AUG 2016


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AUGUST 2016 46 CompositesWorld FOCUS ON DESIGN instead of scattering if the light were pointing into open space. For the SMAP reflector antenna, the radar feed is similarly placed at the focal point of the parabola. e boom holds the reflector out in front of the light bulb, which, in this case, is the radar and radiometer feed, so that all of the radio frequency (RF) energy leaves collimated (in parallel), focused into a fairly tight beam. Lightweight composite solution For the SMAP reflector, Astro Aerospace chose its latest and lightest AstroMesh Lite product line. Optimized for 3-8m aperture reflectors, its surface is a knitted, gold-plated molybdenum wire mesh supported by hundreds of aramid-reinforced poly- etherimide (PEI) tapes. "We approximate a perfect parabolic surface via a supporting lattice of triangular facets made from these composite webs," Ochoa explains. e tapes use Twaron T2200 fiber (2420 dtex), supplied by Teijin Aramid (Arnhem, e Neth- erlands), and Ultem 1010 PEI from SABIC (Pittsfield, MA, US). In a proprietary process, the tape is pultruded by TenCate Advanced Composites (Morgan Hill, CA, US). e composite webs — some as long as 6m — were welded together to form the support lattice, which was then attached to a collapsible circular perimeter truss structure made from carbon fiber-reinforced epoxy tubes and bonded joints (see Fig. 1 and Learn More, above). "When the truss structure is fully deployed," says Ochoa, "it pulls the mesh and composite webs into tension, like a drum or high-performance tennis racket." Astro Aerospace structural engineering manager Michael Beers says the thermoplastic composite offers "a good balance between flexibility — allowing us to compact the whole structure, yet deliver the required stability — and stiffness, once deployed." e composite construction was a key to meeting mass requirements: e reflector weighs in at 25 kg, and the system, with boom, totals a mere 58 kg. Modeling to meet mass balance requirements "We provided very accurate mass data, moments-of-inertia and center-of-gravity specifications to JPL so they'd know the precise characteristics of this reflector," Ochoa recalls. "We modeled and weighed each one of our 15,000 individual parts as we were building them, then again in different phases of construction, through to final assembly." He notes that the mass measurements they needed were so unprecedented, "we had to invent new methods to acquire them." For example, machines used to balance aeroengine blades were brought in for some tests. "ey were so sensitive that breathing near them skewed the results," Ochoa quips. He explains this level of accuracy was imperative, "because the large reflector, designed to function in the microgravity of space, could not be cost-effectively spun-up in the 1G of Earth, so we had to be absolutely certain before launch." e RBA mass had to be kept within a 99g window, and the center of mass within 12.7 mm. e boom had to extend the reflector 3.35m off the deck of the spacecraft at a 40° angle and be stiff enough to maintain expected alignment without distorting the reflector surface when spun up to 14.6 rpm. "e shape of the reflective surface defined by the composite webs must be exact to provide the required radar performance," explains Ochoa. "e CFRP boom also would deflect with the load of the dish, so then the reflector moves as a result," he adds. To compensate for the deflection, says Beers, "we had to predict deflections for spinning, using computer modeling, and also Read this article online | Read more online in " In-orbit- deployable radar dish" | Fig. 1 Collapsible, but supportive and precision-shaped The AstroMesh Lite reflector for SMAP uses aramid fiber/PEI composite webs to support and shape its radar-signal-reflective, gold- plated molybdenum mesh surface, attached to a CFRP circular perimeter truss. Source | Northrop Grumman Astro Aerospace

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