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University of Houston • University of Houston-Clear Lake • ISSO Annual Report Y2003 • 116-119
Investigation of Plasma-Flow Characteristics in a Magneto-Plasma Rocket
THE VARIABLE SPECIFIC IMPULSE Magneto-Plasma Rocket (VASIMR) is a project under development at the Advanced Space Propulsion Laboratory (ASPL) at JSC. Dr. Franklin Chang Díaz has been working on the development of the VASIMR concept since 1979 and founded ASPL in 1993.
The project has a long history beginning at the Charles Stark Draper Laboratory before moving to the MIT Plasma Fusion Center and, finally, to JSC. The project is based and directed in NASA-JSC and has contracts with several government research centers, industrial companies, and universities. In addition, researchers from universities and institutes around the world collaborate with ASPL.
The Magneto-plasma rocket engine provides propulsion by ionizing and heating neutral gases to high temperatures and then guiding them out of a magnetic nozzle in order to produce thrust, much like a chemical rocket engine. However, the essential difference between VASIMR and a chemical rocket engine is that VASIMR will produce a very high specific impulse at relatively low thrust (i.e., a low density, high velocity exhaust), while a chemical rocket engine produces high thrust at relatively low specific impulse (i.e., a high density, low velocity exhaust). The roles of the two engines are complementary for interplanetary exploration: a chemical rocket provides the high thrust needed to escape a planetary surface, while VASIMR provides the high specific impulse required for efficient interplanetary transit.
In fact, electric propulsion systems such as VASIMR enable interplanetary space-flight missions that would not be feasible if chemical propulsion were the only option. The particular niche filled by VASIMR in the electric propulsion community is that of a relatively high-power plasma propulsion system that is focused on human space flight, rather than on less massive unmanned, robotic space flight missions. If we are to fulfill the goal of human exploration of other worlds, the VASIMR engine is an absolute necessity, for the efficiency of the engine permits a favorable ratio of payload mass to spacecraft mass, one that allows human beings to contemplate realistically long-duration space exploration missions.
In its research configuration, VASIMR utilizes four co-axial magnetic coils and two co-axial antennas to achieve its purpose. The first antenna is a so-called helicon antenna, which serves as a plasma generator in that it ionizes an injected neutral gas (typically hydrogen, deuterium, or helium). The second antenna is known as the ion cyclotron resonance heating (ICRH) antenna. It boosts the energy of the plasma by feeding electro-magnetic energy preferentially into the ions. While the helicon antenna is primarily responsible for creating the plasma, the second antenna is used to increase ion energy and exhaust velocity, thus, the specific impulse of the rocket engine. The magnetic coils work in concert to shape the strong axial magnetic field that guides the strongly magnetized plasma (i.e., magneto-plasma). The fourth magnetic coil (or a smaller auxiliary coil) serves as a magnetic nozzle, by which the specific impulse and thrust of the plasma exhaust may be varied. When components are operating together, the result is the Variable Specific Impulse Magneto-Plasma Rocket, or VASIMR.
The magnetic nozzle gives VASIMR the unique ability to modulate the plasma exhaust so as to maintain maximum power and efficiency. This technique is termed "Constant Power Throttling" and is similar to adjusting the transmission on an automobile. The VASIMR engine (specific impulse, I sp ~ 15,000 sec) is designed to run continuously, so that, although it has low thrust, any interplanetary transit time is considerably reduced. In contrast, a chemical rocket, such as the space shuttle main engine (Isp ~ 450 sec), is designed to provide very high thrust, but only for about eight minutes. A traditional chemical rocket lifts a space ship off a planet and gives it an initial velocity, after which it is in free flight toward its objective. The role of VASIMR is to provide thrust during what would have been unpowered free flight, thereby shortening travel time. For example, a chemical rocket alone would require a transit time of about 300 days to reach Mars. Adding VASIMR for the interplanetary section of the journey would reduce the trip to as little as 39 days carrying 20 tons of cargo, or 115 days for a larger 61-ton cargo load, equipped as it is with a nuclear power generation system. By minimizing transit time, the crew is likely to suffer minimized physical stress and risk.
