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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,135 records · Page 63
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Optimal thruster configurations for the GP-B spacecraft

Thruster locations and control schemes which make the most efficient use of the available propellant of a constant flow propulsion system are investigated. A set of three necessary and sufficient conditions are established for a thruster system to be optimal in the sense of maximizing the smallest magnitude force which can be generated by the thrusters. Analytical expressions are derived for the 'the envelope of least authority' which is defined as the smallest force magnitude versus moment magnitude which can be generated by any given thruster system. Several thruster configurations are proposed for the GP-B spacecraft and their performance is compared with respect to their envelopes of least authority. The results are applicable to any system composed of thrusters which saturate at some finite value.

Wiktor, P.↗
NASA NTRS1991

How much hydrogen is there in a white dwarf?

Stratified hydrogen/helium envelope models in diffusive equilibrium are calculated for a 0.6-solar-mass white dwarf for effective temperatures between 10,000 and 80,000 K in order to investigate the observational constraints placed on the total hydrogen mass. Convective mixing is included ab initio in the calculations, and synthetic spectra are used for comparing these models with observational materials. It is shown that evolutionary changes in the surface composition of white dwarfs cannot be explained by a model in which a small amount of hydrogen floats to the surface from initially being mixed in the outer parts of a helium envelope. It is pointed out that the shape of the hydrogen lines can be used for constraining theories of convective overshoot.

Macdonald, James↗
NASA NTRS1991

Steady spherical hypercritical accretion onto neutron stars

The present study of hypercritical accretion flows onto neutron stars considers steady-state, spherically symmetric flows whose accretion rate range is characterized by the carrying away of gravitational-accretion energy by neutrinos. The models used encompass pair production, radiation diffusion, and general relativistic effects. While pair pressure dominates throughout the accretion envelope when accretion rates above about 100 solar masses/yr, radiation diffusion becomes important when the accretion rate falls below about 0.001 solar masses/yr. At the lower accretion rates, free fall toward the neutron-star surface stops and an extended, quasi-static, radiation pressure-supported envelope emerges which is probably dynamically unstable.

Houck, John C.↗
NASA NTRS1991

Collisions of giant stars with compact objects - Hydrodynamical calculations

Smooth particle hydrodynamics are used here to model encounters between an 0.8 solar mass giant star and a 1.4 solar mass compact object on a parabolic orbit. It is found that all encounters with periastron distance r(p) less than about 2.5 stellar radii R(G) lead to significant disruption of the stellar envelope, including some mass loss. When r(p)/R(G) is less than about 0.5, the envelope is completely destroyed during the collision. About 40 percent of the gas escapes, while the rest forms a massive disk around the compact object, leaving behind the bare stellar core on a highly eccentric orbit. Subsequent close passages lead to continuous extraction of gas from the disk at a large rate, and the binary orbit may eventually stabilize while still retaining a large eccentricity. It is suggested that many globular cluster pulsars may have been spun up by accretion from a massive disk rather than from a binary companion, thereby providing a possible solution to the birthrate problem.

Rasio, Frederic A.↗
NASA NTRS1991

Cosmic-ray transport and gamma-ray emission in supernova shells

The mixing and transport of cosmic rays accelerated by a pulsar inside an expanding supernova remnant are examined, and the resulting high-energy gamma-ray emission from nuclear interactions of these accelerated particles in the shell is investigated. Rayleigh-Taylor instability at the interface between a pulsar wind cavity and the inner supernova envelope is assumed to be the mixing mechanism. The analysis is applied to the model of Gaisser, Harding, and Stanev (1987), where protons are accelerated at the reverse shock in the pulsar wind. The instability time-scale is estimated from the dynamics of the pulsar wind cavity, and model the injection, diffusion, and interaction of protons in the shell. The resulting gamma-ray flux is lower than previous estimates due to proton adiabatic losses in the expanding pulsar wind. The protons mix and diffuse only into the innermost regions of the envelope before interacting. Energy-dependent diffusion causes the higher energy gamma-ray light curves to decay faster than those at lower energy.

