Electron beam charging of Space Shuttle thermal protection system tiles

Six space shuttle reusable surface insulation tiles were tested in the NASA Lewis Research Center's electron bombardment test facility. The 30-cm-square specimens were assembled by using the same materials and techniques used to apply the tiles to the space shuttle and were composed to 15-cm- and 20-cm-square tiles and pieces on 0.6-cm-thick aluminum substrates. There were two specimens of each of three thicknesses. One specimen of each thickness had gaps of less than 0.1 cm between tiles, and the other had gaps of approximately 0.15 cm. The specimens were exposed to monoenergetic electron beams (2 to 25 keV) with nominal fluxes of 0.1 and 1 nA/sq. cm. Tests were conducted with both grounded and floating substrates. The data presented include charging rates, equilibrium potentials, and substrate currents. There is evidence that discharging occurred

Testing of a spacecraft model in a combined environment simulator

A scale model of a satellite was tested in a large vacuum facility under electron bombardment and vacuum ultraviolet radiation to investigate the charging of dielectric materials on curved surfaces. The model was tested both stationary and rotating relative to the electron sources as well as grounded through one megohm and floating relative to the chamber. Surface potential measurements are presented and compared with the predictions of computer modelling of the stationary tests. Discharge activity observed during the stationary tests is discussed and signals from sensing devices located inside and outside of the model are presented

Charging of flexible solar array substrates in kilovolt electron beams

A series of survey tests were conducted to evaluate samples of flexible solar arrays. The samples used woven carbon fibers and/or coatings to increase the surface conductivity of the KAPTON substrate and thereby reduce surface charging. Four different samples were evaluated by exposing them to monoenergetic electron beams of 2 to 20 KeV at a current density of 1 nA sq cm. Simulated eclipse tests were also conducted. The results were as expected; the more continuous the conductive pattern, the lower the surface charging

Test program for transmitter experiment package and heat pipe system for the communications technology satellite

The test program is described for the 200 watt transmitter experiment package and the variable conductance heat pipe system which are components of the high-power transponder aboard the Communications Technology Satellite. The program includes qualification tests to demonstrate design adequacy, acceptance tests to expose latent defects in flight hardware, and development tests to integrate the components into the transponder system and to demonstrate compatibility

The Space Station Photovoltaic Panels Plasma Interaction Test Program: Test plan and results

The Plasma Interaction Test performed on two space station solar array panels is addressed. This includes a discussion of the test requirements, test plan, experimental set-up, and test results. It was found that parasitic current collection was insignificant (0.3 percent of the solar array delivered power). The measured arcing threshold ranged from -210 to -457 V with respect to the plasma potential. Furthermore, the dynamic response of the panels showed the panel time constant to range between 1 and 5 microsec, and the panel capacitance to be between .01 and .02 microF

Testing of typical spacecraft materials in a simulated substorm environment

The test specimens were spacecraft paints, silvered Teflon, thermal blankets, and solar array segments. The samples, ranging in size from 300 to 1000 sq cm were exposed to monoenergetic electron energies from 2 to 20 keV at a current density of 1 NA/sq cm. The samples generally behaved as capacitors with strong voltage gradient at their edges. The charging characteristics of the silvered Teflon, Kapton, and solar cell covers were controlled by the secondary emission characteristics. Insulators that did not discharge were the spacecraft paints and the quartz fiber cloth thermal blanket sample. All other samples did experience discharges when the surface voltage reached -8 to -16kV. The discharges were photographed. The breakdown voltage for each sample was determined and the average energy lost in the discharge was computed

Robotic Hand Controlled by Myoelectric Signals

In this project we developed a prosthetic hand that is controlled by myoelectric signals from the forearm. The forearm signals were filtered and amplified to be easier read by a microcontroller. The microcontroller then generated signals to control the mechanical hand based on these myoelectric input signals. This is done by the microcontroller increasing the pulse width sent to several servos making up the hand. The larger the pulse width the more flexed the hand becomes until it reaches the maximum programmed into it. In order to open the hand the high input to the microcontroller must be deactivated and then reactivated for the pulse width to begin decreasing. Several designs and products are currently available online as complicated limbs and hands for amputees, however many of these can be costly and heavy. This project has taken inspiration from these designs with the goal of reducing cost and weight without sacrificing functionality. One major advantage of our project is the dry reusable electrodes used to capture the myoelectric signals in the forearm. The silver coated cloth functions as an electrode with the help of a pushbutton from a tarp making kit. This electrode is reusable and remains in the same place in the sleeve it is sewn into for ease of use

Investigation of high voltage spacecraft system interactions with plasma environments

An experimental investigation was undertaken for insulator and conductor test surfaces biased up to + or - 1kV in a simulated low earth orbit charged particle environment. It was found that these interactions are controlled by the insulator surfaces surrounding the biased conductors. For positive applied voltages the electron current collection can be enhanced by the insulators. For negative applied voltages the insulator surface confines the voltage to the conductor region. Understanding these interactions and the technology to control their impact on system operation is essential to the design of solar cell arrays for ion drive propulsion applications that use direct drive power processing

A double-stranded secondary structure for hyaluronic acid in aqueous-organic solvent as revealed by vacuum ultraviolet circular dichroism

The association of hyaluronic acid (HA) into a double stranded structure has been investigated by circular dichroism (CD) spectroscopy into the vacuum ultraviolet region. The CD of HA changes dramatically, monitoring a cooperative transition as the dielectric constant of an aqueous solution is reduced by adding organic solvents. This structural transition results in a high intensity CD band at 190 nm, indicating an ordered structure in the mixed solvent. Heating hyaluronic acid in the mixed solvent also causes a cooperative transition reducing the CD to that found for the polymer in aqueous solution. The CD for the polymer in aqueous solution is not very different from some of the CD's for the constituent monomers, and the CD of the monomers is similar in both aqueous solution and mixed solvent. Furthermore, heating HA in aqueous solution results in small, noncooperative changes in the CD spectrum. The CD as dielectric constant is reduced exhibits isodichroic points, showing that there are two states. Singular value decomposition of the CD as a function of organic solvent has only two principal components, confirming that there are only two states. Oligomers of HA show a chain length effect with approximately nine disaccharides required for the structural transition as a function of organic solvent. This proves that the transition is indeed cooperative. The concentration dependence for the transition was investigated in detail for the 12 mer and 16 mer, and it was possible to prevent completely the transition in mixed solvent with sufficient dilution of the oligomer. The CD data in mixed solvent as a function of oligomer concentration were fit with various models for association of two and more strands. Simplex directed search methods were used to investigate the vector space of unknown model parameters, and two strand models were shown to consistently give a better fit. A cooperative two-strand zipper model which allows relative sliding of the chains gave the smallest error and produced the following thermodynamic parameters for the ordered structure: enthalpy of growth, -1000 ± 300 cal/mole; entropy of growth, -2.25 ± 1.30 eu/mole: enthalpy of initiation, -20,000 ± 3000 cal/mole; entropy of initiation, -71 ± 15 eu/mole. These results are consistent with a double-stranded and helical structure for hyaluronic acid in solutions of reduced dielectric constant

First results of material charging in the space environment

A satellite experiment, designed to measure potential charging of typical thermal control materials at near geosynchronous altitude, was flown as part of the SCATHA program. Direct observations of charging of typical satellite materials in a natural charging event ( 5 keV) are presented. The results show some features which differ significantly from previous laboratory simulations of the environment