SABRE (rocket engine)
A model of SABRE
|Country of origin||United Kingdom|
|Designer||Reaction Engines Limited|
|Status||Research and development|
|Propellant||Air and LO2 / liquid hydrogen[dead link]|
|Cycle||Combined cycle precooled jet engine and closed cycle rocket engine|
|Thrust (vac.)||Approx. 2,940 kN (660,000 lbf)|
|Thrust (SL)||Approx. 1,960 kN (440,000 lbf)|
|Thrust-to-weight ratio||Up to 14 (atmospheric)|
|Isp (vac.)||460 seconds (4.5 km/s)|
|Isp (SL)||3,600 seconds (35 km/s)|
SABRE (Synergistic Air-Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air-breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE is an evolution of Alan Bond's series of liquid air cycle engine (LACE) and LACE-like designs that started in the early/mid-1980s for the HOTOL project.
The design comprises a single combined cycle rocket engine with two modes of operation. The air-breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram-compressed air leading to a very high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited along with stored liquid hydrogen. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE's precooler does not liquefy the air, letting it run more efficiently.
After shutting the inlet cone off at Mach 5.14, 28.5 km altitude, the system continues 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.
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 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 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, excess liquid hydrogen consumption, patents and the 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.
Like the RB545, the SABRE design is neither a conventional rocket engine nor jet engine, but a hybrid that uses air from the environment at low speeds/altitudes, and stored liquid oxygen (LOX) at higher altitude. The SABRE engine "relies on a heat exchanger capable of cooling incoming air to −150 °C (−238 °F), to provide oxygen for mixing with hydrogen and provide jet thrust during atmospheric flight before switching to tanked liquid oxygen when in space."
At the front of the engine, a simple translating axisymmetric shock cone inlet slows the air to subsonic speeds using two shock reflections. Part of the air then passes through a precooler into the central core, with the remainder passing directly through a ring of bypass ramjets. The central core of SABRE behind the precooler uses a turbo-compressor run off the same gaseous helium loop Brayton cycle which compresses the air and feeds it into four high pressure combined cycle rocket engine combustion chambers. The oxygen is also fed to the combustion unit, using a turbopump.
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 November 2012, Reaction Engines announced it had successfully concluded a series of tests that prove the cooling technology of the engine, one of the main obstacles towards the completion of the project. The European Space Agency (ESA) evaluated the SABRE engine's pre-cooler heat exchanger, and accepted claims that the technologies required to proceed with the engine's development had been fully demonstrated.
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 2014-2016 with the ESA providing an additional £7M. The total cost of developing a test rig is estimated at £200M.
By June 2015, SABRE's development continued with The Advanced Nozzle Project in Westcott, UK. The test engine, operated by Airborne Engineering Ltd., is being used to analyze the aerodynamics and performance of the advanced nozzles that the SABRE engine will use, in addition to new manufacturing technologies such as the 3D-printed propellant injection system.
In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. Air Force Research Laboratory, the lab will reveal two-stage-to-orbit SABRE concepts in the near future, they believe that a single-stage-to-orbit Skylon space plane is "technically very risky as a first application of SABRE engine ," and this is why they are developing two-stage-to-orbit concepts. 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. Then 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.
As the air enters the engine at supersonic/hypersonic speeds, it becomes very hot due to compression effects. The high temperatures are traditionally dealt with in jet engines 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 dramatically cools the air from 1000 °C down to −150 °C in a heat exchanger while avoiding liquefaction of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler. SABRE inserts 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 is necessary to prevent blocking the precooler from frozen water vapour and other air fractions. On October 2012, the cooling solution was demonstrated for 6 minutes using freezing air. The cooler consists of a fine pipework heat exchanger and cools the hot in-rushing atmospheric air down to the required −150 °C 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.
