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.

Comparison of thermal properties of metals

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
Required thickness to withstand a heat flux of 100kW/m2 for 20s without failure anywhere, assuming perfect conduction (mm)	2.7	1.2	2.1
Mass per m2 to do the above (kg/m2)	7.6	9.4	9.3

CFRP

CFRP is the material of choice for CUSF hypersonic vehicles, due to high yield stress and Young's modulus as well as the ability to carefully tune the properties. However, the default carbon fibre-epoxy composite is not especially suitable for high temperature applications. Typical epoxy has a glass transition temperature around 100°C and a decomposition temperature around 350°C. Furthermore, the strength of the composite drops off dramatically. The exact magnitude is hard to quantify and varies between different properties - tensile strength is dependent largely on carbon fibre performance and so does not degrade much, whereas shear strength is more a function of epoxy matrix performance.

For all epoxies, the safe operating temperature (below which pyrolysis will not occur) is around 350°C. The glass transition temperature varies between mixes^[6] - from around 80°C for a standard room-temperature cure epoxy to 230°C for EP46HT-1^[7] high temperature mix (which must be oven-cured for full strength. For a custom layup, it may be wise to pursue epoxies with a higher transition temperature in order to provide better strength in the actual highest-stress operating conditions of the vehicle.

Carbon-phenolic composite

Phenolic resin (PR) is a far better choice for the matrix of an ablative composite. It decomposes at higher temperatures than epoxy (in the 500-700°C range) and resists weakening effects up to higher temperatures. Phenolic is the matrix of choice for all modern ablative heat shields and thus should certainly be the one we use.

Woven carbon fibre sheets can be used to make CF-PR into a structural material with comparable properties to epoxy-based CFRP. This has superior thermal properties - soaking around 160MJ/kg^[8] when decomposing totally. The CF-PR retains the majority of its base shear strength above 500°C and does not experience shear modulus decrease to temperatures above 1000°C. The combination of high strength and temperature performance means this is likely the superior choice for the Aquila fins. If the layup process proves relatively easy it could also be viable to coat the airframe in a single layer of CF-PR for thermal resistance.

For the ultimate in thermal resistance, woven carbon fibre sheets can be replaced with carbon felt to produce PICA: phenolic-impregnated carbon ablator. This is among the highest-functioning non-commercially sensitive ablators in existence, and is used in evolved forms on SpaceX's Dragon capsule. However the preparation is an intensive process requiring several days of vacuum chamber curing and the use of liberal quantities of formaldehyde. Additionally it has little in the way of mechanical properties, as the felt forms an aerogel-like structure which is relatively flexible. For more details on PICA performance and preparation, see the relevant page.

Silica-phenolic composite

Silica-phenolic composite is glass fibre composite using phenolic resin: a direct parallel to CF-PR. It has very comparable properties to CF-PR - with lower ablation energy of "just" 27MJ/kg but a higher operational thermal resistance due to the survival of the glass fibres after matrix pyrolysis.

Authors

- Sam Ross