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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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NASA NTRS1986

Dust counter and mass analyser (DUCMA) measurements of comet Halley's coma from Vega spacecraft

Measurements of dust particles in comet Halley's coma from Vega spacecraft made with instruments using a new principle of dust detection and having a high time resolution over a large range of dust fluxes and masses are reported. The dust coma, whether quiescent (as seen by Vega 2) or containing a major jet structure, (as seen by Vega 1) displays large, short-term variations throughout which are at times quasi-periodic. The integral mass spectra increase in intensity to the lowest masses measured, and the flux levels lie approximately in the ranges estimated previously from ground-based observations. The coma is highly dynamical on all spatial and temporal scales, suggesting a complex structure of localized regions of dust emission from the nucleus.

Simpson, J. A.↗

NASA NTRS1987

Properties of planetary fluids at high pressure and temperature

In order to derive models of the interiors of Uranus, Neptune, Jupiter and Saturn, researchers studied equations of state and electrical conductivities of molecules at high dynamic pressures and temperatures. Results are given for shock temperature measurements of N2 and CH4. Temperature data allowed demonstration of shock induced cooling in the the transition region and the existence of crossing isotherms in P-V space.

Nellis, W. J.↗

NASA NTRS1986

Evidence for intrinsically asymmetrical radio structure in the nucleus of NGC 6251

High dynamic range VLBI hybrid maps of the nucleus of NGC 6251 at 18 cm have been made in order to study its structure. The maps are consistent with a simple two-sided relativistic beaming model of the nucleus only if the radio axis is aligned within 10 deg of the line of sight and the flow velocity in the two jets is greater than 0.7c. This implies an unreasonably large physical size for the extended radio emission associated with this galaxy. The most plausible alternative explanation is that both jets are transporting energy to the radio lobes continuously, but that the counterjet is much less efficient at converting a fraction of this energy into radio emission than the brighter jet is.

Jones, Dayton L.↗

NASA NTRS1986

The solar wind interaction with Venus

The relation between Venus and the solar wind is analyzed. The effects of the intrinsic field, neutral atmosphere, and ionosphere of Venus on the solar wind are examined. The solar wind interaction phenomena is studied; consideration is given to the free-stream solar wind, bow shock, magnetosheath, boundary layer, ionospheric features, neutral atmosphere features, and wake and magnetotail. Further research on the boundary layer and tail formation, global models, and high-dynamic pressure interaction is proposed.

Luhmann, J. G.↗

NASA NTRS1987

The thermal environment of transatmospheric vehicles

A transatmospheric vehicle using primarily air-breathing propulsion must fly in the denser part of the atmosphere to achieve adequate acceleration to reach orbital speed. The potentially long ascent times, combined with the need for a low-drag configuration, result in a severe aerothermodynamics environment. To achieve low drag, the vehicle must have a relatively sharp nose and wing leading edges. The ascent peak stagnation point and equilibrium wall temperatures for the wing leading edge can reach values of 4000 K and 3000 K, respectively, for high dynamic pressure trajectories, making some form of mass addition cooling mandatory. The corresponding temperatures during entry are about 1500 K lower. The vehicle windward centerline temperatures are more moderate, however, with values peaking around 1500 K. Therefore, radiative cooling should be effective over large areas of the vehicle. The windward, centerline heat loads are relatively insensitive to the dynamic pressure of the ascent trajectory, in contrast to the stagnation point and wing leading edge. The windward surface entry heat loads are much lower, but depend strongly on the flightpath.

Tauber, Michael E.↗

NASA NTRS1987

VLBI observations of the 3C 111 jet

VLBI observations have been made of the central component of the broad-line radio galaxy 3C 111 at 1.7 GHz and 5.0 GHz. High-dynamic-range maps at both frequencies show a one-sided jet in PA 62 + or - 1 deg but no counterjet. The lower limit to the jet/counterjet ratio is 30:1 at 5 GHz and 15:1 at 1.7 GHz. The jet is initially straight, but beyond 15 marcsec from the core it exhibits small bends or wiggles. Their onset corresponds to a sharp reduction in the rate of intensity falloff along the jet, suggesting that an instability in the jet is causing substantial particle acceleration.

