Abstract

Objective

The goal of this electricity and electronics science fair project is to understand how ions are used to propel spacecraft in space and to use an online NASA simulator to design your own ion engine.

Introduction

Humans have always looked to space and asked questions like, "How big is the universe?", "How old is the universe?", "Is there life anywhere else in the universe?", and "How did our solar system form?". The job of the National Aeronautics and Space Administration (NASA) is to seek answers to these questions. One of NASA's missions is to advance and communicate scientific knowledge and understanding of Earth, the solar system, and the universe, and to use the environment of space for their research. For instance, NASA's Rover vehicle missions have given scientists a wealth of information regarding the probability of water and life on Mars. By studying Mars, scientists get clues about Earth and possible futures for the planet Earth.

	Figure 1. Testing an ion engine. (Courtesy of NASA, Glenn Research Center, 2008.)

On September 27, 2007 NASA launched mission Dawn. The purpose of Dawn is to study the asteroid Vesta and the dwarf planet Ceres. These bodies are believed to have each formed at the beginning of the solar system. By studying these bodies, scientists hope to learn more about the early solar system and how the solar system was formed. Dawn is expected to reach Vesta in August 2011 and reach Ceres in February 2015. The end of the primary mission is slated to be July 2015. This mission will last a little over 8 years!

One of the components of the spacecraft that is making this mission possible is the ion engine. The ion engine propulsion system is replacing the standard chemical propulsion system that has been used in past missions. Standard chemical propulsion systems use liquid oxygen and liquid hydrazine (a highly toxic, dangerously unstable, colorless liquid that is used in different applications, including rocket fuel) as fuel. The combination of hydrazine and oxygen is explosive and provides the spacecraft with the thrust it needs to move forward. An ion engine, however, uses electric fields instead of chemical reactions to move the spacecraft. See Figure 2, below. The ion engine propulsion system is made up of a discharge chamber and two plates, which are located at one end of the discharge chamber. In an ion engine, the gas xenon is ionized, or given an electric charge. The xenon gas enters the discharge chamber, where it is ionized by high-energy electron bombardment. A high-energy electron collides with a xenon atom and knocks off an electron from the xenon atom. This process yields two electrons (including the original electron) and one positively charged xenon ion. Each of the plates is charged such that the electric field across the plates attracts and accelerates the positively charged xenon ions. The xenon ions are electrically accelerated to a speed of 30 kilometers (km) per second to the exhaust of the spacecraft. Due to conservation of momentum, the spacecraft then moves forward. The thrust that the xenon ions provide is very gentle, though, and can't be used for rapid acceleration, such as launching a spacecraft from Earth. Thus, the Dawn spacecraft actually has both types of engines—the chemical propulsion system to propel it into space, and the ion engine to accelerate it while it is in space.

Chemical propulsion systems provide thrust to a spacecraft, which then coasts at a constant speed until the chemical propulsion system provides the next boost. In contrast, the ion engine propulsion system provides a small thrust continually to the spacecraft, giving it almost constant acceleration. This means that a spacecraft with an ion engine propulsion system can get to its destination faster. An ion engine system is 10 times more efficient than a chemical propulsion system. A more-efficient system requires less fuel and can last longer without running out of fuel. A spacecraft carrying less fuel can thus, be smaller and lighter and therefore, cost less to launch.

Figure 2. This schematic shows how an ion engine works. (Courtesy of NASA, Glenn Research Center, 2008.)

In this electricity and electronics science fair project, you will design an ion engine on NASA's Solar System Exploration website, using their ion engine propulsion simulator. You'll also practice with some exercises prior to using the ion engine simulator to learn more about how electric charges interact with each other. The simulator consists of a spacecraft with an ion engine, and two parameters (which you will control) that affect how far the spacecraft travels. Experiment with how these two parameters interact and find out how far your ion engine will go in the design simulator!

Figure 3. This is an image of the Dawn mission spacecraft at Kennedy Space Center (Courtesy of NASA, 2007.)

Terms, Concepts, and Questions to Start Background Research

• Ion engine
• Propulsion
• Thrust
• Ionization
• Momentum
• Conservation of momentum
• Acceleration
• Electric charge
• Voltage

Questions

• What is an ion? How are ions created?
• What is conservation of momentum and how is this concept used to move spacecraft in outer space?
• By how much can the ion propulsion system increase the speed of the Dawn spacecraft? What is the limit of the maximum speed?
• What are the forces on Earth that affect a body in motion? Are these forces present in outer space?

Bibliography

To learn more about NASA's Dawn mission, visit the Dawn mission website. Check out the section on ion propulsion.

• Jet Propulsion Laboratory, California Institute of Technology. (2009, March 30). Dawn. Retrieved April 10, 2009, from http://dawn.jpl.nasa.gov/mission/index.asp

These sites explain ion propulsion in depth.