Below 5 times the speed of sound and 25 kilometres of altitude, which is 20% of the speed and 20% of the altitude needed to reach orbit, the cooled air from the precooler passes 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 feeds the compressed air at 140 atmospheres into the combustion chambers of the main engines.
The turbo-compressor is powered by a gas turbine running on a helium loop, rather than by combustion gases as in a conventional jet engine. The turbo-compressor is powered by waste heat collected by the helium loop.
The 'hot' helium from the air precooler is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel. The loop forms a self-starting Brayton cycle engine, cooling critical parts of the engine and powering turbines. The heat passes from the air into the helium. This heat energy is used to power various parts of the engine and to vaporise hydrogen, which is then burnt in ramjets.
Due to the static thrust capability of the hybrid rocket engine, the vehicle can take off under air-breathing mode, much like a conventional turbojet. As the craft ascends and the outside air pressure drops, more and more air is passed into the compressor as the effectiveness of the ram compression drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.
At Mach 5.5 the air-breathing system becomes inefficient and is powered down, replaced by the on-board stored oxygen which allows the engine to accelerate to orbital velocities (around Mach 25).
The combustion chambers in the SABRE engine are cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen to further reduce the system's use of liquid hydrogen compared to stoichiometric systems.
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. An SSTO engine must use a single set of nozzles. Successful tests were carried out in 2010 on an expansion deflection nozzle called STERN that varies the nozzle output to overcome the problem of non-dynamic exhaust expansion.
Avoiding liquefaction improves the efficiency of the engine since less entropy is generated and therefore less liquid hydrogen is boiled off. However, simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is expelled through a series of burners called "spill duct ramjet burners", that are arranged in a ring around the central core. These are fed air that bypasses the precooler. This bypass ramjet system is designed to reduce the negative effects of drag resulting from air that passes into the intakes but is not 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 can feed to the combustion chamber is at its highest, requiring the bypassed air to be accelerated to maintain efficiency at these low speeds. This distinguishes 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.
The designed thrust-to-weight ratio of SABRE is 14 compared to about 5 for conventional jet engines, and 2 for scramjets. This high performance is a combination of the denser, cooled air, requiring less compression, and, more importantly, the low air temperatures permitting lighter alloys to be used in much of the engine. Overall performance is much better than the RB545 engine or scramjets.
Fuel efficiency (known as specific impulse in rocket engines) peaks at about 3500 seconds within the atmosphere. Typical all-rocket systems peak around 450 seconds and even "typical" nuclear thermal rockets at about 900 seconds.
The combination of high fuel efficiency and low-mass engines permits a single-stage-to-orbit (SSTO) approach, with air-breathing to Mach 5.14+ at 28.5 km (17.7 mi) altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear launch vehicle ever proposed.
The precooler adds mass and complexity to the system, and is the most aggressive and difficult part of the design, but the mass of this heat exchanger is an order of magnitude lower than has been achieved previously. The experimental device achieved heat exchange of almost 1 GW/m3. The losses from carrying the added weight of systems shut down during the closed cycle mode (namely the precooler and turbo-compressor) as well as the added weight of Skylon's wings are offset by the gains in overall efficiency and the proposed flight plan. Conventional launch vehicles such as the Space Shuttle spend about one minute climbing almost vertically at relatively low speeds; this is inefficient, but optimal for pure-rocket vehicles. In contrast, the SABRE engine permits a much slower, shallower climb, breathing air and using its wings to support the vehicle therefore increasing payload fraction.
A hybrid jet engine like SABRE needs only reach low hypersonic speeds inside the lower atmosphere before engaging its closed cycle mode, whilst climbing, to build speed. Unlike ramjet or scramjet engines, the design is able to provide high thrust from zero speed up to Mach 5.5, with excellent thrust over the entire flight, from the ground to very high altitude, with high efficiency throughout. In addition, this static thrust capability means the engine can be realistically tested on the ground, which drastically cuts testing costs.
In 2012, REL expected test flights by 2020, and operational flights by 2030.
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