Publications

This paper describes experiments on the wettability of tin on oxygen free, high conductivity (OFHC) copper using a ″point source″ ultrasonic horn. Ultrasonics are used on such metals as aluminum or stainless steel which are difficult to wet without the use of very strong corrosives. These experiments explore the behavior of acoustic energy transmission in the horn-solder-substrate systems indicated by the solder film generated and explore the use of ultrasonics in actual electronic systems component fabrication and assembly processes.

An evaluation of substitutes for tin-lead alloy solders is discribed. The first part of the evaluation studies the wettability of tin-based, lead free solders. The second part evaluates the solderability. The solders evaluated were commercially available.

This paper presents the results of a set of structural analyses performed to investigate the effects of internal gas generation on the extension of pre-existing fractures around disposal rooms at the Waste Isolation Pilot Plant. The response of a room and its contents is computed for this scenario to establish the condition of the room at any point in time. The development of the capability to perform these analyses represents an additional step in the development of an overall model for the disposal room.

Acid vapors have been used to fluxlessly reduce metal oxides and enhance wetting of solder on metallizations. Dilute solutions of hydrogen, acetic acid and formic acid in an inert carrier gas of nitrogen or argon were used with the sessile drop technique for 60Sn-40Pb solder on Cu and Au/Ni metallizations. The time to reduce metal oxides and degree of wetting as a function of acid vapor concentration were characterized. Acetic and formic acids reduce the surface metal oxides sufficiently to form metallurgically sound solder joints. Hydrogen did not reduce oxides rapidly enough at 220°C to be suitable for soldering applications. The optimum conditions for oxide reduction with formic acid was with an acid vapor concentration in nitrogen carrier gas of 4% for Cu metallizations and 1.6% on Au/Ni. The acetic acid vapor concentration, also in nitrogen, was optimized at 1.5% for both metallizations. Above a vapor concentration of 1.5%, the acetic acid combined with the bare metal to form acetates which increased the wetting time. These results indicate that acid vapor fluxless soldering is a viable alternative to traditional flux soldering.

The measurement of surface pressures on a body which is submerged in flowing water involves several problems which are not encountered when the test medium is air. Many of these problems exist even if the water velocity is low, and become more severe at higher velocitics (45-65 ft/sec) where the surface pressure may be low enough for cavitation to occur. Problem areas which are discussed include:hydrostatic pressure, surface tension, orifice errors, thermal effects on surface-mounted transducers, electrical fields, two-phase phenomena and air content.

Deconing controllers are developed for a spinning spacecraft, where the control mechanism is that of axial or radial moving masses that are used to produce intentional, transient principal axis misalignments. A single mass axial controller is used to motivate the concept, and then axial and radial dual mass controllers are described. The two mass problem is of particular interest since spacecraft imbalances can be simultaneously removed with the same control logic. Each controller is tested via simulation for its ability to eliminate existing coning motion for a range of spin rates. Both controllers are developed via a linear-quadratic-regulator synthesis procedure, which is motivated by their multi-input/multi-output nature. The dynamic coupling in the radial two mass control problem introduces some particularly interesting design complications.

Many complex physical processes are modeled by coupled systems of partial differential equations (PDEs). Often, the numerical approximation of these PDEs requires the solution of large sparse nonsymmetric systems of equations. In this paper we compare the parallel performance of a number of preconditioned Krylov subspace methods on a large-scale MIMD machine. These methods are among the most robust and efficient iterative algorithms for the solution of large sparse linear systems. They are easy to implement on various architectures and work well on a wide variety of important problems. In this comparison we focus on the parallel issues associated with both local preconditioners (those that combine information from the entire domain). The various preconditioners are applied to a variety of PDE problems within the GMRES, CCGS, BiCGSTAB, and QMRCGS methods. Conclusions are drawn on the effectiveness of the different schemes based on results obtained from a 1024 processor a nCUBE 2 hypercube.

A two-stage self-organizing neural network architecture has been applied to object recognition in Synthetic Aperture Radar imagery. The first stage performs feature extraction and implements a two-layer Neocognitron. The resulting feature vectors are presented to the second stage, an ART 2-A classifier network, which clusters the features into multiple target categories. Training is performed off-line in two steps. First, the Neocognitron self-organizes in response to repeated presentations of an object to recognize. During this training process, discovered features and the mechanisms for their extraction are captured in the excitatory weight patterns. In the second step, Neocognitron learning is inhibited and the ART 2-A classifier forms categories in response to the feature vectors generated by additional presentations of the object to recognize. Finally, all training is inhibited and the system tested against a variety of objects and background clutter. In this paper we report the results of our initial experiments. The architecture recognizes a simulated tank vehicle at arbitrary azimuthal orientations at a single depression angle while rejecting clutter and other object returns. The neural architecture has achieved excellent classification performance using 20 clusters.

This paper presents results of a set of numerical experiments performed bo benchmark the Cell-Centered Implicit Continuous-fluid Eulerian (CCICE), and to determine their limitations as flow solvers for water entry and water exit simulations.

