Michael Patterson’s talk on June 21, 2012, began with a discussion of electric propulsion and its comparison to conventional chemical propulsion. Chemical rocket propulsion encompasses every type of typical propulsion system that we think of when it comes to a rocket: from the solid rocket boosters to the liquid Hydrogen/liquid Oxygen main engines on the Space Shuttle. This is the type of propulsion used in every ascent stage because it is capable of extremely high thrust. Unfortunately, the exhaust velocity, which is a main component in the rocket thrust equation, is limited by the chemical energy contained within the fuel, and this limits the specific impulse, a measure of thrust to weight of fuel required, to about 400 seconds.
If chemical propulsion is the hare, then electric propulsion is the slow but steady tortoise. Instead of using the chemical energy in a propellant, electric propulsion uses a large voltage gap to accelerate ionized molecules out of the thruster. Because the voltage difference is not dependent on the chemical energy in the propellant, which is usually a heavy element like Xenon, its exhaust velocity can far exceed a chemical engine. This means that an electric propulsion unit can have a specific impulse, Isp, in the tens of thousands, meaning lower propellant weight and more payload. This all comes at a price, and that price is thrust. Because the thrusters use such a small amount of propellant, they are unable to produce much thrust, about 1/10th of a pound, even with the high exhaust velocities. However, because they are so efficient, electric propulsion systems are ideal for station keeping as well as high Isp, planetary exploration missions. In fact, most communication satellites use electric propulsion to maintain orbit, and the Dawn mission uses three Xenon ion thrusters to accelerate out to the asteroid Vesta.
After going through the above basic concepts, along with some additional in-depth discussions on different electric propulsion techniques, Mr. Patterson began to discuss his new concept in electric propulsion. There are two proven concepts in electric propulsion: ion thrusters and Hall effect thrusters. While they are both under the broad category of electric propulsion, they use different methods to accelerate a spacecraft. Because of this, they operate in different power regimes and, therefore, are efficient at different thrusts that are applicable to different situations. In response to this problem, Mr. Patterson envisioned a thruster that is basically a combination of both. In general, electric thrusters utilize circular tubes; however there is no reason why they cannot be implemented as co-annular devices. Since there are no technical problems with this configuration, instead of trying to devise a completely new system to cover both power regimes, Mr. Patterson suggested an engine that takes a typical Hall Effect thruster and wraps an ion thruster around it. In this configuration, the thrusters can be fired independently when the mission situation calls for that thrust. Additionally, since this is such a modular design, life testing can become easier because the argument can be made that a small section can be life tested, and then a scaled up version can be qualified by the small section’s results. This discussion was very interesting in that it gave a good overview of electric propulsion, as well as, showed that sometimes the somewhat obvious ideas can be the best.