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NASA NTRS2002

High-Strength Composite Fabric Tested at Structural Benchmark Test Facility

Large sheets of ultrahigh strength fabric were put to the test at NASA Glenn Research Center's Structural Benchmark Test Facility. The material was stretched like a snare drum head until the last ounce of strength was reached, when it burst with a cacophonous release of tension. Along the way, the 3-ft square samples were also pulled, warped, tweaked, pinched, and yanked to predict the material's physical reactions to the many loads that it will experience during its proposed use. The material tested was a unique multi-ply composite fabric, reinforced with fibers that had a tensile strength eight times that of common carbon steel. The fiber plies were oriented at 0 and 90 to provide great membrane stiffness, as well as oriented at 45 to provide an unusually high resistance to shear distortion. The fabric's heritage is in astronaut space suits and other NASA programs.

Krause, David L.↗

NASA NTRS2014

The Neutron Star Zoo

Neutron stars are a very diverse population, both in their observational and their physical properties. They prefer to radiate most of their energy at X-ray and gamma-ray wavelengths. But whether their emission is powered by rotation, accretion, heat, magnetic fields or nuclear reactions, they are all different species of the same animal whose magnetic field evolution and interior composition remain a mystery. This article will broadly review the properties of inhabitants of the neutron star zoo, with emphasis on their high-energy emission. XXX Neutron stars are found in a wide variety of sources, displaying an amazing array of behavior. They can be isolated or in binary systems, accreting, heating, cooling, spinning down, spinning up, pulsing, flaring and bursting. The one property that seems to determine their behavior most strongly is their magnetic field strength, structure and evolution. The hot polar caps, bursts and flares of magnetars are likely due to the rapid decay and twisting of their superstrong magnetic fields, whose very existence requires some kind of early dynamo activity. The intermediate-strength magnetic fields of RPPs determines their spin-down behavior and radiation properties. However, the overlap of the magnetar and RPP populations is not understood at present. Why don't high-field RPPs burst or flare? Why don't lower-field magnetars sometimes behave more like RPPs? INS may be old magnetars whose high fields have decayed, but they do not account for the existence of younger RPPs with magnetar-strength fields. Not only the strength of the magnetic field but also its configuration may be important in making a NS a magnetar or a RPP. Magnetic field decay is a critical link between other NS populations as well. "Decay" of the magnetic field is necessary for normal RPPs to evolve into MSPs through accretion and spin up in LMXBs. Some kind of accretion-driven field reduction is the most likely mechanism, but it is controversial since it is not clear how effective it is or on what timescale a buried field might re-emerge. One piece of evidence in favor of accretion-driven field reduction is the fact that NSs in LMXBs, which are older systems (> 108 yr), have mostly low fields and NSs in HMXBs, which are younger systems (107 − 108 yr), have higher fields. This may be an indication that accretion-driven field reduction or decay has not had enough time to operate in HMXBs but has in LMXBs. However, there does not seem to be any evidence of decaying fields in either the LMXB or HMXB populations; e.g. smaller magnetic fields in older systems. On the other hand, CCOs are very young so if they acquired their low fields through mass fallback accretion, the field submergence would have had to operate on much faster timescales than it apparently does in LMXBs. But as we continue to find new species in the NS zoo, one of these may someday be the "Rosetta Stone" that will give us the clues for solving these puzzles.

Zoo↗

NASA NTRS1986

Possible evidence for beaming in flares from microwave and hard X-ray imaging and spectra

The magnetic field strength and number of burst-producing energetic electrons are presently deduced for the impulsive phase of a solar flare at microwave wavelengths, with the VLA, and hard X-rays, with the SMM Hard X-ray Burst Spectrometer. The combined data indicate that the number of microwave-emitting electrons is at least three orders of magnitude smaller than the number of thick target electrons producing the hard X-rays; this is suggested to be due to the high beaming and inefficient radiation of gyrosynchrotron emission by comparison with isotropically distributed electrons.

Schmahl, E. J.↗

NASA NTRS1988

Burst vortex/boundary layer interaction

Several configurations of delta wing vortex generator and boundary layer test plate were tested, and two final ones selected. Sample measurements and flow visualizations in the candidate configurations, together with more detailed measurements in one of the two final arrangements, which were selected so that a pure vortex bursts repeatably and then interacts, in as simple fashion as possible, with a simple turbulent boundary layer, are included. It is concluded that different intensities of bursting or breakdown, like different strengths of shock wave or hydraulic jump, can be produced by minor changes of configuration. The weaker breakdowns do not produce flow reversal. The initial measurements were done with a fairly weak, but repeatable, breakdown. Basic measurements on the second final arrangement, with a stronger breakdown, are in progress.

