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By Greg Clark
Staff Writer
posted: 09:45 am ET
02 March 2000

carbonsail_000302

A remarkable new carbon-fiber material is causing a revolution in the way scientists are thinking about laser and solar sails, according to engineers at NASA's Jet Propulsion Laboratory (JPL).

The fiber is a departure from solar sails of the past: it is about 200 times thicker than the thinnest solar sail materials, but so porous that it weighs the same.

"It's one of those things that comes along every so often that's kind of a technology breakthrough and at the same time, an honest-to-goodness paradigm shift," said Robert Frisbee, who analyzes advanced propulsion mission concepts at JPL.

"Always, I mean at least since the 1920s, people have thought of solar sails, laser sails, as basically being thinner and thinner sheets of solid material."

In space, solar sails would reflect photons of sunlight, thus harnessing their momentum. The sails, which could also be propelled by lasers, must be super-lightweight in order to benefit from the massless particles bouncing off their surfaces.

In addition, the sail surfaces must be tremendously reflective to maximize the propulsive force from the particles of sunlight.

Only in the past decade has it even been possible to manufacture sails that are thin enough, light enough and able to be coated with the proper reflective coating that sails require. But recent advances have finally made solar sails an option for certain featherweight spacecraft of the future.

Solar sails are now the propulsion method of choice for proposed NASA programs that strive to travel past the boundaries of the solar system, and to keep a gargantuan telescope hovering in space. Still, there is more work to be done.

The new carbon fiber mesh, which is developed by Energy Science Laboratories, Inc. in San Diego, California, could be the first step. The mesh is composed of a network of carbon fibers crisscross linked into a matrix that is mostly empty space. One hundred of these carbon fibers bunched together would make up a strand the size of a human hair.

A piece of rigid carbon sail material, made by Energy Science Laboratories, Inc. "floats" in the air above a model's hand. At 3 grams per square meter, the fiber is 25 times lighter than standard copier paper. (Credit: ELSI)

The finished product, which is about as thick as the cover of a hardback book, is lighter than all but the thinnest plastic sheeting. Even the thin plastic that wraps a cigarette pack, or a batch of index cards is heavier than this carbon fiber material, said Tim Knowles, president of Energy Science Laboratories.

"You know how when you finally get that crap off the index cards, it's clinging to your fingers, and you've got to sort of shake your hand to get rid of the wrapping? Well, that stuff has an area mass of 15 grams per square meter," Knowles said. "Our stuff has an area mass of about 5 [grams per square meter]."

There are a few problems with traditional sail membranes: these sheets tear easily, they require a relatively heavy support structure to stretch them out and keep them tensioned, and they can build up static electricity. Moreover, ultraviolet light degrades these membranes and they melt easily at high temperatures, so they can never get too close to the sun. Another drawback: they can't be propelled by high-energy lasers.

However, the carbon fiber mesh is 25 times lighter than a sheet of typical office paper the same size, yet rigid enough that you can pick up a piece at one corner and the sheet will not bend, Knowles said.



"It's really my candidate for magic."


"We're making things that are 10 times lighter than that little plastic, but they stand in your hands like a stiff cardboard plate. You can take the material and then pleat it -- zigzag pleat it -- and then hold this sort of stack with your hand on the table," he said. "But when you move your hand, it jumps up like a jack-in-the-box, and falls flat on the table and you can't even see where it was creased."

The fabric is receiving tremendous notice because of its promise to self-deploy. Instead of having complicated and cumbersome deployment mechanisms to unpack and stretch out films in space, this carbon scrub-pad material could be packed so that it pops out flat once it is released.

"It's really my candidate for magic," Frisbee said. "Now we've sort of turned everything on top of its head It's like in the cartoons when the little light bulb goes on over the guy's head. It's got people thinking in new ways about an old problem."

The carbon fiber is also a great leap because it can tolerate temperatures as high as 4,500 degrees Fahrenheit (2,500 degrees Celsius). This gives it a great versatility and durability that other materials lack, said Charles Garner, a JPL senior engineer who works with solar sails.

"The dream would be then you can make these ultra-, ultra-lightweight solar sails out of carbon fiber that can be either taken very, very close to the sun to get a big kick, or can be hit with large amounts of laser power or microwave power," Garner said.

Recent experiments at JPL have demonstrated that lasers can indeed push this carbon material, which glows hot orange and white from the power of the laser beam, but keeps its integrity as a sail.

Henry Harris, a JPL engineer who is managing research into laser and microwave propulsion, said the materials are a huge advance.

"We got the temperature of the sail up to 2,600 degrees Centigrade (4,712 degrees Fahrenheit), which is really good news," Harris explained. "What it means is that it's like any kind of engine: we can run the engine at much higher power levels than we had previously anticipated because we're using advanced materials. And so that has tremendous implications for the future of interstellar spaceflight."

Implications, Knowles said, that do not end with solar sails.

"When you see the stiffness of these things you don't have to limit yourself to thinking about sails. They can be antennas, they can be propulsion systems like sails, or they can be support structures for other things that a spacecraft needs," such as solar cells, or optical sensors, or even flexible electronics circuits, he said.

 

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