The third factor is that the North Sea aircraft looks unlike anything else. No aircraft other than a high-supersonic vehicle, or a test aircraft for such a vehicle, has ever been built or studied with a similiar planform. At hypersonic speeds, traditional aerodynamic design gives way to aero-thermodynamic design. In order for a hypersonic vehicle to remain structurally intact at such high speeds and stresses, the vehicle must produce minimum drag and be free of design features that give rise to concentrations of heat. The aircraft design must be able to spread the heat over the surface of the structure.
Thermal management is critical to high-speed aircraft, especially hypersonic vehicles.
Skin friction releases heat energy into the aircraft and must be pumped out again if the vehicle is to have any endurance. The only way to do this is to heat the fuel before it enters the engine, and dump the heat through the exhaust. On a hypersonic vehicle, thermal management is very critical, the cooling capacity of the fuel must be used carefully and efficiently or else the range and endurance of the aircraft will be limited by heating rather than the actual fuel tank capacity. Conventional turbojet engines won't be able to handle the incoming airstreams at such speeds, they can barely handle transonic speeds. In the case of hypersonic propulsion, an aero-thermodynamic duct, or ramjet, is the only engine proven to work efficiently at such speeds.