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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 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Sensory redundancy management: The development of a design methodology for determining threshold values through a statistical analysis of sensor output data

Sensor redundancy management (SRM) requires a system which will detect failures and reconstruct avionics accordingly. A probability density function to determine false alarm rates, using an algorithmic approach was generated. Microcomputer software was developed which will print out tables of values for the cummulative probability of being in the domain of failure; system reliability; and false alarm probability, given a signal is in the domain of failure. The microcomputer software was applied to the sensor output data for various AFT1 F-16 flights and sensor parameters. Practical recommendations for further research were made.

Scalzo, F.↗
NASA NTRS1984

s-wave threshold in electron attachment - Results in 2-C4F6 and CFCl3 at ultra-low electron energies

Electron attachment lineshapes and cross sections are reported for the processes 2-C4F6(-)/2-C4F6 and Cl(-)/CFCl3 at electron energies of 0-120 and 0-140 meV, and at resolutions of 6 and 7 meV (FWHM), respectively. As in previous measurements in CCl4 and SF6, the results show resolution-limited narrow structure in the cross section at electron energies below 15 meV. This structure arises from the divergence of the s-wave cross section in the limit of zero electron energy. Comparisons are given with swarm-measured results, and with collisional ionization (high-Rydberg attachment) data in this energy range.

Chutjian, A.↗
NASA NTRS1985

Quantum noise and the threshold of hearing

It is argued that the sensitivity of the ear reaches a limit imposed by the uncertainty principle. This is possible only if the receptor cell holds the detector elements in a special nonequilibrium state which has the same noise characteristics as a ground (T = 0 K) state. To accomplish this 'active cooling' the molecular dynamics of the system must maintain quantum mechanical coherence over the time scale of the measurement.

Bialek, W.↗
NASA NTRS1985

Changes of instability thresholds of rotor due to bearing misalignments

The influence of bearing misalignment upon the dynamic characteristics of statistically indeterminant rotor bearing systems is investigated. Both bearing loads and stability speed limits of a rotor may be changed significantly by magnitude and direction of bearing misalignment. The useful theory of short journal bearings is introduced and simple analytical expressions, governing the misalignment problem, are carried out. Polar plots for the bearing load capacities and stability maps, describing the speed limit in terms of misalignment, are presented. These plots can be used by the designer to estimate deviations between calculation and experimental data due to misalignment effects.

Springer, H.↗
NASA NTRS1985

Powered-lift technology on the threshold

A design configuration study and comparative performance evaluation is presented for current and planned V/STOL military aircraft and their powerplant and control systems, with a view to the effect of the operational capabilities uniquely furnished by such aircraft for battlefield tactics. Attention is given to V/STOL naval aircraft and the carrier or ship's deck conditions under which they must be able to operate. Thrust-vectoring and tilt rotor configurations are the most prominent among successful experimental and operational aircraft of this type; cargo aircraft have most often incorporated short/rough airfield operation capabilities rather than full VTOL. A major development thrust has been mounted for a supersonic short takeoff/vertical landing aircraft.

Deckert, W. H.↗
NASA NTRS1985

Radiation thresholds for synthetic elastomers

Synthetic elastomeric materials, when used in systems that must operate in radiation fields, frequently comprise the weakest link for materials. The results of a comprehensive tabulation of mechanical response data are summarized in graphical form useful to designers. General information on elastomers is given along with electrical, outgassing and thermal response data.

Bouquet, F. L.↗
NASA NTRS1986

Low threshold solar-pumped iodine laser

Solar-pumped lasing of i-C3F7I and n-C4F9I at lower solar concentrations (170 solar constants) and longer gain lengths than previous solar lasers is demonstrated, with potential application to the lasing needs of space. Two xenon arc solar simulators provide an AM0 spectrum over the 60-cm gain length, and output pulse energies of 70 mJ and an average power of 550 mW are achieved. Low pressure lasing times of 600 ms are reached, and the observed 0.074 percent slope efficiency could approach the 0.2 percent maximum theoretical efficiency. Due to less quenching of I(asterisk), n-C4F9I is found to be a superior lasant to i-C3F7I.

Deyoung, R. J.↗
NASA NTRS1987

Frequency and load ratio effects on critical strain energy release rate Gc thresholds of graphite/epoxy composites

Graphite/epoxy composite laminates of T300/BP907 and AS6/HST-7 were axial-tension fatigue tested. Tests were conducted at 5 and 10 Hz, and at loading ratios of R = 0.1 and R = 0.5. Edge delamination was monitored as a function of number of fatigue cycles by monitoring stiffness reduction during fatigue testing. Delamination was confirmed and documented using dye-enhanced X-ray and optical photography. Critical strain energy release rates were then calculated. The composites delaminated readily, with loading ratio having a significant influence. Frequency effects were negligible.

Adams, Donald F.↗
NASA NTRS1988

Total photoionization cross sections of atomic oxygen from threshold to 44.3 A

Synchrotron radiation was used to obtain the relative photoionization cross section of atomic oxygen for the production of singly charged ions over the 44.3-910.5-A wavelength range. Measurement of the contribution of multiple ionization to the cross sections has made possible the determination of total photoionization cross sections below 250 A. The series of autoionizing resonances leading to the 4P state of the oxygen ion has been observed using an ionization-type experimental procedure for the first time.