The importance of VASIMR for human exploration of space should be apparent at this point. Having realized this importance, we also realize the necessity of ensuring that its development proceed expeditiously. There are many exciting technical challenges in the VASIMR project, all of which require attention. For this project, we chose one particular challenge: to understand the fluid dynamics and thermodynamics of plasma flow in the VASIMR engine and in its exhaust, with respect to variation of such system parameters as magnetic coil current values and magnetic field structure. This work will be primarily experimental with numerical models and data analysis procedures employed, as necessary.
Scope of workThe proposed experimental study of VASIMR flow dynamics involves utilizing existing sensors (i.e., Langmuir probes and "B-dot sensors" for magnetic field fluctuations) and building new ones (i.e., magnetic "Rogowski coils," which measure total current passing through the coil) to monitor interior and exterior plasma dynamics. Variables to be measured include plasma particle density, momentum flux, and local magnetic field fluctuations, as well as temperature and ionization level variation. The development of any new sensors will be based on technology designed to complement the suite of sensors that already exist.
The ability to accurately measure these variables at appropriate locations within and outside of the VASIMR engine will allow us to determine the precise response of the plasma flow to variations in identified system parameters, as well as to better ascertain the effects of design changes on VASIMR engine performance. The capability to perform these measurements will add to the experimental database that is essential for developing and validating plasma kinetic and magneto-gas-dynamic numerical models that are critical for under-standing and predicting VASIMR engine performance, both in the laboratory and in space environments. Thus, the research team will establish and maintain a close working relationship with theoretical and computational modeling efforts.
The ISSO Post-Doctoral Aerospace Fellow is the central and unifying person within this proposal. The VASIMR team that currently exists has done very well in its research to date and has been moving steadily forward in the attainment of its goal—a human space flight-qualified, high-performance plasma rocket engine. The team is, however, small and will greatly benefit in the expansion of its scientific staff. A basic challenge, in addition to the technical issues that are being faced, is the challenge of building a team that has sufficient "critical mass" in the face of NASA’s current budgetary and personnel limitations. This Fellow, whose selection is imminent, along with other personnel who will be brought aboard when possible, will provide us the expertise and critical mass required to meet the needs of our nation’s human space exploration endeavor.
The post-doctoral fellow will have access to the VX-10 prototype, associated diagnostic equipment, the new axial diag-nostic translation stage, the MSFC momentum flux probe, UH and UT diagnostic probes installed on the VX-10, digital data acquisition system and the panoply of associated hardware, as needed.
The Advanced Space Propulsion Laboratory (ASPL) was founded at the NASA Johnson Space Center by astronaut Franklin Chang-Díaz in 1993. The purpose of ASPL has been to develop the plasma propulsion systems that are required for future interplanetary human space flight missions. Thus, research and development of the Variable Specific Impulse Magneto-plasma Rocket (VASIMR) is being carried out as the primary effort at ASPL. The VASIMR project began at the Charles Stark Draper Laboratory in Cambridge, Mass., before moving to the MIT Plasma Fusion Center and, finally, to JSC. The current stage of the project is centered on an experiment called VX-10, a 10 kW experimental version of the VASIMR engine. To support VX-10, ASPL has contracts with Oak Ridge and Los Alamos National Laboratories, as well as several companies and universities. A future 50 kW experiment VX-50 is being planned.
In addition to Dr. Chang-Díaz, ASPL locally employs the efforts of about two dozen people, including civil servants, contractors, undergraduate and graduate students, post-doctoral fellows and university professors. All are enthusiastic about participating in the adventure of space exploration. There is no end to this journey and the fruits of the effort are to be found not just in the immediate development of essential aspects of space propulsion, but also in the inspiration that people derive from original research.
Shebalin, J. V. "Størmer Regions for Axisymmetric Magnetic Multipole
Fields," Phys. Plasmas (forthcoming).
Shebalin, J. V. "A Spectral Algorithm for Solving the Relativistic Vlasov-Maxwell
Equations," Computer Physics Communications 156 (2003): 86-94.
Shebalin, J. V. "Theory and Simulation of Real and Ideal Magnetohydrodynamic
Turbulence," Discrete & Continuous Dynamical Systems B (forthcoming ).
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Institute for Space Systems Operations - Y2003 Annual Report
Copyright © 2004
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