David G. Drewry

Unsteady Airflows and Their Impact on Small Unmanned Air Systems in Urban Environments

Existing unmanned air system platforms currently do not lend themselves well to autonomous operation within complex, highly variable aerodynamic environments. As such, there is a need for accurate high-fidelity urban airflow models to help reduce the risk of failure of urban unmanned air system surveillance and engagement missions. Although urban aerodynamics are exceptionally complicated because of complex interactions between geometry, physical conditions, and varying meteorology, high-fidelity computational fluid dynamics models exist that capture these interactions effectively. Using sufficient resolution, these large-eddy simulation models provide a viable means to characterize urban airflow environments when wind-tunnel testing and field trials are too expensive or impossible. This paper presents a simulation tool that captures unsteady aerodynamics of aircraft flight in an urban environment for the study of vehicle–environment interactions. By combining a high-resolution model of the terrain/buildi...

Unsteady Urban Airflows and Their Impact on Small Unmanned Air System Operations

Existing Unmanned Air System (UAS) platforms currently do not lend themselves well to autonomous operation within complex, highly-variable aerodynamic environments. As such, there is a strong need for accurate, high-fidelity urban air flow models to help reduce risk of failure of urban UAS surveillance and engagement missions. Although urban aerodynamics are exceptionally complicated because of complex interactions between geometry, physical conditions, and varying meteorology, research-caliber Computational Fluid Dynamics (CFD) models exist that capture these interactions effectively. Using sufficient resolution, these Large Eddy Simulation models provide a viable means to characterize urban environments when wind tunnel testing and field trials are too expensive and/or impossible. We have successfully integrated these extant CFD models and their real-time effects on embedded UAS within a synthetic modeling environment. By further incorporating urban terrain, UAS aerodynamic data, and a UAS vehicle dynamics model, a real-time physicsbased simulation tool has been created for the study of vehicle-environment interactions. This paper will discuss how this tool could revolutionize our understanding of these interactions in urban environments, giving unprecedented capability and flexibility to UAS designers, acquisition agents, mission planners, and operators.

Charging of Ceramic Coatings in Space

[Abstract] Conventional materials used for spacecraft thermal protection are conductive, thereby preventing the buildup of differential charge, as it can dissipate across the entire surface assuming proper grounding. The extremely high temperatures that NASA’s Solar Probe will encounter during solar approach require the use of refractory ceramic materials as a component of the passive thermal management system. Since ceramics are typically electrically insulating and therefore tend to collect surface charge, special attention has been devoted to understanding how ceramics’ electronic properties vary at selected points along the spacecraft trajectory and how their interaction with the radiation environment could potentially affect the overall spacecraft charging. These studies are relevant for any mission encountering extreme environments. Spacecraft charging can be investigated and characterized using a combination of modeling and experimental measurements. Charging behaviors of three ceramic materials in near-solar radiation environments were studied: alumina, pyrolytic boron nitride, and barium zirconium phosphate. Charging simulations were performed, as well as electron emission studies in order to more accurately model the charging-relevant material properties. Mild differential charging of approximately tens of volts is predicted for thin (~100 micron) ceramic coatings between Earth and approximately 0.3 AU. As the spacecraft moves nearer to the sun and its temperature increases, the charging behavior changes from differential to very mild absolute charging of a few volts. These results depend sensitively on spacecraft geometry, material properties, and radiation environment.

Passive Optical-Based Thermal Management Approach for Spacecraft Operating in the Near Solar Environment

[Abstract] The Johns Hopkins University Applied Physics Laboratory (JHU/APL) and NASA’s Goddard Space Flight Center are currently evaluating optical coatings made from ceramics as a means of passive thermal management for spacecraft operating in the solar environment. Ceramics were selected based on chemical stability; and inertness to radiation damage and hydrogen degradation. Investigations have focused on “white” ceramics such as aluminum oxide, pyrolytic boron nitride, and barium zirconium phosphate which have been shown to also possess desirable optical characteristics making them ideal candidates.

Combined High Temperature Aerothermal-Structural Physical Property Testing of Ceramic Matrix Composites

Abstract : The objective was to integrate a mechanical test apparatus into a wind tunnel flow field capable of generating controlled multi-directional stress/strain states in flat coupons exposed to appropriate chemical, thermal, and aerodynamic shear flow environments. Displacement control enables imposition of stress/strain fields (in the flat coupons) similar to those expected in complex flight component geometries. Control and measurement of these variables and the material response in a controlled environment enables assessment of material performance for hypersonic missile and re-entry vehicle flight trajectories. Requirements for the test apparatus were developed and the apparatus was designed. Unfortunately, closure of the proposed wind tunnel facility led to increased costs and imposed a major schedule slip. The programs contract was not extended and the test apparatus was not assembled.