Harding, A. K.↗
NASA NTRS1991

Positron annihilation in the interstellar medium

Positronium formation and annihilation are studied in a model for the interstellar medium consisting of cold cloud cores, warm partially ionized cloud envelopes, and hot intercloud gas. The gamma-ray spectra resulting from positron annihilation in these components of the interstellar medium are calculated. The spectra from the individual components are then combined, using two limiting assumptions for the propagation of the positrons, namely, that the positrons propagate freely throughout the interstellar medium, and that the positrons are excluded from the cold cloud cores. In the first case, the bulk of the positrons annihilate in the cloud cores and the annihilation line exhibits broad wings resulting from the annihilation of positronium formed by charge exchange in flight. In the second case, the positrons annihilate mainly in the warm envelopes, and the line wings are suppressed.

Guessoum, Nidhal↗
NASA NTRS1991

Neutral matter in planetary nebulae

A review of current studies of neutral envelopes is presented with particular attention given to the use of the envelopes as test cases for understanding the ionization and thermal structure of photodissociation regions. The study of near-IR H2 emission is discussed with detailed spectra given for a few planetary nebulae, and airborne observations of far-IR atomic lines are discussed. These two methods can discern photodissociation regions with warm gas and UV flux is fairly prominent. The use of resonance-absorption-line spectroscopy is also reviewed with respect to the analysis of the Na D lines, and thereby allows the measurement of integrated columns of material through the shell. The methods provide evidence for the notion that planetary nebulae consist of more than just ionized material; large amounts of neutral and molecular material are being confirmed, which has important implications for the mass-loss episode of the nebulae.

Dinerstein, Harriet L.↗
NASA NTRS1991

Evaluation of the longitudinal stability and control characteristics of a mixed-flow remote-lift STOVL aircraft in transition and hover

An evaluation of the longitudinal stability and control characteristics of a mixed-flow remote-lift (MFRL) STOVL aircraft in the powered-lift portion of the flight envelope is presented. A stabilization and command augmentation system was implemented on the MFRL aircraft to meet the requirements for satisfactory flying qualities. The pitch portion of this control system uses a state-rate feedback implicit model following controller to achieve the desired flying qualities and to suppress the effects of external variations and disturbances in the aircrafts characteristics over the low speed envelope.

Engelland, Shawn A.↗
NASA NTRS1991

An aircraft model for the AIAA controls design challenge

A generic, state-of-the-art, high-performance aircraft model, including detailed, full-envelope, nonlinear aerodynamics, and full-envelope thrust and first-order engine response data is described. While this model was primarily developed Controls Design Challenge, the availability of such a model provides a common focus for research in aeronautical control theory and methodology. An implementation of this model using the FORTRAN computer language, associated routines furnished with the aircraft model, and techniques for interfacing these routines to external procedures is also described. Figures showing vehicle geometry, surfaces, and sign conventions are included.

Brumbaugh, Randal W.↗
NASA NTRS1992

Linearized aerodynamic and control law models of the X-29A airplane and comparison with flight data

Flight control system design and analysis for aircraft rely on mathematical models of the vehicle dynamics. In addition to a six degree of freedom nonlinear simulation, the X-29A flight controls group developed a set of programs that calculate linear perturbation models throughout the X-29A flight envelope. The models include the aerodynamics as well as flight control system dynamics and were used for stability, controllability, and handling qualities analysis. These linear models were compared to flight test results to help provide a safe flight envelope expansion. A description is given of the linear models at three flight conditions and two flight control system modes. The models are presented with a level of detail that would allow the reader to reproduce the linear results if desired. Comparison between the response of the linear model and flight measured responses are presented to demonstrate the strengths and weaknesses of the linear models' ability to predict flight dynamics.

