High-Performance Composites

JUL 2014

High-Performance Composites is read by qualified composites industry professionals in the fields of continuous carbon fiber and other high-performance composites as well as the associated end-markets of aerospace, military, and automotive.

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J U L Y 2 0 1 4 | 3 7 the Arecibo telescope, in Puerto Rico. However, even a dish of this size can- not match the resolution of an optical telescope. To produce photo-equivalent detail and resolution, a radio telescope's re- fector dish must have an area measured in kilometers, not meters. This can be accomplished in one of two ways. One is to manufacture one large dish that measures about 1 km 2 /0.39 mi 2 . Such a large dish, however, would have to be in a fxed position and thus could not be repositioned to receive signals from different regions of space. Ideally, then, radio signal exploration would require a large adjustable dish, able to maintain po- sitional accuracy within ±1 mm/0.04 inch. A more feasible alternative is to manu- facture a series of smaller dishes in an ar- ray known as an interferometer. Practically, these individual dishes may be located anywhere, as long as each can "see" the same portion of the sky. The goal of an interferometer, then, is a combined dish area of about 1 km 2 . Signals received by each dish in an interferometer are centrally collected and processed by a correlator, which combines the signals (adjusting for each telescope's position in the array) to simulate a much larger telescope. This is the premise of the Square Kilo- metre Array program (SKA, Manchester, U.K., www.skatelescope.org). A multi- national effort, its participants hope, by 2030, to have in place thousands of radio telescopes clustered in remote regions of South Africa and Australia. SKA envi- sions the manufacture of three types of radio telescope: A conventional circular dish, a low-frequency aperture array and a mid-frequency aperture array. SKA has gathered consortia of manufacturers and software developers to create the tele- scope systems and the other equipment that the project requires. One of the frst deliverables is a dish type, with a 15m/49.2-ft diameter primary refector, which is being competitively prototyped by three organizations, one in Canada (called DVA-1), one in China (DVA-C) and one in South Africa (Meer- KAT 1). Of these, only the Chinese and Canadian versions employ carbon fber composites in signifcant amounts. The Canadian fabrication marks one of the most ambitious applications of carbon fber composites in a dish structure of any kind. Building the perfect dish The Canadian DVA-1 dish is an offset Gregorian type. Its mostly composite 15m primary refector refects captured radio signals up to a 4m/13.1-ft diameter secondary refector dish, positioned on support legs over the main dish. From the secondary refector, radio signals are received by hardware on a feed support platform located between the second- ary and primary refectors (see opening image). Of the other critical structures, some are fabricated from metals and some are fabricated from composites. The main dish's support truss, as well as the pedestal on which it rests, for exam- ple, are metallic. However, the feedlegs that support the secondary refector and feed support platform are manufactured with carbon fber composites. DVA-1 was built by the National Re- search Council Canada (NRC-Canada, Ottawa, Ontario, Canada). Most produc- tion work was done at NRC's facility in Penticton, British Columbia. Gordon Lacy, an NRC-Canada mechanical en- gineer and DVA-1's lead engineer, has been working on dish design and devel- opment since 2006, adjusting to shift- ing political and fnancial winds as a variety of governments and government agencies around the world entered and exited the SKA project. He admits that the decision to use carbon fber might be questioned because of the material's cost, but when NRC-Canada looked at the mechanical requirements, he says carbon fber was the logical choice. "The dish requires very high preci- sion of the refective surfaces under wind, gravity and temperature changes," he notes. "For the best radio-frequency performance, we need the refectors to retain their design shape under all con- ditions. We also need the refector to have a high refectivity to radio waves. For this we have developed an embed- ded metallic layer. Carbon fber has a very high stiffness-to-weight ratio, which enables us to more easily meet the grav- itational distortion requirements. Car- bon fber composites also have very low coeffcients of thermal expansion [CTE], which helps us to meet our thermal sta- bility requirements." To meet all of DVA-1's tolerance re- quirements, NRC partnered with com- posites manufacturers that were ac- customed to working to that level of precision. Among the frst was Janicki In- dustries (Sedro-Woolley, Wash.), which supplied the massive mold for the main refector. The tool is possibly the most critical component, says Lacy, because its accuracy dictates the dimensional tolerance of the fnished product. "The tool had to keep the shape of the mold, or we had to be able to predict very ac- curately what the shape change was go- ing to be," he recalls. "We opted just for keeping the shape under control, so it's very close to the mold shape." Janicki manufactured a fve-piece, pro- totype-type mold of fberglass, topped by a putty layer, on a steel substructure. Lacy says NRC-Canada used a FARO (Lake Mary, Fla.) laser tracker to ac- Prototype telescope dish The Square Kilometre Array is a multinational effort to deploy thousands of high-performance radio telescopes in remote regions of South Africa and Australia to receive space- based radio signals. One design possibility is a 15m/49.2-ft diameter Gregorian offset type (illustrated here), which features a primary and secondary dish-type reflectors and secondary support structures made with carbon fiber composites. A prototype is being manufactured by the National Research Council Canada (Ottawa, Ontario, Canada). Source: NRC-Canada 0714HPC IM_NRC-OK.indd 37 6/17/2014 10:54:55 AM

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