Engineering topics

Stewart, David A.

Publications and source records attributed to Stewart, David A..

At least 19 records

NASA NTRS2017

Thermal Protection Supplement for Reducing Interface Thermal Mismatch

A thermal protection system that reduces a mismatch of thermal expansion coefficients CTE between a first material layer (CTE1) and a second material layer (CTE2) at a first layer-second layer interface. A portion of aluminum borosilicate (abs) or another suitable additive (add), whose CTE value, CTE(add), satisfies (CTE(add)-CTE1)(CTE(add)-CTE2)<0, is distributed with variable additive density,.rho.(z;add), in the first material layer and/or in the second material layer, with.rho.(z;add) near the materials interface being relatively high (alternatively, relatively low) and.rho.(z;add) in a region spaced apart from the interface being relatively low (alternatively, relatively high).

Stewart, David A.↗

NASA NTRS2010

High efficiency tantalum-based ceramic composite structures

Tantalum-based ceramics are suitable for use in thermal protection systems. These composite structures have high efficiency surfaces (low catalytic efficiency and high emittance), thereby reducing heat flux to a spacecraft during planetary re-entry. These ceramics contain tantalum disilicide, molybdenum disilicide and borosilicate glass. The components are milled, along with a processing aid, then applied to a surface of a porous substrate, such as a fibrous silica or carbon substrate. Following application, the coating is then sintered on the substrate. The composite structure is substantially impervious to hot gas penetration and capable of surviving high heat fluxes at temperatures approaching 3000.degree. F. and above.

Stewart, David A.↗

NASA NTRS2010

Development of a Catalytic Coating for a Shuttle Flight Experiment

A spray-on coating was developed for use on the shuttle wing tiles to obtain data that could be correlated with computational fluid dynamics (CFD) solutions to better understand the effect of chemical heating on a fore-body heat shield having a turbulent boundary layer during planetary entry at hypersonic speed. The selection of a spray-on coating was conducted in two Phases 1) screening tests to select the catalytic coating formulation and 2) surface property determination using both arc-jet and side-arm facilities at NASA Ames Research Center. Comparison of the predicted surface temperature profile over a flat-plate with measured values obtained during arc-jet exposure (Phase I study) was used to validate the surface properties obtained during Phase II.

Stewart, David A.↗

NASA NTRS2008

Toughened uni-piece, fibrous, reinforced, oxidization-resistant composite

A composite thermal protection structure, for applications such as atmospheric re-entry vehicles, that can withstand temperatures as high as 3600.degree F. The structure includes an exposed surface cap having a specially formulated coating, an insulator base adjacent to the cap with another specially formulated coating, and one or more pins that extend from the cap through the insulator base to tie the cap and base together, through ceramic bonding and mechanical attachment. The cap and insulator base have corresponding depressions and projections that mate and allow for differences in thermal expansion of the cap and base.

Stewart, David A.↗

NASA NTRS2008

Toughened uni-piece, fibrous, reinforced, oxidization-resistant composite

A composite thermal protection structure, for applications such as atmospheric re-entry vehicles, that can withstand temperatures as high as 3600.degree. F. The structure includes an exposed surface cap having a specially formulated coating, an insulator base adjacent to the cap with another specially formulated coating, and one or more pins that extend from the cap through the insulator base to tie the cap and base together, through ceramic bonding and mechanical attachment. The cap and insulator base have corresponding depressions and projections that mate and allow for differences in thermal expansion of the cap and base. A thin coating of a reaction cured glass formulation is optionally provided on the structure to allow reduce oxidization and/or to reduce catalytic efficiency.

Stewart, David A.↗

NASA NTRS2006

Tantalum-Based Ceramics for Refractory Composites

A family of tantalum-based ceramics has been invented as ingredients of high-temperature composite insulating tiles. These materials are suitable for coating and/or permeating the outer layers of rigid porous (foam-like or fibrous) ceramic substrates to (1) render the resulting composite ceramic tiles impervious to hot gases and (2) enable the tiles to survive high heat fluxes at temperatures that can exceed 3,000 F ( 1,600 C).

