Transitional Correction to the Drag of a Sphere in Free Molecule Flow

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Abstract
The mechanics of collisions between molecules in front of a sphere moving at hypervelocity through a rarefied atmosphere are analyzed. The study is concerned with the ``transitional'' regime in which molecules emitted by the surface begin to shield the body from the oncoming Newtonian stream and the net drag decreases from its free-molecular value. It is found that a simplified but physically significant model can be formulated, and that it leads to a dependence of the drag coefficient on two transition parameters. These can be interpreted as the Reynolds number and the surface-to-free stream temperature ratio. The predictions of this theory agree qualitatively and in order of magnitude with observations.

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