This paper will include a brief overview of the components of the QUICKSILVER suite and its current modeling capabilities. As time permits, results from sample applications will be shown, including time animations of simulation results.

Technologies that use carbon and mixed metal oxides as the electrode material have been pursued for the purpose of producing high-reliability double-layer capacitors (DLCs). The author demonstrates their environmental stability in temperature, shock, vibration, and linear acceleration. She reviews the available test data for both types of DLCs under these stress conditions. This study suggests that mixed metal oxides and carbon-based double-layer capacitors can survive robust environments if packaged properly, and that temperature decreases performance of double-layer capacitors.

We describe a simple engineering model applicable to stand-off “Whipple bumper” shields, which are used to protect space-based assets from impacts by orbital debris particles. The model provides a framework for analyzing: 1) the parameter limits governing the penetration and breakup or decomposition of the hypervelocity debris particle; 2) the behavior of the induced debris cloud, including its velocity and divergence; and 3) the design and optimization of the stand-off shield for a specific threat and level of protection required. The model is normalized to actual stand-off debris shield experiments and multi-dimensional numerical simulations at impact velocities of ~10 km/s. The subsequent analysis of a current space station shield design suggests that: 1) for acceptable levels of protection, stand-off shields can be significantly thinner than previously thought; and 2) with the proper balance between shield thickness and stand-off distance, the total shield mass can be reduced substantially.

A series of experiments has been performed on the Sandia Hypervelocity Launcher to determine the performance limits of conventional Whipple shields against representative 0.8 g aluminum orbital debris plate-like fragments with velocities of 7 and 10 km/s. Supporting diagnostics include flash X-rays, high speed photography and transient digitizers for timing correlation. Two Whipple shield designs were tested with either a 0.030 cm or a 0.127 cm thick front sheet and a 0.407 cm thick backsheet separated by 30.5 cm. These two designs bracket the ballistic penetration limit curve for protection against these debris simulants for 7 km/s impacts.

In this paper we take a new look at the tomographic formulation of spotlight mode synthetic aperture radar (SAR), so as to include the case of targets having three-dimensional structure. This bridges the work of David C. Munson and his colleagues, who first described SAR in terms of two-dimensional tomography, with Jack Walker`s original derivation of spotlight mode SAR imaging via Doppler analysis. The main result is to demonstrate that the demodulated radar return data from a spotlight mode collection represent a certain set of samples of the three-dimensional Fourier transform of the target reflectivity function, and to do so using tomographic principles instead of traditional Doppler arguments. We then show that the tomographic approach is useful in interpreting the two-dimensional SAR image of a three-dimensional scene. In particular, the well-known SAR imaging phenomenon commonly referred to as layover is easily explained in terms of tomographic projection. 4 refs.

Short communication.

The Sandia Airborne Computer (SANDAC) is a small, modular, high performance, multi-processor computer originally designed for aerospace applications. It can use a combination of Motorola 68020 and 68040 based processor modules along with AT&T DSP32C based signal processing modules. The system is designed to use up to 15 processors in almost any combination and a complete system can include up to 20 modules. Depending on the mix of processors, total computational throughput can range from 2.5 to greater than 225 million instructions per second (MIPS). The system is designed so that processors can access all resources in the machine and the inter-processor communication details are completely transparent to the software. In addition to processors, the system includes input/output, memory, and special function modules. Because of its ease of use, small size, durability, and configuration flexibility, SANDAC has been used on applications ranging from missile navigation, guidance and control systems to medical imaging systems. © 1992.

The unit cell shape of thick frequency selective surfaces, or dichroic plate, is dependent on its frequency requirements. One aperture shape may be chosen to give wider bandwidths, and another chosen for sharper frequency roll-off. This is analogous to circuits where the need for differing frequency response determines the circuit topology. Acting as spatial frequency filters, dichroics are a critical component in supporting the Deep Space Network (DSN) for spacecraft command a control up links as well as spacecraft down links. Currently these dichroic plates separate S-band at 2.0--232 GHz from X-band at 8.4--8.45 GHz. But new spacecraft communication requirements are also calling for an up link frequency at 7.165 GHz. In addition future spacecraft such as Craft/Casssini will require dichroics effectively separating K{sub a}-band frequencies in the 31--35 GHz range. The requirements for these surfaces are low transmission loss of < 0.1 dB at high power levels. Also is important to maintain a minimal relative phase shift between polarizations for circular polarization transmission. More current work has shown the successful demonstration of design techniques for straight, rectangular apertures at an incident angle of 30{degrees}. The plates are air-filled due to power dissipation and noise temperature considerations. Up-link frequency powers approach 100 kW making dielectrics undesirable. Here we address some of the cases in which the straight rectangular shape may have limited usefulness. For example, grating lobes become a consideration when the bandwidth required to include the new frequency of 7.165 GHz conflicts with the desired incident angle of 30{degrees}. For this case, the cross shape`s increased packing density and bandwidth could make it desirable. When a sharp frequency response is required to separate two closely space K{sub a}-band frequencies, the stepped rectangular aperture might be advantageous. 5 refs.

