Clark W. Hawk

A Plasmoid Thruster for Space Propulsion

There are a number of possible advantages to using accelerated plasmoids for in-space propulsion. A plasmoid is a compact plasma structure with an integral magnetic field. They have been studied extensively in controlled fusion research and are classified according to the relative strength of the poloidal and toroidal magnetic field (B(sub p), and B(sub t), respectively). An object with B(sub p), / B(sub t) much greater than 1 is classified as a Field Reversed Configuration (FRC); if B(sub p) approximately equal to B(sub t), it is called a Spheromak. The plasmoid thruster operates by producing FRC-like plasmoids and subsequently ejecting them from the device at a high velocity. The plasmoid is formed inside of a single-turn conical theta-pinch coil. As this process is inductive, there are no electrodes. Similar experiments have yielded plasmoid velocities of at least 50 km/s, and calculations indicate that velocities in excess of 100 km/s should be possible. This concept should be capable of producing Isps in the range of 5,000 - 15,000 s with thrust densities on the order of 10(exp 5) N per square meters. The current experiment is designed to produce jet powers in the range of 5 - 10 kW, although the concept should be scalable to several MWs. The plasmoid mass and velocity will be measured with a variety of diagnostics, including internal and external B-dot probes, flux loops, Langmuir probes, high-speed cameras and a laser interferometer. Also of key importance will be measurements of the efficiency and mass utilization. Simulations of the plasmoid thruster using MOQUI, a time-dependent MHD code, will be carried out concurrently with experimental testing.

Magnetic flux compression reactor concepts for spacecraft propulsion and power

This technical publication (TP) examines performance and design issues associated with magnetic flux compression reactor concepts for nuclear/chemical pulse propulsion and power. Assuming that low-yield microfusion detonations or chemical detonations using high-energy density matter can eventually be realized in practice, various magnetic flux compression concepts are conceivable. In particular, reactors in which a magnetic field would be compressed between an expanding detonation-driven plasma cloud and a stationary structure formed from a high-temperature superconductor are envisioned. Primary interest is accomplishing two important functions: (1) Collimation and reflection of a hot diamagnetic plasma for direct thrust production, and (2) electric power generation for fusion standoff drivers and/or dense plasma formation. In this TP, performance potential is examined, major technical uncertainties related to this concept accessed, and a simple performance model for a radial-mode reactor developed. Flux trapping effectiveness is analyzed using a skin layer methodology, which accounts for magnetic diffusion losses into the plasma armature and the stationary stator. The results of laboratory-scale experiments on magnetic diffusion in bulk-processed type II superconductors are also presented.

Inductive measurement of plasma jet electrical conductivity

An inductive probing scheme, originally developed for shock tube studies, has been adapted to measure explosive plasma jet conductivities. In this method, the perturbation of an applied magnetic field by a plasma jet induces av oltage in a search coil, which, in turn, can be used to infer electrical conductivity through the inversion of a Fredholm integral equation of the first kind. A 1-in. (25.4-mm)-diam probe was designed and constructed, and calibration was accomplished by firing an aluminum slug through the probe using a light-gas gun. Exploratory laboratory experiments were carried out using plasma jets expelled from 15-g high-explosive shaped charges. Measured conductivities were in the range of 3 kS/m for unseeded octol charges and 20 kS/m for seeded octol charges containing 2% potassium carbonate by mass.

Magnetic and Langmuir Probe Measurements on the Plasmoid Thruster Experiment (PTX)

The Plasmoid Thruster Experiment (PTX) operates by inductively producing plasmoids in a conical theta-pinch coil and subsequently ejecting them at high velocity. An overview of PTX is described in a companion paper. The shape and magnetic field structure of the translating plasmoids will be measured with of an array of inductive magnetic field probes. Six sets of two B-dot probes (for a total of twelve probes) have been constructed for measuring B(sub z) and B(sub theta), the axial and azimuthal components of the magnetic field. The probes were calibrated with a Helmholtz coil, driven alternately by a high-voltage pulser or a signal generator. The probes are wound on a G-10 form, and have an average (calibrated) NA of 9.37 x 10(exp -5) square meters, where N is the number of turns and A is cross-sectional area. The frequency response of the probes was measured over the range from 1 kHz to 10 MHZ. The electron number density n(sub e), electron temperature T(sub e) and velocity v will be determined from measurements taken with a quadruple Langmuir probe, situated in the exhaust chamber. Three of the four probes on the quadruple probe sample the current-voltage characteristic, and from this yield measurements of T(sub e) and n(sub e). The fourth probe provides a measurement of plasma flow velocity. A 6-inch long alumina rod, hollowed with four holes to house the probe wires, is being used to construct the quadruple probe. A variety of propellants will be used, including hydrogen, nitrogen and argon. From the measurements of the plasmoid mass, density, temperature, and velocity, the basic propulsion characteristics of PTX will be evaluated.

