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About this Article
Written by: Christopher Shelner
Written on: October 18th, 2004
Tags: aerospace engineering, physics
Thumbnail by: NASA
About the Author
In the fall of 2004, Chris was a second year aerospace engineering major. His primary research and career interests include propulsion technologies and the commercial mining of asteroids.
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Volume V Issue V > Ion Propulsion: Exploring Space in the 21st Century
Electric or ion propulsion is the newest propulsion system that NASA has put into successful operation. The Deep Space 1 mission used the ion engine as its primary propulsion system and tested its capabilities for the 21st century. Its advantages over conventional propulsion include lower fuel weight, much higher fuel efficiency, and longer operational life. The NSTAR engine that operated on Deep Space 1 used electromagnetic forces to accelerate positively charged xenon ions through a potential difference. The potential difference was produced by circular electromagnets that pushed the xenon ions resulting in an ion stream that exited at the exhaust end of the spacecraft at high speed. The thrust output of the ion engine is very small, but the fuel efficiency is an order of a magnitude higher than chemical rockets. In propulsion systems, fuel efficiency is technically referred to as specific impulse or the amount of momentum increase for a given amount of fuel consumption. Given a sufficiently long mission time, an ion engine is able to achieve speeds far greater than any chemical rocket. The use of the ion engine will undoubtedly be the best choice of propulsion for space probes in the 21st century.

Introduction

Ion propulsion is a new technology that has been fully tested and implemented in experimental spacecraft. Research into this unique type of propulsion began in the 1950s. Commonly referred to as electric propulsion, ion propulsion systems are a particular type of electric propulsion. The most noticeable difference between a fully loaded conventional rocket and an electric propulsion system would be the mass of fuel required to produce thrust. While conventional chemically fueled rockets require millions of kilograms of propellant, ion propulsion systems require only a miniscule amount of propellant by comparison [1]. Also, all electric engines are highly efficient and reliable, making them excellent choices for long, unattended operation. One of the most applicable areas for electric propulsion is space exploration. The spacecraft designated Deep Space 1 was launched by NASA on October 24, 1998. The purpose of this spacecraft was to test 12 different experimental technologies; one of which was the ion engine developed under the NASA Solar Electric Propulsion Technology Application Readiness program, or NSTAR program. The engine was, therefore, referred to as the NSTAR engine. The mission was an unprecedented success and spurred further study into the new technologies that may arise through ion propulsion [2].
NASA
Figure 1: Ion-propulsive thruster.

Thrust of an Engine

Under most circumstances, operations in space can only be accomplished by imparting some sort of speed gain or loss on a spacecraft. There are two possibilities for this action: either the mass of the spacecraft is decreased or the exhaust speed provided by a thruster is increased [3] (see Fig. 1). A chemically fueled rocket does both of these tasks simultaneously. As the exhaust speed is increased, fuel is being consumed, and the weight is being reduced. The force imparted by this exhaust speed is called the thrust.

Design of the NSTAR engine

The concept behind the ion propulsion system is that ionized particles are used to create thrust. The thrust is produced through a series of stages. First, electrical power is generated from the solar arrays, which convert light energy from the sun into electrical energy that powers the navigation, communication, and propulsion systems. The electrical power that is sent to the propulsion system is directed to two different locations. Some electricity is used to heat a metallic plate on which the solid fuel is attached. The heat causes the solid fuel to be converted to a gaseous fluid composed of neutrally charged atoms. These neutral atoms are then released into a cylindrical, electromagnet-ringed​ chamber where they float freely because they are uncharged. The rest of the electricity sent to the propulsion system charges the electromagnets. The current flowing through the electromagnets produces a magnetic field that pushes positively charged particles perpendicularly to the direction of current flow.