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At least 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Axisymmetric bubble profiles in a slowly rotating helium dewar under low and microgravity environments

The Gravity Probe-B (GP-B) Spacecraft is designed to test the General Theory of Relativity through long-term monitoring the precession of a set of gyros in free-fall around the earth. The potential problems for GP-B Spacecraft design requirements could be due to asymmetry in the static liquid helium distribution, or to perturbations in free surface which is present in the partially-filled liquid helium dewar. In this study, doughnut-shaped helium bubble equilibrium profiles in rotating dewar, under the conditions during the period of GP-B experiment instrument calibration and also during the period of normal operation stages of GP-B Spacecraft, have been numerically calculated. For academic interest, the range of coverage has been extended beyond the conditions assigned by the GP-B Spacecraft experiment.

Hung, R. J.↗
NASA NTRS1989

Analysis of dewar and transfer line cooldown in Superfluid Helium On-Orbit Transfer Flight Experiment (SHOOT)

The Superfluid Helium On-Orbit Transfer Flight Experiment (SHOOT) is designed to demonstrate the techniques and components required for orbital superfluid (He II) replenishment of observatories and satellites. One of the tasks planned in the experiment is to cool a warm cryogen tank and a warm transfer line to liquid helium temperature. A math model, based on single-phase vapor flow heat transfer, has been developed to predict the cooldown time, component temperature histories, and helium consumption rate, for various initial conditions of the components and for the thermomechanical pump heater powers of 2 W and 0.5 W. This paper discusses the model and the analytical results, which can be used for planning the experiment operations and determining the pump heater power required for the cooldown operation.

Ng, Y. S.↗
NASA NTRS1990

Helium and neon isotopes in deep Pacific Ocean sediments

Helium and neon concentration measurements, along with isotope ratio determinations, have been made for particles collected in the deep Pacific with a magnetic sled, and they are believed to be of extraterrestrial origin. Analyses were made for samples consisting of composites of many extremely fine particles and for several individual particles large enough to contain sufficient gas for analysis but small enough to escape melting in their passage through the atmosphere. Step-heating was employed to extract the gas. Cosmic-ray spallation products or solar-wind helium and neon, if present, were not abundant enough to account for the isotopic compositions measured. In the case of the samples of magnetic fines, the low temperature extractions provided elemental and isotopic ratios in the general range found for the primordial gas in carbonaceous chondrites and gas-rich meteorites. The isotopic ratios found in the high temperature extractions suggest the presence of solar-flare helium and neon.

Nier, A. O.↗
NASA NTRS1990

Thermal performance of a five year lifetime superfluid helium dewar for SIRTF

The Space Infrared Telescope Facility (SIRTF) is a 1 m class cryogenically cooled observatory for infrared astronomy. The SIRTF cryogenic system has to satisfy the five year mission lifetime requirement as well as to provide sufficient cooling for the science instruments and optical system. A 4000 cu dm superfluid helium dewar has been selected for the current baseline cryogenic system which represents the largest superfluid helium dewar proposed to date for a long lifetime space-borne application. This paper discusses the design and predicted performance of the current cryogenic system, as well as its comparison with IRAS and other space-borne superfluid helium dewars currently under development.

Lee, J. H.↗
NASA NTRS1990

Moments of dipole oscillator-strength distribution for the helium sequence

The moments S(mu) for mu at least -6 but no more than 2 and L(mu) for mu = 0, 1, and 2 are calculated for the helium sequence for atomic numbers (Z) up to 30 under a screened hydrogenic model. The model describes the atom by single-particle hydrogenic wave functions and treats the initial and the final state as characterized by two different effective charge parameters Zi and Zf, respectively. The differential oscillator strength of the screened hydrogenic model is asymptotically expanded. Assuming the value of 287.6 for the coefficient of the term epsilon to the -7/2 for helium atoms, the parameter Zf is determined for the helium sequence.

Khan, F.↗
NASA NTRS1991

Helium abundances on the moon: Assumptions and estimates

Nuclear energy is a highly desirable source of energy, and He-3 is the most prized of the fusion reactants. As the Wisconsin Group has emphasized, He-3 may be the only true economic ore on the Moon. The lack of a shielding atmosphere on the Moon permits solar-wind alpha particles to impinge upon the lunar regolith and become implanted into the various solid components. In particular, large quantities of helium (5 to 50 ppm) are presented. The measured parameter of I(sub s)/FeO, a direct indicator of maturity and exposure age, can be used as a first approximation to predict the abundances of many solar-wind components in the soils. However, because ilmenite has a much higher retentivity for helium than the other phases, the TiO2 contents of the soils are better indicators of helium contents (Taylor, Space 90). High-Ti mare bassalt regions, such as at the Apollo 17 locale, appear to be the best areas for He mining (15 to 50 ppm He(sub T)), versus 3 to 9 ppm in the Highlands. However, the relationships between I(sub s)/FeO, TiO2 and He-3 contents are complicated - e.g., many of the most He-rich soils are immature to submature. The amount of He-3 in the regolith of the moon is estimated at 220,000 tons in the outer 2 m of the Maria.

Taylor, Lawrence A.↗
NASA NTRS1990

Superfluid and normal fluid helium II in a rotating tank under low and microgravity environments

The physical behavior of superfluid and normal fluid helium II is discussed. It is shown that the centrifugal force due to a rotating container can never separate the superfluid from the normal fluid helium in a large rotating cylindrical spacecraft. A classical approach is appropriate for treating superfluid and normal fluid cryogenic helium as a mixture without separation in a large rotating cylinder in a spacecraft propellant systems.

Hung, R. J.↗
NASA NTRS1989

Superfluid Helium On-Orbit Transfer (SHOOT) operations

The in-flight tests and the operational sequences of the Superfluid Helium On-Orbit Transfer (SHOOT) experiment are outlined. These tests include the transfer of superfluid helium at a variety of rates, the transfer into cold and warm receivers, the operation of an extravehicular activity coupling, and tests of a liquid acquisition device. A variety of different types of instrumentation will be required for these tests. These include pressure sensors and liquid flow meters that must operate in liquid helium, accurate thermometry, two types of quantity gauges, and liquid-vapor sensors.

Kittel, P.↗
NASA NTRS1989

Fluid acquisition system for superfluid helium

To enable the resupply of liquid helium users on orbit, the technology for helium transfer in space is being developed. A key element of the resupply process is the fluid acquisition in the supply dewar; feeding the liquid to the pump. For the Superfluid Helium On-Orbit Transfer (SHOOT) flight experiment a number of fluid acquisition techniques have been examined. Subscale tests performed in one gravity on two of the candidates are described. The hope is that these results may be scaled to much larger systems in low gravity. These two types are a screened channel device and a capillary device. Flow rates versus negative head height are presented. Difficulties with each device and the test environment are explained. Some of the measurements to be made on orbit on the fluid acquisition for SHOOT are described.

Dipirro, M. J.↗
NASA NTRS1991

Measurement of cosmic-ray proton and helium spectra during the 1987 solar minimum

The differential cosmic-ray proton and helium spectra have been measured during the 1987 solar minimum using a balloon-borne superconducting magnetic spectrometer launched from Prince Albert, Canada. The changing geomagnetic cutoff along the balloon trajectory was observed in the low-energy proton data to be about 25 percent below the nominal calculated values. The absolute particle fluxes were approximately equal to the highest fluxes observed at the previous solar minimum in 1977. Above 10 GV the observed spectra are represented by a power law in rigidity with spectral indices of 2.74 + or - 0.02 for protons and 2.68 + or - 0.03 for helium. The measurements above 200 MeV per nucleon are consistent with rigidity power-law interstellar spectra modulated with the solar modulation parameter phi = 500 MV. The energy dependence of the proton-to-helium ratio is consistent with rigidity power-law injection spectra and rigidity-dependent propagation without reacceleration.

Seo, E. S.↗
NASA NTRS1991

Performance of all-metal demountable cryogenic seals at superfluid helium temperatures

Two all-metal demountable cryogenic seals with an outside diameter of 36.6 mm, inside diameter of 27.2 mm and thickness of 0.51 mm were leak-tested at room temperature (300 K), liquid nitrogen temperature (21 cycles at 77 K), liquid helium temperature (nine cycles at 4.2K) and superfluid helium temperature (four cycles at 1.6 K). Each seal was mounted and demounted for 13 cycles. Thickness measurements at 90 deg intervals along the circumference showed a maximum seal compression of 0.038 mm. Leak rate measurements at all temperatures showed no detectable leak above the helium background level, typically 0.1 x 10 exp -9 std cu cm/s, during testing.

Salerno, L. J.↗
NASA NTRS1992

Electron-impact excitation of the n 1P levels of helium - Theory and experiment

New experimental electron-energy-loss data have been used to extract differential and integral cross sections for excitation of the 2 1P level, and for the overlapping (3 1P, 3 1D, 3 3D) levels of helium, at 30-, 50-, and 100-eV incident electron energies. First-order many-body theory (FOMBT) has been used to calculate the differential and integral cross sections for excitation of the n 1P (n = 2,...,6) levels of helium by electron impact, for incident electron energies from threshold to 500 eV. Detailed comparisons between these two new sets of data are made as well as comparisons with appropriate published experimental and theoretical results. A simple scaling relationship is derived from the FOMBT results for n = 2,...,6 that provides differential and integral cross sections for all symmetry final levels of helium with n = 6 or greater.

