Forces on Convex Bodies in Free Molecular Flow.
by J. Pike.
AIAA Journal, Vol 13, No. 11, pp. 1454-1459, Nov. 1975.

Summary.
In this paper general expressions are derived for the aerodynamic forces on a body when the momentum transfered to the surface depends on the local surface inclination only. Applied to free molecular flow conditions, an added advantage of this technique is to separate the derivation of the forces from the uncertain physics of the gas surface interaction. The general expressions obtained for the forces contain constants of integration which are determined by reference to the particular body shape. As an example these constants are evaluated for any axisymmetric segment, and the expressions are used to find the drag of slowly tumbling axisymmetric bodies for a range of conditions, including typical Earth satellite conditions.

Comment.
Suppose we have a convex body in a free molecular flow. Then the calculation of the aerodynamic forces which act on the body is simplified in this paper to the evaluation of a series of integation constants. The constants separate the treatment of the body shape from the way in which the molecules react with the surface, allowing these effects to be studied independently. The analytical method allows the influence of the shape and molecular reflection constants on the results to be easily assessed, such that the significance of the various assumptions and components of the shape are apparent. In the case of optimisation studies, this understanding is important to try and ensure that the optimum found (by whatever means), is not some local optimum which could be improved upon for some different set of conditions.

Availability.
Obtainable from the AIAA Journal
or email j a c k @ j a c k p i k e . c o . u k

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Last amended: Dec 2013.