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The Pioneer Venus probes approached Venus with high relative velocity. As they entered the atmosphere, they were rapidly decelerated by aerodynamic drag, and a great deal of heat was generated. To protect the probe structure and the scientific instruments, a carbon phenolic heat shield was placed on the front of the probes. Because the design of heat shields for planetary entry is a developing technology, thermocouples were placed in the heat shields so that actual and predicted heat shield performance could be compared. The function of the heat shield is discussed, the probe environments during entry into the Venusian atmosphere are described, and some results from the heat shield experiment are presented. It was found that for the most part, the heat shields performed better than expected.

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The purpose of the investigation was to determine the effects of various modes and levels of heat transfer on the effectiveness of the material (phenolic nylon) in providing protection against entry heating. To this end, detailed measurements were made and assessed of 1) the degradation of the material and 2) the interactions of the degradation products (char and vapors) with the external environment. The degradation was evaluated in terms of the velocities of the internal interfaces in the material and the mass rates of production of char and vapor. The interactions between the degradation products and the environment were convective-heat-transfer blockage by the vapors, gas-phase combustion, and surface combustion. Correlations were developed for these interactions as well as for the material degradation. The correlations are used to develop a method of predicting the effectiveness of the material in providing protection in heating environments other than those investigated. Material effectiveness is predicted for variations in the mode and level of heat transfer. NomenclatureB = mass-transfer parameter, (m^Aftapp/^J C = constant in Eq. (7) E = activation energy, Btu/lb-mole / = dimensionless stream function G = defined by Eq. (21) ht = stream total enthalpy, Btu/lb A/iap P = applied enthalpy potential, (ht -h s ), Btu/lb AJi cg = enthalpy potential due to gas-phase combustion, Btu/lb Heft = effective heat of ablation, Btu/lb H rc -heat of reaction of surface material, Btu/lb H r v = heat of reaction of vapors, Btu/lb k = thermal conductivity or mass flux of air per unit mass of vapors for stoichiometric mixture K 0 = frequency factor, Ib/ft 2 (atm) w sec m p -pyrolysis mass loss per unit area, lb/ft 2 mp = pyrolysis rate, lb/ft 2 sec m cp = char-production rate, lb/ft 2 sec ih cr = char-removal rate, lb/ft 2 sec m vp = vapor-production rate, lb/ft 2 sec n = pressure exponent p = pressure, atm Qapp = applied heating rate, Btu/ft 2 sec qb = convective heating rate blocked by mass transfer, Btu/ft 2 sec q chw = hot-wall convective heating rate, Btu/ft 2 sec q cg = gas-phase combustion heating rate, Btu/ft 2 sec q c s = surface combustion heating rate, Btu/ft 2 sec q n = net heating rate, Btu/ft 2 sec q r = radiative heating rate, Btu/ft 2 sec R = universal gas constant, Btu/lb-mole °R R n -body nose radius, ft T = temperature, °R V = interface velocity, ft/sec v = stream velocity component normal to body, ft/sec X = distance along body centerline from original position of unablated surface, ft x = distance normal to body surface, ft Z = compressibility factor a = surf ace absorptance A = increment of change e = surf ace emittance p = density, lb/ft 3 Presented at the AIAA Entry Technology Conference, Williamsburg/Hampton, Va., October 10-12, 1964 (no preprint number; published in bound volume of preprints of the meeting); revision received May 14,1965. * Research scientist. Member AIAA. t Research scientist. a--Stefan-Boltzmann constant \l/ = heat blockage factor (1 -qb/q chw ) Subscripts c = char e = boundary-la...