Running cold, yet flammable, jet fuel through heat exchangers—in much the same way coolants like ethylene glycol flow through a car radiator to carry heat away from a car engine—is old hat. That kind of “conventional” jet cooling worked for aircraft such as the SR-71 Blackbird, a now-retired U.S. Air Force reconnaissance jet that flew for 30-some years and reached top speeds of just over Mach 3 (more than 3,500 km/hour). Its fuel cooled the aircraft, then went on to burn, propelling the jet in the usual way once the fuel had heated up.
Hypersonic planes need a more advanced fuel. Scientists believe that a hydrocarbon fuel that additionally sucks up heat via heat-absorbing, or endothermic, chemical reactions might do the trick.
The bulk of research in endothermic fuels focuses on cracking chemistry typically mediated by zeolites. But there are other promising options. Dehydrogenations, for example, can also soak up a lot of heat, and platinum catalysts drive those reactions enthusiastically. The problem is, platinum’s activity tends to fall quickly as coke accumulates and blocks the catalysts’ active sites.
For more than half a century, researchers have searched for ways to cleverly exploit the thermodynamic properties of a hydrocarbon fuel so that it can double as a coolant for hypersonic aircraft. The idea that an onboard fuel could mitigate the extreme heat buildup associated with ultrafast aviation is tantalizing. The concept is simple. Taming the complex chemistry is another story entirely.
There is a lot of research from the USA and China indicating progress in the use of fuel to cool hypersonic planes.