Plasma sheath transducer for axisymmetric re- entry vehicles
F aerodynamic stability, the center of gravity must be forward of the center of pressure Frequently, conical and other axisymmetric re-entry vehicles require highdensity ballast (e g , lead) located as far forward as possible to achieve stability Ballast preempts useful payload This note describes a transducer that serves two functions: it substitutes for ballast, and it provides useful information concerning the plasma sheath The transducer described herein is similar to one developed for measurement of electrical conductivityvelocity all in arc plasma jets and rocket exhausts l It is an extension of a transducer described by Shercliff and by Lehde and Lang 3 It consists of three circular coils with a common axis The two outer coils are driven at audio frequencies and are connected so that the magnetic fields are opposing Between the primary coils is a sensing coil The voltage that appears at the terminals of the sensing coil can be related to a U of the flowThe transducer will be located at the forward tip of the re-entry vehicle, if it is to serve as ballast In the stagnation region and just beyond the sonic line, the flow is changing rapidly The question is, how is the sensing coil voltage related to the flow properties? This note answers this question and briefly describes an experiment conducted in an arc plasmajet A re-entry vehicle with ablation cooling is considered The tip of the vehicle requires a thick layer of ablation material as shown in Fig 1 The coil arrangement is also shown in Fig 1 Calculations were made for the geometry illustrated The emf induced in the sensing coil is