Smith chapter 1 problem 6, space shuttle heat shield materials

a) Needed material properties

· low mass, low density to minimize size of rocket engine needed to launch it

· low thermal conductivity

· high temperature of melting or sublimation

· low coefficient of thermal expansion, needed because the metal structure under the heat shield will stay relatively cool and not be experiencing much thermal expansion and contraction.

· Chemically rather inert so it won’t react much with air at an expected temp. of 3000F.

· Strong enough so it can resist abrasion and indentation during fabrication and use.

· Tough enough so it will not fracture catastrophically (like glass) during fabrication or use.

· Some areas need to be a dark color so they are a better black-body radiator of heat. Not only do black bodies absorb radiation better, they also radiate better too.

· Material needs to be repairable in orbit to some extent. This was a lesson learned from the recent shuttle crash.

b) Identify suitable materials – below are discussed the materials actually used.

The shuttle has been flying for about 25 years during which time improvements have been made. At present three main types of heat shield material are in use, each for a specific region on the orbiter. The entire orbiter surface must have some thermal insulation, not just to keep the heat of re-entry out of the crew quarters, but also to keep the extreme cold of space out too.

· Carbon-carbon composite coated with alumina, silicon and silicon carbide for oxidation resistance. This is used on the nose cone and leading edges of the wings where temp. is highest, up to 3000F. The carbon-carbon composite is made from layers of graphite cloth glued together with an organic polymer. This composite is heated in an oven so that all the hydrogen is driven out of the polymer leaving just charred carbon in its place, as the binder for the graphite cloth.

· Two different types of silica tiles, coated with borosilicate glass to seal the surface. One type of tile is colored black and is OK up to 2300F while the other is a light color, good up to 1200F.

· A nylon felt “Nomex” material to seal the gaps in between the tiles of silica and the larger sections of carbon-carbon composite. This gap filler material is also good for vibration damping and also compliant to serve as an expansion joint during thermal expansion & contraction. The Nomex is some kind of heat resistant material.

· RTV (room temperature vulcanizing) silicon rubber adhesive is used to glue the tiles on.

c) Why not titanium alloys?

· Ti conducts heat too well.

· Ti melts at 1668C=3034F, way too close to the re-entry temperatures. Close to that temp. it softens. Not good.

· Ti oxidizes quite a bit in air at temp. near 3000F.

Note and disclaimer: The space shuttle is the most complicated machine ever built, so some of the discussion is a bit of a simplification.

Bibliography: http://www-pao.ksc.nasa.gov/kscpao/nasafact/tps.htm

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts_sys.html

www.floridatoday.com/apps/pbcs.dll/article?AID=/20051219/NEWS02/512190318/1007/news02