M. Foygel

Temperature Stratification in a Cryogenic Fuel Tank

A reduced dynamic model describing temperature stratification effects driven by natural convection in a liquid hydrogen cryogenic fuel tank has been developed. It accounts for cryogenic propellant loading, storage, and unloading in the conditions of normal, increased, and microgravity. The model involves multiple horizontal control volumes in both liquid and ullage spaces. Temperature and velocity boundary layers at the tank walls are taken into account by using correlation relations. Heat exchange involving the tank wall is considered by means of the lumped-parameter method. By employing basic conservation laws, the model takes into consideration the major multiphase mass and energy exchange processes involved, such as condensation–evaporation of the hydrogen, as well as flows of hydrogen liquid and vapor in the presence of pressurizing helium gas. The model involves a liquid hydrogen feed line and a tank ullage vent valve for pressure control. The temperature stratification effects are investigated, inc...

Model-based diagnostics for propellant loading systems

The loading of spacecraft propellants is a complex, risky operation. Therefore, diagnostic solutions are necessary to quickly identify when a fault occurs, so that recovery actions can be taken or an abort procedure can be initiated. Model-based diagnosis solutions, established using an in-depth analysis and understanding of the underlying physical processes, offer the advanced capability to quickly detect and isolate faults, identify their severity, and predict their effects on system performance. We develop a physics-based model of a cryogenic propellant loading system, which describes the complex dynamics of liquid hydrogen filling from a storage tank to an external vehicle tank, as well as the influence of different faults on this process. The model takes into account the main physical processes such as highly non-equilibrium condensation and evaporation of the hydrogen vapor, pressurization, and also the dynamics of liquid hydrogen and vapor flows inside the system in the presence of helium gas. Since the model incorporates multiple faults in the system, it provides a suitable framework for model-based diagnostics and prognostics algorithms. Using this model, we analyze the effects of faults on the system, derive symbolic fault signatures for the purposes of fault isolation, and perform fault identification using a particle filter approach. We demonstrate the detection, isolation, and identification of a number of faults using simulation-based experiments.

Dynamical Model of Rocket Propellant Loading with Liquid Hydrogen

Viatcheslav V. Osipov MCT, Inc., Moffett Field, California 94035 Matthew J. Daigle University of California, Santa Cruz, Moffett Field, California 94035 Cyrill B. Muratov New Jersey Institute of Technology, Newark, New Jersey 07102 Michael Foygel SGT, Inc., Newark, New Jersey 07102 Vadim N. Smelyanskiy NASA Ames Research Center, Moffett Field, California 94035 and Michael D. Watson NASA Marshall Space Flight Center, Huntsville, Alabama 35805

AUGER ELECTRON SPECTROSCOPY OF THE HYDROGEN TERMINATED CHEMICAL VAPOR DEPOSITED DIAMOND SURFACE

A simultaneous study of Auger spectra and secondary electron emission from the chemical vapor deposited diamond films under extended electron beam exposure is presented. Up to a 1.2 eV increase in energy of the carbon Auger peak accompanied by the decrease of the total secondary electron yield has been found. Exposure to hydrogen has resulted in recovery of the both Auger peak position and secondary yield. First‐principles electronic structure calculations have shown that the effect is due to a change in the surface upper‐valence band local density of states of the diamond crystal which is dependent on the extent of hydrogen coverage of the surface.

Atomic Spin Scattering and Giant Magnetoresistance in Magnetic Semiconductors

We studied electrical transport in dilute magnetic semiconductors, which is determined by scattering free carriers by localized magnetic moments. While calculating the scattering time and the mobility of the majority and minority-spin carriers, we took into account both the effects of thermal spin fluctuations and of the spatial disorder of magnetic atoms. These effects are responsible for the magnetic-field dependence of electrical resistivity. Namely, the application of the external magnetic field suppresses the thermodynamic spin fluctuations, thus promoting negative magnetoresistance (MR). Simultaneously, scattering by built-in spatial fluctuations of the atomic spins increases with the magnetic field. The latter effect is due to the growth of the magnitude of random local Zeeman splittings with the magnetic field. It promotes positive MR. We discuss the role of the above effects on MR of semiconductors where magnetic impurities are isoelectronic

Moving-Boundary Model of Cryogenic Fuel Loading, I: Two-Phase Flow in a Pipe

A moving-boundary model of two-phase flow in a cross-country cryogenic fuel supply line has been developed. It is based on time-dependent one-dimensional ordinary differential equations that describe mass and energy conservation of the flowing cryogen that exchanges heat with the tubes’ walls. Momentum conservation is taken into consideration by relating the pressure drop across the boundaries of the control volumes with the corresponding inlet and outlet mass flow rates through the boundaries of these volumes. With a relatively small computational effort, compared to full-scale schemes, the model describes pressure and temperature variations together with the kinetics of vapor void fraction and of the interphase boundary motion in the different parts of the spatially distributed system. In this paper, special attention is given to the detailed study of the transient and steady-state two-phase cryogenic movement in a long horizontal pipe with different regimes of flow: with and without heat exchange betwe...

Variable-Range Hopping in the Array of Magnetic Quantum Dots

We analyzed the spin-dependent conductivity in the system of paramagnetic quantum dots embedded in semi-insulating matrix, which is due to bound magnetic polaron (BMP) inter-dot hopping. If such a system is characterized by wide distributions of the “bare” electron energies and BMP shifts, variable-range and variable-polaron-barrier hopping can be observed at low temperatures T . It results in the giant magnetoresistance,ρ( H, T ) governed by a non-activation law, lnρ /α [ T 0 ( H )/ T ] p , where T 0 ( H ) drops with magnetic field, H . Depending on the conditions, parameters of the material, and the dimensionality of the system, the exponent 0.25 T -dependence has been observed in GaMnAs and MnGe magnetic semiconductors.