Special Coverage

Supercomputer Cooling System Uses Refrigerant to Replace Water
Computer Chips Calculate and Store in an Integrated Unit
Electron-to-Photon Communication for Quantum Computing
Mechanoresponsive Healing Polymers
Variable Permeability Magnetometer Systems and Methods for Aerospace Applications
Evaluation Standard for Robotic Research
Small Robot Has Outstanding Vertical Agility
Smart Optical Material Characterization System and Method
Lightweight, Flexible Thermal Protection System for Fire Protection

Low-Power, Zero-Vibration Sorption Coolers

A report discusses three designs of proposed low-power, zero-vibration coolers for infrared instruments planned to be flown aboard spacecraft to perform astrophysical observations far from Earth. The designs take advantage of the radiative precooling available in the projected deep-space operational environments: such precooling makes it possible to reach radiator temperatures as low as tens of kelvins. The working fluids would be helium and hydrogen, and vibration would be eliminated by the choice of thermally cycled gas-sorption (hydrogen/metal hydride and helium/charcoal) units instead of mechanical compressors.

Posted in: Briefs, TSP, Physical Sciences, Cooling, Test equipment and instrumentation, Spacecraft


Inhibited Carrier Transfer in Ensembles of Quantum Dots

A report presents an experimental study of time-resolved, temperature-dependent photoluminescence in InxGa1–x As/GaAs specimens containing In0.6Ga0.4As quantum dots (QDs) distributed at several different areal densities on a GaAs surface and capped with GaAs. The specimens were fabricated by metal-organic vapor deposition of InxGa1–x As on slightly misoriented, semi-insulating GaAs(100) substrates. At high areal densities, the intensities of photoluminescence from the QDs were found to exhibit Arrhenius temperature dependence, attributed to thermal emission of charge carriers and recapture of the charge carriers into neighboring QDs. At low densities, it was found that the temperature dependence is more complex, the thermal transfer of charge carriers between neighboring QDs plays no significant role in the temperature dependence, and the efficiency of transfer of charge carriers into isolated QDs is limited by the rate of carrier transport in InxGa1–x As wetting layers.

Posted in: Briefs, TSP, Physical Sciences, Research and development


Area Production in Supercritical, Transitional Mixing Layers

This paper presents a study of area production in mixing layers undergoing transition to turbulence. These layers evolve from the mixing of two initially segregated counterflowing streams under supercritical conditions. The study may contribute to development of means to control area production in order to increase disintegration of fluids and enhance combustion in diesel, gas turbine, and liquid rocket engines. As used here, “area production” signifies the fractional rate of change of surface area oriented perpendicular to the mass-fraction gradient in a mixing layer. In the study, a database of transitional states obtained from direct numerical simulations of temporal three-dimensional supercritical mixing layers for heptane/nitrogen and oxygen/hydrogen systems was analyzed. A few of the many conclusions drawn from the analysis are that area production is determined more by strain than by compressibility; area is produced by strain and convective effects; area is destroyed by species mass flux, rotational effects, and pressure gradients; area can be either produced or destroyed by pressure gradients; and effects of viscosity on area production are negligible. Effects of departure from perfect-gas and ideal-mixture behavior were found to be important. Smaller-wavelength initial perturbations were found to lead to greater area production: this observation could be a guide to initial development of control of area production.

Posted in: Briefs, TSP, Physical Sciences, Engines, Turbulence


Paraffin-Actuated Heat Switch for Mars Surface Applications

Missions to the surface of Mars pose unique thermal-control challenges to rover and lander systems. With diurnal temperature changes greater than 100 °C, the presence of a Mars atmosphere, and limited power for night-time heating, the thermal-control engineer is faced with a fundamental problem: how to successfully keep components above their survival or operating temperatures at night while managing higher environmental temperatures and dissipation rates during the day. A report describes such a paraffin-actuated heat switch as part of the thermal-control system for a robotic exploratory vehicle on Mars. Over a predetermined temperature range, the switch heat conductance varies by nearly two orders of magnitude to regulate temperatures. The actuation of the heat switch is entirely mechanical and autonomous, relying on the temperature based expansion and contraction of paraffin contained in a seal boot.

Posted in: Briefs, TSP, Physical Sciences, Switches, Thermal management


Making Three-Dimensional Holograms Visible From All Sides

Three-dimensional virtual reality displays could be viewed without visual aids.A technique for projecting holographic images to make both still and moving three-dimensional displays is undergoing development. Unlike older techniques based on stereoscopy to give the appearance of three-dimensionality, the developmental technique would not involve the use of polarizing goggles, goggles equipped with miniature video cameras, or other visual aids. Unlike in holographic display as practiced until now, visibility of the image would not be restricted to a narrow range of directions about a specified line of sight to a holographic projection plate. Instead, the image would be visible from any side or from the top; that is, from any position with a clear line of sight to the projection apparatus. In other words, the display could be viewed as though it were an ordinary three-dimensional object. The technique has obvious potential value for the entertainment industry, and for military uses like displaying battlefield scenes overlaid on three-dimensional terrain maps.

Posted in: Briefs, TSP, Physical Sciences, Imaging and visualization, Displays, Performance upgrades


Electrometer for Triboelectric Evaluation of Materials

Some materials should be distinguishable by their triboelectric responses.An electrometer developed for measuring the triboelectric responses of soils on Mars is also potentially useful on Earth for identifying some materials via their triboelectric responses. In operation, an array of triboelectric sensors is rubbed against the material of interest for a predetermined distance, then withdrawn from the material of interest at a predetermined speed. During this operation, the electrometer circuitry measures the time-varying sensor output voltages, which are proportional to the electric charges induced on the sensors by the rubbing. The resulting voltage-vs.-time data constitute the desired triboelectric-response data. If materials that one seeks to identify or to distinguish from each other exhibit unique, known triboelectric responses that have been catalogued, then a sample of unknown material can be identified by seeking a match between its triboelectric response and one of the catalogued responses.

Posted in: Briefs, TSP, Physical Sciences, Sensors and actuators, Soils, Materials identification, Test equipment and instrumentation


Characteristics of Supercritical Transitional Mixing Layers

This report describes a study of threedimensional, temporal mixing layers between different fluids initially flowing at different velocities under supercritical conditions. The study involved direct numerical simulations by use of a validated mathematical model of high-pressurefluid behavior that has been described in a number of prior NASA Tech Briefs articles. In some cases, the fluids were heptane and nitrogen; in other cases, they were hydrogen and oxygen. In all these simulations, the mixing layers underwent transition to turbulence.

Posted in: Briefs, TSP, Physical Sciences, Computational fluid dynamics, Mathematical models


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