Abstract: A radar window for a reentry vehicle where the window is made of a composite material of boron nitride, silica, and boron nitride yarn. Silica in the range of 2 to 18 percent is the preferred range in the composite material and the boron nitride matrix ranges from 5 to 15 percent.

Abstract: Heat exchangers (2) with particular use as solar receivers are mass-produced by weaving on a textile loom a yarn of high modulus fibers (18, 20) into a repeating series of substantially flat preforms (4), forming a set of interconnected passageways (26, 30, 32) connecting one side of each preforms (4) with an opposite side thereof. The separated preform (4) are each wrapped around a mandrel to give each the shape of a cylindrical sleeve (2). The sleeve (2) is impregnated in a low modulus matrix, dried, and sintered for rigidization. In a first sealing embodiment, the outer sleeve surface (24) is sealed or weatherproofed, and the surfaces of the internal passageways (26, 30, 32) are sealed, but the radiation absorbing inner sleeve surface (22) is not sealed. In a second sealing embodiment, the outer surface (24), the radiation absorbing inner surface (22), and the exchanger ends (14, 34) are sealed, but the surfaces of inner passageways (26, 30, 32) are not sealed.