Bosworth, John T.↗
NASA NTRS1992

Coded 16-CPFSK for downlink applications

A bandwidth-efficient constant-envelope Proof-of-Concept (POC) modem is described. The modem was developed on an Advanced Modulation Techniques Development contract for the NASA/Lewis Research Center. The POC modem employs 16-ary Continuous Phase Frequency Shift Keying (16-CPFSK) modulation. The 16 frequencies are spaced every 1/16th baud rate which produces a compact spectrum allowing 2 bits/sec/Hz operation. The modem is designed for 200 mb/s Time Division Multiple Access (TDMA) application with 100 MHz adjacent channel spacing. Overall rate 3.4 convolutional coding is incorporated. The demodulator differs significantly from typical quadrature phase detector approaches in that phase is coherently measured by processing the baseband output of a frequency discriminator. Baud rate phase samples for the baseband processor are subsequently decoded to yield the original data stream. The method of encoding onto the 16-ary symbol-ending phase nodes, together with convolutional coding gain, results in near Quadrature Phase Shift Keying (QPSK) theoretical performance. The modulated signal is of constant envelope; thus the power amplifier can be saturated for peak performance. The spectrum is inherently bandlimited and requires no RF filter for sidelobe containment. Two novel theoretical techniques are used in this 16-CPFSK modem: (1) coherent phase measurements are obtained by processing an FM discriminator baseband output; and (2) modulation is accomplished via a closed-loop-linearized voltage controlled oscillator (VCO).

Davis, Robert↗
NASA NTRS1991

Derated ion thruster design issues

Preliminary activities to develop and refine a lightweight 30 cm engineering model ion thruster are discussed. The approach is to develop a 'derated' ion thruster capable of performing both auxiliary and primary propulsion roles over an input power range of at least 0.5 to 5.0 kilo-W. Design modifications to a baseline thruster to reduce mass and volume are discussed. Performance data over an order of magnitude input power range are presented, with emphasis on the performance impact of engine throttling. Thruster design modifications to optimize performance over specific power envelopes are discussed. Additionally, lifetime estimates based on wear test measurements are made for the operation envelope of the engine.

Patterson, Michael J.↗
NASA NTRS1992

The F-18 High Alpha Research Vehicle: A High-Angle-of-Attack Testbed Aircraft

The F-18 High Alpha Research Vehicle is the first thrust-vectoring testbed aircraft used to study the aerodynamics and maneuvering available in the poststall flight regime and to provide the data for validating ground prediction techniques. The aircraft includes a flexible research flight control system and full research instrumentation. The capability to control the vehicle at angles of attack up to 70 degrees is also included. This aircraft was modified by adding a pitch and yaw thrust-vectoring system. No significant problems occurred during the envelope expansion phase of the program. This aircraft has demonstrated excellent control in the wing rock region and increased rolling performance at high angles of attack. Initial pilot reports indicate that the increased capability is desirable although some difficulty in judging the size and timing of control inputs was observed. The aircraft, preflight ground testing and envelope expansion flight tests are described.

Regenie, Victoria↗
NASA NTRS1991

A geometric approach to regulator and tracker design for an aerospace plane

The paper presents a nonlinear design approach drawing from singular perturbations, feedback linearization, and variable structure control, that leads to regulators with automatic gain scheduling which exhibit similar dynamic behavior over the entire flight envelope of the aerospace plane. Additionally, design approach provides for a systematic way to counter disturbance effects as well as modeling uncertainties. The unifying feature of the three nonlinear feedback control methodologies is that they all have a geometric interpretation. First, the translational dynamics are decomposed into reduced-order slow and fast dynamics by way of a formal singular perturbation analysis. After feedback linearization the fast dynamics are robustly stabilized via a variable structure control approach. The slow dynamics are stabilized using conventional proportional-integral compensation based on the nominal slow dynamics. A number of sample command and disturbance responses at opposite ends of the flight envelope are presented for a nonlinear aerospace plane model.

