Thruster Components Research Articles

Cathode and Insulator Materials for a 30 kW Arcjet Thruster

This paper describes past experience with 30 kW arcjet engines, emphasizing the operational parameters pertinent to the selection of new/alternate erosion-resistant materials to enhance lifetime. Mechanisms of mass loss from the electrodes and insulators are suggested, and ways of identifying potential processes for controlling cathode erosion and insulator degradation are proposed. The limitations of present materials used in critical arcjet thruster components are described. An outline is given of the criteria and the figure of merit on which the selection of candidate materials can be based. Potentially useful new/alternate materials are listed based on available thermophysical and other material properties.

Measurement of MPD thruster erosion using surface layer activation

An in-situ method for measuring MPD thruster component erosion has been developed and tested on two multimegawatt thruster configurations. The technique involves activation of selected areas on the components to be studied by precisely controlled high-energy ion beam bombardment. Monitoring the decrease in activity during thruster operation provides a precise, quantitative measure of the amount of material removed from the surface. In preliminary tests, erosion of the tungsten cathode occurred at all operating conditions, but a number of factors contributed to the lack of detectable erosion of the copper anode or the boron nitride insulator. These results indicate that cathode erosion is linearly related to the total charge transfer, but scatter in the individual test sequence erosion rate data prevents a more definitive conclusion. Nevertheless, a comparison of erosion data obtained at two different pulse lengths indicates that cathode material loss is generally independent of the number of pulse cycles. Work is continuing in a broad effort to refine the surface layer activation technique and to obtain a data base for additional thrusters.

Cyclic Life-Test of an Ion Tfaruster Hollow Cathode

To assist in assessing the durability of the U.K. T4A and T5 10-cm mercury ion thrusters when used for NorthSouth stationkeeping, cyclic life-tests of thruster components have been carried out. This paper reports a cyclic discharge test of a hollow cathode in a diode configuration, together with thermal cycling tests of cathode heaters. The automatically controlled discharge test consisted of repetitive on and off periods with normal durations of 3 h and 40 min, respectively. The test was terminated voluntarily after 4200 h of discharge operation and 1343 starts. The rate of degradation of performance, as indicated by keeper and plasma potentials, was fully acceptable for the envisaged missions, as were the time, power, and energy required for discharge initiation. The thermal cycling tests demonstrated that the heater technology is satisfactory. It was concluded from a consideration of various aspects of the long-term operation of hollow cathodes that diode tests are adequately representative of thruster conditions.

Design and Operational Aspects of All-Electric Thruster Control Systems for Geostationary Satellites

Results of an all-electric thruster control concept study for geostationary satellites are presented. Early work on this type of system emphasized feasibility rather than design and operational aspects. Conservative estimates of 1975 technology were used as guides in selecting possible thruster combinations and component sites to satisfy basic mission requirements and constraints. The present work goes beyond 1975 technology to that of the late 1970's and considers innovations in hardware and operational techniques which may permit significant reductions in control system weight and components. Finally, recommendations regarding thruster development and satellite design compatible with optimum all-electric control systems are offered. It is concluded that, for systems without momentum wheels or gyros, three orthogonal components of control torque and continuous yaw sensing are required to ensure a stable automatic control system. Furthermore, alternating use of different inclination control thrusters will be highly desirable if these thrusters are canted away from the orbit normal direction due to plume impingement problems.

All-Electric Thruster Control of a Geostationary Communications Satellite

Investigation of an all-electric thruster combination for precise, long-life control of a nonspinning geostationary satellite without momentum exchange devices is presented. Electric thrusting devices offer certain unique advantages for this application and are assumed for both orbit and attitude control. Maneuvers associated with deployment, acquisition, and on-station operations are described in detail. A specific satellite configuration is assumed as a realistic model for use during the 1977-84 period, and thruster components and required performance parameters are selected based on estimated 1975 technology. Mercury electron bombardment engines were chosen for large-impulse maneuvers, and solid Teflon pulsed plasma thrusters were picked for small impulse-bit functions. Restrictions on thruster site selections resulted in a nonoptimum propulsion subsystem configuration. Nevertheless, adverse interaction with communications operations appear minimal, and full thruster redundancy is available. It is concluded that an all-electric thruster control system is feasible and can satisfy mission requirements for a large class of nonspinning satellites.

Plasma separator thruster.

The plasma separator thruster is an advanced concept in ion thruster design which is based on independent operation and optimization of the plasma source and plasma extraction system. The plasma source consists of a propellant feed tube which functions as a hollow cathode. Cesium vapor is fed from the cathode through a simple converging-diverging nozzle, and is ionized by a coaxial discharge between the hollow cathode and a downstream ring anode. The dense plasma is then expanded aerodynamically to a pressure suitable for ion extraction and acceleration. Surface recombination is nearly eliminated by proper nozzle and anode design, and volume recombination is avoided by keeping the electron temperature high in the continuum region. A modified theory of plasma extraction is presented to explain the experimentally observed total extraction of plasma ions. Considerable improvement in efficiency and lifetime is predicted for a fully developed thruster of this type, based on the measured performance of the thruster components. Several complete but unoptimized thruster systems have been operated to demonstrate compatibility of components.