SABRE (Synergetic Air Breathing Rocket Engine) was a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine was designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE was an evolution of Alan Bond's series of LACE-like designs that started in the early/mid-1980s for the HOTOL project. Reaction Engines went into bankruptcy in 2024 before completing the project.
The design comprised a single combined cycle rocket engine with two modes of operation. The air-breathing mode combined a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler would cool the hot, ram-compressed air, which would otherwise reach a temperature that the engine could not withstand, leading to a very high pressure ratio within the engine. The compressed air would subsequently be fed into the rocket combustion chamber where it would be ignited along with stored liquid hydrogen. The high pressure ratio would allow the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air would permit light alloy construction to be employed and thus allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE's precooler would not liquefy the air, thus letting it run more efficiently.
After shutting the inlet cone off at Mach 5.14, and at an altitude of , the system would continue as a closed-cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.
An engine derived from the SABRE concept called Scimitar has been designed for the company's A2 hypersonic passenger jet proposal for the European Union-funded LAPCAT study.
History The precooler concept evolved from an idea originated by Robert P. Carmichael in 1955. This was followed by the liquid air cycle engine (LACE) idea which was originally explored by General Dynamics in the 1960s as part of the US Air Force's aerospaceplane efforts.
The liquid air cycle engine (LACE) system was to be placed behind a supersonic air intake which would compress the air through ram compression, then a heat exchanger would rapidly cool it using some of the liquid hydrogen fuel stored on board. The resulting liquid air was then processed to separate the liquid oxygen for combustion. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be expelled, giving useful thrust, but greatly reducing the potential efficiency.
Instead, as part of the HOTOL project, the LACE-based RB545 engine was developed with more efficient cycle. The engine was given the Rolls-Royce name "Swallow". In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited to continue research. The RB545's precooler had issues with embrittlement and excess liquid hydrogen consumption, and was encumbered by both patents and the UK's Official Secrets Act, so Bond developed SABRE instead.
In 2016 the project received £60m in funds from the UK government and ESA for a demonstrator involving the full cycle. In July 2021 the UK Space Agency provided a further £3.9m for continued development.
Concept Like the RB545, the SABRE design was neither a conventional rocket engine nor a conventional jet engine, but a hybrid that used air from the environment at low speeds/altitudes, and stored liquid oxygen at higher altitude. The SABRE engine "relies on a heat exchanger capable of cooling incoming air to , to provide oxygen for mixing with hydrogen and provide jet thrust during atmospheric flight before switching to tanked liquid oxygen when in space."
In air-breathing mode, air would enter the engine through an inlet. A bypass system would then direct some of the air through a precooler into a compressor, which would inject it into a combustion chamber where it would be burnt with fuel, the exhaust products then accelerated through nozzles to provide thrust. The remainder of the intake air would continue through the bypass system to a ring of flame holders which act as a ramjet for part of the air breathing flight regime. A helium loop would be used to transfer the heat from the precooler to the fuel and drive the engine pumps and compressors.
Inlet At the front of the engine, the concept designs proposed a simple translating axisymmetric shock cone inlet which would compress and slow the air (relative to the engine) to subsonic speeds using two shock reflections. Accelerating the air to the speed of the engine would incur ram drag. As a result of the shocks, compression, and acceleration the intake air would be heated, reaching around at Mach5.5.
Bayern-Chemie, through ESA, had undertaken work to refine and test the intake and bypass systems
Precooler As the air would enter the engine at supersonic or hypersonic speeds, it would become hotter than the engine can withstand due to compression effects. Jet engines, which have the same problem but to a lesser degree, solve it by using heavy copper or nickel-based materials, by reducing the engine's pressure ratio, and by throttling back the engine at the higher airspeeds to avoid melting. However, for a single-stage-to-orbit (SSTO) spaceplane, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses. Instead, using a gaseous helium coolant loop, SABRE would dramatically cool the air from down to in a counterflow heat exchanger while avoiding liquefaction of the air or blockage from freezing water vapour. The counterflow heat exchanger would also allow the helium to exit the engine at a sufficiently high temperature to drive pumps and compressors for the liquid hydrogen fuel and helium working fluid itself.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler. SABRE would insert a helium cooling loop between the air and the cold fuel to avoid problems with hydrogen embrittlement in the precooler.