Stewart, David A.↗

NASA NTRS2002

Thermal Protection System Evaluation Using Arc-jet Flows: Flight Simulation or Research Tool?

The arc-jet has been used to evaluate thermal protection systems (TPS) and materials for the past forty years. Systems that have been studied in this environmerd include ablators, active, and passive TPS concepts designed for vehicles entering planetary and Earth atmospheres. The question of whether arc-jet flow can simulate a flight environment or is it a research tool that provides an aero-thermodynamic heating environment to obtain critical material properties will be addressed. Stagnation point tests in arc-jets are commonly used to obtain material properties such as mass loss rates, thermal chemical stability data, optical properties, and surface catalytic efficiency. These properties are required in computational fluid dynamic codes to accurately predict the performance of a TPS during flight. Special facilities have been developed at NASA Ames Research Center to approximate the flow environment over the mid-fuselage and body flap regions of proposed space-planes type vehicles. This paper compares flow environments generated in flight over a vehicle with those created over an arc-jet test articles in terms of scale, chemistry, and fluid dynamic properties. Flight experiments are essential in order to validate the material properties obtained from arc-jet tests and used to predict flight performance of any TPS being considered for use on a vehicle entering the Earth atmosphere at hypersonic speed.

Stewart, David A.↗

NASA NTRS2002

Reusable Surface Insulation Tile Thermal Protection Materials: Past, Present and the Future

Silica (LI-900) Reusable Surface Insulation (RSI) tile have been used on the majority of the Shuttle since its initial flight. Its overall performance with Reaction Cured Glass (RCG) coating applied will be reviewed. Improvements in insulations, Fibrous Refractory Composite Insulation (FRCI-12) and Alumina Enhanced Thermal Barrier (AETB-8) and coatings/surface treatments such as Toughened Uni-Piece Fibrous Insulation (TUFI) have been developed and successfully applied. The performance of these enhancements on the Shuttle Orbiters over the past few years along with the next version of tile materials, High Efficiency Tantalum-based Ceramic (HETC) with even broader applicability will also be discussed.

Leiser, Daniel B.↗

NASA NTRS2001

Flight Performance of a Functionally Gradient Material, TUFI, on Shuttle Orbiter

TUFI, Toughened Uni-Piece Fibrous Insulation, a functionally gradient material has been successfully flying on the Shuttle Orbiters in several locations on two insulation substrates over the past few years. TUFI is composed of insulation and a gradated surface treatment. The locations it has flown include the base heat shield where damage had been observed after every flight before its application. It was also applied to the to of the body flap, the bottom of the body flap and around selected windows and doors where damage had been observed in the past. A description of the types of processing used including substrates will be presented and its overall performance will be reviewed.

Leiser, Daniel B.↗

NASA NTRS2001

Flight Performance of a Functionally Gradient Material, TUFI, on Shuttle Orbiter

TUFI (Toughened Uni-Piece Fibrous Insulation), a functionally gradient material has been successfully flying on the Shuttle Orbiters in several locations on two insulation substrates over the past few years. TUFI is composed of insulation and a gradated surface treatment. The locations it has flown include the base heat shield where damage had been observed after every flight before its application. It was also applied to the body flap, the bottom of the body flap and around selected windows and doors where damage had been observed in the past. A description of the types of processing used including substrates will be presented and its overall performance will be reviewed.

Leister, Daniel B.↗

NASA NTRS2000

Surface Characterization of LMMS Molybdenum Disilicide Coated HTP-8 Using Arc- Jet Hypersonic Flow

Surface properties for an advanced Lockheed Martin Missile and Space (LMMS) molybdenum disilicide coated insulation (HTP-8) were determined using arc-jet flow to simulate Earth entry at hypersonic speeds. The catalytic efficiency (atom recombination coefficients) for this advanced thermal protection system was determined from arc-jet data taken in both oxygen and nitrogen streams at temperatures ranging from 1255 K to roughly 1600 K. In addition, optical and chemical stability data were obtained from these test samples.

