Reaction Engines’ Skylon reusable spaceplane project has been given a boost, with analysis confirming the feasibility of the SABRE engine cycle concept that lies at its heart.The feasibility study conducted looked at the thermodynamic cycle of the SABRE concept. That is, whether the engine is able to do what Reaction Engine claims it can do.
According to US Air Force study, there’s no theoretical problem with the concept if the engine is properly built and integrated. The SABRE (Synergetic Air-Breathing Rocket Engine) is a scramjet. That is, it reduces the propellant load because it acts as a jet while in the atmosphere and a rocket in space, so it doesn’t have to carry as much oxygen to burn the liquid hydrogen fuel. It does so at velocities above Mach 5 (4,500 mph, 7,200 km/h) before flying into space, when it switches to rocket mode to achieve the even faster speeds needed to reach orbit.
The limit of the engine is how hot it gets. Above a certain point, even the best metal alloys soften and melt. At hypersonic speeds, the air is coming into the engine at 25 times more force than that of a Category 5 hurricane and the heat is like something blasting out of a cutting torch.
Before it can be burned, the air needs to be cooled dramatically, so as it enters the SABRE it passes over a series of heat exchangers that use the cryogenic hydrogen fuel to cool it down from 1,000° C (1,832° F) to minus 150° C (minus 302° F) in 1/100th of a second. Previously, this sort of heat exchanger was the size of a factory, but the SABRE uses one that’s small and light enough to be installed inside the scramjet.
Reaction Engines and the US Air Force are currently collaborating on vehicle concepts that can use the SABRE engine. These not only include space launch vehicles, but also hypersonic aircraft and military applications of the Reaction Engines heat exchanger technologies.