A More Efficient Spacecraft Engine

NASA engineers have finished testing a new ion-propulsion system for earth-orbiting and interplanetary spacecraft. The system is more powerful and fuel-efficient than its predecessors, enabling it to travel farther than ever before.

Ion propulsion works by electrically charging, or ionizing, a gas using power from solar panels and emitting the ionized gas to propel the spacecraft in the opposite direction. The concept was first developed over 50 years ago, and the first spacecraft to use the technology was Deep Space 1 (DS1) in 1998. Since then, there have only been a few other non-commercial spacecraft that have used ion propulsion: NASA’s Dawn mission to the outer solar system, launched in 2007; the Japanese deep space asteroid sample return mission called Hayabusa, launched in 2003; and the European Space Agency launched the SMART-1 spacecraft in 2003, it crashed on the moon in 2006. (There are many commercial communication satellites that use ion thrusters.)*

To build the new ion-propulsion system under NASA’s Evolutionary Xenon Thruster (NEXT) program, engineers at NASA’s Glenn Research Center in Cleveland, OH, modified and improved the design of the engines used for DS1 and Dawn. “We made it physically bigger, but lighter, reduced the system’s complexity to extend its lifetime, and, overall, improved its efficiency,” says Michael Patterson, the principal investigator on the project.

Patterson presented a paper describing the engine at the Joint Propulsion Conference and Exhibit held this week in Denver. He says that his team could start building a mission-ready version of the engine by January 2010, which would take about 36 months to complete.

Chemical propulsion systems are most commonly used for spacecraft, but they require large amounts of fuel and are inefficient for deep-space missions. “You are limited in what you can bring to space because you have to carry a rocket that is mostly fuel,” says Alexander Bruccoleri, a researcher in the aeronautics and astronautics department at MIT. In addition, he says, “you have to compensate for the weight and size of the propellant tanks by building a spacecraft that is flimsy or does not have many structures to reinforce it.”

As an alternative, several research groups are exploring electric propulsion systems. While these engines produce much less thrust than chemical engines, they are very efficient, making them ideal for long-distance missions to asteroids, comets, or planets like Jupiter and Mercury. However, “one of the biggest challenges in electric propulsion is the high power and lifetime of the system,” says Daniel Brent White, another researcher in aeronautics and astronautics at MIT.

*Thanks to readers comments, this information was corrected to include the European and Japanese missions.

The new ion engine builds upon the electric propulsion systems used by both DS1 and Dawn, says Patterson. It uses the same method to achieve thrust: xenon gas flows into a reaction chamber inside the engine and is ionized by electrons; electromagnets positioned around this chamber enhance the efficiency of ionization. Electrodes positioned near the engine’s thrusters (known as ion optics) are then used to accelerate the ions electrostatically and shoot them out of the exhaust to push the spacecraft forward.

The Glenn Research Center engineers optimized the mechanical design of the engine’s magnets and ion optics, and made other modifications, including reducing the number of thrusters, to make the system more powerful and more efficient. “The engine has a higher power level and a larger throttling dynamic range–it can go from very high power to very low power–so it can operate for longer periods of time and better execute its mission,” says Patterson.

Michael Huggins, the space and missile propulsion directorate in the Air Force Research Laboratory at Edwards Air Force Base in California, says it is important to find ways to make propulsion systems more efficient, smaller, and more economical. The fact that NASA is looking at more-efficient devices for interplanetary missions “is definitely the right answer,” he says.

However, there are potential drawbacks to ionic propulsion. For example, solar energy cannot be used too far from the sun. “Solar just won’t work out to distances like Neptune,” says White, who presented a paper at the same conference on using nuclear energy as a power source for deep-space missions. While this would provide plenty of power in deep space, safety concerns would make it politically challenging to launch a nuclear-powered spacecraft.

“The only competitor we really have is advanced chemical technology,” says Patterson. “The advantage that we have is that we are very fuel efficient.” Thus, for complex planetary missions that require lots of energy, says Patterson, the US and its international partners, including Japan and European nations, are transitioning to ion propulsion engines.