Stewart, David A.↗

NASA NTRS1998

Leading Edge Heat Shield for Wings of Spacecraft

A heat shield for thermally insulating the leading edge of a wing of a spacecraft during ascent and reentry includes a plurality of rigid tiles. Each tile is formed with a pie-shaped element which interlocks with the complementarily-formed element of another tile. The combination of structure afforded by the pie-shaped elements substantially impedes hypersonic flow of any gases that might enter the gaps between tiles.

Stewart, David A.↗

NASA NTRS1997

Surface Catalysis and Characterization of Proposed Candidate TPS for Access-to-Space Vehicles

Surface properties have been obtained on several classes of thermal protection systems (TPS) using data from both side-arm-reactor and arc-jet facilities. Thermochemical stability, optical properties, and coefficients for atom recombination were determined for candidate TPS proposed for single-stage-to-orbit vehicles. The systems included rigid fibrous insulations, blankets, reinforced carbon carbon, and metals. Test techniques, theories used to define arc-jet and side-arm-reactor flow, and material surface properties are described. Total hemispherical emittance and atom recombination coefficients for each candidate TPS are summarized in the form of polynomial and Arrhenius expressions.

Stewart, David A.↗

NASA NTRS1996

Surface Catalytic Efficiency of Advanced Carbon Carbon Candidate Thermal Protection Materials for SSTO Vehicles

The catalytic efficiency (atom recombination coefficients) for advanced ceramic thermal protection systems was calculated using arc-jet data. Coefficients for both oxygen and nitrogen atom recombination on the surfaces of these systems were obtained to temperatures of 1650 K. Optical and chemical stability of the candidate systems to the high energy hypersonic flow was also demonstrated during these tests.

Stewart, David A.↗

NASA NTRS1995

Characterization of Arcjet Flows Using Laser-Induced Fluorescence

A sensor based on laser-induced fluorescence has been installed at the 20-MW NASA Ames Aerodynamic Heating Facility. The sensor has provided new, quantitative, real-time information about properties of the arcjet flow in the highly dissociated, partially ionized, nonequilibrium regime. Number densities of atomic oxygen, flow velocities, heavy particle translational temperatures, and collisional quenching rates have been measured. These results have been used to test and refine computational models of the arcjet flow. The calculated number densities, translational temperatures, and flow velocities are in moderately good agreement with experiment

Bamford, Douglas J.↗

NASA NTRS1994

Hyflite TPS Domain Test Results Ames 2x9 Turbulent Flow Duct

Detailed experimental data have been obtained on several advanced TUFI systems during their exposure to high temperature high shear supersonic turbulent flow. Dimensional stability of these systems were determined at surface temperatures above 3000 F. Effect of step gap configuration on the thermal performance of the TUFI systems were also evaluated.

Stewart, David A.↗

NASA NTRS1994

Effect of Porosity on Surface Catalytic Efficiency

This paper describes the effect of surface porosity of thermal protection materials on surface catalytic efficiency using test data taken from both arc-jet and side-arm reactor facilities. Relative surface porosity of the samples varied from 6% to 50%. Surface porosity was measured using a flow apparatus and Bernoulli equation. The surface catalytic efficiency of the materials was calculated using aerothermodynamic, and kinetic theories. The catalytic efficiency of the materials are compared at surface temperatures between room temperature and 2500 F. The data are presented in the form of graphs and tables.

Stewart, David A.↗

NASA NTRS1993

Characterization of the thermal conductivity for Advanced Toughened Uni-piece Fibrous Insulations

Advanced Toughened Uni-piece Fibrous Insulations (TUFI) is discussed in terms of their thermal response to an arc-jet air stream. A modification of the existing Ames thermal conductivity program to predict the thermal response of these functionally gradient materials is described in the paper. The modified program was used to evaluate the effect of density, surface porosity, and density gradient through the TUFI materials on the thermal response of these insulations. Predictions using a finite-difference code and calculated thermal conductivity values from the modified program were compared with in-depth temperature measurements taken from TUFI insulations during short exposures to arc-jet hypersonic air streams.

Stewart, David A.↗