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NASA NTRS1975

Optimum exhaust velocity for laser-driven rockets

The laser-driven rocket in which remotely generated laser power is used to heat propellant belongs basically to the class of specific-impulse limited propulsion systems if difficult missions are considered. It was previously established that trip time reaches a minimum as specific impulse is varied for payload transfers from low earth orbit to synchronous orbit and return via laser-driven rocket propulsion, the computations being based on the perigee-propulsion laser drive described by Minovitch (1972). The present study shows that such minimum occur for all missions and that optimum specific impulse is primarily determined by the mission difficulty. More generally, this optimum specific impulse maximizes payload kinetic energy achievable with a fixed jet power and propulsion time. A formula relating propulsion time parameter to payload ratio is obtained for estimating mission capabilities of laser-driven rockets.

Moeckel, W. E.↗

NASA NTRS1979

Light-driven solute transport in Halobacterium halobium

The cell membrane of Halobacterium halobium exhibits differential regions which contain crystalline arrays of a single kind of protein, termed bacteriorhodopsin. This bacterial retinal-protein complex resembles the visual pigment and, after the absorption of protons, translocates H(+) across the cell membrane, leading to an electrochemical gradient for protons between the inside and the outside of the cell. Thus, light is an alternate source of energy in these bacteria, in addition to terminal oxidation. The paper deals with work on light-driven transport in H. halobium with cell envelope vesicles. The discussion covers light-driven movements of H(+), Na(+), and K(+); light-driven amino acid transport; and apparent allosteric control of amino acid transport. The scheme of energy coupling in H. halobium vesicles appears simple, its quantitative details are quite complex and reveal regulatory phenomena. More knowledge is required of the way the coupling components are regulated by the ion gradients present.

Lanyi, J. K.↗

NASA NTRS1982

Evaluation of lightning accommodation systems for wind-driven turbine rotors

Wind-driven turbine generators are being evaluated as an alternative source of electric energy. Areas of favorable location for the wind-driven turbines (high wind density) coincide with areas of high incidence of thunderstorm activity. These locations, coupled with the 30-m or larger diameter rotor blades, make the wind-driven turbine blades probable terminations for lightning strikes. Several candidate systems of lightning accommodation for composite-structural-material blades were designed and their effectiveness evaluated by submitting the systems to simulated lightning strikes. The test data were analyzed and system design were reviewed on the basis of the analysis.

Bankaitis, H.↗

NASA NTRS1981

Centrifugally driven diffusion of Iogenic plasma

The plasma distribution around Io as measured by Voyager 1 displays an asymmetric discontinuity at Io's orbit that has been suggested to be the signature of centrifugally driven interchange diffusion fed by plasma derived from Io. This hypothesis is explored further and found to be valid. The particular form for the diffusion coefficient appropriate to centrifugally driven turbulence is derived. The nonlinear character of this kind of diffusion is thereby made explicit. Solutions to the nonlinear, time-independent and linearized, time-dependent diffusion equations are given. These display a markedly conservative behavior. The nonlinear, steady state solutions are identical in form to the solutions of the previously studied equation of linear, atmospherically driven diffusion. The linearized, time-dependent solutions exhibit a negative feed-back quality that buffers the response of the density to changes in the source strength. Estimates of the source strength, the diffusion coefficient, and the signal propagation speed are also given.

Siscoe, G. L.↗

NASA NTRS1981

Parametric interaction and spatial collapse of beam-driven Langmuir waves in the solar wind

Observations are presented of the parametric decay and spatial collapse of Langmuir waves driven by an electron beam streaming into the solar wind from the Jovian bow shock. Long wavelength Langmuir waves upstream of the bow shock are effectively converted into short wavelength waves no longer in resonance with the beam. The conversion is shown to be the result of a nonlinear interaction involving the beam-driven pump, a sideband emission, and a low level of ion-acoustic turbulence. The beam-driven Langmuir wave emission breaks up into a complex sideband structure with both positive and negative Doppler shifts. In some cases, the sideband emission consists of isolated wave packets with very short duration bursts, which are very intense and are thought to consist of envelope solitons which have collapsed to spatial scales of only a few Debye lengths.

Gurnett, D. A.↗

NASA NTRS1984

Instabilities in line-driven stellar winds. I - Dependence on perturbation wavelength

An analytical study is presented of the stability of absorption line-driven flows, such as found in stellar winds, in the presence of small-amplitude disturbances. A generalized calculation of the perturbed direct extinction force is performed and the evolution of the perturbation into the nonlinear regime and the dynamical results of perturbation in the scattered radiation field are examined. An expression is derived for the wavenumber variation of the perturbed line force from the milieu of nonoverlapping lines which have a power-law distribution in opacity. A linear dispersion analysis is carried out to model the growth and propagation of radiative-acoustic waves in absorption line-driven flows, which are found unstable to perturbations that may be smaller than the Sobolev length. No damping mechanism was found that would eliminate the absorption line-driven flows.

