A series of reusable external insulation (REI) materials are being developed by the Re-Entry and Environmental Systems Division of the General Electric Company (RESD) for application to the Space Shuttle. These materials are bonded to a substrate in the form of panels resulting in numerous panel-to-panel joints. A plasmaarc ground-test facility has been used to evaluate the effects of the local heat transfer in these interface areas. Models with transverse, axial, and "tee" joints, both filled and open, have been tested. The major results of the test program are that the unfilled gaps run hotter than comparable undisturbed areas, and that for transverse and axial gaps a trend of decreasing temperatures with decreasing gap width is evident. The major conclusion is that unfilled butt joints are acceptable for the Orbiter if H/L > 17, but a conservative design approach would be to partially fill all gaps. NomenclatureH = depth of gap, enthalpy L -width of gap M = Mach number q = heat flux Qs = total energy flux crossing the shear layer dividing streamline over an Area A l Q A = total energy flux to the wall over the gap projected area A 1 if attached flow existed S* = dimensionless parameter, L/X 1 X 1 = wetted length e = emittance Subscripts A = projected area c = convection e = local m = model o = orbiter R = radiation RR = reradiation S = shear layer W =wall

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should be taken, however, in regions where large variations in flux or temperature are expected to occur.The present results may be extended by observing that the term "local" may be interpreted in the sense of a specific nodal surface rather than a point on a nodal surface, while "mean" refers to a collection or set of nodal surfaces. When this interpretation is made, it is possible to combine several nodal surfaces into subsets and formulate mean reflectances for the subsets, mean exchange factors among subsets, and mean-to-local exchange factors between nodes and subsets. In this way, a full set of N surfaces may be reduced to a smaller number of subsets, M < N. The advantage of this reduction lies in decreasing the size of the matrix implicit in Eqs. (20) and (22) and the economy of inverting the matrix to obtain Eqs. (21) and (23), respectively. Preliminary results indicate that this approach yields a sizable saving in computer time with accuracies suitable for most design applications. The analytical details and evaluation of this alternate interpretation will be the subject of a future communication.A qualitative evaluation of candidate thermal protection systems for a Mars-entry vehicle indicates that low-density charring ablators are most likely to meet all the requirements. Consequently, two such materials, foamed silicone and epoxy silicone low-density elastomers, were developed and characterized for this application. These materials, in addition to possessing good chemical resistance, are shown to have low thermal conductivities, high decomposition temperatures, and excellent low-temperature properties.

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