New Mini Microwave Thruster For Satellites Is Most Powerful In Its Class

Only 2 inches long and 1.25 inches in diameter, the new mini-thruster depends on a microwave generator used in weather radar, aircraft radios and other communications applications instead of the larger kitchen oven magnetron used in the earlier prototype. Performance tested under simulated space vacuum conditions using as little as 80 Watts of power, the new mini-thruster produced the highest thrust for a continuously-operating low power electrothermal thruster.

Dr. Michael Micci, professor of aerospace engineering, led both the original and current thruster projects. He says electric propulsion thrusters currently used to position and maneuver satellites in space operate inefficiently or not at all in the 100-Watt range. The new mini- microwave thruster has the potential not only to reduce on-board power requirements but also to extend a satellite's productive life since it requires only one third the amount of propellant used by other systems. In addition, its small size makes it appropriate for the next generation of mini and microsatellites.

"Many commercial communication satellites in orbit today, for example, were still operational when they ran out of maneuvering propellant," Micci adds. "The increased capacity of the mini-thruster could extend satellite life by three times the number of years."

The idea for a microwave thruster has been around for over a decade. However, currently, Micci's thruster is the only prototype undergoing active ground testing. His ambition is to see the system tested in space.

The thruster concept is based on the fact that microwaves can be used to create and maintain a free-floating plasma or superheated, electrically charged gas within a cavity, Micci says. If a cold "propellant" gas is passed through or around the hot plasma in the cavity, the cold gas will become heated and create thrust when allowed to flow out through a nozzle.

Since the plasma creates temperatures higher than those possible by chemical combustion, the plasma creates more thrust from the same amount of cold "propellant" gas than chemical combustion, notes the Penn State engineer.

In addition to being more propellent-efficient than chemical systems, the microwave-powered thruster is inherently safer as well, he adds. The thruster operates only when the magnetron is generating microwaves. If the magnetron is turned off, so is the plasma that heats the fuel.

In recent tests, Micci operated the system using nitrogen, helium and ammonia as propellant gases. The thruster can also potentially be operated with water as propellant. By means of spectroscopy, the helium velocity at the nozzle exit was found to be about 13,000 meters per second. That is the highest measured specific impulse for a continuously operating low power electrothermal thruster.

Micci will detail the results of the recent tests Friday, Nov. 19, at the Penn State Propulsion Engineering Research Center annual symposium in State College. He also presented the findings at the 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit June 20-24 in Los Angeles, California.

His paper, "Low-Power Microwave Arcjet Testing: Plasma and Plume Diagnostics and Performance Evaluation," is scheduled to be published in the Proceedings of that conference on CD-ROM. His co-authors are F. J. Souliez, doctoral candidate; S. G. Chianese, master's candidate; and G. H. Dizac, Penn State master's graduate. The research was supported, in part, by a grant from the Air Force Office of Scientific Research.