Simplified Spacecraft Design Tools For Evaluating Architecture Concepts
First published May 2013, Crosslink® magazine
As space system architecture capabilities mature, the tools used to analyze them must be modified and enhanced. The Aerospace Corporation is placing an increasing emphasis on its corporate architecture and systems-of-systems capabilities, which supports its customers in the analysis of options as they decide how best to spend their limited resources.
Current corporate tools include the decision support framework and the concurrent program definition environment. The Air Force Space and Missile Systems Center’s (SMC) cross-enterprise architecting tool is another method used in the analysis of options. These tools support activities such as the current spaced-based environmental monitoring assessment of alternatives, and the NavSat study (a satellite navigation system demonstration) for the SMC commander.
A research effort is under way at Aerospace to address updating, completing, and expanding some of these existing tools, as well as developing new ones. Investigating or generating spacecraft concepts inevitably becomes part of the architecture analysis process, and the tools that support these activities also need updating. Spacecraft concepts and designs are developed to gather a basic understanding of the required capabilities (i.e., size, weight, and power) for the mission. While Aerospace’s Modular Concurrent Engineering Methodology (ModCEM) (system-level fidelity) and the Concept Design Center’s (subsystem-level fidelity) capabilities are well-suited to generating tens of discrete point designs within a few hours to a few days, neither is conducive to rapid exploration of vast trade spaces (perhaps hundreds of thousands of point designs).
The team updating and creating these architecture analysis and spacecraft design tools includes Richard Gong, systems director, Developmental Planning and Projects, along with coinvestigators Joseph Aguilar, O’brian Rossi, and Dan Judnick, Vehicle Concept Department; Daniel Nigg, Concept Design Center Office; and John Evans, Space Architecture Department.
Four existing corporate tools are being upgraded and enhanced: historical mass/power fractions (HMPF), design estimation relationship (DER), simplified concurrent engineering methodology (CEM), and rapid tradespace exploration (RTE). These spacecraft design tools each employ a different approach to generating data. “These domains often have large uncertainties associated with them, making high-fidelity modeling fruitless. With these tools, thousands of spacecraft designs can be quickly created, making efficient exploration of large solutions spaces possible,” Gong said.
HMPF generates mass and power fractions from Aerospace’s small satellite database by retrieving historical information on design life, orbit, stabilization type, and propulsion systems. The size of the supporting spacecraft bus can be determined using historical data about a particular payload’s mass and power. “The usefulness of this tool lies in its ability to calculate mass and power fractions based on satellites in the database. For example, a three-year, low Earth orbit mission will use only 44 of the 139 spacecraft available in the database to calculate the mass and power of the supporting spacecraft bus because those are the only spacecraft with attributes that are applicable to the mission,” said Rossi. The tool allows engineers to easily compare newly proposed space vehicle designs to those that have historically worked for similar payloads. Having such a reference allows engineers to better identify high-risk subsystems too. This is particularly relevant for conceptual designs where subsystems such as structures, thermal, and harness masses are often estimated and therefore susceptible to miscalculation.
The DER tool creates spacecraft mass and power using estimation relationships. It generates information similar to what is calculated by the cost-estimation relationships used in costing, but is based on design parameters such as design life, data rate, and pointing knowledge stored in Aerospace’s small satellite database. “Significant effort was devoted to developing unique mass and power design estimation relationships for each subsystem and the total spacecraft. As with the HMPF tool, this tool helps to identify out-of-family subsystems for further study and review,” Rossi said.
The CEM tool is derived from the ModCEM tool and requires 23 high-level inputs such as payload mass, design life, and orbit. The tool was compared to 13 different satellites as it was updated. Judnick, a senior member of the technical staff, said that good agreement (error under 26 percent) was achieved between actual missions and the simplified CEM tool. “It is interesting to note that while the subsystem masses may vary widely, the overall space vehicle mass matches rather well,” he said. The tool has been used to support a number of national security space programs. It is increasingly used in areas that do not require the level of fidelity of the ModCEM tool, or for those conducted by the Concept Design Center, but do require more fidelity than the HMPF or DER tools.
The RTE tool is used to quickly create zeroth-order (coarse approximation) spacecraft designs and has produced suitable results comparable to other more complex capabilities. “The tool is flexible enough to accommodate a variety of mission types in numerous operational orbit regimes with some attention to coarse and fine interaction between subsystem sizing algorithms,” Nigg said. “All candidate point designs presented are capable of meeting their respective mission requirements; trades are made only with respect to the application of technology development in each spacecraft subsystem and result in numerous aggregate system solutions,” he said.
“Ultimately, the goal is to offer engineering tools that allow the exploration of a wide variety of alternatives leading to better, more capable architectures at a reduced cost,” Gong said. Being able to quickly generate first-order spacecraft designs based on limited mission and payload information can help to better focus time and resources on those designs that warrant further examination. “For example, thousands of potential solutions were created in RTE in a few minutes and mapped against lifecycle costs during one recent study,” Nigg said.
“The tools we are developing provide a means to generate efficiently and quickly thousands of spacecraft designs, helping engineers eliminate alternatives that do not meet a particular set of criteria, such as low cost or high agility,” Gong said. “Further refinement using the CDC or the ModCEM can then be used to focus on fewer alternatives, saving resources and allowing a deeper exploration of those culled alternatives,” he said.
This independent research and development work attempts to identify areas within these tradespaces where rigorous, high-fidelity modeling activities should be investigated to determine optimal solutions. Trend information based on subsystem technology and mass-cost relation-ships, which determine system-level impacts, can also be developed for any modeled space mission with these tools. Studying these trends helps to identify opportunities to lower development costs and launch mass, and offers information on how to increase design life or optimize any combination of these characteristics.
