Product Design & Galactic Thermal Management
From bearings to computer systems to galactic hardware, rising temperatures have a tendency to make life a nightmare for design engineers.
A prominent enemy of efficiency and functionality is heat. From bearings to computer systems to intergalactic hardware, rising temperatures have a tendency to make life a nightmare for design engineers. Thermacore recently experienced the rigors of temperature management while contributing to the design of the NASA’s Landsat Data Continuity mission, which launched on February 11, 2013.
The Thermacore team provided mission-critical components for what will be dubbed Landsat 8 once the craft is thoroughly tested in orbit. The Landsat 8 was launched to monitor natural and man-made changes to the Earth’s surface using two sensitive instruments, the Operational Land Imager (OLI) and the Thermal Infrared Sensor (TIRS). High-tech devices of this nature require finite thermal management to preserve measurement accuracy and a shelf-life, expected to last several years.
NASA sought out the Thermacore’s k-Core® technology, a system of encapsulated graphite to help alleviate heat and keep temperatures under control. The space organization has worked with Thermacore in the past, and already utilizes some of the technology in its aerospace exploits. Mark Montesano explains, “We have parts on the International Space Station, and we’ve made other hardware for NASA before. We know them pretty well, so that helped us get involved early-on in this project.”
Thermacore provided three components to the Landsat 8: a thermal spreader, a thermal bracket, and a large cryocooler supporting bracket. “The most complicated part was the bracket that was used to crowd-cool the cylindrical compressors that needed to be structurally supported, because they’re pretty massive devices,” explains Montesano. The heat extracted needed to be pulled away and conducted out to an array that would, essentially, be a radiator. Due to the weight support and heat transference it was responsible for, the bracket had strict conductance and stiffness parameters, Montesano adds, “and of course they had a pretty challenging weight budget.”
NASA provided a design that laid out the footprint and the requirements in terms of thermal and structural parameters. “We turned that solid design in to a composite. We went in and designed where the graphite would be placed and the scheme on manufacturing it,” explains Montesano.
The Thermacore engineers fashioned a lightweight, high-integrity 20-pound bracket from 400 pounds of encapsulated Annealed Pyrolytic Graphite (APG). “We built this fairly elaborate bracket with our [APG] in two planes, so it provided a highly conductive path to an array where they had heat pipes, which dumped it to a radiator that dumped it out into space,” says Montesano.
Part of the challenge when designing brackets and support structures that conduct is the give-and-take that occurs between stiffness and conductance. An area that requires tensile-type loading might need to be replaced with graphite for conductance, which doesn’t have the same stiffness as aluminum. Montesano says, “With this situation, in terms of natural frequency, because the material was less dense, the little bit they did lose with stiffness, they got back because things were less dense.”
The Ever-Present Enemy, Time
NASA is infamous for taking ages to get from concept to reality, but the k-Core® team found themselves under tight deadlines. Montesano explains, “The problem with this mission was that the thermal infrared system was basically going along for a free ride on the Landsat.” NASA had a hard launch date for the satellite and an area available with some mass budget left over. “If this wasn’t ready to go in time, NASA would have just put up a ballast, a mass model, to keep it balanced,” he adds.
The company was provided with the spatial design requirements and what the three thermal systems had to do, in terms of thermal and structural parameters. Thermacore took NASA’s set design and turned it into a composite. They went in and designed where the graphite would be placed, and the scheme on manufacturing it. “We had weekly meetings with the team, so they were involved every step of the way.” After some iterations and a pre-flight model were completed, NASA ran it through a gauntlet of tests.
These tests are standard procedure for Thermacore, but the sign-off process with the space agency added some friction to a commonly smooth process. Montesano explains, “A lot of the hard inspection points were a little more elaborate than what we typically do. They would inspect the pre-bond materials, the post-bond materials, and the pre-machined materials, and each one was a sign-off, which makes it a challenge in schedules.” These paper-trails are commonplace for the company, but with space applications that paper trail’s accuracy is vital. “Just dealing with NASA, in general, there’s a lot of paperwork. The paperwork weighed as much as the part,” Montesano says. “In terms of the overall complexity of the part, it was high, and the documentation trail that we had to support made it a bit of a challenge.”
Simulation software seems to be written for the likes of composite testing that Thermacore executes. Throughout the design phase, the team used a SolidWorks model. “It’s pretty easy to do a finite element mesh and ready some steady-state thermals,” says Montesano. “After we build something, we’ll often run and mesh conductance values, and run parts through to determine what the natural frequencies actually are.”
After NASA set up the layout of the overall part, they passed the design to Thermacore who modeled the composite. Thermacore then provided NASA with some strength and stiffness data, and the space agency performed modeling and analysis. Utilizing that information, NASA came up with the vibration requirements for the thermal brackets. “They knew what they could expect from the material. Some customers have no idea what this material is capable of, but with NASA, they are up on the learning curve,” explains Montesano. Though time was tight, Thermacore and NASA have a familiarity that allowed for a quick time-to-market, or in this case, time-to-launch.
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