The dramatic cooling of the air created a potential problem: it would be necessary to prevent blocking the precooler from frozen water vapour and other air fractions. In October 2012, the cooling solution was demonstrated for 6 minutes using freezing air. In English The cooler would consist of a fine pipework heat exchanger with 16,800 thin-walled tubes, and would cool the hot in-rushing atmospheric air down to the required in 0.01s. The ice prevention system had been a closely guarded secret, but REL disclosed a methanol-injecting 3D-printed de-icer in 2015 through patents, as they needed partner companies and could not keep the secret while working closely with outsiders.Norris, Guy. "Reaction Engines Reveals Secret Of Sabre Frost Control Technology" Aerospace Daily & Defense Report, 8 July 2015, p. 3 Similar article "The Skylon Spaceplane's 3D Printed Injector""Helping the Skylon Spaceplane Reach Orbit with a 3D Printed Injector Mechanism"
Compressor Below five times the speed of sound and 25 kilometres of altitude, which are 20% of the speed and 20% of the altitude needed to reach orbit, the cooled air from the precooler would pass into a modified turbo-compressor, similar in design to those used on conventional jet engines but running at an unusually high pressure ratio made possible by the low temperature of the inlet air. The compressor would feed the compressed air at 140 atmospheres into the combustion chambers of the main engines.
In a conventional jet engine, the turbo-compressor is driven by a gas turbine powered by combustion gases. SABRE would drive the turbine with a helium loop, which would be powered by heat captured in the precooler and a preburner.
Helium loop The 'hot' helium from the air precooler would be recycled by cooling it in a heat exchanger with the liquid hydrogen fuel. The loop would form a self-starting Brayton cycle engine, cooling critical parts of the engine and powering turbines. The heat would pass from the air into the helium. This heat energy would then be used to power various parts of the engine and to vaporise hydrogen, which would then be burnt in ramjets.
Combustion chambers The combustion chambers in the SABRE engine would be cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen to further reduce the system's use of liquid hydrogen compared with stoichiometric systems.
Nozzles The most efficient atmospheric pressure at which a conventional propelling nozzle works is set by the geometry of the nozzle bell. While the geometry of the conventional bell remains static the atmospheric pressure changes with altitude and therefore nozzles designed for high performance in the lower atmosphere lose efficiency as they reach higher altitudes. In traditional rockets this is overcome by using multiple stages designed for the atmospheric pressures they encounter.
The SABRE engine would have to operate at both low and high altitude scenarios. To ensure efficiency at all altitudes a sort of moving, expanding nozzle would be used. First at low altitude, air-breathing flight the bell would be located rearwards, connected to a toroidal combustion chamber surrounding the top part of the nozzle, together forming an expansion deflection nozzle. When SABRE later transitions into rocket mode, the bell would be moved forwards, extending the length of the bell of the inner rocket combustion chamber, creating a much larger, high altitude nozzle for more efficient flight.
Bypass burners Avoiding liquefaction would improve the efficiency of the engine since less entropy would be generated and therefore less liquid hydrogen would be boiled off. However, simply cooling the air would need more liquid hydrogen than could be burnt in the engine core. The excess would be expelled through a series of burners called "spill duct ramjet burners", that would be arranged in a ring around the central core. These would be fed air that bypasses the precooler. This bypass ramjet system was designed to reduce the negative effects of drag resulting from air that would pass into the intakes but would not be fed into the main rocket engine, rather than generating thrust. At low speeds the ratio of the volume of air entering the intake to the volume that the compressor could feed to the combustion Chambers would be at its highest, requiring the bypassed air to be accelerated to maintain efficiency at these low speeds. This distinguished the system from a turboramjet where a turbine-cycle's exhaust is used to increase air-flow for the ramjet to become efficient enough to take over the role of primary propulsion.
Development
Tests were carried out in 2008 by Airborne Engineering Ltd on an expansion deflection nozzle called STERN to provide the data needed to develop an accurate engineering model to overcome the problem of non-dynamic exhaust expansion. This research continued with the STRICT nozzle in 2011.
Successful tests of an oxidiser (both air and oxygen) cooled combustion chamber were conducted by EADS-Astrium at Institute of Space Propulsion in 2010.
In 2011, hardware testing of the heat exchanger technology "crucial to [the] hybrid air- and liquid oxygen-breathing [SABRE] rocket motor" was completed, demonstrating that the technology is viable. The tests validated that the heat exchanger could perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.