Hi, I’m Marley, the astronomer here at the Space Centre. With this month’s theme being on propulsion, I wanted to take a look at how we move spacecraft, and the new types of engines being developed.
Propulsion is the action of driving or pushing something forward. In a propulsion system, thrust, a force, is produced that pushes an object forward. We experience this in everyday situations. The engine in a bus or a car, for example, provides the force necessary to turn the wheels and propel the vehicle forward.
This stems from Newton’s 3rd law: for every action, there is an equal and opposite reaction. We can see it happening when watching a rocket launch. What we see as fire from the engine is pushed outwards, down towards the ground. The equal and opposite force is the thrust, pushing the rocket into the sky. The type of propulsion used in rockets is known as chemical propulsion.
In chemical propulsion, the exhaust is produced through the combustion of fuel and an oxidizer. The combustion of these chemicals converts the energy stored in the chemical bonds into the powerful thrust that we see as fire. The initial burst is very powerful, but there are a few problems with using chemical propulsion as our main form of space travel – especially if we want to do routine trips to Mars.
To produce the thrust, you need two ingredients, and once you’re out of ingredients you cannot produce anymore thrust. You could bring more fuel – but that will increase the mass of your rocket, and take away space that could be used for something else. Once you’re out of propellant, you become unable to course-correct. This leads to missions being locked into specific launch windows, with the lifetime of missions like orbiters, or even the JWST, being tied to how much propellant is onboard.
A solution that covers all of these problems is using a different type of propulsion all together: ion propulsion. Instead of using fuel and an oxidizer, ions are ejected to create thrust. An ion is an atom or a molecule that has an electric charge – either positive or negative. In ion propulsion, the ions have lost electrons, making them be positively charged. This type of technology was first tested with the launch of Deep Space 1 in 1998.
This type of technology was used with amazing success in NASA’s Dawn mission. The Dawn spacecraft has three ion engines, and it used one at a time. Each ion thruster used an electrical charge to accelerate xenon ions to speeds that were up to 10 times that of chemical engines. The electrical charge is created by the energy collected by Dawn’s solar arrays. Each engine only used about 3.25 milligrams of xenon per second when operating at maximum thrust. (For comparison, 1 aspirin tablet weighs about 340 mg.) The force at maximum thrust was about the same as the force necessary to hold a piece of paper in the palm of your hand. This does not seem like a lot, but on the scale of travelling through the solar system it is. While it took Dawn 4 days to accelerate from 0 to 97 km/h at maximum thrust, over the entirety of Dawn’s mission the total change in velocity is similar to the thrust of the Delta rocket that carried Dawn into space. All nine solid-fuel boosters, plus the three stages of Dawn – comparable to a single ion propulsion engine on a spacecraft.
Ion propulsion engines are incredibly efficient. They reduce the amount of fuel needed by up to 90% when compared to chemical propulsion systems. This also saves money. However, they are not without their problems. Xenon gas is rare and expensive. An alternative, iodine, is much more prevalent, but it is corrosive and could destroy sensitive electronics over time.
Scientists and engineers at NASA’s Glenn Research Center are currently developing and testing new technologies in Solar Electric Propulsion. This is a type of propulsion that uses solar arrays to generate electricity that will power the ion thrusters. In March of last year, the propulsion system that will keep the Lunar Gateway in orbit around the Moon was tested – and passed! So, while ion propulsion sounds like something out of a sci-fi movie, it may become a reality much sooner than we think. Check out some activities and videos below to learn more.
Beginners Guide to Propulsion
Ask yourself: What might be some of the issues with using such explosive chemicals to launch rockets?
A balloon is like a very simple rocket engine. Experiment with how the amount of air in a balloon affects how far it will travel with this activity.
Build your own rocket launcher out of PVC pipe and then use it to launch a paper rocket. Record the altitude your rocket reaches. Are there ways to make your rocket fly higher?
Ask yourself: Where else do you experience Newton’s 3rd law as you go about your day?
Ions in Action
Then test the power of ions with this fun activity , using a balloon and small pieces of paper.
Ask yourself: Would you rather travel on a spaceship powered by an ion drive or by chemical propellant?