Van Buren, Mark A.↗
NASA NTRS1992

The bow shock structure of IRS 7 - Wind-wind collision near the Galactic center

New structural details of ionized gas associated with IRS 7, a mass-losing supergiant near the Galactic center, are reported, and radio continuum and theoretical maps are compared. High-resolution multiconfiguration VLA observations of IRS 7 reveal ionized gas with a bow shock structure at a wavelength of 2 cm. The apex of the bow shock is facing more toward the cluster of blue stellar objects, known as IRS 16, than Sgr A*, the compact nonthermal source near the Galaxy's dynamical center. It is inferred that the shape of the mass-losing envelope of IRS 7 is influenced by Galactic winds from IRS 16. It is concluded from a comparison of the morphology of the ionized envelope of IRS 7 with simple theoretical modeling of the bow shock structure that the shape of the shock results from the collision between the Galactic center and stellar winds rather than from the motion of IRS 7 through the interstellar medium.

Yusef-Zadeh, Farhad↗
NASA NTRS1992

Hydrogen recombination at high optical depth and the spectrum of SN 1987A

A general theory is presented for hydrogen recombination line formation in an expanding medium in which some of the lines are optically thick. This theory is used to calculate the time evolution of the hydrogen lines of SN 1987A at t equal to or greater than 150 days, assuming that the supernova envelope is a homologously expanding uniform sphere. The theoretical luminosities and ratios of the recombination lines agree remarkably well with the observations. For the first 2 yr, the supernova envelope is optically thick to Balmer continuum. For t equal to or less than 400 days, hydrogen is ionized primarily from the n = 2 level by Balmer continuum photons, which are provided partly by the two-photon decay of the 2s state and partly by emission lines of heavy elements.

Xu, Yueming↗
NASA NTRS1992

Atmospheric analysis for airdata calibration on research aircraft

In-flight airdata calibrations are used to determine the aerodynamic influence of an airplane on pitot-static pressure measurements of altitude and speed. Conventional flight-test calibration techniques are briefly reviewed and meteorological analysis methods for estimating calibration reference values of atmospheric conditions are described. There are cases where some conventional in-flight techniques are not entirely satisfactory for research aircraft because of added equipment requirements or flight envelope and location limitations. In these cases, atmospheric wind and pressure information can be used to complement conventional techniques. Accuracy of the atmospheric measurements and the variability of upper-air winds and pressure values are discussed. Results from several flight research aircraft show that wind reference calibration is generally less accurate than calibration accuracy standards for civil and research aircraft. Examples of pressure reference altimetry derived from meteorological analyses are also presented for a variety of flight research programs. These flight data show that the reference pressure accuracy provided by meteorological analyses is usually within civil aircraft and flight research airdata calibration accuracy standards. Meteorological analyses altimetry is particularly useful when it is not feasible to restrict the test airplane altitude, location, or maneuver envelope.

Ehernberger, L. J.↗
NASA NTRS1992

Analytical aeroelastic stability considerations and conversion loads for an XV-15 tilt-rotor in a wind tunnel simulation

A rotorcraft analysis is conducted to assess tilt-rotor stability and conversion loads for the XV-15 rotor with metal blades within its specified test envelope. A 38-DOF flutter analysis based on the code by Johnson (1988) is developed to simulate a wind-tunnel test in which the rotor torque is constant and thereby study stability. The same analytical model provides the simulated loads including hub loads, blade loads, and oscillatory pitch-link loads with attention given to the nonuniform inflow through the proprotor in the presence of the wing. Tilt-rotor stability during the cruise mode is found to be sensitive to coupling effects in the control system stiffness, and a stability problem is identified in the XV-15 Advanced Technology Blades. The present analysis demonstrates that the tilt-rotor is stable within the specified test envelope of the NASA 40 x 80-ft wind tunnel.

Kottapalli, Sesi↗