Monday's Lesson is Hot!

This time of year, I'm not usually worried about it, but when I do need a layer of thermal protection, it is protection against the cold. Not so the space shuttle - it needs thermal protection against the heat of re-entry.

You have probably heard in the news that the space shuttle's thermal protection system has once again been damaged during launch. Why is that such a big deal? To answer that, let's take a look at where the heat comes from and the thermal protection system that is supposed to protect against that heat.

Why does it get so hot on re-entry? Basically all the potential energy that it had at a height of 320 km gets converted into thermal energy. How much energy is that? About 6X10¹² J or 6 terajoules or approximately the amount of energy you would have used if you had left a 100-watt lightbulb on since the birth of Jesus.

(Note that if you did that calculation using the approximation mgh for potential energy, which is accurate only near Earth's surface, instead of Newton's Universal Law of Gravity, which is, of course, universal, you would be off by about 4.4%.)

That's a lot of energy, and it can raise an object's temperature very high - like the meteors that burn up in the atmosphere, only to be obscured from my vision by clouds (so sad). We don't want the shuttle to burn up in the atmosphere, so it is protected by a complicated system of insulators:

1) Felt reusable surface insulation (FRSI) is used in a few places that won't get very hot. FRSI only protects up to about 370^oC - that's not even glowing hot. Your furnace chimney might get that hot.

2) Low-temperature reusable surface insulation (white tiles) and advanced flexible reusable surface insulation (a "quilted blanket") are used to protect parts of the shuttle to a temperature of about 650^oC. At that temperature the surface would be just barely glowing red.


3) High-temperature reusable surface insulation and fibrous refractory composite insulation (black tiles) will protect up to about 1260 ^oC. Anything that hot glows a bright red, like an electric stove burner set on high. It is these inch-thick black tiles that have a 3 1/2 inch gouge in them (see photo). If the aluminum framework below the tiles were to reach a temperature of 1260^oC, the shuttle would be in trouble. The melting temperature of aluminum is only 660^oC!

4) Some parts of the shuttle take the full hit of re-entry and can get as hot as 1500^oC (approximately the melting temperature of iron). These parts are protected by reinforced carbon-carbon panels. I don't know the actual heat characteristics of this material, but carbon itself remains solid up to 3700^oC, at which point it sublimates.

Oh, and did you notice the printing on the tiles in the photo above? Those are individual tile ID's, painted in Sperex VHT (very high temperature) paint, developed for automobile manifolds. Sperex is only rated to 700^oC. Clearly since the paint remains from launch to launch on the black tiles, the paint must actually withstand temperatures more like 1260^oC. Sometimes you get more than you pay for!

8/18/07 Update
Apparently NASA has decided not to try to repair the damage to the thermal protection system. "(T)he surface of the gouged tile on Endeavour's belly will experience between 2,000 and 2,100 degrees Fahrenheit (1,093-1,148 degrees Celsius), with temperatures on the inner felt strip near the shuttle's aluminum skin reaching about 325 degrees Fahrenheit (162 degrees Celsius)."

So tests and simulations seem to indicate that the tile won't get any hotter than it can handle, and the felt won't get hotter than it can handle. However, some are still concerned that the adhesive holding the tiles on may get too hot and break down, releasing more tiles than was assumed when testing. If that happens, things could get hotter than the tests indicate. Let's hope NASA is right.

Also, it turns out that bad weather due to Hurricane Dean will force the shuttle to come back sooner than expected. No time for repairs, anyway.

Update Aug 22: In case you haven't heard, the space shuttle made it back safely yesterday. The experts made the right call.