cold/warm gas thrusters. These thrusters accelerate a propellant gas, sometimes heated to increase efficiency, through a rocket nozzle to create thrust force capable of controlling a spacecraft’s movement.

They are the simplest form of thruster, much easier to design, manufacture and operate than thrusters that rely on combustion or a chemical reaction to work.

However, this simplicity means the choice of which propellant to use is paramount.

UNSW Canberra researcher, Dr Trevor Lafleur, together with collaborator Javier Martinez Martinez based in Germany, recently completed one of the most comprehensive evaluations to date looking at   almost 5000 possible propellants to determine which are the most effective for use in space missions.

Dr Lafleur said the evaluation could have important consequences for the space industry.

“A wide range of propellants have been used in previous space missions,” Dr Lafleur said.

“Nitrogen has been used frequently, as well as more unusual and rare substances like xenon, krypton, argon, and even iodine.

“However, recently the space industry has become increasingly interested in alternative propellants.

"Selecting a propellant is not an easy task as it can affect the design, operation, and performance of the propulsion system, and consequently also affect the mission.

“There is a broad range of considerations when choosing a propellant, such as cost, toxicity, safety and whether the substance is easily available.

“For example, some gases need to be stored at very high pressure which can be an explosion risk or require specialized equipment and trained personnel. But other propellants, like many that we studied, can be safely stored unpressurised as a liquid or solid.”

The space industry commonly uses a metric called ‘specific impulse’ which provides a measure of the fuel efficiency of a propulsion system; but it has its flaws.

While it gives an indication of fuel efficiency, it ignores some other critical factors, such as the actual amount of propellant needed to effectively complete manoeuvres and therefore the size and mass of the fuel tank required.

“So, you might pick a propellant with a good fuel efficiency, but the size and mass of the storage tank may be so large that it completely cancels out any fuel efficiency benefit you thought you had,” Dr Lafleur said.

To account for this limitation, Dr Lafleur and his colleagues introduced a novel and little-known metric which they called ‘effective specific impulse’. This provides a more comprehensive assessment of the propellant and entire propulsion system, as it accounts for not only the mass of the propellant, but also the mass of all the other hardware (such as the storage tank) needed to successfully use the substance.

Using the new effective specific impulse measurement Dr Lafleur and Mr Martinez Martinez determined that water was one of the most attractive and useful propellants. Several other common substances, such as ammonia and propane, were found to be among the top 10 most effective.

“In this project we ranked almost 5000 different propellants in the hope it can offer government space agencies like NASA, private space operators, and other researchers a clear picture of the most useful propellant for their particular mission,” Dr Lafleur said.

“The industry can now consult our rankings and easily use this as a basis to determine which substance will suit their needs best and hopefully take some of the guesswork out of picking the most suitable propellant.”

The research paper and rankings were recently published in space science journal Acta Astronautica, available here: https://doi.org/10.1016/j.actaastro.2023.07.031