Andrew D. Williams

Development of a satellite structural architecture for operationally responsive space

The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing a satellite structural architecture in support of the Department of Defenses Operationally Responsive Space (ORS) initiative. Such a structural architecture must enable rapid Assembly, Integration, and Test (AI&T) of the satellite, accommodate multiple configurations (to include structural configurations, components, and payloads), and incorporate structurally integrated thermal management and electronics, while providing sufficient strength, stiffness, and alignment accuracy. The chosen approach will allow a wide range of satellite structures to be assembled from a relatively small set of structural components. This paper details the efforts of AFRL, and its contractors, to develop the technology necessary to realize these goals.

Chamber Core Structures for Fairing Acoustic Mitigation

Abstract : The U.S. Air Force Research Laboratory is pursuing an innovative composite structure design called chamber core for constructing launch vehicle payload fairings. A composite chamber core fairing consists of many axial tubes sandwiched between face sheets, tubes that can be used as acoustic dampers to reduce low-frequency interior noise with virtually no added mass. This paper presents the results of experimental studies of noise transmission through a 1.51 m diameter x 1.42 m tall chamber core cylinder. It was tested in a semireverberant acoustics laboratory using band-limited random noise at sound pressure levels up to 110 dB. The bare cylinder provided approximately 12.7 dB of attenuation over the 0-500 Hz bandwidth and 15.3 dB over 0-2000 Hz. The noise reduction increased to over 18 dB for both bandwidths with the axial tubes acting as acoustic dampers. Narrowband reductions in excess of 15 dB were measured around specific acoustic resonances. This was accomplished with virtually no added mass to the composite cylinder. Results were compared with the performance provided by a 2.5 cm acoustic blanket treatment. The acoustic dampers were as effective as the acoustic blanket at low frequency, but not at higher frequencies. The acoustic dampers were better able to couple with and damp the low-frequency acoustic modes. Together, the acoustic blanket and dampers provided over 10 dB more noise reduction over the 2000 Hz bandwidth than the bare cylinder.

Review of Modern Spacecraft Thermal Control Technologies

Originally created and developed for space applications, several commercial terrestrial technologies still permeate our society today. Examples include solar cells, Global Positioning Systems, and less expensive methods of carbon nanotube manufacture. Given a long and successful history of spinoffs, there might exist opportunities for the transfer of modern spacecraft thermal control technologies to terrestrial HVAC&R applications. First, this paper presents a broad overview of spacecraft thermal control. Next, a review of several modern spacecraft thermal control subsystem technologies is provided, each including an assessment of their potential use for terrestrial applications.

Optimal Placement of Electronic Components to Minimize Heat Flux Nonuniformities

Development of electronic devices and systems with increased capability and good reliability will require improved thermal management techniques. Technology improvements such as embedded heat pipes, integrated pumped fluid loops, and integrated high conductivity thermal spreaders such as annealed pyrolytic graphite provide advances that can enable more powerful devices in ever-decreasing package sizes. Although technology innovations provide one solution path, an alternative method that has not received much attention is simply optimized component placement. The present approach provides a fast method for determining optimized component placement that approaches a uniform distribution of heat flux. The result is improved thermal performance of electronic systems. A tool was developed which can optimally place any number of components within a rectangular domain. Optimized results were obtained for 18 uniform and 11 non-uniform components within 20 s and 7 s, respectively, using a 2.5GHz Core™ Duo processor. The approach presented in this paper is especially useful in situations where limited or no thermophysical and environmental conditions are readily available for the problem at hand and can be utilized in a variety of industries including microelectronics and satellite development.

Modular Thermal Control Architecture for Modular Spacecraft

Development of modular spacecraft requires modularization of interfaces between modules as well as the functions of the modules themselves. Although many of the functions and interfaces of a spacecraft are already well adapted to modularization, the traditional thermal control architecture is not. The thermal control system is the most significant hurdle to modularization yet to be overcome. This paper provides an overview of the issues related to creating a modular thermal control subsystem and how this last major hurdle can be overcome. Principles of product architecture and modularity are used to define the functional elements, interfaces, and physical components and develop a modular thermal control architecture. A means of decoupling the thermal control subsystem, perhaps the most difficult characteristic necessary to implement, is presented. The process of developing the architecture is summarized with illustrations.

Biologically Inspired Multifunctional Composite Panel with Integrated Thermal Control

Recently, there has been a surge in interest in biologically inspired or bio-mimetic structures where living organisms are used to provide guidance to improve overall performance of aerospace systems. In this study, an isogrid architecture for satellite electronics panels was investigated. Fluidic channels were incorporated into the structure, thus providing thermal management capability in addition to structural support. To provide the surface area for heat transfer, a thin face-sheet was mounted on the outside of the isogrid, also containing fluidic channels. The fluidic channels are analogous to a circulatory system in a biological organism. The primary supporting ribs act as arteries by providing the main flow supply and channels in the face-sheet act as capillaries by providing the area coverage for heat transfer. The challenge with this concept is the requirement for complex, leak-free internal geometries to transport fluid throughout the panel as needed to cool the components and still provide a high stiffness-to-weight as required for satellite structures. Using a novel fabrication approach, the channels can be integrated directly into an isogrid composite panel during build up. This approach also eliminates the traditional failure point of composite isogrid structures by forming the rib structure and face-sheet into a single, multi-directional laminate. As a result of the fabrication approach, the size of the primary arterial fluid channels must be carefully designed to balance heat transfer, pressure losses, and face-sheet deflection. This paper reviews the design and optimization of the channel as well as the overall thermal design of the panel.

