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

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Figure 11 shows the lift-to-drag ratio curve. A positive lift to drag ratio occurs when pitch is at zero degrees orhigher. Positive pitch will allow StarLight to climb above neutral buoyancy. In comparison, the results ofcalculations from a laminar assumption results in lift-to-drag ratios of higher than 5 for zero degrees of pitch angle,going up to about 10 at pitch angle of greater than 8 degrees.Lift to Drag RatioWind Velocity = 30 knots64Cl / Cd_20-10 -5-20 5 10 15 20-4-6Pitch Angle (deg.)Figure 11. Lift to Drag Ratio Vs. Pitch Angle at 30 knotsC. Envelope Material Selection and SizingHistorically, one of the greatest technical challenges to LTA design is developing an acceptable hull material thatmeets the programs mass, life, and cost constraints 8 . The envelope material must be able to withstand the day/nightand seasonal temperature changes at the operating altitude, in conjunction with the stresses and loads associated withoperation. It is also important to note that the material must be able to survive the dynamic effects of deployment,launch and ascent. For these reasons, and considering the harsh space-like environment at the altitudes between65,000 and 110,000 feet, our design approach is to use a commercially proven, multi-layered polyesterfilm/polyester fabric. Our team also chose to decline to use photovoltaics on any part of the balloon surface becauseof the thermal impact on the material. The other driver for selecting a polyester film/fabric for the envelope materialis cost, since the upper stage is disposable. Higher tenacity fibers such as Vectran or UHMWPE are available for usein the construction or lower mass materials, but at significantly higher cost. These higher cost fibers and laminatesare still in consideration, especially if they can provide a system level performance benefit such as longer endurance,but at this time polyester is the baseline fiber in the laminate.A parametric super-pressure hull sizing model was created for sensitivity studies. One important parametriccomponent of the sizing model is material selection. Model inputs include a table of potential material candidatesincluding their strengths and areal weight. The sizing model selects the best candidate material based on thestructural margin goals. An early investigation was to determine if commercial off the shelf (COTS) materials couldbe used for the upper stage envelope. COTS material laminates with a polyester tenacity of 4.5 gm/denier wereevaluated versus modified COTS laminates with a higher polyester tenacity of 8.3 gm/denier. The Modified-COTSmaterial design points were superior in terms of envelope weight by a large margin. The 3 layer lay-up of thebaseline polyester laminate material is shown in Figure 12.10American Institute of Aeronautics and Astronautics
Polyester Film, Clear, 0.5 milAdhesiveFabric, PolyesterFigure 12. Baseline Envelope Material Lay-upIn addition to cost and strength considerations, the hull material has also been selected for its low areal weight,low permeability, and its compatible with state-of-the-art airship envelope manufacturing processes. A polyesterfabric is used to provide strength in the warp and fill directions while a polyester film layer is bonded to the fabricusing an adhesive to create a barrier layer to contain the helium inflation gas. The baseline coated fabric hullmaterial has a tensile strength of 60-lbf/in in the warp direction and 120-lbf/in in the fill direction. Related UVexposure testing indicates that this hull material will have the endurance to withstand the UV environment above80,000 feet for the 6 week mission, as a similar material was originally selected as a candidate hull material for theAlternate Ultra-Long Duration Balloon (AULDB) concept 9 . The AULDB program had an intended mission durationof 100 days at 100,000 feet.One immediate outcome of the initial sizing model sensitivity runs was the strong influence that the maximumtemperature increase has on the size and weight of the envelope. Accordingly, one of the main objectives for thisinitial model was to estimate the maximum diurnal increase in temperature. The thermal analyses were run at vernalequinox with latitudes between 0 and 45 degrees. The estimate for maximum temperature increase wasapproximately 32C and came from a run with 0 degree latitude. A plot of the helium temperature for this case isshown in Figure 13. This value of diurnal temperature increase was an important input for subsequent sizing andstress analysis modeling. It was found that the temperature results from the thermal analysis were in good agreementwith those from the trajectory modeling. As part of the trajectory modeling, the diurnal temperature result was usedto calculate the pressure change in the balloon envelope as shown in Figure 14.The sizing model contains a first order for maximum stress in the upper stage under a uniform inflation pressure.A more detailed stress prediction is needed to support the design effort of the upper stage. A finite element modelwas constructed for the upper stage with the intent of evaluating stresses due to maximum super pressure. A quartersymmetry model was constructed for this task. The mesh for this model is shown in Figure 15 along with someprinciple stress contour results. The results indicated that the first order calculation in the preliminary sizing modelwas over predicting stress by a small margin. The sizing model for the upper stage was updated with this moredetailed stress prediction.11American Institute of Aeronautics and Astronautics
Page 5 and 6: does not utilize a ballonet. The la
Page 8 and 9: Figure 7. Inflated Table Top Model
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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