Effects of various re-entry vehicle parameters on radio frequency attenuation

Abstract

The effects of antenna location, cone angle, nose radius, signal frequency, vehicle velocity, and wall temperature on rf signal attenuation are examined. For the assumptions of a clean air equilibrium flow and a thin slab electromagnetic model, the results indicate that a great reduction in rf signal attenuation can be achieved by proper aerodynamic shaping of the vehicle. Sample calculations are made for a transmission frequency of 600 me for nose bluntness varying from J-in. to 1-ft radius, cone angles from 10° to 60°, wall enthalpies from 300 to 1200 Btu/lb, and velocities from 20,000 to 24,000 fps for a range of altitudes of 50,000 to 200,000 ft. Altitude-velocity maps for lines of constant attenuation are also presented. Results indicate that, for sharp slender bodies, rf attenuation can be limited to 10 db throughout the re-entry envelope. Nomenclature a = speed of sound C Q = speed of light in a vacuum db = attenuation dS/dn = entropy gradient normal to streamline e = charge E = electric field f t = transmission frequency, cps g = mass conversion factor, 32.2 Ibm/slug hi = enthalpy h w = wall enthalpy / = energy conversion factor, 778 ft-lb/Btu J = current density L = plasma thickness m = electron mass N = number density of electrons HO = £0/Co = 27T/X0 Ri = radius of curvature in the hodograph plane R n = nose radius S = specific entropy T = transmission coefficient u = velocity component in axial direction v = velocity component in radial direction v = electron drift velocity v ni = velocity normal to ray v ri -velocity along ray V = freestream velocity V e = re-entry velocity, kft/sec V\ira = limiting velocity W = F/F lim X = (cop/co) 2 y = ratio of specific heats A0 = angular increment from ray to ray rj e = electron density, electrons/cm 3 0 = flow angle B c -cone half angle, deg X 0 = free space wavelength fj. = Mach anglê = stream function co = transmission frequency, rad/sec co c = collision frequency, collision/sec cop = plasma frequency, rad/sec