Linfield, Roger↗

NASA NTRS1987

Simplified solution for a class of fading memory filters

A fading memory filter is a least squares estimator (LSE) that applies an exponentially decaying weight to past measurements. When compared with a standard Kalman filter, its key advantages are asymptotic stability and reduced sensitivity to modeling errors. This paper derives a simple solution for a class of fading memory filters, resulting in a reduction in computational complexity. Steady state filter solutions are obtained for second- and third-order filters used in a global positioning system (GPS) receiver for high dynamic vehicles.

Statman, Joseph I.↗

NASA NTRS1987

Characteristics of the Marslike limit of the Venus-solar wind interaction

The response of the Venus ionosphere to the solar wind when the latter exceeds the peak thermal pressure in the Venus ionosphere is characterized on the basis of observations from the Pioneer Venus Orbiter. At these times, the Venus ionosphere becomes increasingly magnetized until the total ionospheric pressure matches that of the solar wind. The ionopause becomes less distinct and asymptotically approaches a limiting altitude of about 220 km for extremely high pressures. When the ionopause is low, the hot regions of electrons which occurs within the ionopause gradient extends to lower altitudes. The ion temperatures show little change, and there is no discernible increase in plasma wave activity at the ionopause. The Venus observations at high dynamic pressure provide a framework for reassessing the available Mars observations. In particular, this study shows that solar wind standoff without sufficient ionospheric plasma pressure cannot by itself be used as evidence for a planetary magnetic field.

Luhmann, J. G.↗

NASA NTRS1987

The interaction of quasiperpendicular shock waves in a collisionless plasma

The interaction between collisionless shocks is examined from a kinetic viewpoint using computer simulations of shock collisions spanning the range of ambient plasma and shock parameters typical of the interplanetary medium. It is found that the interaction between two subcritical, perpendicular shocks is largely fluidlike with no evidence for accelerated particles. The electromagnetic fields associated with each shock, but not the plasmas, pass through each other during the collision. This interaction is due to self-consistently generated electric fields which adjust to the value needed to bring to rest the two incoming streams. The interaction between two supercritical perpendicular collisionless shocks is similar to the subcritical case, in that the electromagnetic fields associated with each shock are transmitted whereas particles are not. However, the interaction is also highly dynamic with substantial acceleration of ions. Increasing the Mach number gives rise to an increasing number of energetic particles.

Cargill, P. J.↗

NASA NTRS1988

Power supply conditioning circuit

A conditioning circuit is provided with a constant current diode in series with a zener diode, the former having a high dynamic impedance and the latter a low dynamic impedance. The constant current diode can receive an input voltage with PARD. In conjunction with the zener diode fixed to a ground, a voltage divider is provided which can give an output voltage whose PARD was significantly reduced. The conditioning circuit is effective down to dc.

Primas, Lori E.↗

NASA NTRS1987

Hypersonic structures and materials - A progress report

The weight of a hypersonic, airbreathing SSTO vehicle may be more critical than for any previous aerospacecraft; an evaluation is accordingly made of the development status and applicability of intermetallic compounds, metal-matrix composites, carbon-carbon composites, ceramics, and ceramic-matrix composites applicable to SSTO craft primary structures. Aerothermal, aerothermoelastic, and acoustic loads are high because the airbreathing SSTO vehicle must follow a high dynamic pressure trajectory in order to achieve the requisite propulsive efficiency. Attention is given to the prospects for integral cryogenic tankage and actively hydrogen-cooled airframe and engine structures.

Jackson, L. Robert↗

NASA NTRS1987

Spectral differential imaging detection of planets about nearby stars

Direct ground-based optical imaging of planets in orbit about nearby stars may be accomplished by spectral differential imaging using multiple passband acoustooptic filters with a CCD. This technique provides two essential results. First, it provides a means to modulate the stellar flux reflected from a planet while leaving the flux from the star and other sources in the same field of view unmodulated. Second, spectral differential imaging enables the CCD detector to achieve a sufficiently high dynamic range to locate planets near a star in spite of an integrated brightness differential of 5 x 10 to the 8th. Spectral differential imaging at nearby diffraction limited imaging conditions with telescope apodization can reduce the time to conduct a sensitive planetary search to a few hours in some cases. The feasibility of this idea is discussed here and shown to provide, in principle, the discrimination and sensitivity to detect a Jovian-class planet about stars at distances of about 10 parsecs. The detection of brown dwarfs is shown to be feasible as well.