Owocki, S. P.↗

NASA NTRS1984

Externally driven magnetic reconnection

A model is presented in which externaly driven reconnection is simulated by solving the MHD equations in an initially plane current sheet. Magnetic reconnection in the earth's magnetotail is widely believed to be the direct cause of magnetospheric substorms. Both 2-D and 3-D versions of the model have been developed. It is postulated that connection in the tail is triggered by a local compression of the plasma sheet which results from an invasion of the solar wind into the magnetotail. Thus, the simulation is started by introducing flow from the lobes normal to the plasma sheet. When resistivity is generated in a local region of the neutral sheet, reconnection develops and magnetic energy is converted into plasma bulk flow. Although the driven reconnection model is highly simplified, it can aid in understanding many features of substorms in the tail; in particular, results show that rapid flows both earthward and tailward of the neutral line and the nightside substorm current system are natural consequences of driven magnetic reconnection.

Walker, R. J.↗

NASA NTRS1986

Space-based laser-driven MHD generator: Feasibility study

The feasibility of a laser-driven MHD generator, as a candidate receiver for a space-based laser power transmission system, was investigated. On the basis of reasonable parameters obtained in the literature, a model of the laser-driven MHD generator was developed with the assumptions of a steady, turbulent, two-dimensional flow. These assumptions were based on the continuous and steady generation of plasmas by the exposure of the continuous wave laser beam thus inducing a steady back pressure that enables the medium to flow steadily. The model considered here took the turbulent nature of plasmas into account in the two-dimensional geometry of the generator. For these conditions with the plasma parameters defining the thermal conductivity, viscosity, electrical conductivity for the plasma flow, a generator efficiency of 53.3% was calculated. If turbulent effects and nonequilibrium ionization are taken into account, the efficiency is 43.2%. The study shows that the laser-driven MHD system has potential as a laser power receiver for space applications because of its high energy conversion efficiency, high energy density and relatively simple mechanism as compared to other energy conversion cycles.

Choi, S. H.↗

NASA NTRS1986

Response of an emerging flux tube to a current-driven instability

Emerging magnetic flux plays an important role in the development of active regions on the sun and, perhaps, in the subsequent activation of flares. However, the energy input that produces preflare brightenings and flares probably does not come from the flux emergence itself but from one or more associated energy-releasing processes - likely candidates include magnetic reconnection and various current-driven plasma micro-instabilities. Here the interplay between the changing physical characteristics of an emerging magnetic-flux tube and the onset and evolution of a representative 'bump-on-tail' plasma current-driven instability is investigated. The microinstability heats the ambient material, thus changing the macroscopic characteristics of the plasma in which the model and current-driven instability occurs.

Karpen, J. T.↗

NASA NTRS1990

Automated control of hierarchical systems using value-driven methods

An introduction is given to the Value-driven methodology, which has been successfully applied to solve a variety of difficult decision, control, and optimization problems. Many real-world decision processes (e.g., those encountered in scheduling, allocation, and command and control) involve a hierarchy of complex planning considerations. For such problems it is virtually impossible to define a fixed set of rules that will operate satisfactorily over the full range of probable contingencies. Decision Science Applications' value-driven methodology offers a systematic way of automating the intuitive, common-sense approach used by human planners. The inherent responsiveness of value-driven systems to user-controlled priorities makes them particularly suitable for semi-automated applications in which the user must remain in command of the systems operation. Three examples of the practical application of the approach in the automation of hierarchical decision processes are discussed: the TAC Brawler air-to-air combat simulation is a four-level computerized hierarchy; the autonomous underwater vehicle mission planning system is a three-level control system; and the Space Station Freedom electrical power control and scheduling system is designed as a two-level hierarchy. The methodology is compared with rule-based systems and with other more widely-known optimization techniques.

Pugh, George E.↗

NASA NTRS1990

The steady state solutions of radiatively driven stellar winds for a non-Sobolev, pure absorption model

The steady state solution topology for absorption line-driven flows is investigated for the condition that the Sobolev approximation is not used to compute the line force. The solution topology near the sonic point is of the nodal type with two positive slope solutions. The shallower of these slopes applies to reasonable lower boundary conditions and realistic ion thermal speed v(th) and to the Sobolev limit of zero of the usual Castor, Abbott, and Klein model. At finite v(th), this solution consists of a family of very similar solutions converging on the sonic point. It is concluded that a non-Sobolev, absorption line-driven flow with a realistic values of v(th) has no uniquely defined steady state. To the extent that a pure absorption model of the outflow of stellar winds is applicable, radiatively driven winds should be intrinsically variable.

