Arc jet diagnostics tests

Two objectives were addressed during a 10 week 1988 NASA/ASEE summer faculty fellowship at the Johnson Space Center Atmospheric Reentry Materials Structures Evaluation Facility (ARMSEF). These objectives were the evaluation of mass spectrometry for the measurement of atomic and molecular species in an arc jet environment, and the determination of atomic recombination coefficients for reaction cured glass (RCG) coated high temperature surface insulation (HRSI) materials subjected to simulated reentry conditions. Evaluation of mass spectrometry for the measurement of atomic and molecular species provided some of the first measurements of point compositions in arc jet tunnel environments. A major objective of this project centered around the sampling residence time. A three staged vacuum sampling system pulled the molecules and atoms from the arc jet to a quadrupole ionization mass spectrometer in 400 milliseconds. Conditions investigated included a composition survey across the nozzle exit at 3 cm z-distance from the nozzle exit for 3 different currents. Also, a point composition survey was taken around a shock created by the presence of a blunt body

Raman spectra of adsorbed layers on space shuttle and AOTV thermal protection system surface

Surfaces of interest to space vehicle heat shield design were struck by a 2 W argon ion laser line while subjected to supersonic arc jet flow conditions. Emission spectra were taken at 90 deg to the angle of laser incidence on the test object. Results showed possible weak Raman shifts which could not be directly tied to any particular parameter such as surface temperature or gas composition. The investigation must be considered exploratory in terms of findings. Many undesirable effects were found and corrected as the project progressed. For instance, initial spectra settings led to ghosts which were eliminated by closing the intermediate of filter slit of the Spex from 8 to 3 mm. Further, under certain conditions, plasma lines from the laser were observed. Several materials were also investigated at room temperature for Raman shifts. Results showed Raman shifts for RCC and TEOS coated materials. The HRSI materials showed only weak Raman shifts, however, substantial efforts were made in studying these materials. Baseline materials showed the technique to be sound. The original goal was to find a Raman shift for the High-temperature Reusable Surface Insulation (HRSI) Reaction Cured borosilicate Glass (RCG) coated material and tie the amplitude of this peak to Arc jet conditions. Weak Raman shifts may be present, however, time limitations prevented confirmation

Optical System for Measuring Position in Space

There exist certain distinct advantages in accuracy and economy in the use of systems to measure vehicle position which operate at optical wavelengths. The positions measured are usually of critical importance to the success of a space mission. There are several types of navigational and tracking systems which produce data in an immediately usable form. Such real time systems, however, sacrifice some relative accuracy to gain speed of data availability. Stellar metric camera systems presently offer the most accurate means of determining position in space; however, there is an unavoidable time delay in reducing the data to a usable form. There is room for significant improvement in the errors of presently used stellar metric cameras, particularly in the geometrical optical characteristics of the lenses in both design and fabrication. The usefulness of such cameras can be extended by daylight use with the proper designs and techniques. Lunar-based cameras should also offer several advantages over Earth-based systems, but only at a high price. Recent developments in optical design techniques using high speed digital computers have brought fcjr greater sophistication and economy than had previously been possible. Vastly improved new optical designs are just waiting to be executed for application to the problems\u27of Measuring Positions in Space

Design and monitoring of narrow bandpass filters composed of non-quarter-wave thicknesses

Narrow bandpass filters have historically been designs of quarter waves at the passband wavelength, and have been monitored at the turning points using the passband wavelength. By direct monitoring at the passband wavelength, errors have been shown to be primarily self compensated, and have allowed much better performance than could otherwise be expected. The turning points are difficult to detect precisely and accurately because the change in transmittance with thickness becomes zero at the desired termination point. By proper design with non-quarter-wave layers, essentially the same spectral performance can be achieved by layer terminations that are far enough from turning points to be significantly more sensitive termination points. The design approach is to maintain the optical thickness of the reflector layer pairs at one half-wave of the passband wavelength, but change the ratio of the optical thicknesses of the high and low index layers. These can be adjusted enough so that the thicker layers contain two turning points and the last turning point in the layer can be more accurately and precisely determined. The error compensation benefit from the historic method should be maintained. This leads to potentially improved control during deposition and monitoring of narrow bandpass filters

Witness sample preparation for measuring antireflection

coating

Simulation comparisons of monitoring strategies in narrow bandpass filters and

antireflection coating

Limitations on Wide Passbands in Short Wavelength Pass Edge Filters

There are differences in the behavior of wide passband edge filters with short wavelength passbands and of those with long wavelength passbands. The bandwidth of the pass band is here defined by the longest wavelength in the band divided by the shortest wavelength. This is virtually unlimited in the case of a long wave pass filter. However, it is significantly limited in the case of the usual approach of using quarter wavelength layer thickness stacks for short wave pass filters. This limitation is encountered because the third and higher harmonics of the blocking band appear at the short wavelength position where the quarter wave optical thicknesses of the layers for the blocking band stack of layers become three (3) quarter waves, 5, 7, 9, etc., at the wavelength of that harmonic. It appears that bandwidths of over 2 start to have increasingly higher reflection losses, and bandwidths of 2.5 become virtually impractical for QWOT stacks. When band-passes broader than about 2 are needed for edge filters with a short wave passband, recourse to rugate-like designs is needed. Such designs can be achieved with only two homogeneous materials by employing the concept of the Herpin approximation, although many layers may be required. The influence of the indices of refraction of the materials, number of layers, and design approach on the bandwidth, average reflectance in the passband, band edge steepness, blocking density, and “squareness ” at the transition from the pass to blocking band are discussed

Mechanistic model for catalytic recombination during aerobraking maneuvers

Several mechanistic models are developed to predict recombination coefficients for use in heat shield design for reusable surface insulation (RSI) on aerobraking vehicles such as space shuttles. The models are applied over a temperature range of 300 to 1800 K and a stagnation pressure range of 0 to 3,000 Pa. A four parameter model in temperature was found to work best; however, several models (including those with atom concentrations at the surface) were also investigated. Mechanistic models developed with atom concentration terms may be applicable when sufficient data becomes available. The requirement is shown for recombination experiments in the 300 to 1000 K and 1500 to 1850 K temperature range, with deliberate concentration variations

The identification of excited species in arc jet flow

Spectrographic work done at the Atmospheric Reentry Material and Structures Facility (arc jet) located at the Johnson Space Center has led to the identification of several excited molecular and atomic states. The excited molecular states identified are: first positive nitrogen system, second positive nitrogen system, the first negative nitrogen system, the gamma system for nitric oxide, and the 306.4 nm system of OH. Excited atoms identified were nitrogen, oxygen, hydrogen, silicon, copper, sodium, barium, potassium, and calcium. The latter five are considered contaminants. Excited molecular states of oxygen were not seen, suggesting full dissociation of oxygen molecules to oxygen atoms within the arc column and nozzle. Further, evidence exists that O(-) may be present since a background continuum is seen, and because of the existence of positive species (first negative system of N2(+)). Interpretation of spectrographic plates was enhanced by the use of a microdensitometer, and by the application of a second order least squares routine which determined wavelength as a function of plate location. Results of this work will ultimately improve models used in the calculation of heat transfer rates to the space shuttle and the aerobraking orbit transfer vehicles

Ways that designers and fabricators can help each other

We show that, when designers and fabricators understand each other's art, there are ways to combine their techniques to achieve the best results with the minimum difficulty. We share some problems that we have encountered, and sometimes caused ourselves, in hopes of helping the reader avoid the same pitfalls.