Chemical rocket engines are used to provide vehicles with adequate thrust to propel them fast enough to escape earth's gravitational field. However, maneuvers in orbit can be achieved with a far more efficient technology - ion propulsion. Ion thrusters require far less propellant mass compared to a chemical rocket. They are capable of generating low thrust over extended periods of time. Hence the vehicle accelerates over a long period and can achieve very high velocities. Another advantage is the maneuvering flexibility that these thrusters allow.
These rocket engines use arrays of solar panels to power electrode grids. The large electrostatic fields created by the grids accelerate xenon ions to very high exhaust velocities. A characteristic of these engines is high specific impulse (and hence high exhaust velocity) and low propellant mass flow rate.
For example, the NASA Solar Electric Propulsion Technology Applications Readiness (NSTAR) thruster has a specific impulse of 1700 - 3300 seconds (an exhaust velocity in the order of 30 KILOMETERS per second) compared to a chemical rocket which has a specific impulse of around 300 seconds (an exhaust velocity in the order of 3 kilometers per second). The NSTAR thruster has a propellant mass flow rate of a few milligrams per second compared to a chemical rocket's mass flow rate of a few kilograms per second. Thus the ion thruster can be employed for maneuvers over extended periods of time while still using a low amount of propellant. The cost savings due to the reduced weight of these thrusters can be several million dollars.
Our research is directed at an accurate prediction of the exhaust plume characteristics of the NSTAR thruster. The behavior of the plume plays a very important role in the design of the thruster. The plume mainly consists of xenon ions and electrons that create an electric field. This field may form so as to accelerate heavy metallic ions such as molybdenum (sputtered from the thruster grid) onto fragile spacecraft surfaces causing degradation of scientific instruments and solar arrays.
The exhaust plume is being modeled using the direct simulation Monte Carlo (DSMC) and Particle In Cell (PIC) techniques. An effort is being made to integrate these two approaches to facilitate greater accuracy in modeling. These methods simulate the behavior of uncharged and charged particles respectively. The modeling will be validated through direct comparison of simulation results with experimental data being generated at NASA Lewis Research Center.
This work is funded by NASA Lewis Research Center and the Air Force Office
of Scientific Research.
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