Short communication.

Several closed form trajectory solutions have been developed for low-thrust interplanetary flight and used with patched conies for analysis of combined propulsion systems. The solutions provide insight into alternative types of Mars missions, and show considerable mass savings for fast crewed missions with outbound trip times on the order of 90-100 days.

Nuclear Thermal Propulsion (NTP) has been identified as a critical technology in support of the NASA Space Exploration Initiative (SEI). In order to safely develop a reliable, reusable, long-lived flight engine, facilities are required that will support ground tests to qualify the nuclear rocket engine design. Initial nuclear fuel element testing will need to be performed in a facility that supports a realistic thermal and neutronic environment in which the fuel elements will operate at a fraction of the power of a flight weight reactor/engine. Ground testing of nuclear rocket engines is not new. New restrictions mandated by the National Environmental Protection Act of 1970, however, now require major changes to be made in the manner in which reactor engines are now tested. These new restrictions now preclude the types of nuclear rocket engine tests that were performed in the past from being done today, A major attribute of a safely operating ground test facility is its ability to prevent fission products from being released in appreciable amounts to the environment. Details of the intricacies and complications involved with the design of a fuel element ground test facility are presented in this report with a strong emphasis on safety and economy.

Several new neutron cross section libraries, such as ENDF/ B-VI and IRDF-90, have recently been made available to the dosimetry community. Recommendations are made for the source selection of reaction cross sections that vary significantly among the libraries. In general, integral parameters from spectra obtained from unfold/adjustment codes using the new cross sections will not significantly change. A 61-reaction compendium of dosimetry cross sections drawn from existing evaluations has been compiled for use at the Sandia National Laboratories Radiation Metrology Laboratory. This dosimetry library (SNLRML) is recommended for use in spectrum determination with unfold/ adjustment methods. © 1992 IEEE

A rapid deployment access delay system (RAPADS) has been designed to provide high security protection of valued assets. The system or vault is transportable, modular, and utilizes a pin connection design. Individual panels are attached together to construct the vault. The pin connection allows for quick assembly and disassembly, and makes it possible to construct vaults of various sizes to meet a specific application. Because of the unique pin connection and overlapping joint arrangement, a sequence of assembly steps are required to assembly the vault. As a result, once the door is closed and locked, all pin connections are concealed and inaccessible. This provides a high level of protection in that no one panel or connection is vulnerable. This paper presents the RAPADS concept, design, fabrication, and construction.

There is an emerging interest in using thermal IR to automatically detect human intruders over wide areas. Such a capability could provide early warning beyond the perimeter at fixed sites, and could be used for portable security around mobile military assets. Sandia National Laboratories has been working on automatic detection systems based on the thermal contrast and motion of human intruders for several years, and has found that detection is sometimes difficult, depending on solar and other environmental conditions. Solar heating can dominate human thermal radiation by 100 fold, and dynamic background temperature changes can limit detector sensitivity. This paper explains those conditions and energy transfer mechanisms that lead to difficult thermal detection. We will not cover those adverse conditions that are more widely understood and previously reported on, such as fog, smoke, rain and falling snow. This work was sponsored by the Defense Nuclear Agency.

In the wavenumber-domain method of SAR imaging, frequencydomain radar data are used to reconstruct a portion of the 2-D Fourier transform of the scene, which is then inverted to create the image. The method suffers no inherent limits on aperture length or scene size. This paper extends the concept to the case where the synthetic aperture is not a straight line and the samples are unevenly spaced. An accumulation formula for wavenumberdomain reconstruction is derived and shown to be equivalent to earlier algorithms in the uniform-aperture case. It is then shown how data with three-dimensional irregularity in the aperture can be processed using height correction and mapping into the slant plane.

CIRCE2 is a cone-optics computer code for determining the flux distribution and total incident power upon a receiver, given concentrator and receiver geometries, sunshape (angular distribution of incident rays from the sun-disk), and concentrator imperfections such as surface roughness and random deviation in slope. Statistical methods are used to evaluate the directional distribution of reflected rays from any given point on the concentrator, whence the contribution to any point on the target can be obtained. DEKGEN2 is an interactive preprocessor which facilitates specification of geometry, sun models, and error distributions. The CIRCE2/DEKGEN2 package equips solar energy engineers with a quick, user-friendly design and analysis tool for study/optimization of dish-type distributed receiver systems. The package exhibits convenient features for analysis of 'conventional' concentrators, and has the generality required to investigate complex and unconventional designs. Among the more advanced features are the ability to model dish or faceted concentrators and stretched-membrane reflectors, and to analyze 3-D flux distributions on internal or external receivers with 3-D geometries. Facets of rectangular, triangular, or circular projected shape, with profiles of parabolic, spherical, flat, or custom curvature can be handled. Provisions for shading, blocking, and aperture specification are also included. This paper outlines the features and capabilities of the new package, as well as the theory and numerical models employed in CIRCE2.