Thermal management

At speeds above Mach 2, the thermal effects from atmospheric compression start to become significant. These effects grow with Mach number, and can become very severe - on the order of 100s of kW/m2 for a Mach 5 vehicle like Aquila. The air temperature around a nosecone or fin can exceed 1000K in these regimes, which is thoroughly non-survivable for a vast majority of rocketry materials. Since the features with peak heating rate are also the most critical to aerodynamic performance (the nosecone and fin leading edges), any failure could cause a severe performance degradation at best or a total vehicle disintegration at worst. Thus, extremely careful attention must be given to the thermal performance and material selection of a high-supersonic or hypersonic vehicle.

There are three broad classes of material behaviour up to high temperature in high-speed flow: unsuitables, ablators and refractories.

Unsuitable materials suffer a catastrophic or unpredictable failure mode when undergoing heating, and are thus immediately ruled out for high-temperature use. Examples include all woods and low-temperature metals like aluminium.

Ablators undergo a two-step chemical process on heating, which serves to absorb energy without causing a temperature increase. The first of these is pyrolysis, in which a fraction of the ablator degrades to gas and is carried outwards by the incident airstream. This leaves a (usually low density) layer of char material which serves as a highly effective thermal insulator. This layer either persists for the duration of the heating, is sheared away by the airstream or decomposes and pyrolyses at some higher temperature. This is the class of material used on almost all orbital-class re-entry vehicles as it can withstand both high overall heat loads and peak incident heat fluxes. They are generally a composite material, of a high-temperature "fibre" in a phenolic resin matrix. The phenolic is pyrolysed around 500°C, leaving a fairly fragile char layer.

Refractories are materials which can effectively withstand very high temperatures without degradation. Instead they experience a high peak surface temperature which allows for a radiative/convective heat balance. These materials are almost always ceramics (oxide, nitride or ceramic). They have poor mechanical properties but excellent thermal ones.

Metals

There are no almost metals with sufficient thermal performance to withstand hypersonic heating in the steady state without some form of cooling (which we do not have available to us). All either melt or soften substantially at intermediate temperatures. The high thermal conductivity of metals means it may be viable to use a large thermal mass to simply capacitively withstand the short heating period, but this clearly carries a very large mass penalty.

Metal	Aluminium 7075-T6 [1] ,	Stainless steel 410, 650C temper[2][3][4]	Titanium 6-4 [5]
Room temperature yield stress (MPa)	503	721	910
Density (kg/m3)	2810	7800	4420
Room temperature Young's modulus (GPa)	71.7	200	115
316°C yield stress (MPa)	45.0	696	625
540°C yield stress (MPa)	N/A	470	570
Maximum service temperature (K)	600	795	620 (limited by oxidation not weaking)
Specific heat capacity (J/kgK)	960	460	670