• National Aeronautics and Space Administration, Glenn Research Center. (2008, May 2). Ion Propulsion: Farther, Faster , Cheaper. Retrieved April 12, 2009, from http://www.nasa.gov/centers/glenn/technology/Ion_Propulsion1.html
• University of California at Los Angeles. (2008, March 17). DAWN: A Journey to the Beginning of the Solar System. Retrieved April 16, 2009, from http://dawn.ucla.edu/dawn/mission.html

You will use the following website for this science fair project:

• National Aeronautics and Space Administration. (n.d.). Solar System Exploration: Design an Ion Engine. Retrieved April 27, 2009, from http://solarsystem.nasa.gov/kids/ionengines.cfm

Materials and Equipment

• Computer with an Internet connection
• Lab notebook
• Graph paper

Experimental Procedure

1. After doing your background research, go to NASA's Design an Ion Engine website. Notice that there are four games available for you to click on. Play the first game, called Positive and Negative Charges, to learn more about electric charges and how they interact with each other.
2. After you are confident that you understand how electric charges move and interact, try the second game, called Charge Control Game, and learn how to manipulate electric charges.
3. Now you are ready to design an ion engine. Click on the last game, Design Your Own Ion Engine. You will come to a page that goes over the instructions for the game. The goal of the game is to adjust the distance between the plates and the voltage (charge) between the plates in order to provide the most thrust to the spacecraft. Both plates are electrically charged and this results in a voltage between the plates.
4. After reading the instructions, go to the simulator on the next screen. The simulator screen depicts an ionization chamber, along with two plates next to it. The blue electrode sends electrons (colored red) into the ionization chamber, which contains xenon atoms (colored green). The electrons ionize the xenon particles, resulting in the blue-colored, positively charged xenon ions. The xenon ions are then accelerated by the plates—from the positively charged plate to the negatively charged plate—next to the ionization chamber.
5. Below the chamber and plates are the two parameters that you will vary to maximize the thrust, plate location, and plate charge. Plate location is the distance between the two plates that provide the accelerating electric field. You can click on the arrows to change the plate location. The plate location can vary from 0 to 90. Although there are no units associated with the plate location, you will have to determine which value represents the Dawn spacecraft's optimum value of 1,000 um.
6. Plate charge varies from 0 to 100 and again, in this simulator, has no units. You will have to find the value of charge that results in the Dawn spacecraft's optimum voltage of 1280 V.
7. Below these are indicators that show the amount of fuel left for ionization and the amount of energy left for charging the plates. As the simulation progresses, the indicators change. Ideally, you want to maximize the thrust provided by the ion engine and minimize fuel and energy usage.
8. Next to the plate location and plate charge parameters are the "Launch" button and the comment box. The "Launch" button allows you to start and stop the simulation, and the comment box shows the final thrust value.
9. Above the ionization chamber is the thrust indicator. As thrust is provided by the ion engine, the little rocket moves to the left, accordingly. Again, thrust is depicted in relative numbers, with no units.
10. The simulator is set to default values for plate location and plate charge. Click on the launch button to see the thrust at this setting. Record the thrust value, plate location, plate charge, the approximate amount of fuel left, and the approximate amount of energy left, in a data table in your lab notebook.
11. Now minimize the plate location to 1 (not 0, which would mean the plates would be touching and would discharge) and maximize the plate charge to 100. Hit the "Launch" button. Record the thrust value, plate location, plate charge, the approximate amount of fuel left, and the approximate amount of energy left in your lab notebook. What kind of thrust value did this setting provide to the rocket?
12. Now start to systematically change the plate location and the plate charge to maximize thrust. Try setting the plate location to 45 and slowly varying the plate charge to see if you can spot any trends in thrust. Remember to record all of your data in a data table in your lab notebook. Does one parameter affect the thrust more than the other? In a real ion engine, increasing the plate location allows more xenon ions to arrive between the electrically charged plates. However, each of the xenon ions has its own electric field. The ion's electric field repels other xenon ions, but it also acts to shield the ion from the accelerating electric field of the plates. So the attractive force that the xenon ion feels from the plates can be reduced or blocked.
13. Once you have collected a significant amount of data, plot it on a 3-D scatter plot. Label the x-axis Plate Location, the y-axis Plate Charge, and the z-axis Thrust. What is the maximum thrust your rocket reached? Do you see any trends in the plot you made? Does either one of the parameters have a larger affect on the thrust, or do both parameters equally affect the thrust?
14. Further investigate the effect on the magnitude of thrust if you keep the plate location constant and vary the plate charge. Choose at least three different plate locations and vary the plate charge for each of the locations. Record all of your data in a data table in your lab notebook. Make a plot for each plate location where the x axis is labeled Plate Charge and the y axis is labeled Thrust.
15. Further investigate the effect on the magnitude of thrust if you keep the plate charge constant and vary the plate location. Choose at least three different values of plate charge and vary the plate location for each of the values of plate charge. Record all of your data in a data table in your lab notebook. Make a plot for each value of plate charge where the x axis is labeled Plate Location and the y axis is labeled Thrust.

Variations

• As stated in the Introduction, the Dawn mission needs both a conventional propulsion system and an ion propulsion system to accomplish its goals. If you would like to learn more about conventional propulsion systems check out this Science Buddies physics science fair project: Solid Motor Rocket Propulsion.

• For more science project ideas in this area of science, see Electricity & Electronics Project Ideas.

Credits

Michelle Maranowski, PhD, Science Buddies

This science fair project is based on one found at the Jet Propulsion Laboratory's website:

• NASA, Jet Propulsion Laboratory. (n.d.). Design an Ion Engine: Teacher Guide—Assessment. Retrieved April 13, 2009, from http://dawn.jpl.nasa.gov/DawnClassrooms/2_ion_prop/assess/1_tg_assessment.pdf

Last edit date: 2011-03-17 12:00:00