Electric Propulsion involves the acceleration of ionized gases through the application of electromagnetic forces. It also represents the only practical method for rapid interplanetary travel currently known to mankind. While conventional rockets produce large amounts of thrust through the highly inefficient process of chemical combustion, such high thrust levels are only important for rapid planetary lift-off scenarios. A much more important characteristic for interplanetary travel is a quantity called specific impulse. Specific impulse is essentially an indication of how much a rocket's velocity can be increased by using a given quantity of fuel. Electrically propelled spacecraft generally possess very high values of specific impulse, allowing them to travel 10 to 100 times faster than chemical rockets with the same amount of fuel.
Why was Electric Propulsion chosen as a research topic?
A number of compelling advantages dictated the choice of this topic over several others that were considered. While the immediate applicability of this technology to space exploration captured the imaginations of all of the students involved, it was also apparent that the design and construction of such devices would entail an interdisciplinary approach involving the application of classical mechanics, electromagnetism, modern physics, fluid dynamics and plasma physics. Additionally, by working together as a team to turn a theoretical design into a working electric thruster, it was apparent that students could develop technical expertise in such areas as soldering, the use of basic handtools, drilling, grinding, milling, lathing, tapping and polishing. Students would also learn about vacuum system construction and operation, the use of adhesives and lubricants, the testing and evaluation of electronic circuits, the implementation of computerized data acquisition techniques and the properties of various transducers and detectors.
What meaningful research can be done in the
field of Electric Propulsion?
Formidable engineering challenges must still be addressed before the advantages of electric thrusters can be fully exploited. Most notably, thrust levels must be increased, power supply weights must be reduced and operational lifetimes must be extended. While extensive research was performed in the 1950's and 1960's on the use of electric propulsion for interplanetary spaceflight, many promising concepts had to be abandoned due to the technological limitations of the power conditioning systems in use at the time. To date no consistent effort has been made to reevaluate these approaches in light of modern power processing technologies. In fact, research into the development of electric thrusters for interplanetary travel has been largely abandoned by NASA and the scientific community since the late 1960's. It was at that time that the major focus of electric propulsion shifted from interplanetary missions to low-power near-earth applications such as orbit stabilization and directional control. This change arose when plans for large space power systems were abandoned in favor of the gradual development of large solar panels. By the end of the 1990's, however, megawatt space power systems will be readily available through the imminent declassification of systems developed for the Strategic Defense Initiative and the Department of Defense. Finally, many of the analytical problems that could not be solved with the computer technology available in the 1960's are now tractable using modern computer modeling techniques and hardware.
What is SAS doing with Electric Propulsion?
We have decided to focus on two aspects of electric propulsion: mission-enabling concepts for interplanetary travel and the development of high thrust electric propulsion launch systems that utilize microwave beamed power. Towards this end, we are interested in investigating a wide assortment of electrostatic, electromagnetic and hybrid electrothermal thrusters. We would also like to reassess some of the more speculative electric propulsion concepts abandoned in the 50's and 60's in light of modern power processing technologies and new technological developments. Naturally, we do not expect to become the first college students in history to revolutionize the nature of interplanetary travel in a few semesters. With the help of interested graduate students and professors, however, undergraduate students can construct electric thrusters based on designs previously published in the relevant literature. While this will not involve original research, it will acquaint students with the process of researching, building and testing a relatively complicated experimental device. We have found that, as students participate in this process, they try to find ways to modify the published design to improve the thruster's performance. While these ideas may not always bear fruit, by building an electric thruster to known specifications, the students will have prepared themselves to construct a fairly sophisticated experiment to test their own original ideas. With proper guidance from professors and graduate students, this can result in an immensely exciting undergraduate research experience.
Members of the Electric Propulsion team have set out to design, build and operate a radiation-cooled electrothermal arcjet. Their goal is to measure thrust as a function of input current and voltage, and to deduce specific impulse and power conversion efficiency from this data in conjunction with measured values of fuel mass flow rate.
Following an extensive literature search, these students have evolved a design that is based on the work of several groups, especially that of B. Glocker and M. Auweter-Kurtz (University of Stuttgart) and F. Curran and T. Haag (NASA Lewis Research Center). The device that they have designed consists of 8 major components:
Several Grafoil gaskets may also be needed on either side of the fuel injection ring to prevent leakage.
Gaseous fuel is injected into the upstream (left) end of the cathode feed tube from a pressurized gas cylinder. The fuel enters a small regenerative cooling chamber in the boron nitride insulator through a hole just upstream of the cathode, thereby cooling the rear end of the cathode. The cathode is press-fitted into the cathode feed tube which will be threaded to allow adjustment of the gap between the anode and cathode. (Two nuts, one on either side of the retaining plate, will be used to anchor the cathode feed tube/cathode assembly.) After entering the regenerative cooling chamber, fuel passes through four channels in the boron nitride insulator to reach the fuel injection ring. The four passages in the injection ring are angled so as to introduce the fuel tangentially, thereby creating a swirling vortex of gaseous fuelfuel. This gas flow is necessary to stabilize the arc which passes through the constrictor channel in the anode/ nozzle after being emitted from the sharpened tip of the cathode. During steady state operation, the arc terminates on the downstream surface of the anode in a diffuse axially symmetric pattern. As it passes through the electric arc in the anode nozzle constrictor channel, the gaseous fuel is ionized and the resulting plasma is accelerated by thermal expansion through the nozzle, producing thrust.
Several of the components are presently in various stages of fabrication, with material costs posing a small challenge. Funding for the two most expensive items, the tungsten nozzle and molybdenum housing, has been conditionally obtained from the UA Department of Physics, pending successful demonstration of a proof of concept model. Funding for the other requisite materials has already been provided. SAS students have obtained a four kilowatt, high current power supply from UA Physicist Dr. Theodore Bowen to operate the thruster and they are currently working to locate the necessary power conditioning equipment. Completion of the proof of concept model is anticipated soon.
Teaching an Electric Propulsion Class at Your School
The Electric Propulsion Development Tracking Project
This page is still under construction. Our goal is to electronically archive an extensive database of technical papers and notes tracing the history and development of each of the many different types of electric thrusters that have been developed since the 1950's. According to Dr. R.G. Jahn of Princeton University's Electric Propulsion Laboratory, "... some provisions for systematic archival recording of results [of electric propulsion research] and some management strategies for preserving the stream of cognizance, both in the major agencies and in the professional societies, is critically needed." With input and guidance from professional researchers across the country, we hope to work with students in other SAS chapters nationwide to create such an archive. By participating in this effort, students will learn how to use their local library resources to locate and obtain technical papers on highly specific topics, while simultaneously gaining experience in a cooperative research effort using the Internet. Postscript versions of journal articles, technical reports and (hopefully) comments from senior professional Electric Propulsion researchers will soon be archived in each of the following categories:
