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A numerical technique has been used to determine the free-molecule drag on a straight chain of uniform spheres moving parallel and perpendicular to its axis. For such a concave-shaped body, certain surface elements are inaccessible to molecules having certain velocity components because these areas are screened by neighboring elements. Likewise, reflected molecules do not always escape directly but may experience more than one reflection due to screening by neighboring elements. Because an analytical solution for the drag seems hopelessly complex, we have used a Monte Carlo method to determine the rate of momentum transfer (drag force) due to the molecular collisions and reflections of large numbers of molecules by following their individual dynamics. The results are discussed in terms of the translation of straight chains having fixed orientation, straight chains undergoing Brownian rotations, and branched and kinked chains.

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