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At least 271 records · Page 15

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NASA NTRS1972

The design of asymmetric 4 pi shields for space reactors

A one dimensional shield optimization program based on the method of discrete ordinates has been developed and is used to determine material thicknesses used in asymmetric 4 pion shields for space power reactors. The two dimensional discrete ordinates program DOT is used to check the design, and the information generated in the DOT calculation is used as a guide in shaping the shield which may be considered a first step in two dimensional shield optimization.

Engle, W. W., Jr.↗

NASA NTRS1972

Space reactor shielding fabrication

The fabrication of space reactor neutron shielding by a melting and casting process utilizing lithium hydride is described. The first neutron shield fabricated is a large pancake shape 86 inches in diameter, containing about 1700 pounds of lithium hydride. This shield, fabricated by the unique melting and casting process, is the largest lithium hydride shield ever built.

Welch, F. H.↗

NASA NTRS1972

Application of transport techniques to the analysis of NERVA shadow shields

A radiation shield internal to the NERVA nuclear rocket reactor required to limit the neutron and photon radiation levels at critical components located external to the reactor was evaluated. Two significantly different shield mockups were analyzed: BATH, a composite mixture of boron carbide, aluminum and titanium hydride, and a borated steel-liquid hydrogen system. Based on the comparisons between experimental and calculated neutron and photon radiation levels, the following conclusions were noted: (1) The ability of two-dimensional discrete ordinates code to predict the radiation levels internal to and at the surface of the shield mockups was clearly demonstrated. (2) Internal to the BATH shield mockups, the one-dimensional technique predicted the axial variation of neutron fluxes and photon dose rates; however, the magnitude of the neutron fluxes was about a factor of 1.8 lower than the two-dimensional analysis and the photon dose rate was a factor of 1.3 lower.

Capo, M. A.↗

NASA NTRS1972

The DOPEX Code: an Application of the Method of Steepest Descent to Laminated-Shield-Weight Optimization with Several Constraints

A two- or three-constraint, two-dimensional radiation shield weight optimization procedure and a computer program, DOPEX, is described. The DOPEX code uses the steepest descent method to alter a set of initial (input) thicknesses for a shield configuration to achieve a minimum weight while simultaneously satisfying dose constaints. The code assumes an exponential dose-shield thickness relation with parameters specified by the user. The code also assumes that dose rates in each principal direction are dependent only on thicknesses in that direction. Code input instructions, FORTRAN 4 listing, and a sample problem are given. Typical computer time required to optimize a seven-layer shield is about 0.1 minute on an IBM 7094-2.

Lahti, G. P.↗

NASA NTRS1971

Effect of angular quadrature on results of two-dimensional space power reactor shield calculations.

Study of the effect of angular quadrature on the results of two-dimensional transport calculations made to evaluate radiation environment in the vicinity of a shielded space power reactor. The application considered is for a manned space-station; crew shielding must be provided within a 25 deg cone angle. The asymmetric shield layout is evaluated using the two-dimensional transport code DOT. Within the shield, an S sub 6 angular quadrature yields satisfactory results. For regions where the ray effect may occur it is not apparent that an S sub 10 quadrature is adequate. Anomalies in results due to transport of particles in a void and necessary geometric approximations were observed.

Connolley, D. J.↗

NASA NTRS1973

Heat shielding for Venus entry probes.

Two contrasted approaches to ablative thermal protection of Venus entry probes are presented - a typical carbonaceous charring ablator and a dielectric reflective ablator. The interesting observation in this study is that mass loss is not the controlling variable in heat-shield sizing. A heat-soak problem determines the carbon phenolic sizing. For Teflon, the material thickness required to accomplish reflection is the sizing factor. The total heat-shield weight required to handle either steep or shallow entry is computed to be 13% less for a Teflon shield (if at least 3.2 mm are required for reflection) than for a charring ablator shield. If an efficient reflective backing is used with Teflon, the thickness can be reduced 1.0 mm and the computed weight is 31% less for Teflon than for the charring ablator. Such weight reductions may significantly increase the science payload weight of the miniprobes.

Peterson, D. L.↗

NASA NTRS1974

On thermal stress failure of the SNAP-19A RTG heat shield

Results of a study on thermal stress problems in an amorphous graphite heat shield that is part of the launch-abort protect system for the SNAP-19A radio-isotope thermoelectric generators (RTG) that will be used on the Viking Mars Lander are presended. The first result is from a thermal stress analysis of a full-scale RTG heat source that failed to survive a suborbital entry flight test, possibly due to thermal stress failure. It was calculated that the maximum stress in the heat shield was only 50 percent of the ultimate strength of the material. To provide information on the stress failure criterion used for this calculation, some heat shield specimens were fractured under abort entry conditions in a plasma arc facility. It was found that in regions free of stress concentrations the POCO graphite heat shield material did fracture when the local stress reached the ultimate uniaxial stress of the material.

Pitts, W. C.↗

NASA NTRS1974

Lithium hydride - A space age shielding material

Men and materials performing in the environment of an operating nuclear reactor require shielding from the escaping neutron particles and gamma rays. For efficient shielding from gamma rays, dense, high atomic number elements such as iron, lead, or tungsten are required, whereas light, low atomic number elements such as hydrogen, lithium, or beryllium are required for efficient neutron shielding. The use of lithium hydride (LiH) as a highly efficient neutron-shielding material is considered. It contains, combined into a single, stable compound, two of the elements most effective in attenuating and absorbing neutrons.

Welch, F. H.↗

NASA NTRS1975

Influence of multitube mixer nozzle geometry on CTOL-OTW jet noise shielding

Acoustic shielding benefits for CTOL over-the-wing (OTW) applications were obtained experimentally with various multitube nozzles using a simple board to represent a wing. Eight nozzles consisting of three to thirteen 2.36-cm diameter tubes were tested. The nozzles included single and double rings of tubes. Shielding surface lengths of 15.0 to 54.4 cm were used with each nozzle. Far-field noise data were obtained at 90 deg from the jet axis and with a nominal jet exhaust velocity of 200 m/sec. The jet noise shielding benefits for the nozzles with double rows of tubes, in terms of sound pressure level spectra, are correlated successfully as a function of an earlier developed parameter for nozzles with a single ring of tubes that includes consideration of the number of tubes and the local peak velocity in the flow field at the trailing edge of the shielding surface.

Vonglahn, U.↗

NASA NTRS1974

Carbon phenolic heat shields for Jupiter/Saturn/Uranus entry probes

Carbon phenolic heat shield technology is reviewed. Heat shield results from the outer planetary probe mission studies are summarized along with results of plasma jet testing of carbon phenolic conducted in a ten megawatt facility. Missile flight data is applied to planetary entry conditions. A carbon phenolic heat shield material is utilized and tailored to accommodate each of the probe missions. An integral heat shield approach is selected over in order to eliminate a high temperature interface problem and permit direct bonding of the carbon phenolic to the structural honeycomb sandwich. The sandwich is filled with a very fine powder to minimize degradation of its insulation properties by the high conductive hydrogen/helium gases during the long atmospheric descent phase.

Mezines, S.↗

NASA NTRS1975

Performance of solar shields

The loss of the micrometeoroid shield from the Orbital Workshop section of Skylab I, about 63 seconds after lift-off, proved to be the harbinger of a prodigious effort to quickly develop a workable substitute for the carefully tailored passive portion of the thermal-control system. The paper describes the intensive ten-day around-the-clock effort in which numerous potential thermal-shield materials were assessed, and during which period ten specific shield designs were developed and carried through various stages of development and test. Thermal-shield materials data are discussed, including optical, strength, fatigue, outgassing, tackiness, ultraviolet radiation, and material 'memory' properties.

Schwinghamer, R. J.↗

NASA NTRS1975

Influence of multitube mixer nozzle geometry on CTOL-OTW jet noise shielding

Von Glahn, U.↗

NASA NTRS1975

Electromagnetic fields and torque for a rotating gyroscope with a superconducting shield

In a proposed experiment, a measurement is to be made of the angular precession of a rotating superconducting gyroscope for the purpose of testing different general-relativity theories. For various reasons having to do with the design of the experiment, the superconducting shield surrounding the gyroscope is not spherically symmetric and produces a torque. There are two distinct features of the shield which lead to a torque on the gyroscope. First, its shape is a sphere intersected by a plane. If the angular momentum of the gyroscope is not parallel to the rotational symmetry axis of the shield, there is a torque which is calculated. Second, there are small holes in the spherical portion of the shield. The earth's field can penetrate through these holes and give an additional torque which is also calculated. In the actual experiment, these torques must be accurately known or made very small in order to obtain meaningful results. The present calculation is sufficiently general for application over a wide range of experimental design parameters.

Ebner, C.↗

NASA NTRS1976

Evaluation of a method to shield a welding electron beam from magnetic interference

It is known that electron beams are easily deflected by magnetic and electrostatic fields. Therefore, to prevent weld defects, stray electromagnetic fields are avoided in electron beam welding chambers if at all possible. The successful results of tests conducted at MSFC to evaluate a simple magnetic shield made from steel tubing are reported. Tests indicate that this shield was up to 85 percent effective in reducing magnetic effects on the electron beam of a welding machine. In addition, residual magnetic fields within the shield were so nearly uniform that the net effect on the beam alignment was negligible. It is concluded that the shield, with the addition of a tungsten liner, could be used in production welding.

Wall, W. A.↗

NASA NTRS1976

Molecular shield - An orbiting low-density materials laboratory

Analysis of a molecular shield orbited at 200 km utilizes the kinetic theory of a drifting Maxwellian gas, applied to a hemispherical shell geometry containing internal sources. The molecular shield provides very low gas density conditions for materials experiments at low gravity, while the hemispherical geometry minimizes the internal surface/volume ratio. Deployment of the shield in orbit is described. Contributions to density by shield outgassing, by experiment outgassing, and by interaction with the orbiter are discussed separately. A jettisonable closure plate sealing the hemisphere minimizes any risk of experiment contamination during deployment.

Melfi, L. T., Jr.↗

NASA NTRS1977

Design, fabrication, and structural testing of a lightweight shadow shield for deep-space application

Two full-scale, lightweight, double-sheeted shadow shields were developed as the primary element of a deep-space thermal protection system for liquid-hydrogen propellant tankage. The thermal and mechanical considerations used in s, the method of fabrication, and the environmental testing results on a prototype shield are discussed. Testing consisted of a transient cooldown period, a prolonged cold soak, and a transient warmup. The mechanical and thermal analyses used in the shield design are sufficient to produce a lightweight rugged shadow shield assembly that is structurally adequate for its intended application.

Miao, D.↗

NASA NTRS1977

Accuracy of the Kirchoff formula in determining acoustic shielding with the use of a flat plate

It has been suggested that if jet engines of aircraft were placed at above the wing instead of below it, the wing would provide a partial shielding of the noise generated by the engines relative to observers on the ground. The shielding effects of an idealized three-dimensional barrier in the presence of an idealized engine noise source was predicted by the Kirchoff formula. Based on the good agreement between experimental measurements and the numerical results of the current study, it was concluded that the Kirchoff approximation provides a good qualitative estimate of the acoustic shielding of a point source by a rectangular flat plate for measurements taken in the far field of the flat plate at frequencies ranging from 1 kHz to 20 kHz. At frequencies greater than 4 kHz the Kirchoff approximation provides accurate quantitative predictions of acoustic shielding.

Gabrielsen, R. E.↗

NASA NTRS1979

Investigation of wing shielding effects on CTOL engine noise

A full scale engine wing shielding investigation was conducted at the Lewis Research Center using a 97,900-N (22,000 lb) thrust turbofan engine and a simulated wing section sized around a conventional-take-off type four-engine narrow body airplane. Sound data were obtained for the wing placed at seven positions in a plane parallel to the engine axis, and were compared to data obtained without the wing at both take off and approach power. In addition, the engine was operated with and without extensive acoustic treatment, including a sonic inlet in order to evaluate wing shielding effectiveness with a highly suppressed engine. The wing shielding effectiveness was also calibrated using a 3.8 cm diam air nozzle as a second source. Results indicated that even though about 10 dB broad band shielding was achieved, the equivalent flyover noise reduction was less than 3.0 EPNdB for most configurations.

Bloomer, H. E.↗