Cartwright, David C.↗
NASA NTRS1992

Analysis of the Cosmic Background Explorer (COBE) helium venting through effects on the orbit

An analysis is described which shows that a helium venting thrust level that is 60 percent higher than the predicted 2.3 millinewtons may be the cause of the unusually high premission-analysis and on-orbit support orbital decay rates and drag correction factors found for the Cosmic Background Explorer satellite. Depletion of the helium allowed a determination of the average mass flow rate and specific impulse. The average mass flow rate of 3.3 mg/sec confirmed the thrust levels indicated by the venting thrust analysis. Liquid helium is used for cooling instruments aboard the satellite.

Cox, C. M.↗
NASA NTRS1992

Ages of globular clusters and helium diffusion

Evolutionary tracks have been calculated with alpha-enhanced compositions which cover the entire globular cluster metallicity range and have constructed isochrones which include the effects of microscopic diffusion of helium. The turnoff magnitudes from the isochrones were combined with the theoretical RR Lyrae magnitudes from Lee to determine the ages of 32 Galactic globular clusters using the magnitude difference between the turnoff and horizontal branch. It is found that including the effects of helium diffusion has a negligible effect on the derived ages of globular clusters. Regardless of the inclusion of helium diffusion, a significant age spread of 5 Gyr among the globular clusters is found. The oldest globular clusters studied here are 17 +/- 2 Gyr old.

Chaboyer, Brian↗
NASA NTRS1991

Welding of the superfluid helium on-orbit transfer (SHOOT) Flight Demonstration tanks and hardware

The SHOOT Flight Demonstration will provide the technology necessary to resupply superfluid helium in space. The helium dewars in SHOOT have unique requirements, including no detectable leaks, demonstrated structural integrity to allow flight aboard the Space Shuttle, and low parasitic heat leak. The requirements, materials, and processes used to fabricate the two superfluid helium dewars, particularly as they apply to welding, are presented.

Mcclure, Sidney↗
NASA NTRS1988

Measurement of C-12, O-16, and Fe-56 charge changing cross sections in helium at high energy, comparison with cross sections in hydrogen, and application to cosmic-ray propagation

We present measurements of the spallation cross sections of carbon, oxygen, and iron in helium and hydrogen, at beam energies from 540 to 1600 MeV/nucleon, performed by exposing liquid helium, CH2, and C targets. Charge changing cross sections are reported for fragments down to Ne for Fe + alpha and Fe + p reactions, and down to B for O + alpha, O + p, C + alpha, and C + p reactions. Alpha- to p-induced cross section ratios (sigma(sub alpha)/sigma(sub p)) are determined at the same energy per nucleon. From these measurements an empirical formula for the (sigma(sub alpha)/sigma(sub p)) ratios is derived and is found in good agreement with available isotopic cross sections data from radioactivity and radiochemical techniques. These results are applied to the propagation of heavy charged cosmic rays in an interstellar medium with a helium to hydrogen abundance ratio of 0.10. It is shown that the Sc-Mn/Fe ratio prediction is decreased relative to the B/C ratio when compared to propagation calculations in a pure hydrogen interstellar medium.

Ferrando, P.↗
NASA NTRS1993

Limits to the primordial helium abundance in the baryon-inhomogeneous big bang

The parameter space for baryon inhomogeneous big bang models is explored with the goal of determining the minimum helium abundance obtainable in such models while still satisfying the other light-element constraints. We find that the constraint of (D + He-3)/H less than 10 exp -4 restricts the primordial helium mass fraction from baryon-inhomogeneous big bang models to be greater than 0.231 even for a scenario which optimizes the effects of the inhomogeneities and destroys the excess lithium production. Thus, this modification to the standard big bang as well as the standard homogeneous big bang model itself would be falsifiable by observation if the primordial He-4 abundance were observed to be less than 0.231. Furthermore, a present upper limit to the observed helium mass fraction of Y(obs)(p) less than 0.24 implies that the maximum baryon-to-photon ratio allowable in the inhomogeneous models corresponds to eta less than 2.3 x 10 exp -9 (omega(b) h-squared less than 0.088) even if all conditions are optimized.

Mathews, G. J.↗
NASA NTRS1937

Some effects of argon and helium upon explosions of carbon monoxide and oxygen

Report presents the results of an investigation conducted to study the effects of the inert gases, argon and helium, upon flame speed and expansion ratio in exploding mixtures of carbon monoxide, oxygen and water.For the particular gas mixtures investigated the results show that: (1) With the possible exception of helium in small amounts the addition of inert gas always produces decreased flame speed and expansion ratio; (2) like volumes of argon and helium have very different effects upon flame speed but practically the same effect upon expansion ratio; and (3) the difference in the effect of these two gases upon speed is independent of the ratio of carbon monoxide to oxygen. A discussion of some possible modes by which inert gases may produce the observed effects is included.

Fiock, Ernst F↗