Smith, W. Hayden↗

NASA NTRS1988

Signal-to-noise limitations in white light holography

A simple derivation is given for the SNR in images reconstructed from incoherent holograms. Dependence is shown to be on the hologram SNR, object complexity, and the number of pixels in the detector. Reconstruction of involved objects becomes possible with high-dynamic-range detectors such as CCDs. White-light holograms have been produced by means of a rotational shear interferometer combined with a chromatic corrector. A digital inverse transform recreated the object.

Ribak, Erez↗

NASA NTRS1988

Construction and evaluation of an imaging focussed-mesh electron multiplier for space instrumentation

A focused-mesh electron multiplier (FMEM) has been combined with a wedge and strip position-sensitive anode to create a novel detector that is considered likely to have a number of applications in space astronomy and laboratory image sensing. Results from the first application of a wedge and strip anode to an FMEM are presented including spatial resolution, spatial linearity, background rate, and pulse risetime. The converted multiplier retains the attributes of an FMEM that made it attractive in the first place, such as low background, high speed, and high dynamic range.

Vallerga, J.↗

NASA NTRS1988

An extended Kalman filter based automatic frequency control loop

An Automatic Frequency Control (AFC) loop based on an Extended Kalman Filter (EKF) is introduced and analyzed in detail. The scheme involves an EKF which operates on a modified set of data in order to track the frequency of the incoming signal. The algorithm can also be viewed as a modification to the well known cross-product AFC loop. A low carrier-to-noise ratio (CNR), high-dynamic environment is used to test the algorithm and the probability of loss-of-lock is assessed via computer simulations. The scheme is best suited for scenarios in which the frequency error variance can be compromised to achieve a very low operating CNR threshold. This technique can easily be incorporated in the Advanced Receiver (ARX), requiring minimum software modifications.

Hinedi, S.↗

NASA NTRS1988

High spatial resolution 10 micron imaging of IRC + 10216

Precise high-resolution 10-micron images of the carbon star IRC + 10216 have been obtained with a scanned linear array. The low noise and high dynamic range of these images permit deconvolution of the telescope point-spread function, revealing the radial brightness distribution of the circumstellar dust shell: approximate reflection symmetry is found in west-east scans, with a distinct division into two components of diameter about 0.40 and 2.2 arcsec. It is shown that this morphology is consistent with published interferometric data that had cast doubt upon an earlier, idealized two-component model. The observed brightness distribution implies that the circumstellar dust density may deviate substantially from the 1/r squared radial dependence expected for spherically symmetric outflow with constant velocity and constant rate of mass loss.

Bloemhof, E. E.↗

NASA NTRS1989

Solar wind interaction with the ionosphere of Venus inferred from radio scintillation measurements

The observation of S-band (2.3 GHz) radio scintillations in the ionosphere of Venus by the Pioneer Venus Orbiter is reported. In situ plasma measurements and propagation calculations show that the scintillations are caused by electron density irregularities in the topside ionosphere of Venus below the ionopause. It is suggested that these topside plasma irregularities are associated with the penetration of large-scale magnetic fields in the ionosphere. It is found that the disturbed plasma and the scintillations are a manifestation of high-dynamic solar wind interaction with the ionosphere.

Woo, Richard↗

NASA NTRS1989

Near-field acoustic characteristics of a single-rotor propfan

The near-field noise characteristics of the SR-7L, an eight-blade, single-rotor, wing-mounted, tractor propfan have been determined. It is found that the noise is dominated by discrete tones, usually at the first order (and occasionally at the second or third order) of the blade-passage frequency. The highest noise levels were noted at conditions of high tip helical speeds and high dynamic pressures.

Bartel, H. W.↗