Hydrogen Fueled Engines


Your question implies that you are asking about the operation of the Aurora, a hypothetical spy plane that many believe has been developed by the US Air Force to replace the SR-71 Blackbird. Since we don't know if this aircraft even exists, let alone whether or not it is powered by hydrogen engines, we can only provide some general discussions on how such engines might work.

NASA and the Air Force have conducted at least some basic research into hydrogen engines, but very little information is publicly available. However, as a general rule, no engine can function using hydrogen alone. Every form of combustion, from a simple fire to an automobile engine or a jet engine to a rocket, requires two substances to function: a fuel and an oxidizer. In most cases, the oxidizer is air. For example, an automobile engine creates a mixture of gasoline (the fuel) and air (the oxidizer) that is burned to release energy while a jet engine mixes jet fuel and air to produce the same result. A rocket is somewhat different since it carries both the fuel and oxidizer aboard. A liquid rocket typically burns a mixture of liquid hydrogen and liquid oxygen to generate thrust. While many potential mixtures exist for solid rockets, a common propellant is made up of a solidified piece of hydrocarbon with small chunks of oxidizing salts mixed in. Once ignited, the hydrogen and oxygen are released from their chemical bonds, mix, and are burned to produce energy.

So in answer to your original question, a hydrogen engine does not use only hydrogen. The hydrogen is only the fuel, and it requires some form of oxidizer to undergo combustion. Depending on the application, the hydrogen may be mixed with air and combusted similarly to a jet engine, or the hydrogen could be carried in liquid form and mixed with liquid oxygen that would also be carried aboard the vehicle.

Your final question on the structure and operation of such an engine is more difficult to answer since it takes us even further into the realm of hypotheticals. However, many have proposed that the Aurora may be powered by what is known as a pulse detonation engine (PDE), sometimes also called a pulse detonation wave engine (PDWE). Even less information on PDEs is publicly available than on hydrogen fuels, but the basic concept of the PDE is substantially different than that of a jet engine. The operation of a jet engine is a rather steady process. Air enters through an inlet, is compressed, mixed with fuel, burned, and exhausted through a nozzle. A PDE, on the other hand, relies on repeated detonations that generate thrust in separate, individual chunks rather than in a steady stream.

Little is known about the operation of a pulse detonation engine, but it may work something like this. First, a measured amount of liquid hydrogen or some other fuel (perhaps liquid methane) is ejected into a chamber where the fuel mist mixes with air and is ignited creating a small, contained explosion. This controlled detonation provides a small nudge that propels the vehicle forward. When repeated at some high frequency, the continuous detonations generate a large thrust force that in theory can accelerate an aircraft to very high supersonic or hypersonic speeds on the order of Mach 4 to Mach 10. While the concept sounds similar to that used in a pulse jet (i.e. the engine used on the German V-1 buzz bomb of World War II), a pulse jet is limited to rather low speeds. The pulse jet also relies on a series of valves to create and contain the fuel-air explosion and direct that force to generate thrust. A PDE instead uses the shock waves generated by the detonation itself and by the vehicle in supersonic flight to compress the mixture and direct the explosion to produce thrust.

However, PDEs are quite a new area of research and it seems somewhat unlikely that enough experience with them has been gained to have already employed them on an advanced, hypersonic, manned spy plane. If the Aurora or some similar high-speed aircraft does indeed exist, perhaps it is propelled by ramjets or scramjets, both being variants of traditional jet engines optimized for high-supersonic flight. To read more about these engines, see previous questions on the jet engine and hypersonic aircraft.
- answer by Jeff Scott, 7 October 2001

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