Some characteristics of exhaust plume rarefaction

In this paper the rarefaction of a free jet expanding into a region of finite background pressure is considered. It is found that the rarefaction process can be described by a simple rarefaction parameter D(P SP B^1^ / T . Here D is the sonic orifice diameter, P8 is the reservoir pressure, PBW is the background pressure and T is the background and reservoir temperatures which are considered to be the same. A simple scattering formulation of the complex physical problem is proposed. Comparison of the scattering prediction and previous molecular beam flux measurements by Fenn and Anderson are presented. A consistent physical description of a plume's approach to the limit of expansion into a perfect vacuum is discussed.