Summary
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Abstract
Ion thrusters, also called ion engines (Figure 1), are a type of electrically-powered spacecraft propulsion. While they provide very low thrusts (and thus low accelerations), they can do so for very long periods of time using a very small amount of fuel. So, while not appropriate for escaping Earth's gravity like chemical rockets which are less efficient but generate higher thrusts, they are useful for deep-space probes or making small adjustments to satellite orbits. You can read more about ion thrusters in the NASA reference in the bibliography.
Figure 1. An ion thruster at NASA's Jet Propulsion Laboratory.
You can build a model of an ion thruster called an ion wind rotor using a Van de Graaff generator. A Van de Graaff generator generates positive electrostatic charge on its dome. This charge can be concentrated at the tip of an electrode (see the Physics Classroom reference in the bibliography and read the section about electric fields and surface curvature), which then ionizes nearby air molecules by stripping away electrons. The now positively-charged air molecules are repelled from the positively-charged electrode. According to Newton's third law of motion, this generates a reaction force that pushes on the electrode. When two electrodes that point in opposite directions are placed on a rotor, the resulting forces cause the rotor to spin (Figures 2 and 3).
Figure 2. A basic ion wind rotor made from a piece of aluminum foil mounted on a nail.
Figure 3. Diagram showing operation of the ion wind rotor. Reaction forces on the electrodes from the repelled air molecules cause the rotor to spin.
While the rotor's functionality is not identical to that of a true ion engine, it demonstrates a similar working principle of generating thrust by expelling a stream of ions. Watch this video for an overview of how to make the basic ion wind rotor shown in Figure 2, and an explanation of how it works:
While the design in Figure 2 works as a simple demonstration, friction between the aluminum rotor and the nail can often occur. Sometimes the rotor can get stuck, and the very lightweight aluminum foil can even be repelled upward, away from the Van de Graaff generator. A better design uses a piece of copper pipe with a hole drilled in it, balanced on a nail (Figure 4).
Figure 4. An ion wind rotor made from a piece of copper pipe balanced on a nail, with two more nails (attached to the pipe with rubber bands) as electrodes.
Watch this video for instructions to build this improved version of the ion wind rotor. You will need tools including a vise, hacksaw or reciprocating saw, a drill, and safety goggles.
Can you use this ion wind rotor design in a science project? While you cannot measure the thrust generated by an electrode directly, you can compare the relative magnitudes of thrust generated by two electrodes by using the rotor as a balance. If you point the electrodes in the same direction, then the rotor should not spin when the torques generated by each electrode about the pivot point cancel each other out (Figure 5).
Figure 5. When the torques exerted about the pivot point by the two electrodes are equal, the rotor will not spin.
This can be expressed mathematically using Equation 1:
Equation 1:Which can be rearranged to show the ratio of the forces generated by the two electrodes:
Equation 2:This will let you compare the relative forces generated by different electrodes by sliding the electrodes back and forth (closer to or farther away from the center) as needed until the torques balance and the rotor no longer spins in either direction. For example, you could compare different size nails or nails with different surface coatings. You can also examine the effect of electrode tip geometry (e.g. cut off or file down the tip of a nail), or even try making your own electrodes from other materials.
It may take some effort and experimentation on your part to get this project to work well. Differences in thrusts generated by different electrodes may be very small and difficult to measure. The analysis above also neglects friction between the center nail and the copper pipe, which will affect your results. You can start out by comparing very large differences between electrodes, e.g. flip one nail around so its head is pointing outward, acting as the "tip" of the electrode. Can you refine your results to measure differences between more similar electrodes, or even arrive at an optimal electrode design?
Bibliography
- Henderson, T. (n.d.). Electric Fields and Conductors. The Physics Classroom. Retrieved February 25, 2022.
- NASA (Aug 7, 2017). NASA - Ion Propulsion. Retrieved February 25, 2022.
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