PAET Entry Heating and Heat Protection Experiment

A description is given of the aerothermo environment and associated thermal response of two low-density (270 and 450 kg/cm3), elastomeric silicone ablators determined during the entry of the NASA Ames Planetary Atmosphere Experiments Test (PAET) vehicle into the Earth's atmosphere. The vehicle configuration, entry parameters, and associated trajectory variables are presented. Measurements of both the surface convective heat-transfer rate and pressure on the beryllium nose cap are shown to be within 5 % of theory. The predicted and measured internal temperatures are compared at various depths in the forebody (Martin ESA-3560HF) and afterbody (Martin SLA-220) heat shields. Significant differences observed between experiment and theory for the afterbody thermal response are examined in relation to existing laboratory data. A a / h h. Nomenclature = surface area, m2 = acceleration, Earth g = drag coefficient = frequency, GHz = total enthalpy, Mjoule/kg = altitude, km = scale height, p/psl = e~hlh\ km 82.54 w cm1 *' = C0nstant K2 = ratio of local to zero-angle-of -attack stagnation point heating rate = 4/4s>a=0 K3 = ratio of local to zero-angle-of-attack stagnation point pressure K4 = constant used in Eq. (6), = 9.86 atm/[(kg/m3)(km/sec)2] M = Mach number m = mass, kg n — distance normal to surface, cm p — pressure, atm Q = total convective heat load, Q = qdi, Mjoule/m2 q = convective heat-transfer rate, w/cm2 Re — Reynolds number based on free stream conditions and body diameter Rn = nose radius, cm S = distance along surface, cm T — temperature, °C V — velocity, km/sec a = angle of attack, deg P = ballistic coefficient, m/CdA, kg/m2 A = change in quantity 9 — flight-path angle, degrees \L — molecular weight p — density, kg/m3 i — time from launch, sec — angle measured from axis of symmetry Subscripts e = entry s = stagnation point si = sea level w = wall of an unknown planetary atmosphere. The successful operation of the instrument complement throughout the entire entry required heat shielding to protect the probe from the convective heating encountered during the high-speed phase of re-entry. Since two of the instruments, a radiometer and a mass spectrometer, required a contamination-free stagnation region, a beryllium heat sink was selected for the spherical nose region to avoid ablation products. The remainder of the vehicle was to be protected by efficient low-density ablators. It was recognized in the early planning stages of the project that this heat-shield configuration would permit heat-transfer measurements to be made on the beryllium while the external flow property variations were monitored with time. This opportunity would provide an excellent and heretofore unavailable set of conditions for performing a meaningful heat-shield performance experiment. Accordingly, the design and implementation of such an experiment was adopted as an auxiliary PAET objective. This paper is a comprehensive review of the various heat-shield experimental elements including 1) a description of the probe and its entry trajectory; 2) details of the heat sink and heat-shield instrumentation ; 3) measurements and predictions of the surface pressure, convective heating rate, and related thermal response of both the forebody and afterbody heat shields; and 4) results from two special tests undertaken to qualify forebody heat-shield penetration and afterbody heat-shield radio-frequency (RF) transparency.