Aurora - Secret Hypersonic Spyplane

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.

So how will an aircraft reach such speeds? 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. Even ramjets have drawbacks though, such as drag created in the process of slowing down and compressing a Mach 6 airstream.

To make a ramjet engine efficient is to spread the air over the entire length of the body. In a hypersonic ramjet aircraft, the entire underside of the forward body acts as a ramp that compresses the air, and the entire underside of the tail is an exhaust nozzle. So much air underneath the aircraft serves another purpose, it keeps the plane up. The ramjets need a large inlet area to provide the high thrust needed for Mach 6 cruise. As a result, the engines occupy a large area beneath vehicle and the need to accomodate a large quantity of fuel means that an all-body shape is most feasible.

Structurally, the all-body shape is highly efficient. As well as being extremely aerodynamic, the average cross-sectional area being very large provides a great deal of space for load, equipment and fuel. This being inside a structure that is light and compact having a relatively small surface area to generate frictional drag.

The spyplane's airframe may incorporate stealth technology, but it doesn't really require it should its mission simply involve high altitude reconnaissance. Hypersonic aircraft are much harder to shoot down than a ballistic missile. Although a hypersonic plane isn't very maneuverable, its velocity is such that even a small turn puts it miles away from a SAM's projected interception point.

Choosing The Right Fuel

Choosing the right type of fuel is crucial to the success of Aurora. Because various sections of the craft will reach cruising-speed temperatures ranging from 1,000 degrees fahrenheit to more than 1,400 degrees fahrenheit, its fuel must both provide energy for the engines and act as a structural coolant extracting destructive heat from the plane's surface.

At hypersonic speeds, even exotic kerosene such as the special high-flashpoint JP-7 fuel used by the SR-71 Blackbird can't absorb enough heat. The plausible solution is cryogenic fuel.