Buran was covered with approximately 38,800 heat-resistant tiles (compared with nearly 31,000 tiles on Columbia for STS-1). Each tile consisted of a substrate and a coating. The substrate came in two types with different densities. One was called TZMK-10 (with a density of 0.15g/cm3) and the other TZMK-25 (density 0.25 g/cm3). These were more or less comparable in characteristics and performance with the two basic types of Shuttle tile substrate (Li-900 and Li-2200). They were used in regions where Buran was exposed to temperatures of anywhere between 700°C and 1,250°C. The tiles were made of high-purity 98-99 percent amorphous silica fibres derived from common sand (SiO2—silica) with minimum amounts of natrium, potassium, and calcium oxides to lower the melting point of the fibers. The thickness of the tiles depended on where they were attached to the aluminum skin and the temperatures and aerodynamic stresses that any particular part of Buran was exposed to.
Both the TZMK-10 and 25 had special 0.3 mm thick glass coatings to reject heat and protect the tiles against wind loads and moisture penetration. This was very
Thermal protection 111
Post-flight picture of black and white tiles near Buran’s entry hatch. Note “smearing” of some tiles (B. Vis).
similar to the Reaction-Cured Glass (RCG) coating on the Shuttle’s tiles. Chemicals were added to the coating to give the tiles different colors and heat rejection capabilities. Black coating (both for TZMK-10 and 25) was mainly needed to protect the underside of Buran against the high temperatures of re-entry, with the higher-density TZMK-25 only being used in regions exposed to the highest stresses. The black – coated tiles on the belly could not be permanently exposed to sunlight for more than 6 hours. White coating (only applied to TZMK-10) mainly served the purpose of protecting the upper surfaces of the vehicle against solar radiation in orbit.
Although the coating provided some protection against moisture penetration, any cracks in the coating would easily let moisture through. Therefore, additional measures had to be taken to make the tiles waterproof. During manufacture the tiles were treated with a special silicon polymer solution, but that burned out during the first flight in all areas where temperatures exceeded +450° C. Therefore, the tiles would have needed to be rewaterproofed for any subsequent missions (had they been flown). For that purpose the Russians developed a varnish-like coating as well as a technique to permeate the tile with a substance known as hexamethyl disilazane. NASA uses a similar substance (dimethylethoxysilane) for rewaterproofing Shuttle tiles, but injects the material into the tiles, whereas the Russians planned to use a gas diffusion technique.
Since the fragile tiles could not withstand structural deflections and expansions of the aluminum skin, they were not attached directly to the skin, but to 4 mm thick felt
Buran sitting atop Mriya at the Paris Air Show in 1989. Square-shaped ATM-19PKP panels are visible on the mid fuselage (surrounding the name “Buran”) and on the upper portions of the payload bay doors (source: Luc van den Abeelen).
pads, which then in turn were bonded to the actual skin. Similar to the Shuttle’s Strain Isolation Pads, they were attached to the tiles as well as to the skin of Buran with an adhesive based on silicon rubber, ensuring a reliable bond in a temperature range of -130°C to +300°C.
Since the tiles thermally expanded or contracted very little, small gaps were left between them to permit relative motion and allow for the deformation of the aluminum structure under them due to thermal effects. The gaps were filled with a special felt-type material based on organic fibers and capable of withstanding temperatures of up to 430° C.
Tests showed that, if a tile was lost but the underlying felt pad remained in place, the temperature of the aluminum skin would not reach its 500°C melting point, even in areas where temperatures reached 1,250° C. If the felt pad was also lost, there could be damage to the skin, but only in regions close to where carbon-carbon panels were used.
About 28,000 of the tiles were trapezoidal in shape with sizes ranging from about 150 x 150 mm to 200 x 200 mm. Approximately 6,000 tiles were irregular and formed complex patterns on the hatches, around the nozzles of the engines, and on certain edges. Approximately 4,800 tiles had even more complex shapes. Although the distribution of black and white tiles over Buran’s surface was very similar to that on the Orbiter, there were different layout patterns. A fan-type pattern was used on the nose section, elevons, and the vertical stabilizer to avoid the use of triangular and sharply angular tiles of low strength.