NASA Psyche Mission

Introduction to Electric Propulsion

What would you think if you were told that today’s space travel relies on non-combustible fuel? You may be tempted to think this is science fiction, however this is exactly how the Hall Effect Thruster functions. Once a spacecraft is in space, this invention applies about the same amount of force that you would feel holding a stack of 10 quarters or a single tennis ball. (10 16 18) Over time in the frictionless void of space, the spacecraft is slowly and continuously accelerated to extreme velocities, up to 200,000 mph. (18) Typically a noble gas (like xenon) is used as a fuel in a process that is over twice as efficient as chemical thrusters. (18) Welcome to the world of electric propulsion.

Introduction to the Hall Effect Thruster

The Hall Effect Thruster is a type of electric propulsion, meaning it uses electric and magnetic fields to accelerate gaseous Xenon propellant to incredible speeds creating thrust for the spacecraft. This differs from traditional chemical thrusters which burn propellant to create thrust for the spacecraft.

Xenon Propellant

The choice to use a noble gas like xenon as propellant is three-fold. First, Xenon is a large, heavy atom which produces high thrust. Second, it is easily ionized. Lastly noble gasses are inert meaning that they are safe to use with a wide variety of other spacecraft materials. (12 18)

Benefits of Electric Propulsion

Improved efficiency:
Electric propulsion systems (like Hall Effect Thrusters) have high specific impulse allowing for them to use significantly less propellant on a mission than with chemical thrusters. (12) For example, the Dawn spacecraft held only 937 lbs of Xenon propellant. (23) The space shuttle's orbiter holds roughly 11,000 lbs of liquid MMH fuel alone and a single space shuttle rocket booster has a solid propellant weight of 1,000,000 lbs. (20 24)

Increased lifetime:
The Hall Effect thrusters used on the NASA PSYCHE mission have lifetimes of around 5,000 hours. To compare, space shuttle rocket boosters which use solid chemical propellant have operational times of only 2 minutes. (20)

How do Hall Thrusters Work?

Anode and Cathode

A cathode (negatively charged electrode) at the downstream end of the chamber emits negatively charged particles. An anode (positively charged electrode) at the upstream end of the chamber injects neutral Xenon propellant.

Electromagnets

Electromagnets surrounding the chamber create a radial magnetic field. The 2 electrodes at each end of the chamber are responsible for the axial electric field.

The Electrons Travel

Some of the emitted electrons travel from the cathode to the chamber, and the forces from both the magnetic field and electric field trap the electrons at the downstream end of the chamber along a particular trajectory called an azimuthal drift.

The Xenon Travels

The Xenon propellant travels downstream toward the newly trapped electrons. Upon interaction with the electrons, the Xenon atoms become ionized, losing one of their electrons and converting them to positively charged Xenon ions. More specifically, when the neutral Xenon molecule comes into contact with electrons bound to the hall current (azimuthal drift), an electron becomes freed and travels to the anode in order to complete the discharge circuit.

Xenon

The electric field propels the Xenon ions out of the chamber at immense speeds. Note that if the thruster only propelled positive Xenon ions into space however, the spacecraft’s overall charge would become increasingly negative to compensate for the lost positive charges of Xenon. To resolve this, an equal number of electrons are exhausted along with the Xenon ions so that charge is conserved.

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The Team

Nic
Breuer

Matt
Bradford

William
Kostecki

Sina
Malek

Samuel
Zirbel

Matthew
McKinstry

This work was created in partial fulfillment of Arizona State University Capstone Course “CSE 423”. The work is a result of the Psyche Student Collaborations component of NASA’s Psyche Mission (https://psyche.asu.edu). “Psyche: A Journey to a Metal World” [Contract number NNM16AA09C] is part of the NASA Discovery Program mission to solar system targets. Trade names and trademarks of ASU and NASA are used in this work for identification only. Their usage does not constitute an official endorsement, either expressed or implied, by Arizona State University or National Aeronautics and Space Administration. The content is solely the responsibility of the authors and does not necessarily represent the official views of ASU or NASA.