Poe, C. H.↗

NASA NTRS1991

The wavelength of supercritical surface tension driven Benard convection

The size or the wavelength of moderately supercritical surface tension driven Benard convection has been investigated experimentally in a thin fluid layer of large aspect ratio. It has been found that the number of the hexagonal convection cells increases with increased temperature differences, up to 1.3 times the critical temperature difference. That means that the wavelength of surface tension driven convection decreases after onset of the instability for moderately nonlinear conditions. This result is in striking contrast to the well-known increase of the wavelength of buoyancy driven Rayleigh-Benard convection.

Koschmieder, E. L.↗

NASA NTRS1992

Particle simulations of driven collisionless magnetic reconnection at the dayside magnetopause

Particle simulations (including both ions and electrons) of the driven collisionless magnetic reconnection process at the earth's dayside magnetopause were carried out using a specially developed two-and-one-half-dimensional electromagnetic particle code with a driven inflow boundary and an open outflow boundary. The simulations were used to examine many features associated with the dayside magnetic reconnection process, including the acceleration and heating of particles and the generation of energetic particles and particle heat flux. In addition, the fluctuating electromagnetic field associated with a driven collisionless magnetic reconnection was examined and compared with the satellite observations of flux transfer events at the day-side magnetopause.

Ding, D. Q.↗

NASA NTRS1992

A combustion driven shock tunnel to complement the free piston shock tunnel T5 at GALCIT

A combustion driven shock tunnel was designed and built at GALCIT to supply the hypersonic facility T5 with 'hot' hydrogen for mixing and combustion experiments. This system was chosen over other options for better flexibility and for safety reasons. The shock tunnel is described and the overall efficiency of the system is discussed. The biggest challenge in the design was to synchronize the combustion driven shock tunnel with T5. To do so, the main diaphragm of the combustion driven shock tunnel is locally melted by an electrical discharge. This local melting is rapidly followed by the complete collapse of the diaphragm in a very repeatable way. A first set of experiments on supersonic hydrogen transverse jets over a flat plate have just been completed with the system and some of the preliminary results are presented.

Belanger, Jacques↗

NASA NTRS1993

Capacitive Proximity Sensors With Additional Driven Shields

Improved capacitive proximity sensors constructed by incorporating one or more additional driven shield(s). Sensitivity and range of sensor altered by adjusting driving signal(s) applied to shield(s). Includes sensing electrode and driven isolating shield that correspond to sensing electrode and driven shield.

Mcconnell, Robert L.↗

NASA NTRS1992

Noise-driven radiative shocks. II - Further implications for the quasi-periodic oscillations of the AM Herculis objects

The driven-shock model by Wolff et al. (1992) is extended and used to model the low-amplitude quasiperiodic oscillations (QPOs) driven by accretion noise in AM Her objects. The driven-shock model is discussed qualitatively with calculational details of the revised version. QPO-like features in the optical and X-ray are shown to be produced by bremsstrahlung-dominated shocks from accretion onto white dwarfs. These features are found to be consistent with the range of properties associated with the AM Her QPO sources. The bremsstrahlung-dominated shocks are shown to have linear responses for the conditions associated with QPO sources, and a luminosity expression for QPO sources is derived. The present analysis supports the shock-oscillation model for describing the optical QPOs from these objects.

Wood, Kent S.↗

NASA NTRS1994

Capacitive Sensor With Driven Shields And Bridge Circuit

Like other capaciflectors described in prior articles in NASA Tech Briefs, this one includes sensing electrode driven by alternating voltage, giving rise to electric field in vicinity of electrode; object entering electric field detected by its effect on capacitance between sensing electrode and electrical ground. Also includes shielding electrode (in this case, driven shield 1), excited via voltage follower at same voltage as that applied to sensing electrode to concentrate more of electric field outward from sensing electrode, increasing sensitivity and range of sensor. Because shielding electrode driven via voltage follower, it does not present significant electrical load to source of alternating voltage.

Vranish, John M.↗

NASA NTRS1992

Event-driven simulation in SELMON: An overview of EDSE

EDSE (event-driven simulation engine), a model-based event-driven simulator implemented for SELMON, a tool for sensor selection and anomaly detection in real-time monitoring is described. The simulator is used in conjunction with a causal model to predict future behavior of the model from observed data. The behavior of the causal model is interpreted as equivalent to the behavior of the physical system being modeled. An overview of the functionality of the simulator and the model-based event-driven simulation paradigm on which it is based is provided. Included are high-level descriptions of the following key properties: event consumption and event creation, iterative simulation, synchronization and filtering of monitoring data from the physical system. Finally, how EDSE stands with respect to the relevant open issues of discrete-event and model-based simulation is discussed.

Rouquette, Nicolas F.↗