Why in space rockets the Windows or glasses are made up of diamond
Answers
The high heating experienced by spacecraft when entering the atmosphere is caused by a high-pressure bow wave in front of the ship. This strong shock wave is caused by the craft flying at supersonic speeds, even hypersonic speeds. Hypersonic is roughly greater than Mach 5. The shock wave is where the atmosphere is rapidly compressed by a factor of 50 to 100, depending on the speed of the vehicle. Because of this rapid compression the gas is heated to high temperatures, as high as 6000 K or more. This hot gas then impinges on the front of the spaceship, and transferring heat to the surface. That is why it has to have a heat shield.
One thing you will notice about human spacecraft is that the windows are all located on the back surface, or leeward side, of the spacecraft. This side experiences lower high heat transfer compared with the windward side, and so it does not reach as high a temperature. This is because the pressure is much lower, at least two orders of magnitude lower (1/100 or less pressure) on the back side. The hot gas on the windward side expands to the leeward side, which means the pressure drops quickly, and so does the gas temperature. For this reason the windows are always located on this "cooler" side. You should know that even on the cooler leeward side if there were any exposed metal surface the metal would melt within seconds. So windows must still be able to withstand high temperatures, say about 1000 C. So Shuttle windows are made from a high-temperature quartz glass that can withstand heating and cooling without cracking. The same explanation applies to the Russian Soyuz and to NASA's new spacecraft called Orion that is under development.
It may not seem like the windows on the Space Shuttle are on the leeward side, but remember that as it flies through the peak heating portion of the reentry the nose of the shuttle is pitched up from horizontal by 40 degrees. At that angle the pilots are not looking toward the direction that the spacecraft is moving, but more "up" towards space. Therefore the hot shock-heated gas bears only on the underbelly of the Shuttle where there are no windows, and not on the upper surface where the windows are. After the Shuttle passes through the hottest region of its flight it pitches down so that the pilots can see where they are flying by looking through the windows.