SABRE (rocket engine)

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. 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. The cooler would consist of a fine pipework heat exchanger with 16,800 thin-walled tubes, 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. 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. 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", ==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. In June 2013 the United Kingdom government announced further support for the development of a full-scale prototype of the SABRE engine, providing £60M of funding between 2014 and 2016 with the ESA providing an additional £7M. The total cost of developing a test rig was estimated at £200M. In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. Air Force Research Laboratory. The laboratory was to reveal two-stage-to-orbit SABRE concepts shortly afterwards, as they considered that a single-stage-to-orbit Skylon space plane was "technically very risky as a first application of SABRE engine". In August 2015 the European Commission competition authority approved UK government funding of £50 million for further development of the SABRE project. This was approved on the grounds that money raised from private equity had been insufficient to bring the project to completion. In October 2015 British company BAE Systems agreed to buy a 20% stake in the company for £20.6 million as part of an agreement to help develop the SABRE hypersonic engine. In 2016, Reaction CEO Mark Thomas announced planned to build a quarter-sized ground test engine, given limitations of funding. In September 2016 agents acting on behalf of Reaction Engines applied for planning consent to build a rocket engine test facility at the site of the former Rocket Propulsion Establishment in Westcott, UK which was granted in April 2017, and in May 2017 a groundbreaking ceremony was held to announce the beginning of construction of the SABRE TF1 engine test facility, expected to become active in 2020. However, development of the TF1 facility was since quietly dropped, and the site was taken on by aerospace and defence group Nammo. In September 2017 it was announced the United States Defence Advanced Research Projects Agency (DARPA) had contracted with Reaction Engines Inc. to build a high-temperature airflow test facility at Front Range Airport near Watkins, Colorado. The DARPA contract was to test the Sabre engine's pre-cooler heat exchanger (HTX). Construction of the test facilities and test articles began in 2018 with testing focusing on running the HTX at temperatures simulating air coming through a subsonic intake travelling at Mach5 or around beginning in 2019. The HTX test unit was completed in the UK and sent to Colorado in 2018, where on 25 March 2019 an F-4 GE J79 turbojet exhaust was mixed with ambient air to replicate Mach3.3 inlet conditions, successfully quenching a stream of gases to in less than 1/20 of a second. Further tests simulating Mach5 were planned, with temperature reduction expected from . These further tests were successfully completed by October 2019. The successful HTX test was thought to maybe lead to spin-off precooler applications which could be developed before a scalable SABRE demonstrator was completed; suggested uses were to expand gas turbines capabilities, in advanced turbofans, hypersonic vehicles, and industrial applications. In March 2019, the UKSA and ESA preliminary design review of the demonstrator engine core confirmed the test version to be ready for implementation. In 2019, Airborne Engineering conducted a test campaign on subscale air/hydrogen injectors for the SABRE preburners. In 2020, Airborne Engineering conducted a test campaign on an "HX3 module" (preburner to helium loop heat exchanger). In 2022, a Foreign Comparative Testing of Reaction’s precooler heat exchanger was performed. The testing was successfully completed by the company’s US subsidiary (Reaction Engines Incorporated – REI) and the US Air Force Research Laboratory (AFRL). “The FCT test program greatly expanded the demonstrated capabilities of our engine precooler technology”, said REI’s director of engineering, Andrew Piotti. “During these recent tests, the precooler successfully achieved our objective of over 10 megawatts of transferred thermal energy from the high-temperature airflow, which is three times higher than our previous test program.” Engine Due to the static thrust capability of the hybrid rocket engine, the vehicle could take off under air-breathing mode, much like a conventional turbojet. As the craft would ascend and the outside air pressure drop, more and more air would be passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets would be able to operate to a much higher altitude than would normally be possible. At Mach5.5 the air-breathing system would become inefficient and would be powered down, replaced by the on-board stored oxygen which would allow the engine to accelerate to orbital velocities (around Mach25). ==Evolution==

RB545 Designed for use with HOTOL. The engine had no air-breathing static thrust capability, relying on a rocket trolley to achieve takeoff. SABRE Designed for use with Skylon A4. The engine had no air-breathing static thrust capability, relying on RATO engines. SABRE 2 Designed for use with Skylon C1. The engine had no static thrust capability, using LOX until the air-breathing cycle could take over. SABRE 3 Designed for use with Skylon C2. This engine included a fuel rich preburner to augment the heat recovered from the airstream used to drive the helium loop, giving the engine static thrust capability. SABRE 4 SABRE 4 was no longer a single engine design, but a class of engines, e.g. a instance of this engine would have been used with SKYLON D1.5, a for a USAF study into a partially reusable TSTO. ==Performance==

The designed thrust-to-weight ratio of SABRE is fourteen compared to about five for conventional jet engines, and two for scramjets. ==See also==