Bradshaw, P.↗

NASA NTRS1991

Occurrence characteristics of VLF bursts in the nightside ionosphere of Venus

Many of the impulsive VLF signals observed by the Pioneer Venus orbiter electric field detector (OEFD) in the nightside ionosphere have been interpreted as arising from lightning in the Venus atmosphere. In order to determine the characteristics of the source, and to compare with terrestrial lightning, the normalized occurrence rate for individual bursts at Venus are determined, as opposed to previous studies of activity in 30-s intervals. Burst identification criteria are determined which take into account the decay constant of the instrument to calculate the burst rate. Under the assumption that all OEFD observations in the nightside ionosphere are due to impulsive signals, it is found that the 100-Hz channel has the highest burst rate, about 0.20/s in the altitude range 150-180 km and in the postmidnight local time sector. The burst occurrence rates for all four frequency channels decrease with increasing altitude. Burst rates at frequencies above 100 Hz are greater in the premidnight hours while 100-Hz signals peak after midnight; however, these burst rates vary from orbit to orbit. The 100-Hz low-frequency burst rate has a stronger dependence on the magnetic field strength than the higher-frequency rates. Most 100-Hz bursts are closely spaced and occur independently of signals at higher frequencies. These dependences suggest that the low- and high-frequency signals have different propagation mechanisms. The properties of the bursts are generally consistent with a lightning source, and the planet-wide burst rate at Venus may be comparable to or larger than the terrestrial lightning rate.

Ho, C.-M.↗

NASA NTRS2012

Reliability of COPVs Accounting for Margin of Safety on Design Burst

In this paper, the stress rupture reliability of Carbon/Epoxy Composite Overwrapped Pressure Vessels (COPVs) is examined utilizing the classic Phoenix model and accounting for the differences between the design and the actual burst pressure, and the liner contribution effects. Stress rupture life primarily depends upon the fiber stress ratio which is defined as the ratio of stress in fibers at the maximum expected operating pressure to actual delivered fiber strength. The actual delivered fiber strength is calculated using the actual burst pressures of vessels established through burst tests. However, during the design phase the actual burst pressure is generally not known and to estimate the reliability of the vessels calculations are usually performed based upon the design burst pressure only. Since the design burst is lower than the actual burst, this process yields a much higher value for the stress ratio and consequently a conservative estimate for the reliability. Other complications arise due to the fact that the actual burst pressure and the liner contributions have inherent variability and therefore must be treated as random variables in order to compute the stress rupture reliability. Furthermore, the model parameters, which have to be established based on stress rupture tests of subscale vessels or coupons, have significant variability as well due to limited available data and hence must be properly accounted for. In this work an assessment of reliability of COPVs including both parameter uncertainties and physical variability inherent in liner and overwrap material behavior is made and estimates are provided in terms of degree of uncertainty in the actual burst pressure and the liner load sharing.

Murthy, Pappu L.N.↗

NASA NTRS2022

Space Weather Modeling and Prediction for Intermediate Time-scales

Much of solar activity within a sunspot cycle occurs as bursts, or 'seasons' of strong activity over several months, separated by periods of much less activity. The most important space weather effects occur during these bursts. Previous modeling and forecasting efforts have focused on time-scales of hours-to-days and decades-to-centuries. The recent discovery of Rossby waves in the Sun, together with recently developed global models of solar MHD Rossby waves and their interactions with differential rotation and spot-producing magnetic fields, reveal the opportunity to simulate and predict the occurrence, strength and location of enhanced activity bursts a few weeks up to several months in advance. We now have a golden opportunity to fill in this gap in time-scales of forecasting space weather. This requires a) continuous observations of solar Rossby waves by various techniques; b) development of coupled nonlinear MHD models that simulate both global Rossby waves and the much smaller spatial scale emergence of new active regions; c) application of advanced data assimilation techniques to couple surface observations to update the model-system to integrate forward in time for creating forecasts months ahead. Then it will be possible to build operational prediction models to meet the needs of customers and stakeholders, including support of future NASA missions, regarding what kind and level of space weather to expect a few weeks to several months ahead.

SMD↗

NASA NTRS2015

NASA Engineering and Safety Center Technical Bulletin No. 15-02: Best Practices for Use of Sine Burst Testing

Sine Burst (SB) Testing is used for strength testing of aerospace hardware as an alternate to static pull and centrifuge tests. The main advantage is that testing can be implemented while hardware is on the shaker for other tests. It also imposes a lower number of cycles on the hardware compared with a sine dwell or sweep, and is particularly suitable for relatively stiff components or electronics boxes, instruments, or spacecraft. Identified risks can be mitigated by best practices.