LABORATORY METHODOLOGIES FOR PROPELLANT CORROSION RESEARCH

Storable liquid propellants are stored for extended periods of time in metal tankage prior to usage in rocket engines. Knowing the chemical interaction of the propellant and the tankage material is essential to evaluating the structural integrity of the tankage hi service and determining if the propellant remains within specifications at the time of use. Some of this information has been obtained through long duration storage studies for periods of over 20 years in some cases. It is desirable to establish valid methods to obtain quantitative data to project long-term corrosion rates in lieu of real-time storage experimentation. Experimental methods and techniques currently used in obtaining such corrosion data and their theoretical basis are described in this article. These include 1) electrochemical: dc polarization and ac impedance measurements; 2) weight loss; and 3) surface analytical: x-ray photoelectron spectroscopy, auger electron spectroscopy, and optical microscopy. This article presents a description of the fundamental methods used by two research organizations and a comparison of these methods and equipment. These techniques are valid for evaluation of corrosion rates on various fuel and oxidizer propellants. The results of specific research with nitric acid based oxidizers with various aluminum alloys are presented in a companion article.

Uncertainty Assessment of Performance Evaluation Methods for Solar Thermal Absorber/Thruster Testing

Proposed ground testing of solar thermal upper stage propulsion systems will verify the technology readiness level and provide a higher degree of cone dence for pursuing a e ight experiment. This study applies experimental uncertainty analysis techniques to investigate various performance evaluation methods that could be used to interpret results of solar thermal absorber/thruster ground testing. A general uncertainty analysis is applied to engine specie c impulse determination methods for an open-ended cavity, heat exchanger type, solar thermal engine. Six performance relationships consisting of both thrust- and nonthrust-based evaluation methods are utilized and two nominal operating cases are assessed. Based on the uncertainty estimates made for individual variables in the data reduction equations, overall specie c impulse uncertainties for the nonthrust-based methods range from about 6 to 10%, depending on the method and testing situation. Thrust-based evaluation methods yield specie c impulse uncertainties on the order of 1.5% if thrust and propellant mass e ow rate can be measured with uncertainties of 1% or less.

Aluminum alloy compatibility with gelled inhibited red fuming nitric acid

Michael F. A. Dove* University of Nottingham, Nottingham NG7 2RD, England, United Kingdom Norman Logant University of Alabama in Huntsville, Huntsville, Alabama 35899 Jeremy P. Maugert University of Nottingham, Nottingham NG7 2RD, England, United Kingdom Barry D. Allan§ Redstone Arsenal, Huntsville, Alabama 35898 and Ramona E. ArndtH and Clark W. Hawk** University of Alabama in Huntsville, Huntsville, Alabama 35899

Three-Dimensional Numerical Modeling of the Magnetohydrodynamic Augmented Propulsion Experiment

Over the past several years, NASA Marshall Space Flight Center has engaged in the design and development of an experimental research facility to investigate the use of diagonalized crossed-field magnetohydrodynamic (MHD) accelerators as a possible thrust augmentation device for thermal propulsion systems. In support of this effort, a three-dimensional numerical MHD model has been developed for the purpose of analyzing and optimizing accelerator performance and to aid in understanding critical underlying physical processes and nonideal effects. This Technical Memorandum fully summarizes model development efforts and presents the results of pretest performance optimization analyses. These results indicate that the MHD accelerator should utilize a 45deg diagonalization angle with the applied current evenly distributed over the first five inlet electrode pairs. When powered at 100 A, this configuration is expected to yield a 50% global efficiency with an 80% increase in axial velocity and a 50% increase in centerline total pressure.

Plasma Injection by a Magnetoplasmadynamic Thruster for the Gasdynamic Mirror Fusion Experiment

The design of a magnetoplasmadynamic thruster (MPDT) as a plasma source for the Gasdynamic Mirror (GDM) Fusion Propulsion Engine Experiment is considered in this paper. A highdensity plasma source is desired for the GDM experiment in order to create a highly collisional plasma, such that the probability of occurrence of fusion reactions is increased. MPDT design parameters such as the plasma density, operating power, mass flow rate, current and plasma beam energy are evaluated. Preliminary engineering design issues such as placement of the MPDT on the GDM device are also discussed.

Magnetic Flux Compression Experiments using Plasma Armatures

Magnetic flux compression reaction chambers offer considerable promise for controlling the plasma flow associated with various micronuclear/chemical pulse propulsion and power schemes, primarily because they avoid thermalization with wall structures and permit multicycle operation modes. The major physical effects of concern are the diffusion of magnetic flux into the rapidly expanding plasma cloud and the development of Rayleigh-Taylor instabilities at the plasma surface, both of which can severely degrade reactor efficiency and lead to plasma-wall impact. A physical parameter of critical importance to these underlying magnetohydrodynamic (MHD) processes is the magnetic Reynolds number (R(sub m), the value of which depends upon the product of plasma electrical conductivity and velocity. Efficient flux compression requires R(sub m) less than 1, and a thorough understanding of MHD phenomena at high magnetic Reynolds numbers is essential to the reliable design and operation of practical reactors. As a means of improving this understanding, a simplified laboratory experiment has been constructed in which the plasma jet ejected from an ablative pulse plasma gun is used to investigate plasma armature interaction with magnetic fields. As a prelude to intensive study, exploratory experiments were carried out to quantify the magnetic Reynolds number characteristics of the plasma jet source. Jet velocity was deduced from time-of-flight measurements using optical probes, and electrical conductivity was measured using an inductive probing technique. Using air at 27-inHg vacuum, measured velocities approached 4.5 km/s and measured conductivities were in the range of 30 to 40 kS/m.