Graphene Growth, Characterization, and Applications
Considered the building block for graphite, graphene is a single layer of carbon atoms and consists of a two-dimensional, hexagonal lattice of sp2-bonded carbon. In addition to its one-atom thickness, graphene’s unique properties are extraordinary electron mobility, high electrical current carrying capacity, high thermal conductivity, high optical transparency, mechanical strength, and large specific surface area. Based on such properties, applications for graphene include electronic and photonic devices, solar cells, and energy storage devices.
These diverse applications are driving the need for large, high-quality graphene films. The need to produce large areas of graphene as well as large single-crystal grains of graphene has propelled the development of new deposition techniques. Various methods for graphene growth have been established, such as annealing silicon carbide, reduction of graphene oxide, and growth on metal substrates using ethylene and methane with hydrogen as gas-phase precursors.
Gouri Radhakrishnan, senior scientist, Materials Science Department, is the principal investigator of an independent research and development project at The Aerospace Corporation. The goals are to develop novel and scalable techniques for the growth of single-layer graphene and the full characterization of the material. In addition, there is an interest in understanding the electrochemical performance of graphene to examine its potential application as an anode for lithium ion batteries. The research team includes coinvestigators Paul Adams, Materials Science Department, and Joanna Cardema, Electronics and Photonics Laboratory. Collaborators include Brendan Foran, Hyun Kim, Heinrich Muller, Andrew Stapleton, Miles Brodie, Michael Meshishnek, Martin Ciofalo, and Matthew Mecklenburg.
A novel process for the growth of monolayer graphene has been developed at Aerospace, in which methanol is decomposed on copper at 1050°C in a flow of pure argon gas. This method offers an alternate synthesis route for making high-quality graphene without using hydrogen as a process gas. Eliminating hydrogen as a process gas offers increased safety and facilitates fabrication scaling. This method produces monolayer graphene films with large, single-crystal areas that are 10–30 square microns. “While our process is typically carried out using copper as the underlying substrate for depositing the graphene, we have also developed a process for growing multilayer graphene at the same temperature on other substrates,” Radhakrishnan said. Aerospace was recently awarded a patent on this subject by the USPTO.
One of the challenges involved in graphene growth is applying sophisticated diagnostic tools in-house to confirm that single-layer graphene has actually been produced. An established diagnostic is Raman spectroscopy, a nondestructive technique that measures laser light scattered by phonon modes in a material. Graphene has a very characteristic Raman spectrum that allows a clear distinction between a single layer, two-to-three layers, and multiple layers of graphene, which would comprise bulk graphite. “We were able to identify the growth of single-layer graphene from the specific peaks in the Raman spectrum of our graphene films as well as the ratio of peak intensities,” Radhakrishnan said.
In addition to establishing that a single layer of graphene has been deposited, it is important to determine the grain structure of graphene. To investigate graphene’s grain structure, the Aerospace research team used transmission electron microscopy (TEM) to examine very thin electron transparent samples. The team carefully transferred the single layer of graphene from the copper substrate to a special grid (typically three millimeters in diameter) that comprised an extremely thin amorphous carbon membrane. This allows high-energy electrons to be transmitted through the graphene film placed on the grid membrane. The transmitted electrons create an image on a viewing screen or detector, and provide signatures representing the crystal structure of graphene with very high spatial resolution.
To study the graphene film’s crystalline grain structure, the research team obtained selected area electron diffraction (SAED) patterns, which are an array of hexagonal spots that reflect the graphene sample’s internal crystal structure. The simple hexagonal spot pattern is due to the hexagonal carbon lattice from a single crystal of graphene. Changes in the graphene lattice’s orientation cause rotations in the hexagonal SAED pattern, and multiple grains of graphene produce multiple hexagonal SAED patterns. The areas with identical diffraction patterns were determined by measuring the SAED patterns across all the grid hole locations covered by graphene. The areas in which the spot pattern was identical and nonrotated indicate a single crystal of graphene.
A specific area of research currently being pursued is the application of graphene to lithium ion batteries. Based on its insignificant mass, strong electrical conductivity, and extremely high specific surface area, graphene is a promising candidate for supercapacitor electrodes and an anode material for the uptake of lithium in lithium ion batteries. Compared to the commonly used powder graphite electrodes, graphene electrodes can offer high specific capacity (i.e., capacity per unit mass).
The Aerospace research team compared the electrochemical performance of anodes fabricated from three well-characterized systems with an increasing number of graphene layers. These systems contained single-layer graphene, multilayer graphene with approximately 50 atomic layers of graphene, and well-ordered bulk graphite in the form of highly oriented pyrolytic graphite (HOPG) with a thickness of about 200,000 layers of graphene. For purposes of establishing the electrochemical effects specifically resulting from graphene, the anodes were assembled without the use of a binder. “Not only does a binder-free electrode provide further weight reduction, it also allows us to test the fundamental electrochemical properties of the active graphene layers,” Radhakrishnan said. “In addition, we performed extensive pre- and post-cycling characterizations that have provided insights into the electrochemical performance of these three systems, which also offers valuable diagnostics for failure analysis.”
The research team has also successfully measured the electrochemical capacity from a single atomic layer of carbon. While the capacity is small, the graphene weight that is needed to obtain a capacity similar to commercial graphite would still be 200 times less. The results provide new insights into the mechanism of lithium uptake in a single graphene layer, which is different from the intercalation of lithium between adjacent layers in multilayer graphene. The results also suggest new designs for improving the capacity and performance of these graphene anodes. In contrast to one graphene layer or a few layers, the electrochemical performance of the thicker HOPG layers becomes diffusion limited, and the lithium ions are not able to access all the graphene layers. Work is ongoing to understand the applications of these very novel nanocarbon material systems.
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