Best Practices↗

NASA NTRS1990

Positron annihilation in gamma-ray bursts

Emission features appear at energies of 350 to 450 keV in the spectra of a number of gamma ray burst sources. These features were interpreted as electron-positron annihilation lines, redshifted by the gravitational field near the surface of a neutron star. Evidence that gamma ray bursts originate at neutron stars with magnetic field strengths of approx. 10(exp 12) Gauss came from recent observations of cyclotron scattering harmonics in the spectra of two bursts. Positrons could be produced in gamma ray burst sources either by photon-photon pair production or by one-photon pair production in a strong magnetic field. The annihilation of positrons is affected by the presence of a strong neutron star magnetic field in several ways. The relaxation of transverse momentum conservation causes an intrinsic broadening of the two-photon annihilation line and there is a decrease in the annihilation cross section below the free-space value. An additional channel for one-photon annihilation also becomes possible in high magnetic fields. The physics of pair production and annihilation near strongly magnetized neutron stars will be reviewed. Results from a self-consistent model for non-thermal synchrotron radiation and pair annihilation are beginning to identify the conditions required to produce observable annihilation features from strongly magnetized plasmas.

Harding, Alice K.↗

NASA NTRS1983

Activity and magnetic fields on stars from radio observations

Radio observations explain certain kinds of stellar activity and reliable estimates of magnetic field strengths are derived for the radio emitting regions. Radio observation of flare stars shows rapidly varying bursts of radiation with frequencies of up to 5 GHz. The inferred brightness temperatures of these bursts (10 to the 10th power to 10 to the 14th power) imply that the radiation is coherent. Two mechanisms are proposed to generate such radiation: (1) plasma radiation, and (2) electron cyclotron masers. The high brightness temperatures of the bursts up to 10 to the 20th power K) makes the latter the most plausible. For bursts in the 5 GHz range, the magnetic field strength would be approximately 900 Gauss.

Dulk, G. A.↗

NASA NTRS2012

All-Sky Search for Gravitational-Wave Bursts in the First Joint LIGO-GEO-Virgo Run

We present results from an aU-sky search for unmodeled gravitational-wave bursts in the data collected by the LIGO, GEO 600 and Virgo detectors between November 2006 and October 2007. The search is performed. by three different analysis algorithms over the frequency band 50 - 6000 Hz. Data are analyzed for times with at least two of the four LIGO-Virgo detectors in coincident operation, with a total live time of 266 days, No events produced by the search algorithms survive the selection cuts. We set a frequentist upper limit on the rate of gravitational-wave bursts impinging on our network of detectors. When combined with the previous LIGO search of the data collected between November 2005 and November 2006, the upper limit on the rate of detectable gra.vitational. wave bursts in the 64-2048 Hz band is 2,0 events per year at 90% confidence. We also present event rate versus strength exclusion plots for several types of plausible burst waveforms. The sensitivity of the combined search is expressed in terms of the root-sum-squared strain amplitude for a variety of simulated waveforms and lies in the range 6 X 10(exp -22) Hz(exp - 1/2) to 2 X 10(exp -20) Hz(exp -l/2). This is the first untriggered burst search to use data from the LIGO and Virgo detectors together, and the most sensitive untriggered burst search performed so far.

Camp, J. B.↗

NASA NTRS1976

On the rotational bursting of interplanetary dust particles

A model of the zodiacal cloud is used to evaluate the effects of rotational bursting on interplanetary dust particles caused by the interaction of solar radiation pressure with the irregular surface geometry of the particles. The effectiveness of this mechanism is evaluated using several values for the effective moment arm and tensile strength of the particles. Due to repeated bursting, it is shown that particles spiraling in from the asteroid belt by the Poynting-Robertson effect do not penetrate inside 1.2 AU. Spin rate stabilization under the effect of magnetic damping is found to depend critically on the initial heliocentric distance of the particle and on its radius.

Misconi, N. Y.↗

NASA NTRS1978

The polarization of second harmonic plasma emission

It is shown that second-harmonic plasma emission is partially polarized in the sense of the ordinary mode of magnetoionic theory only when the Langmuir waves are confined to a small range of angles (less than 30 deg) to the magnetic-field lines. Consequently, Suzuki and Sheridan's (1977) observations of the polarization of harmonic Type III emission implies that (at least in the cases reported) the Langmuir waves must be nearly one-dimensional. For a nearly one-dimensional distribution, the degree of polarization and the frequency of observation should be related to the magnetic field. For the observed polarization of Type III bursts, the implied magnetic-field strengths are strong enough for induced scattering to cause the Langmuir waves to become nearly one-dimensional, which is consistent with the observed sense of polarization. In other applications of harmonic plasma emission where the Langmuir waves might be isotropic or in a loss-cone distribution, polarization in the sense of the extraordinary mode is predicted.

