QUESTION:
I was reading over the heat shield document on nasa.gov, and I am curious to know.. space shuttles and moduals usually have some sort of viewing ports or windows, if you will... what kind of material do you use for that? what stops the "glass" from melting in re-entry?

ANSWER:
Thanks for sending in your question, it is a good one. My name is John Balboni and I work in the Thermophysics Facilities Branch at Ames Research Center in Mountain View, California. What my colleagues and I do here is to test samples of heat shield materials in very hot wind tunnels to simulate the heating of atmospheric flight at very high speeds, Mach 25 and greater. Here are some pictures taken during these arc-jet tests: http://www.nasa.gov/centers/ames/multimedia/images/2006/cev_arcjet.html The samples of heat shield materials are about the size of a hockey puck.

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.

I hope that helps in understanding how windows survive the severe environment of reentry.

John Balboni
Aerospace Engineer, NASA

As an Aerospace Engineer for NASA, John Balboni's position includes using or operating experimental facilities (arc jets); designing or developing requirements, modifications, and procedures of arc jet facility operations; establishing safety requirements, and analyzing and validating test data. The arc jet facilities are used to simulate the heating of atmospheric entry on samples of heat shield materials for spacecraft. The test data are used to observe the performance of these materials under the severe thermal environment experienced during atmospheric entry flight. Also to develop and verify computer models of the materials' performance under simulated entry heating conditions. Arc jets will heat air to thousands of degrees by the use of tens of megawatts of electrical energy in a manner somewhat like a huge plasma cutting torch.

NASA Ames Research Center, Moffett Field, CA