FEA Software Enables Design of Science Instruments for Airborne Observatory
NASA Tech Briefs, Mar 2005
COSMOS/M(TM) finite element analysis (FEA) software
Structural Research & Analysis Corp. (SRAC)
Los Angeles, CA
310-207-2800
Co-sponsored by NASA and the German Aerospace Center (DLR), the Stratospheric Observatory for Infrared Astronomy (SOFIA) project is an airborne observatory that will study the universe in the infrared spectrum. The SOFIA project involved designing and building a 2.5-meter telescope and converting a Boeing 747-SP jetliner to house the telescope, which will peer through an open cavity in the side of the plane. The project also involved designing and building science instruments such as the High-Speed Imaging Photometer for Occultations (HIPO) that will mount to the telescope flange and analyze radiation from celestial objects.
A seven-person team, headed by Dr. Edward Dunham at the Lowell Observatory in Flagstaff, AZ, designed and built the 750-pound HIPO science instrument, which has been used for initial ground-based SOFIA tests, and will be used for the flight test program and for subsequent scientific observations.
The HIPO serves as a camera to capture images viewed through the telescope, which will sit in a bay exposed to stratospheric conditions while the science instruments and the mission crew will work in the pressurized aircraft cabin. Since HIPO will form part of the aircraft pressure boundary, its design had to deal with thermal and pressure loads, the turbulence of operating at high altitudes, and Federal Aviation Administration (FAA) requirements for airworthiness, all while weight and space were constrained. The design's ultimate effectiveness is measured by its contribution to image motion and quality.
Another science instrument developed for the SOFIA project is FLITECAM (First Light Infrared Test Experiment CAMera), a 500-pound instrument built at the University of California at Los Angeles (UCLA). FLITECAM can be attached to the HIPO mounting flange as shown in the figures of the finite element model.
Lowell Observatory contracted with engineer Eugene Loverich to perform a finite element analysis (FEA) evaluation of the preliminary design to ensure that the HIPO instrument would meet structural design objectives for weight, displacement, and natural frequencies. Specific objectives were to predict the displacements and the six lowest free-vibration natural frequencies for the initial design of the HIPO, evaluate the predicted performance of the HIPO, and provide appropriate redesign recommendations if necessary.
Loverich used COSMOS/M(TM) FEA software to generate and analyze the finite element model of the HIPO instrument both with and without the FLITECAM instrument attached. The element types used in the finite element analyses included eight-node and nine-node isoparametric quadrilateral shell elements, six-node isoparametric triangular shell elements, two-node uniaxial beam elements, two-node uniaxial truss elements, and one-node concentrated mass elements. The one-node mass elements were used to structurally represent 22 optical components and electronics boxes that attach to the HIPO. The final configuration redesign based on FEA evaluation of the HIPO with the FLITECAM instrument attached was modeled using 29,826 nodes and 7,863 elements.
Analysis determined that the original configuration finite element model of the HIPO did not meet the structural design objectives involving natural frequencies. Subsequent to numerous model trial modifications and FEA iterations, various structural enhancements were made to the original model so that the structural design objectives involving natural frequencies, displacements, and structure weight would be met. These structural modifications involving increased plate thickness dimensions, structure additions, and structure component rearrangement were included in the redesigned finite element model and the associated model versions of the HIPO.
Major prototyping by Lowell Observatory was impossible as a result of the prohibitive cost and time involved. Using FEA software enabled the researchers and designers who developed and built the HIPO instrument to verify performance and optimize material use, while anticipating and correcting flaws before finalizing the designs and building the instrument.
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