The Solar Probe Mission Design Challenges

A probe flying into one of the last unexplored regions of the solar system, the Suns corona, would revolutionize our knowledge of the physics of the origin and evolution of the solar wind. Many studies have been conducted to conceive such a mission and design a spacecraft and payload which could survive the harsh solar environment and still provide the necessary science measurements. This paper summarizes engineering work done over the past seven years in the development of the Solar Probe mission from a concept to a flight ready design.

Terahertz Time -Domain Spectroscopy of Aluminum Oxide for Thermal Protection Applications

In this work we show how terahertz time -domain spectroscopy (THz TDS) can be an effective tool for the characterization of optical and microstructural properties of alumin um oxide. Terahertz spectroscopy was performed on aluminum oxide test samples that ranged in mean pore size from 0 to 15 microns. These experiments were performed with a transmission mode terahertz system and show that there is a strong sensitivity to chan ges in the size and number of pores due to scattering . Results show that the magnitude of the THz transmission obtained using standard Fourier transform methods is reduced at higher freq uencies as porosity increases. A t heoretical coherent scattering model ing approach is also discussed that might assist in the interpr etation of these signal losses.

A Ceramic Coated Thermal Radiation Heat Shield in the Near Solar Environment: Testing Methods and Performance Prediction

(Abstract) Applying white ceramic optical surfaces to the carbon-carbon (C-C) heat shield for a near solar exploration spacecraft significantly reduced its temperature. The coatings are designed to be highly reflective in the visible band (thus reflecting the majority of the visible solar irradiance) yet highly emitting in the infrared band. Optical property testing was performed on C-C coupons coated with either alumina (Al2O3) or pyrolitic boron nitride (PBN). Testing was conducted using lasers at discrete wavelengths (spanning the visible to near-IR (NIR) bands) and at temperatures up to 1773 K (in an inert gas). While uncoated C-C surfaces had solar absorptance to IR emittance ratios ( α α α αS/eIR ) approaching 1, the α α α αS/eIR � of C-C coated with the white ceramic coatings was 0.6 or less. At a distance of 4 solar radii from the Sun, the resulting predicted temperature of an uncoated C-C heat shield is ~2100 K, while that of an ceramic coated heat shields is more than 250 K lower. Such a temperature reduction lowers the potential for mass loss from the C-C substrate, reduces spacecraft weight, and decreases mission risk.

High-Fidelity Simulation Strategy for Aerothermochemistry at Ablating Gas/Solid Interfaces

A simulation strategy is described for problems involving thermal protection systems and aerodynamic heating phenomena. At the core of the analysis methodology are two models, a fluid dynamics model and a material thermal response model, loosely coupled through boundary condition treatments. This highfidelity analysis capability was developed to facilitate the study of the fundamental physical processes that occur at reacting gas/solid interfaces of various aerospace systems. The strategy itself, general in nature, allows model interchange and is applicable to a wide range of thermo-chemical ablation problems. The initial target application is the assessment of a highspeed carbon-carbon nosecone during a thermally stressing trajectory. The time-dependent analysis of a nosecone trajectory is accomplished iteratively through the numerical solution of the compressible reacting Navier-Stokes equations coupled with phenomenologies that describe the surface kinetic constraints. Recent enhancements to the methodology include a chemical kinetics model for air-carbon reactions to account for surface reactions and reactions with the air stream, and inclusion of shape change effects due to surface recession. Also presented is the status of a numerical/experimental performance assessment of a high-speed nosecone. __________________ ∗ Engineer, Senior Professional Staff, Associate Fellow AIAA † Engineer, Senior Professional Staff ‡ Engineer, Professional Staff, Member AIAA Nomenclature a = Blowing constant Cm = Stanton number for mass transfer hrec = Recovery enthalpy hund = Enthalpy of solid exposed to ablation hw = Enthalpy at wall m = Species concentration ab m& m& = Ablative mass flux w = Mass flux off wall Pt = Total pressure q& = Heat flux r& = Surface recession rate t = Time Tref = Reference temperature Tt = Total temperature Tw = Wall temperature ue = Edge velocity x, y, z = Cartesian coordinates ∆t = Time step δx = Grid point displacement, x direction δy = Grid point displacement, y direction e = Emissivity ρ = Gas density σ = Stefan-Boltzmann constant τx = Shear stress, x direction τy = Shear stress, y direction