Atmospheric Re-entry: Techniques and Materials
Heat Management Design Methods
How do we manage such extreme conditions? There are a number of different strategies, and two of them aren’t so different from your own body’s way of handling heat…
The first, called a heat sink, could be considered the “toughing it out” approach; here, you construct your R/V of a material that can soak up the heat you’re experiencing along the way without experiencing structural failure along the way. This particular strategy was used on the North American Aviation X-15 rocket plane. It used a “hot structure” that could tolerate the heat load it experienced under normal conditions.
The second, ablation, is analogous to sweating. Rather than relying on insulation, ablation absorbs energy from the airstream by making it work to do another task - pyrolysing (burning) an expendable heat shield. The phase change effectively removes energy from the proximity of the vehicle. Ablation can be used in conjunction with other heat management methods; in fact, a number of ablative coatings were used on and tested with the X-15 during its program lifespan.
The third, radiation, relies on emitting heat back into the atmosphere at a faster rate than it is absorbed. This strategy is used to an extent on the Space Shuttle - the black tiles are very efficient at both absorption and radiation, allowing the bottom of the Orbiter to get rid of some of the intense heat of re-entry. As mentioned before, some vehicles use multiple approaches - the Shuttle itself uses both # 1 & 3, while the X-15 (during a part of the program) used # 1 & 2. There are other ways to manage heat, as well.
Variances in flight profile also have a strong effect on heating; one strategy is to decrease wing loading (the ratio of mass to wing area), increasing the initial altitude in the descent profile and increasing the proportion of the deceleration that occurs at high altitudes (where the density, and thus the frictional heating, is lower). The opposite of this technique was demonstrated recently when Soyuz capsules incorrectly initiated an emergency descent, one that allowed for a speedier return at the sacrifice of additional heating and deceleration stress on the crewmembers. Another technique is to increase the bank angle in order to descend faster, thus reducing the heat load on the vehicle. This is why the Shuttle executes a number of banks during reentry.
Thermal Protection System materials
Since the construction of the first Space Shuttle Orbiter, NASA has relied on low-density ceramics and carbon composites (reinforced carbon-carbon (RCC) to be precise) to protect space vehicles; this approach relies on the low thermal conductivity of its silica tiles along with high-emissivity coatings to both absorb and radiate the heat absorbed during re-entry. Unfortunately, this system requires extensive refurbishing after each mission to remian spaceworthy.
There are a number of advanced thermal protection systems (TPS) out there on the horizon that would improve upon the existing status quo. Of those, both metallic shingles and zirconium/hafnium diboride ceramics look promising. Both would be more impact resistant and require less refurbishing after use than the Orbiter’s existing TPS, and the ceramics have a significantly higher temperature limit (around 3000 degrees F) than the RCC used in the Shuttle. Of course, both of these technologies have to make it off the drawing board and into a vehicle to be validated, and the replacement of the Shuttle with a rocket may push that time further back on the horizon.