Melrose, D. B.↗

NASA NTRS1983

Advanced Gas Turbine (AGT): Power-train system development

Technical work on the design and effort leading to the testing of a 74.5 kW (100 hp) automotive gas turbine is described. The general effort was concentrated on building an engine for test starting in July. The buildup progressed with only routine problems and the engine was delivered to the test stand 9 July. In addition to the engine build effort, work continued in selected component areas. Ceramic turbine parts were built and tested. Burst tests of ceramic rotors show strengths are approaching that achieved in test bars; proof testing is required for acceptable strength ceramic vanes. Over 25 hours was accumulated on the combustor rig in three test modes: pilot nozzle only, start nozzle, and main nozzle operation. Satisfactory ignition was achieved for a wide range of starting speeds and the lean blowout limit was as low as 0.06 kg/b (0.14 lb/hr). Lean blowout was more a function of nozzle atomization than fuel/air ratio. A variety of cycle points were tested. Transition from start nozzle flow to main nozzle flow was done manually without difficulty. Regenerator parts were qualification tested without incident and the parts were assembled on schedule. Rig based performance matched first build requirements. Repeated failures in the harmonic drive gearbox during rig testing resulted in that concept being abandoned for an alternate scheme.

Helms, H. E.↗

NASA NTRS1984

Nonthermal synchrotron radiation in a strong magnetic field

A synchrotron source model is presented for the origin of gamma-ray bursts from neutron stars. The field strength and length of time needed for electrons to lose sufficient energy in emitting the gamma rays are calculated, together with the efficiency of the process, limiting frequencies, and the energy output. A Monte Carlo simulation of the synchrotron radiation spectrum of an electron undergoing a self-consistent energy loss demonstrates that a process involving thermalized electrons can be sufficiently efficient to produce 30 keV gamma rays. Problems expected in using the current capabilities of detectors to scan for the projected energy spectra from fields of at least 3 x 10 to the 12th Gauss are discussed.

Bussard, R. W.↗

NASA NTRS1995

Constraints on the gamma-ray burst luminosity function from Pioneer Venus Orbiter and BATSE observations

We examine the width of the gamma ray burst luminosity function through the distribution of Gamma Ray Burst (GRB) peak fluxes as detected by the Pioneer Venus Orbiter (PVO) and the Burst and Transient Source Experiment (BATSE). The strength of the analysis is greatly enhanced by using a merged catalog of peak fluxes from both instruments with good cross-calibration of their sensitivities. The range of peak fluxes is increased by approximately a factor of 20 relative to the BATSE catalog. Thus, more sensitive investigations of the log N - log P distribution are possible. We place constraints on the width of the luminosity function of gamma-ray bursts brighter than the BATSE completeness limit by comparing the intensity distribution in the merged catalog with those produced by a variety of spatial density and luminosity functions. For the models examined, 90% of the detectable bursts have peak luminosities within a range of 10, indicating that the peak luminosities of gamma-ray bursts span a markedly less wide range of values than many other of their measurable properties. We also discuss for which slopes of a power-law luminosity function the observed width is at the upper end of the constrained range. This is essential in determining the power-law slopes for which luminosity-duration correlations could be important.

Ulmer, Andrew↗

NASA NTRS1979

Spectral evolution of multiply impulsive solar bursts

Some results from the analysis of a set of multiply impulsive hard X-ray and microwave solar bursts are presented, showing that some bursts can exhibit widely different magnetic-field strengths at different times. Two categories of microwave spectral behavior are identified: those events during which the microwave turnover frequency (MTF) and spectral shape (SS) remain the same from peak to peak, and those during which the MTF and SS change significantly. These categories correspond to two classes of multiply impulsive bursts: those for which the emission can be characterized by a constant magnetic field and therefore a single source region, and those in which groups of component spikes appear to originate in regions of different magnetic-field strengths, corresponding to separate source regions which flare sequentially. With regard to the latter type, examples are presented, the discrete flaring regions are examined, and their spatial separations are estimated.

Karpen, J. T.↗

NASA NTRS1973

Observation of MeV gamma-ray events during May 1967 from ERS-18

Gamma-ray fluxes between 25 keV and 6 MeV were observed by instruments aboard the Environmental Research Satellite-18 (ERS-18)during the great solar flares of May 21 and May 23, 1967. Measurements were made with a 6.35-cm-diameter by 7.65-cm-long Nal scintillation counter with five differential channels between 0.25 and 6.0 MeV. Data obtained during the event at 1945 UT on May 23, 1967, can be fitted by a two-part, power-law, differential-number spectrum. One part is of spectral index 3.2 between 8 keV and 0.6 MeV, and the other is of spectral index 1.2 above 0.6 MeV. An upper limit of 0.52 photons is set for 2.23 MeV line strength for this event. In all three bursts, tentatively associated with white light flares, the MeV-range fluxes declined more slowly than the 25-keV fluxes.

Gruber, D. E.↗