What is Electric Propulsion? – Space Propulsion Laboratory

First, Chemical Propulsion

Chemical propulsion is what most people think of when they hear “rocket engine”. Chemical rockets leverage chemical reactions (usually combustion of 2 rocket fuels) to release energy and generate thrust.

Very high thrust
Low specific impulse
Operate in atmosphere or vacuum

Now, Electric Propulsion

Electric propulsion exhaust is made of charged particles, which feel and react to electric and magnetic fields. These fields accelerate the charged particles to high velocities to generate thrust.

Relatively low thrust
High specific impulse
Operate in vacuum of space

Images courtesy of NASA.

What are “thrust” and “specific impulse”?

Thrust: The amount of force that an engine produces. With more thrust, you can increase your speed at a much faster rate and you can overcome much stronger opposing forces (such as gravity and atmospheric resistance).

Specific Impulse: A measure of the effectiveness of a rocket engine relative to the amount of propellant (fuel) it consumes. In some ways, specific impulse can be thought of as similar to the “miles per gallon” of a car. With higher specific impulse, the engine can do more with less fuel.

Specific impulse is measured in “seconds”. Chemical rockets cannot have specific impulse higher than about 500 seconds, limited by the amount of energy produced by the chemical reactions. Electric propulsion is only limited by the amount of electric power you can generate. While theoretically almost limitless, practical electric rockets have specific impulse as high as 5,000 seconds, up to 10 times higher than chemical propulsion!

If electric rockets are so efficient, why aren’t they used for everything?

The high efficiency of electric propulsion comes at the cost of a much lower thrust. Electric rockets do not generate enough thrust to launch a rocket from ground to orbit or to fight against the atmosphere. Additionally, even though electric rockets can do more with less propellant, the low thrust makes it so that it will take a longer time to do it. Electric propulsion is most useful for systems that are already in space and ones that value saving mass over a short misson duration.

DCF thruster test firing in MIT's Astrovac

Diverging Cusped Field (DCF) thruster test firing in SPL’s Astrovac test facility at MIT.

FAQs

Is electric propulsion a new technology?

Electric propulsion often appears very futuristic, especially because it is less commonly known compared to chemical propulsion. However, some are surprised to hear that the first spacecraft to use an ion engine was launched in 1964! Electric propulsion is now almost commonplace, as thousands of active satellites are making use of it at this very moment.

What is an electrospray thruster?

At MIT, we specialize in researching electrospray thrusters, which is a subset or type of electric propulsion. Different types of electric propulsion distinguish themselves in how ions are produced and accelerated. In an electrospray thrusters, charged particles are pulled directly out of a liquid propellant using a strong electric field and accelerated using charged electrodes.

Who works on electric propulsion

A lot of groups and people! NASA and international space agencies have strong electric propulsion programs, many universities (like MIT) do research in electric propulsion, and dozens of companies make and sell electric thrusters.

How can I become a part of this exciting field?

Electric propulsion is inherently multidisciplinary. Designing, building, and testing electric thrusters requires many people with diverse skillsets, including aerospace engineering, plasma physics, fluid dynamics, electronics, and more.