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  • 1
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    Anode power deposition in quasi-steady MPD thrusters (1990)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Spatially resolved anode heat flux measurements of a pulsed quasi-steady MPD thruster have been made by embedding thermocouples to the inner surface of a hollowed anode. Results obtained using argon propellant at mass flow rates of 4 and 16 g/s with the thruster operating at currents between 8 and 24 kA are presented. The results show that the anode fall voltage increases linearly with thruster current and does not depend on propellant mass flow rate. The fraction of thruster power deposited into the anode is 42 percent at 1 MW and falls to less than 20 percent at 6 MW. At any given operating condition, there is an inverse relationship between the anode fall and anode current density.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 90-2668
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  • 2
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    The manifestation of Alfven's hypothesis of critical ionization velocity in the performance of MPD thrusters (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The role of Alfven's critical ionization velocity in the performance of the self-field MPD thruster has been investigated. The existence of a well defined characteristic velocity can be attributed to an ionization process involving the production of a population of suprathermal electrons by an electrostatic instability. It is shown that for the MPD thruster plasma, suprathermalization of electrons via this electrostatic instability can only happen if ions are initially accelerated to velocities larger than the Alfven critical ionization velocity. When this occurs the mechanism will be initiated and the ions decelerated to velocities near the critical velocity. This mechanism ceases to be limiting when all neutrals are ionized. A model of MPD thruster terminal behavior, incorporating Alfven's hypothesis, is presented. Experiments with three different propellants reveal that operation at values of the current squared to total mass flow ratio corresponding to the Alfven critical velocity is marked by a transition wherein low frequency voltage oscillations and a notable change in the voltage-current dependence occurs. One major result of this study is the demonstration that the Alfven critical velocity is not a fundamental limitation on MPD exhaust velocity.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 85-2037
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  • 3
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    Onset phenomena in MPD thrusters (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: An experimental study has clarified some aspects of MPD thruster onset phenomena. The steep increase in terminal voltage that occurs as the onset current is approached may have different causes, depending on the propellant injection geometry. For propellant injection at the cathode radius, terminal voltage increase corresponds to a growing anode fall voltage; for injection at a larger radius, the increase is related to the back emf in the near-cathode plasma. The formation of the onset current pattern within the arc has been mapped experimentally as the thruster responds to an input current step which rises from below onset to the onset value. The appearance of terminal voltage hash at onset correlates with the extension into the exhaust region of a significant fraction of the arc current.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 85-2038
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  • 4
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    MPD thruster performance - Propellant distribution and species effects (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Previous research has shown that MPD thruster performance, as measured by thrust, specific impulse, and efficiency, is limited by a so-called 'onset' current level. Experimental results are presented which show that the onset current level varies with the radial position of mass injection. Specifically, mass injection into either of two zones results in drastically different performance for both argon and xenon propellants. This difference in performance is evidenced by the onset current levels, the onset V-J signatures, and the overall V-J characteristics. The behavior of the thruster with the total mass flow split between the two zones follows a transition between the zonal behavioral limits. The behavior of propellant mixtures also follows a transition between the behavior of the individual propellant species. The flow region between the two zones is characterized by a particle number density discontinuity, a large axial current density, and a large electron Hall parameter, suggesting a possible correlation between observed onset phenomena and a shear-flow discontinuity located in the region between the two zones.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 85-2022
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  • 5
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    Basic research for future electric propulsion (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: It is pointed out that the evolution of electric propulsion over the past two and a half decades has been constrained by the interaction of three broad factors, including the physics and dynamics of the propellants, the dynamical and logistical requirements of the mission, and the technological realities of materials, power sources, and thermal management. A projection of the future of electric propulsion requires, therefore, a simultaneous reassessment of all three factors. Aspects of mission specification and power systems are discussed, and basic research needed for future electric propulsion applications is considered. Attention is given to electrostatic propulsion, electrothermal propulsion, electromagnetic propulsion, electrothermal/electromagnetic hybrids, novel concepts, and ancillary concerns.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 85-2068
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  • 6
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    Anode power deposition in a MPD thruster with a magnetically annulled Hall parameter anode (1992)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Results from previous studies indicate that the anode fall increases monotonically with the electron Hall parameter. In an attempt to reduce the anode fall by decreasing the local electron Hall parameter, a proof-of-concept test was performed in which an array of 36 permanent magnets were imbedded within the anode of a high power quasi-steady MPD thruster to decrease the local azimuthal component of the induced magnetic field. The modified thruster was operated at power levels between 150 kW and 4 MW with Ar and He propellants. Terminal voltage, triple probe, floating probe, and magnetic probe measurements were made to characterize the performance of the thruster with new anode. Incorporation of the modified anode resulted in a reduction of the anode fall by up to 15 V with Ar and 20 V with He, which corresponded to decreased anode power fractions of 40 and 45 percent with Ar and He, respectively.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 92-3461
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  • 7
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    Cathode phenomena in a low power, steady state MPD thruster (1988)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Mass loss and surface temperature measurements, filter photography and SEM surface characterization are used to study cathode phenomena in a steady state self-field MPD thruster operated at power levels between 15 and 30 kW. The equilibrium cathode temperature is found to be approximately 3200 K over most of its length, for which evaporation and thermionic emission adequately explain measured erosion rates and current levels. The start-up phase is characterized by spot current attachment with associated high erosion rates.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 88-3206
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  • 8
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    Megawatt level electric propulsion perspectives (1987)
    In:  CASI
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    Publication Date: 2019-06-28
    Description: For long range space missions, deliverable payload fraction is an inverse exponential function of the propellant exhaust velocity or specific impulse of the propulsion system. The exhaust velocity of chemical systems are limited by their combustion chemistry and heat transfer to a few km/s. Nuclear rockets may achieve double this range, but are still heat transfer limited and ponderous to develop. Various electric propulsion systems can achieve exhaust velocities in the 10 km/s range, at considerably lower thrust densities, but require an external electrical power source. A general overview is provided of the currently available electric propulsion systems from the perspective of their characteristics as a terminal load for space nuclear systems. A summary of the available electric propulsion options is shown and generally characterized in the power vs. exhaust velocity plot. There are 3 general classes of electric thruster devices: neutral gas heaters, plasma devices, and space charge limited electrostatic or ion thrusters.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: New Mexico Univ., Transactions of the Fourth Symposium on Space Nuclear Power Systems; p 167-171
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  • 9
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    Anode power deposition in an applied-field segmented anode MPD thruster (1991)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Anode heat flux measurements of a water cooled segmented anode applied-field MPD thruster were made to investigate anode heat transfer phenomena. Pure argon and argon-hydrogen mixtures were used as propellants for a variety of thruster currents, propellant mass flow rates, and axial applied magnetic field strengths. The thruster was operated in two modes; with all four segments active, and with two of the segments floating. In addition, thrust and specific impulse were determined for each operating condition. The results show that the heat flux to the anode increases monotonically with axial magnetic field strength and thruster current. Between 50 and 75 percent of the anode heat flux is transported by the current carrying electrons. Convective and radiative heat transfer account for the remaining portion of the power deposited in the anode. The addition of hydrogen to the argon propellant results in the reduction of the fraction of anode power deposited by the anode fall to a level equivalent to that deposited by convection and radiation.
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: AIAA PAPER 91-2343
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  • 10
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    Measured performance of a multimegawatt MPD thruster (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Previously cited in issue 12, p. 1960, Accession no. A81-29538
    Keywords: SPACECRAFT PROPULSION AND POWER
    Type: (ISSN 0022-4560)
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