Thermal shielding
A heat shield is designed to shield a substance from absorbing excessive heat from an outside source by either dissipating, reflecting or simply absorbing the heat. It is often used as a form of exhaust heat management.
Passive cooled protectors were used initially to absorb heat peaks and subsequently irradiate stored heat to the atmosphere. Unfortunately, early versions required a considerable amount of metals such as titanium, beryllium, copper, etc. which greatly increased the mass of the vehicle. Heat absorption and ablative systems became preferable.
In modern vehicles, however, they can be found, but instead of metal, Reinforced carbon–carbon material is used (also called RCC Reinforced carbon-carbon or carbon-carbon ) . This material constitutes the thermal protection system of the nose and the front edges of the Space Shuttle and was proposed for the vehicle X-33. Carbon is the most refractory material known with a sublimation temperature (for graphite) of 3825 °C. These characteristics make it a material particularly suitable for passive cooling, but with the disadvantage of being very expensive and fragile.
Some aircraft at high speed, such as the Concorde and SR-71 Blackbird, must be designed considering similar, but lower, overheating to what occurs in spacecraft . In the case of the Concorde the aluminum nose allowed to reach a maximum operating temperature of 127 °C (which is 180 °C higher than the ambient air outside which is below zero); the metallurgical consequences associated with the peak temperature were a significant factor in determining the maximum aircraft speed.
Recently new materials have been developed that could be superior to '' RCC. The prototype SHARP (Slender Hypervelocity Aerothermodynamic Research Probe) is based on materials Ultra-high-temperature ceramics,(UHTC), such as zirconium diboride (ZrB2) and hafnium diboride (HfB2). The thermal protection system based on these materials would allow to reach a speed of Mach number 7 at sea level, Mach 11 at 35000 meters and significant improvements for vehicles designed for Hypersonic speed. The materials used have thermal protection characteristics in a temperature range from 0 °C to + 2000 °C, with melting point at over 3500 °C. They are also structurally more resistant than RCC, so they do not require additional reinforcements, and are very efficient in re-irradiating the absorbed heat. NASA funded (and subsequently discontinued) a research and development program in 2001 for testing this protection system through the University of Montana.
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