SABRE engine concept passes US Air Force feasibilty test
Reaction Engines' Skylon reusable spaceplane project has been given a boost, with analysis by the United States Air Force Research Laboratory (AFRL) confirming the feasibility of the SABRE engine cycle concept that lies at its heart.
The feasibility study conducted as part of a Cooperative Research and Development Agreement (CRADA) with the AFRL’s Aerospace Systems Directorate (AFRL/RQ) 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 AFRL, 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.
Paradoxically, 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 AFRL 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.
"The activities under the CRADA have allowed AFRL to understand the SABRE engine concept, its pre-cooler heat exchanger technology, and its cycle in more detail," says AFRL/RQ program manager Barry Hellman. "Our analysis has confirmed the feasibility and potential performance of the SABRE engine cycle. While development of the SABRE represents a substantial engineering challenge, the engine cycle is a very innovative approach and warrants further investigation. The question to answer next is what benefit the SABRE could bring to high speed aerospace vehicles compared to other propulsion systems. Although application of the SABRE for single stage to orbit space access remains technically very risky as a first application, the SABRE may provide some unique advantages in more manageable two stage to orbit configurations. Furthermore, the heat exchanger technology also warrants further investigation for applications across the aerospace domain."
Source: Reaction Engines
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In "mild" super-sonic flight, the engine turns into a ramjet. As air is forced into the engines inlet by mere speed of flight, it is compressed and heated just by aerodynamics. The heat of that intake air is then transferred to the helium, so the pre-burner can be switched off. The air compressor continues to run, though, now operating on the heat of the income air as an energy source. The result is a much higher compression of intake air than in a normal ramjet, as the ram effect (supersonic air coming in, which gets compressed by aerodynamics) and conventional compressor turbine are combined.
At very high speeds, the air coming in even carries too much energy, so that it can no longer be adequately cooled with the hydrogen required to burn that air. There are two possible solutions to this problem: One is to also use some oxygen from the tanks to feed the engine, thus reducing the amount of intake air required to be cooled. Alternatively, excess hydrogen could be used to cool the intake air to the main compressor. This hydrogen is then burned off (with low efficiency, but still better than nothing) in conventional ramjets.
Whether Sabre is efficient enough to enable SSTO flight is not yet clear. Therefore, a two stage concept, where Sabre brings a conventional rocket to outer atmosphere, seems more doable for the start.