Payloads requiring insertion into high altitude orbits are delivered using the upper stages of chemical rockets (ex., Delta and Atlas classes) normally employing cryogenic propellants. During the transfer period between orbits, the upper stage may coast for several hours during which time the thermodynamic state of the propellants may vary due to solar heating. At the conclusion of the coast phase, and in preparation for orbital insertion of the payload, the propellants must be within a

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... defined range of temperature and pressure for the engine to resume operation. Buoyancy-driven thermal stratification of the propellant is one of the critical mechanisms taking place during this coast phase. Traditional stratification models are based on velocity and temperature correlations developed for flow along smooth vertical walls. In contrast, actual propellant tanks may have a mass-reducing Isogrid internal surface over which the velocity and temperature profiles differ significantly from smooth-wall correlations. A preliminary study to investigate the impact of Isogrid on the boundary layer has shown that the thickness of the layer adjacent to the wall in a forced freestream flow is substantially thicker (150-700%) than the equivalent flat plate boundary layer thickness. Furthermore, the flow is highly turbulent with many recirculation zones suggesting that the classical idea of a boundary layer may not exist over such geometries.