A Novel Concept for Stratospheric Communications ... - Team-Logic

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Figure 7. Inflated Table Top Model of the StarLightB. Aerodynamic AnalysisHigh fidelity analysis of the platform aerodynamics during development is critical to validating the design andincreasing the probability of successful achievement of performance requirements. For the aerodynamics of theupper stage the lowest possible drag coefficient was sought while maintaining a positive lift to drag ratio throughoutthe flight regime. Near Space Systems conducted internal computational fluid dynamics (CFD) analysis on the upperstage shape throughout the design process to validate or invalidate design characteristic and projected performance.In May, 2006, the United States Air Force approved a project to independently analyze the flight characteristics ofStarLight. Based on current mission needs, the government wanted to validate that the StarLight design couldsuccessfully operate in the cold, rarified environment of the earth’s stratosphere in a persistent manner. Theprocessing capability of the Major Shared Resource Center (MSRC) at Wright-Patterson Air Force Base in Dayton,Ohio allows significantly higher fidelity CFD analysis of StarLight than the earlier lower fidelity CFD modelingcould provide. The MSRC is a computational science facility supporting DoD research, development, test, andevaluation using high-performance computing and visualization. Critical aerodynamic performance factors wereused in the CFD modeling. Extensive resources in terms of CPU time have been devoted to this task. An exampleoutput of the simulations on the chosen upper stage shape is shown in Figure 8. These results, along with themanufacturing considerations, are the main reason that this particular shape concept was chosen. The results ofMSRC CFD analysis is a higher design confidence, an optimized balloon shape for maximum performance, andlower overall project risk.Figure 8. CFD Analysis of the StarLight Upper StageThe MSRC CFD analysis on the StarLight saucer shaped body is focused on how much drag and lift theaerodynamic lifting body presents to the wind as a function of angle of attack or pitch angle, altitude and windvelocity. Once these aerodynamic forces were calculated, codes were developed to estimate the flight profile formaintaining loitering position relative to a fixed geographical location as a function of wind velocity and direction.The results of the CFD codes are critical to the assumption of either a laminar or a turbulent flow. The laminar8American Institute of Aeronautics and Astronautics
assumption results in higher (optimistic) lift to drag ratios for the different pitch angles. For the StarLight design,we used turbulent flow assumptions, the most conservative case. Using 3-D mesh models of the StarLight shape,below are the outputs (Figures 9-11) from the CFD calculations assuming the turbulent flow methodology.Figure 9 shows that pitch angles from zero degrees and higher results in positive lift. This means that theStarLight can pitch up to climb above neutral buoyancy using aerodynamic lift. Altitude gained above neutralbuoyancy is energy that is used to accelerate the StarLight in a dive allowing it to recover ground distance lost inhigh velocity wind surges. StarLight’s vertical maneuver is enabled by the aerodynamics of the upper stageballoon shape and functionality of the vehicle’s flight control systems, separating StarLight from other LTAconcepts.Total Lift ForceWindVelocity= 30knotsLift Force (nts) )150010005000-10 -5 0-5005 10 15 20-1000Pitch Angle (deg.)Figure 9. Lift Force vs. Pitch Angle at 30 KnotsFigure 10 shows the drag component for pitch angles with 30 knots of head wind. Notice that the drag remainslowest between minus 5 degrees nose low to 7 degrees nose high. This information helps to identify pitch changesthat provide the highest lift to drag ratio. The higher the C L -C D ratio, the greater the climb above neutral buoyancy.Zero degrees of pitch provide the least amount of drag to maximize thrust.Total DragForceWindVelocity=30knotsDrag Force (nts)e (350300250200150100500-10 -5 0 5 10 15 20Pitch Angle (deg.)Figure 10. Drag Force Vs. Pitch Angle at 30 knots9American Institute of Aeronautics and Astronautics
Page 5 and 6: does not utilize a ballonet. The la
Page 10 and 11: Figure 11 shows the lift-to-drag ra
Page 12 and 13: -20Temperature (C)-30-40-500 0.5 1
Page 14: VI. ConclusionThe StarLight hybrid

A Novel Concept for Stratospheric Communications ... - Team-Logic

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