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A Boom in Satellite Engineering

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Hari Dharan holds one of the prototype booms. The machine beside him is used to push the molding form out from the center of the rolled carbon fiber tube. (David Pescovitz photo)

Two years from now, a group of five small satellites will ride a spacecraft "bus" into orbit where they'll gather data about Earth's magnetosphere. The satellites' 8-foot long articulated limbs, laden with various sensors, will measure the spectacular auroral eruptions behind such phenomena as the Northern Lights. It's the job of UC Berkeley mechanical engineering professor Hari Dharan and his students to ensure that the satellites' arms deploy properly and remain steady as they spin around the Earth at thousands of miles per hour.

Dharan is the director of the Berkeley Composites Labortaory, a state-of-the-art facility where he and his colleagues design, test, and fabricate new materials that will eventually be used in myriad structures, from buildings to airplanes to space vehicles. Composites, Dharan explains, combine filament and a resin bonding matrix to produce a new material with useful mechanical, electrical, or other high-performance properties. Fiberglass, for example, is one well-known composite.

Dharan's specialty is carbon fiber composites, materials in which carbon threads are woven or braided in specific directions to provide incredible strength with very low weight. These materials, Dharan says, are tailor-made for space applications, where the approximate cost of launching something into orbit is roughly $15,000 per kilogram.

"In addition to being very stiff and strong, the carbon fiber structures we build have one-fifth of the weight of steel," he says.

For three decades, Dharan has championed the use of carbon fiber composites for space vehicles. As an engineer at Ford Aerospace in the 1970s, Dharan helped construct the carbon fiber dish antenna for NASA's Voyager 1 space probe, now the most distant human-made object in space. Since then, he's spearheaded the recent shift from aluminum to carbon fiber in the construction of the International Telecommunications Satellite Organization's Intelsat communications satellites.

Graduate student Tyler Williams with the system he co-designed to test the boom. (David Pescovitz photo)

Right now, Dharan and his students are fabricating essential components for the NASA THEMIS (Time History of Events and Macroscale Interactions) mission, at $173 million program led by the UC Berkeley Space Sciences Lab. A collaboration between NASA, four universities, and seven foreign nations, the THEMIS satellites are slated for launch in 2006. Outfitted with magnetometers, electrostatic analyzers, solid state telescopes, and other instruments, the five identical satellites will give an unprecedented "bird's-eye view" of magnetic substorms. The missions' aim is to help scientists determine how energy from solar wind is transported and explosively released, visible to us in the form of the Aurora Borealis and Aurora Australis.

Inside Dharan's fabrication laboratory at the Richmond Field Station, the researchers are constructing and testing the hinged booms that will unfurl to deploy the sensors once the 200-pound microsatellites are in orbit. This is the last step in a process that began with developing the perfect carbon fiber composite recipe for the job.

"The main challenge here is dimensional stability and stiffness," Dharan explains. "Our carbon composites are designed to have a thermal expansion that is less than 1/100 that of aluminum. Once you deploy an 8-foot long beam, you don't want it wobbling back and forth with the large temperature changes that exist in space."

The design constraints were non-trivial, Dharan says, and much stricter than those applied for terrestrial applications. For starters, the sensitive electronics onboard make iron and other ferromagnetic materials completely off limits. Meanwhile, any new material used in space must undergo an arduous approval process, a problem when the research and development cycle is short.

"Our approach is to invent new combinations of materials that have already been approved by adding ingredients and arranging the fibers in different ways," Dharan says. "After all, the fundamental research in my laboratory involves characterizing new composites, new configurations, and mechanical behavior under loading."

Once the researchers settled on a material, they constructed prototype booms in the machine shop. The carbon fiber "fabric" material is wrapped tightly around an accurate metal form and then cured in an oven into the desired shape. Then, the form is pushed out of the tube using an incredibly strong but precise device. The booms are machined, assembled, and subjected to temperature cycling and vacuum comparable to that of space. Finally, they're tested under conditions that must adequately simulate the zero gravity environment of space.

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To accomplish that, graduate students Tien Tan, Tyler Williams, and Arezki Rahmani designed and built the entire hinged boom assemblies. The boom deployment mechanism is tested on the floor of the laboratory atop a piece of smooth acrylic. Frictionless air bearings pistons enable the boom to glide across the acrylic panel on a cushion of air so that the researchers can measure and tune the system as if it's floating in zero gravity.

In addition to the sensor booms, Dharan's group is building part of the main body structure for each spacecraft. Once the booms, structures, and mechanisms are complete, they'll be integrated into the rest of the space vehicle at the Space Sciences Lab. Then, in October 2006, if all goes as planned, Dharan will watch as his research truly takes off again.

Hari Dharan's home page

THEMIS Mission

"NASA funds $173 million auroral satellite mission" by Robert Sanders (Media Relations, 31 March 2003)

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