Surface Step Induced Gap Heating in the Shuttle Thermal Protection System

Abstract

An analytical study has been performed to investigate the excessive heating in the tile-to-tile gaps of the Shuttle Orbiter Thermal Protection System due to stepped tiles. The excessive heating was evidenced by visible discoloration and charring of the filler bar and strain isolation pad that is used in the attachment of tiles to the aluminum substrate. Two tile locations on the Shuttle Orbiter were considered: one on the lower surface of the fuselage and one on the lower surface of the wing. The gap heating analysis involved the calculation of external and internal gas pressures and temperatures, internal mass flow rates, and the transient thermal response of the Thermal Protection System. The results of the analysis are presented for the fuselage and wing location for several step heights. Nomenclature Ac = cross-sectional area for flow B = width of flow path h = tile step height k = thermal conductivity Kp = permeability constant m = mass flow rate M = Mach number P = pressure q = conduction heat flux Rec = Reynolds number for base pressure correlation s = straight line length between points T = temperature w = tile-to-tile gap width X, Y = Shuttle Orbiter coordinates AJf = longitudinal coordinate measured in the downstream direction from the point of separation kXff = longitudinal coordinate measured in the upstream direction from a forward-facing step <5 = velocity boundary-layer thickness 6* = boundary-layer displacement thickness 6eff = d for laminar flow, 1.56* for turbulent flow A = sweep angle of a tile with respect to the local flow \L = viscosity coefficient p = density