Emissivity modulating electro-chromic device

The Eclipse infrared electro-chromic device (IR-ECD) is an all-solid-state monolithic vacuum deposited thin film system functioning as an electrically controlled dimmable mirror in the IR region. The maximum reflectance corresponding to the bleached condition of the system is around 90% (low-e condition, e=0.1). The minimum reflectance reaches nearly zero in the colored condition of the system (high emmittance, e=1). It is a variable emittance electro-chromic device (VE-ECD). The average emissivity modulation of the Eclipse VE-ECD is 0.7 in the 8-12 micron region, and at 9.7 micron (room temperature) it reaches a value of 0.9. Half and full emissivity modulations occur within 2 and 10 minutes, respectively. Because of its low mass (5 g/m2), low voltage requirement (±1 V), extremely good emissivity control properties (from 0 to 0.9 at 300 K), and highly repeatable deposition process, the VE-ECD technology is very attractive for satellite thermal control applications. The Eclipse VE-ECD has been under evaluation in a real space environment since March 8, 2007. This paper presents recent developments on Eclipses VE-ECD including space test results.

Experimental Analysis of a Biologically Inspired Thermal-Structural Satellite Panel

Recently, there has been a surge in interest in biologically inspired or bio-mimetic structures where living organisms are used to provide guidance to improve overall performance of aerospace systems. In this study, an isogrid architecture for satellite electronics panels was investigated. Fluidic channels were incorporated into the structure, thus providing thermal management capability, in addition to structural support. To provide the surface area for heat transfer, a thin facesheet was mounted on the outside of the isogrid, also containing fluidic channels. The fluidic channels are analogous to a circulatory system in a biological organism. The primary supporting ribs act as arteries by providing the main flow supply and channels in the facesheet act as capillaries by providing the area coverage for heat transfer. Also incorporated into the structure was a passive-reactive valve designed to regulate coolant flow. The base of the valve was fabricated directly onto the thermal-structural panel and could be placed anywhere on the panel. This valve was designed to operate on the same principle as traditional automotive thermostats for regulating engine temperature. In addition, the valve was designed to utilize paraffin expansion properties for actuation. Although designed and built, the valve was not tested in these experiments and the results are focused on the thermal structural panel. Results indicate that this thermal control method provides satisfactory thermal management for standard satellite bus components.

Biologically Inspired Thermal-Structural Satellite Panels

Recently, there has been a surge in interest in biologically-inspired structures where natural structures are used to provide guidance to improve overall performance of aerospace systems. In particular, the incorporation of circulatory systems to significantly improve the material and physical properties of the system or enhance the system by adding a new capability such as thermal management is of interest. This paper details a “symbiotic” solution where thermal management functionality is embedded within the structure of the satellite. This approach is based on the flight proven and structurally efficient isogrid architecture with the addition of fluid channels embedded in the ribs and face-sheet to form an integrated circulatory system. The use of combined fluidic-structural systems has enormous potential for satellite thermal management. The conceptual design, analysis, and optimization of a reconfigurable thermal-structural panel for satellite applications will be presented. The focus will be on the fluidic chambers integrated into the ribs of the panel, the distribution system in the face-sheets of the panel, and the passive micro valve used to control flow within the system.

Hot- and Cold-Case Orbits for Robust Thermal Control

Realizing cheaper, more flexible alternatives to traditional satellites requires robust design approaches. Robust satellite subsystems are designed to meet a broad range of mission requirements; consequently, they drastically reduce nonrecurring engineering costs and greatly diminish design, development, assembly, integration, and test schedules. Robust thermal control subsystems must be capable of handling a broad range of thermal environments, thus reducing design and development costs but can be susceptible to overdesign. Therefore, improved design methodologiesareneededtomaintaintheiradvantageswhileminimizingexcessivedesign.Asa firststep,designhotandcold-case orbits shouldbe examined. Theprimarygoal of the study described in thispaper wasto identify single hot- and cold-case design orbits that work well in the design of robust thermal control subsystems over a wide range of satellite surface properties and likely operating environments. A general approach was developed to identify worst-case orbits that employ a combination of statistical and historical data such that statistically insignificant orbits are disregarded. Using this method, individual hot- and cold-case design orbits were found at beta angle/ inclinationcombinationsof72 deg =52 degand0 deg =28 deg,respectively.Theuseofthesedesignorbitsworkswell for a wide range of different satellite surface properties.