A jet is, for all intents and purposes, an “air-breathing rocket”. Whether the SABRE combined-cycle (from engine proposed for use in the Skylon rocket (and the HOTOL before that) is actually workable is another question (and one for a separate discussion).
A pure jet aircraft takes in air to use as both oxidizer and propellant (e.g. it pushes the resulting fluid out the back in order to generate thrust). In order to develop the pressure necessary to accelerate the incoming fluid, it has to combust it with fuel (usually some kind of liquid petrocarbon, but cryogenic hydrogen, various alcohols and ethers, and other complex fuels have been tried). The problem therein lies in getting enough time for the incoming fluid to combust with the fuel so as to get close to complete combustion and generate net thrust. When moving at supersonic speeds, the incoming air has to be slowed to subsonic speed before entering the combustion chamber so that complete combustion can occur within the combustion chamber, while still being compressed enough to combust efficiently. In turbofan and turbojet engines, the compression is done by driving a turbine with some of the exhaust to drive the compressor (and the fan in a turbofan).
In a ramjet engine, the speed of the aircraft and shape of the incoming duct are sufficient to compress the fuel without the complexities of a turbine and compressor; however, because the pressure at the inlet changes with speed, the duct has to be able to change size or shape (typically done by moving the inlet cone in/out or pitching ramps), but they don’t function efficiently at lower speeds, and essentially not at all at subsonic speed. At higher speeds (around Mach number of 4.5 to 6, depending on air density) it is just not possible to slow down the flow without creating too much drag, so a supersonic ramjet (hence, scramjet) is used; however, because of the high speed of the fluid this requires a really long combustion chamber. Some tricks are used to amplify the combustion rate, but generally speaking scramjets (which are still only in demonstration phase) are terribly inefficient in combustion terms. So practically speaking, from a propulsion standpoint, we’re limited to the speeds at which a ramjet can function.
Heating, at the air intakes and the wing leading surfaces is certainly a problem. If cryogenic fuels or oxidizers are used, they could be routed to provide some amount of active cooling, but at hypersonic speeds there just isn’t going to be enough active convection (the transfer of heat to the ambient environment) to expel excess heat. In conventional vehicles this has required the use of high temperature ‘superalloys’ and ceramic liners to protect against heating in critical locations, but controlling the heat paths, figuring out how to attach and maintain thermal protection systems, and dealing with coefficient of thermal expansion (CTE) mismatches are all complex engineering problems that often defy good solution. (Due to CTE issues, the SR-71 would leak fuel on the ground and had to be fully fueled in flight prior to engaging in missions, which is expensive and risky.) If safety and functional utility are of no concern, then we could probably figure out a way to make the plan survive for at least a short duration to any degree of aerodynamic heating it would see, if by no other means than just making the skin really thick and providing heat sinks. So again, the ability to propel the vehicle is the real limitation.
Stranger