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98 April/May 2018 | Unmanned Systems Technology PS | SABRE rocket engine T here’s no doubt that SpaceX is having a profound effect on space technology with its partially reusable Falcon family of launch vehicles and their brilliant trick of making a soft vertical landing (writes Peter Donaldson). However, British company Reaction Engines’ Synergetic Air Breathing Rocket Engine (SABRE) is even more revolutionary and could be transformative. SABRE is intended to power a single- stage-to-orbit, fully reusable aerospace plane capable of serving, for example, as a satellite launcher or a hypersonic transport, in first unmanned and later manned forms. Fuelled by liquid hydrogen, SABRE operates as an air-breathing engine up to Mach 5, then transforms into a pure rocket burning its hydrogen with stored liquid oxygen. One of the biggest challenges the company has faced involves managing heat. Air entering the engine at supersonic speeds gets very hot as the intake geometry slows and compresses it, reaching temperatures too high for the materials of the compressor. Cooling it down requires a very powerful heat exchanger called a pre-cooler. At Mach 5, the air entering the pre- cooler is at 1000 C, and the pre-cooler is claimed to remove heat equivalent to 400 MW from the intake airflow. The company claims an effectiveness of 95% while minimising the heat exchanger’s mass. It cools the air so much that antifreeze has to be introduced to prevent the water condensing out of the air from blocking the cooler with frost. Made from thousands of thin-walled tubes carrying helium gas as coolant, the pre-cooler combines a very large surface area with very low weight. Reaction Engines describes the pre-cooler as a counter-flow heat exchanger with its tubes arranged in involute spirals to form a hollow cylindrical structure. Turned through 90° after entering the hypersonic intake, the air flows radially inwards over the outside of the tubes while the helium flows in the opposite direction inside them and picks up the heat that passes through the tube walls. The cooled air emerges from the centre of the hollow cylinder on its way to the engine’s compressor. Each tube is joined to an inlet and outlet manifold to allow the coolant to be injected and removed, and manufacturing the heat exchanger involves bonding thousands of joints. Achieving this in a single operation with effectively zero leakage is very demanding, but the company claims that the tubes are hermetically sealed, and the tiny amount of gas that escapes can be measured by the molecule. When it has done its cooling work, the now hot helium gas is used again to drive the contra-rotating turbine that in turn drives the compressor that feeds air into the rocket engine combustion chambers. After that, the helium is passed through another heat exchanger in the fuel system, where it is cooled again before being returned to the pre- cooler to begin the cycle again. Now, here’s a thing “ ” At Mach 5, the air entering the pre-cooler is at 1000 C; the pre-cooler is said to remove heat equivalent to 400 MW