Angel, G. C.↗
NASA NTRS1989

On the threshold - The outlook for supersonic and hypersonic aircraft

A development history and current development status evaluation is presented with a view to the prospective viability of supersonic, hypersonic, and transatmospheric vehicle R&D efforts. It is stressed that such high-speed vehicles will not supplant the current fleet of subsonic aircraft, but simply address the growing need for very long range commercial and military missions. Mach 2-3 airliners could be developed by the year 2000; Mach 5-6 military reconnaissance aircraft could be operational soon after. Transatmospheric, mixed airbreathing/rocket propulsion single-stage vehicles capable of orbital insertion will require several years of additional development beyond hypersonic ones to become operational.

Harris, Roy V., Jr.↗
NASA NTRS1989

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

An investigation of the fatigue growth (FCG) behavior of PWA 1480 single crystal nickel base superalloy was conducted. Typical Paris region behavior was observed above a Delta-K of 8 MPa sq rt m. However, below that stress intensity range, the alloy exhibted highly unusual behavior. The behavior consisted of a region where the crack growth rate became essentially independent of the applied stress intensity. The transition in the FCG behavior was related to a change in the observed crack growth mechanisms.

Telesman, Jack↗
NASA NTRS1990

Measurement of threshold temperature effects in dissociative electron attachment to HI and DI

From accurate spectroscopic constants it is found that the thermal dissociative-attachment process (DA) in DI should be exothermic only for rotational levels J greater than 8 in v = O. Here, measurement of an enhancement of DA with rotational temperature T in the range 298-468 K is reported. The effect is easily accounted for by the increase in total fractional population of excited J levels in DI relative to HI. The effect affords a rotational analog to the use of vibrationally excited molecules (e.g., HCl) in a plasma to control electron conduction.

Chutjian, A.↗
NASA NTRS1990

On the threshold for triggering substorms

The substorm-neutral-line model of Hones (1984) is extended in order to interpret substorm-related effects that have not previosly been linked to model. It is proposed that the level of stress at which the substorm expansion starts is controlled by the tail field geometry and that substorms most easily initiate when the bending of the magnetotail is most extreme. Using this 'bent-tail' (BT) hypothesis, a new interpretation is developed for the annual and diurnal variations of the level of geomagnetic activity, that are independent of the polarity of the IMF but are due to the BT effect. The BT effect leads to predictions regarding annual and diurnal signatures of substorm occurrence frequency and magnitude that can be tested.

Kivelson, Margaret G.↗
NASA NTRS1990

On a class of unsteady three-dimensional Navier Stokes solutions relevant to rotating disc flows: Threshold amplitudes and finite time singularities

A class of exact steady and unsteady solutions of the Navier Stokes equations in cylindrical polar coordinates is given. The flows correspond to the motion induced by an infinite disc rotating with constant angular velocity about the z-axis in a fluid occupying a semi-infinite region which, at large distances from the disc, has velocity field proportional to (x,-y,O) with respect to a Cartesian coordinate system. It is shown that when the rate of rotation is large, Karman's exact solution for a disc rotating in an otherwise motionless fluid is recovered. In the limit of zero rotation rate a particular form of Howarth's exact solution for three-dimensional stagnation point flow is obtained. The unsteady form of the partial differential system describing this class of flow may be generalized to time-periodic equilibrium flows. In addition the unsteady equations are shown to describe a strongly nonlinear instability of Karman's rotating disc flow. It is shown that sufficiently large perturbations lead to a finite time breakdown of that flow whilst smaller disturbances decay to zero. If the stagnation point flow at infinity is sufficiently strong, the steady basic states become linearly unstable. In fact there is then a continuous spectrum of unstable eigenvalues of the stability equations but, if the initial value problem is considered, it is found that, at large values of time, the continuous spectrum leads to a velocity field growing exponentially in time with an amplitude decaying algebraically in time.

Hall, Philip↗
NASA NTRS1991

America at the threshold

The Space Exploration Initiative is a challenge issued by President Bush that will return America to the Moon to stay and onward to Mars by 2019. In doing this, six 'visions' guide and direct the space efforts. These efforts are: Knowledge of the universe; Advancement in science and engineering; U.S. leadership; Technologies for Earth; Commercialization of space; and Strengthened U.S. economy. Why the Moon and Mars were chosen as the objects of exploration is examined. Ten recommendations are provided for the effective implementation of the SPI, along with a detailed examination of each.

Synthesis Group↗
NASA NTRS1991

Saltation threshold reduction due to the electrostatic agglomeration of fine particles

Particles between 80 and 110 microns in diameter are the most easily moved by the wind. As the particle size decreases below 60 microns, they are increasingly more difficult to move by surface winds, and a number of experiments were performed in an attempt to reduce the required wind velocity. These include: (1) the bombardment of a bed of fine particles by particles near the optimum size, the larger particles kicking the fine particles into the windstream where they are entrained; and (2) the electrostatic agglomeration of fine particles into sizes more easily saltated. The results of these experiments are discussed.

Leach, Rodman N.↗
NASA NTRS1990

Single- and double-photoionization cross sections of atomic nitrogen from threshold to 31 A

The relative photoionization cross section of atomic nitrogen for the production of singly and doubly charged ions has been measured from 44.3 to 275 A and from 520 to 852 A. The results have been made absolute by normalization to one-half of the molecular nitrogen cross section at short wavelengths. The smoothed atomic nitrogen cross sections sigma can be accurately represented, at short wavelengths, by the equation sigma(Mb) = 36,700 x (E exp-2.3) as a function of the photon energy E (eV), thereby allowing the cross sections to be extrapolated to the nitrogen K edge at 31 A.

Samson, James A. R.↗