An implicit ablation and thermal response program is presented for simulation of one-dimensional transient thermal energy transport in a multilayer stack of isotropic materials and structure which can ablate from a front surface and decompose in-depth. The governing equations and numerical procedures for solution are summarized. Solutions are compared with those of an existing code, the Aerotherm Charring Material Thermal Response and Ablation Program, and also with arcjet data Numerical experiments show that the new code is numerically more stable and solves a much wider range of problems compared with the older code. To demonstrate its capability, applications for thermal analysis and sizing of aeroshell heatshields for planetary missions, such as Stardust, Mars Microprobe (Deep Space n), Saturn Entry Probe, and Mars 2001, using advanced light-weight ceramic ablators developed at NASA Ames Research Center, are presented and discussed. Nomenclature a B' F g h h I' K k rh = absorption coefficient, m"' = rh / p e u e C M , dimensionless mass blowing rate = pre-exponential constant in Eq.(8), s" 1 = Stanton number for heat transfer = Stanton number for mass transfer = specific heat, J/kg-K = activation energy in Eq.(8), J/kmol = exponential integral function = view factor = outward pyrolysis mass flux, kg/m 2 -s = enthalpy, J/kg = partial heat of charring, defined in Eq.(6), J/kg = radiation source function in Eq.(2), W/m 2 -sr = radiant intensity in +x direction, W/m 2 -sr = radiant intensity in -x direction, W/m 2 -sr = a + o s , extinction coefficient, m" 1 = thermal conductivity, W/m-K = mass flux, kg/m 2 -s * Aerospace Engineer. t Aerospace Engineer. Senior Member AIAA. P = pressure, N/m 2 q c = conductive heat flux, W/m 2 qn = radiative heat flux, W/m 2 R = universal gas constant, J/kmol-K s = surface recession, m s = surface recession rate, m/s T = temperature, K u = velocity, m/s x = y -s, moving coordinate, m y = stationary coordinate, m Z* = coefficient in Eq.(9), defined in Ref. 14 a = surface absorptance e = surface emissivity F = volume fraction of resin K = optical thickness K D = optical thickness for path of length D A, = blowing reduction parameter 9 = time, s p = total density, kg/m 3 a = Stefan-Boltzmann constant, W/m 2 -K 4 a s = scattering coefficient, m" 1 I = mass fraction of virgin material, defined in Eq.(5) T = decomposition reaction order in Eq.(8) subscripts c = char e = boundary-layer edge g = pyrolysis gas i = density component (A, B, and C) j = surface species v = virgin w = wall