Question

In some video games, spacecraft employ ion drives. Write a one page report discussing how an ion drive operates and whether it can efficiently propel a spacecraft. You will be marked on how well you explain scientific principles and the accuracy of your statements. Your report should be understandable by nonscientists.

Answer 1

An ion thruster or ion drive is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes propellant (neutral atoms) by adding or removing electrons to produce ions. In a typical ion thruster, electrons are generated by a hollow discharge cathode, located at the center of the thruster on the upstream end. The charged gas i.e. plasma produced consists of positive ions and negative electrons in proportions and the net charge is zero. Plasma has some of the properties of a gas, but it is affected by electric and magnetic fields. Although less powerful than existing rocket engines, ion thrusters are more efficient and can be used continuously for long periods, making them ideal for deep space missions. To compensate for low thrust, an ion thruster must be operated for a long time for the spacecraft to reach its top speed.

General working principle:

Ion thrusters use beams of ions to create thrust in accordance with momentum conservation and produced acceleration varies with charge/mass ratio of the ions. This ratio means that relatively small potential differences can create high exhaust velocities i.e. able to achieve high specific impulses. Ion thrusters are categorized in two categories - electrostatic or electromagnetic. The main difference is the method for accelerating the ions.

• Electrostatic ion thrusters use the Coulomb force and accelerate the ions in the direction of the electric field.
• Electromagnetic ion thrusters use the Lorentz force to move the ions.

The most common propellant used in ion propulsion is xenon, which is easily ionized and has a high atomic mass, thus generating a desirable level of thrust when ions are accelerated. Operating in the near vacuum of space, ion engines shoot out the propellant gas much faster than the jet of a chemical rocket. They therefore deliver about ten times as much thrust per kilogram of propellant used, making them very 'fuel-efficient'. The ion thrusters being developed now are being designed to operate for 7 to 10 years.