be manufactured either via steam reforming or expensive electrolysis. A few experimental hydrogen-powered aircraft have flown with propellers, and jets have been proposed that may be feasible. Precooled jet engines An idea originated by Robert P. Carmichael in 1955 is that hydrogen-fueled engines could theoretically have much higher performance than hydrocarbon-fueled engines if a heat exchanger were used to cool the incoming air. The low temperature allows lighter materials to be used, a higher mass-flow through the engines, and permits combustors to inject more fuel without overheating the engine. This idea leads to plausible designs like Reaction Engines SABRE, that might permit single-stage-to-orbit launch vehicles, and ATREX, which could permit jet engines to be used up to hypersonic speeds and high altitudes for boosters for launch vehicles. The idea is also being researched by the EU for a concept to achieve non-stop antipodal supersonic passenger travel at Mach 5 (Reaction Engines A2). Turborocket The air turborocket is a form of combined-cycle jet engine. The basic layout includes a gas generator, which produces high pressure gas, that drives a turbine/compressor assembly which compresses atmospheric air into a combustion chamber. This mixture is then combusted before leaving the device through a nozzle and creating thrust. There are many different types of air turborockets. The various types generally differ in how the gas generator section of the engine functions. Air turborockets are often referred to as turboramjets, turboramjet rockets, turborocket expanders, and many others. As there is no consensus on which names apply to which specific concepts, various sources may use the same name for two different concepts. Terminology To specify the RPM, or rotor speeds, of a jet engine, abbreviations are commonly used: For a turboprop engine, Np refers to the RPM of the propeller shaft. For example, a common Np would be about 2200 RPM for a constant speed propeller. N1 or Ng refers to the RPM of the gas generator section. Each engine manufacturer will pick between those two abbreviations. N1 is also used for the fan speed on a turbofan, in which case N2 is the gas generator speed (2 shaft engine). Ng is mainly used for turboprop or turboshaft engines. For example, a common Ng would be on the order of 30,000 RPM. N2 or Nf refers to the speed of the power turbine section. Each engine manufacturer will pick between those two abbreviations but N2 is mainly