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

2. For wind > 30 knotsStarLight pitches upto maximum Cl/Cd.StarLight climbsabove neutral buoyancy3. At max Altitude gain,Lift=Gravity,StarLight will driftdownwind until windsurge passes4. StarLight will pitchdown to dive andaccelerate recoveringground distance lost indrift1. StarLight can resistwinds up to 30 knotsDirection of acceleration during diveNeutral Buoyancy AltitudeWind DirectionFigure 4. StarLight’s vertical maneuver is enabled by the aerodynamics of theupper stage balloon shape and functionality of the vehicle’s flight control systems,separating StarLight from other LTA conceptsA. Envelope DesignThe StarLight upper stage consists of the super pressure balloon envelope with its attachment lines andassociated load patches. The upper stage performance is critical in meeting certain operational system goals.Driving requirements for the upper stage are:1) Balloon shape lift and drag properties2) Inflated shape maintenance in all thermal environments3) Float altitude4) Structural integrity under maximum super-pressure5) Low cost, disposable design6) Materials durability and negation of pinholing (all flight phases)The lift and drag properties directly relate to the station keeping and maneuvering system requirements. Theupper stage balloon envelope must maintain positive differential pressure in order to achieve this ideal shape. Thechallenges for shape maintenance arise from the minimum cold state and the effect of leakage during operationallife. Float altitude is the traditional analysis trade for super pressure balloons. The maximum hot day at floataltitude determines maximum super pressure. This maximum super-pressure is used in the stress analysis of theupper stage envelope material, which would verify the structural integrity.There are several interactions and dependencies in the design calculations for super pressure balloons. In thecase of StarLight, the importance of the aerodynamic shape is added to these. In the performance of these analysesa best in class tool set was used including high fidelity CFD, Abaqus FEA, Navajo Trajectory 7 , and ThermalDesktop.The primary input to the patent pending envelope design was the discovery that ellipsoidal asymmetry plays asignificant role in overall aerodynamic performance. Parametric analysis of two primary functions, the envelopeheight-to-width ratio and the ratio of envelope volume above and below the equatorial chord line, revealed thatasymmetry in both areas contributed to the lowest drag coefficient with the highest lift coefficient. Both factorsrelate directly to performance in the upper atmosphere. A cross section plot of this ellipsoidal shape is shown inFigure 5. The challenge was to develop low stress inflatable shapes that came close to the theoretical shape, while atthe same time had acceptable weight. Design trades were conducted to develop potential envelope concepts. As6American Institute of Aeronautics and Astronautics