AUG 2016


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47 account for the deformations in the structure and mesh due to spinning." "We built the reflector and boom knowing exactly how the spinning shape would differ from the original shape," Ochoa explains. "We had to make sure that the reflector shape changed in a controlled manner, so we attached specific masses, or counter- balances, on the reflector where necessary." ere also was a strict tolerance — 350 millidegrees — on where the RBA would point, i.e., where it would reflect the RF beam. Ochoa translates, "One millidegree rotation of our assembly trans- lates to an offset of about the thickness of a hair at the reflector tip." Details of deployable structure design e minimum resonant frequency for the stowed RBA was 50 Hz in the axial direction and 35 Hz in the lateral. It also had to resist random vibration during launch. Femap with NX Nastran by Siemens PLM Software (Plano, TX, US) was used to develop multiple finite element models (FEMs) and perform various stowed RBA vibrational analyses. Femap also was used to calculate the RBA's mass properties and then re-run the mass properties model throughout the design process to ensure that the effective product of inertia (POI) and center of mass (CM) remained within required limits. Large uncertainties at the beginning of the RBA design process prompted a sensitivity study. A Monte Carlo simulation based on a Femap-created FEM was used to examine the effects of seven sources of uncertainty (see Fig. 2, above) on the RBA mass proper- ties. e simulation ran 10,000 mass cases, using a uniform distri- bution of random inputs for each uncertainty. Results showed the required accuracy for part size, CM and positional measurements, and identified parts critical for overall system POI and CM prop- erties. For these, the center of mass was verified with a measure- ment. e mass from the CAD model was used for all others. Success in space When the satellite was finally launched, positioned in LEO and spun up to operational speed, SMAP's RBA deployed as planned. JPL mission control reported that system alignment was and still is about as close to perfect as it could get. Northrop Grumman Astro Aerospace's lengthy design process and hard work clearly paid off. In late 2016, the company also was awarded the contract to supply the 12m diameter AstroMesh reflector for JPL's NISAR (NASA Isro Synthetic Aperature Radar) mission. What's next? "e market for large mesh deployable reflec- tors is very strong," says Ochoa. "ere are requests for products from all across the space spectrum, such as Starshade, a NASA/ JPL mission to identify Earth-like planets in other star systems. "Composites already feature heavily in our preliminary designs for Starshade," says Ochoa, "as well as for the large aperture deployable antenna for the NISAR spacecraft, which is designed to observe and measure some of the planet's most complex processes." He is confident that composites and Astro Aerospace are up to the challenge. SMAP Deployable Reflector Design CW senior editor Ginger Gardiner has an engineering/ materials background and has more than 20 years in the composites industry. Fig. 3 Furled and ready for launch The RBA, with reflector and hinged boom collapsed and secured for launch, is shown ready for transport to rendezvous with its launch vehicle before it went into space and successfully unfurled in 2015. Source | Northrop Grumman Astro Aerospace CAD Parts Data • CM location • MOI • POI • Mass (initial) Part Measurements • Part mass • CM, critical components • Position • Stiffness • Moisture loss Uncertainties • Mass • Center of mass • Positional • Dynamic distortion • Moisture loss • Thermal distortion Unspun FEM FEM Analysis (Mass, CM, MOI, POI) Spin-deflected FEM Monte Carlo Analyses (CM, MOI, POI, Effective POI, CMx) Compare FEM mass properties to CAD model mass properties RBA mass properties knowledge Fig. 2 Calculating critical mass properties FEM analysis was performed throughout the design process to check RBA mass properties, and a Monte Carlo simulation was used to reduce uncertainty in the mass knowledge process. Source | Northrop Grumman Astro Aerospace

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