The space shuttle has triple paned, optical-quality windows. Thirty-seven window panes in eleven different sizes and shapes are produced for each Orbiter. The %%fused%% %%silica%% outer panes of the forward windshields are designed to withstand high atmosphere reentry temperatures. The inner, tempered alumino-silicate glass %%pane%% is called the pressure pane. It is designed for maximum strength to withstand the shuttle's cabin pressure in the vacuum of space. The thick %%fused%% %%silica%% middle pane is capable of withstanding both the reentry temperature and the cabin pressure.
Heat Tile Frit
Finely powdered glass, called frit, is used to glaze the tiles that protect the space shuttle from burning up during its flight. Two compositions of frit are currently used. One is for the shuttle's underside and small areas on top where the temperature rises to between 1200° and 2300° Fahrenheit during reentry. The other frits are used for areas on the side of the tail section, engine, fuselage, and wing tops where temperatures range from 600° to 1200° Fahrenheit.
Glass-ceramics combine some of the best characteristics of two different materials; glass and ceramics. A special glass-ceramic with high temperature resistance and qualities that make it machinable with ordinary metalworking tools is the material chosen for the space shuttle's tile retainers. The retainers secure the insulation around all hinge points and door opening protecting the Orbiter from the heat of reentry and as boundary retainers that fit accurately between insulation tiles and the vehicle's body.
Ninety-six percent silica glass is the designation given to a type of glass made by a proprietary temperature of up to 900° C, which makes this glass the choice for industrial items such as furnace sight glasses and for outer windows on space vehicles where the glass must withstand the heat of reentry into earth's atmosphere.
%%Fused%% %%silica%% glass consists of a single oxide. This glass consists simply of silica (silicon dioxide) in the noncrystalline or amorphous state. Adding anything to it puts it in another category. %%Fused%% %%silica%% is the most expensive of all glasses and shows the maximum resistance to heat shock as well as the highest permissible operating temperature (900° C for extended periods, 1200° C for short periods). %%Fused%% %%silica%% is clearly superior in a number of respects and is restricted to applications where uncompromising requirements dictate its use such as mirror blanks or astronomical telescopes, optical waveguides and crucibles for growing crystals. Fabrication of %%fused%% %%silica%% is difficult and the number of available shapes is therefore sharply limited. The more silica a glass contains, the lower its thermal expansion rate and the higher the resistance to heat shock.
Alumino-silicate glass, which can be specially heat treated, is another type of heat-shock-resisting glass.
Glass-Ceramics are so named because they begin life as glasses and then are converted into dense, fine-grained crystalline ceramics. Conversion takes place in two stages. In the first, the glass is seeded with nuclei, or centers around which crystals grow. In the second stage, crystals grow around these nuclei. The crystallinity of the resultant ceramic can vary from 50 to 99% depending on the choices of composition and heat treatment. The final product is different from a glass in nearly all of its properties.
The difference between a glass that is intended to remain a glass and a glass-ceramic in the glassy state lies in the nucleating agent which is included in the glass-ceramic batch and the proper ingredients for forming crystals.
The nucleating agent is a substance that is barely soluble in the glass. At high temperatures it will remain in solution, but at low temperatures it will precipitate and furnish the nuclei which are necessary as centers for %%crystal growth%%. When the glass is raised to the correct temperature and held there for a long enough time, a %%crystal%% will grow around each nucleus.
To start the conversion procedure, the formed glass-ceramic article may be lowered to the nucleating temperature after forming, or it may be lowered to room temperature, then re-heated to the nucleating temperature. Following nucleation the temperature is then raised to the %%crystal%%-growth range.