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Reaction Engines SABRE

The engine SABRE (Synergic Air breathing engine) is designed by Reaction Engines Limited for a hypersonic air hydrogen as fuel engine breathing combined cycle rocket / turboramjet precooled air to propel the project Skylon vehicle to low Earth orbit (LEO). SABRE is the logical continuation of Alan Bond's series of air cycle engine fluid (LACE) and designs such as lace began in HOTOL early/mid-1980s for the project.

The SABRE design combines a turbine engine light cycle air jet with a Precooler placed just behind the inlet cone. At high speeds the Precooler cools the heat, a compressed air ram, allowing the jet engine to continue to provide high thrust at high speeds and altitudes. In addition, low air temperature allows the construction of light alloy to be employed which gives a very light engine - essential to achieve orbit.

The engine also includes features of rocket engines, which allow the vehicle to reach low-Earth orbit after leaving the atmosphere after closing off the entrance cone at Mach 5.5, 26 km of altitude.

Alan Bond, said the level of technological readiness of the machine is, in May 2009, 2-3.


Precooler The concept is an idea originated by Robert P. Carmichael in 1955. [2] This was followed by the liquid air cycle engine (LACE) idea that was explored initially by Marquardt and General Dynamics in 1960 as part of the U.S. efforts aerospaceplane Air Force. This work finally culminated in average thrust engines that lasted several minutes at a time. Although the program was generally satisfactory, the change of priorities and the loss of funding from the USAF led to the idea of being abandoned.

In an operational environment with lace, the system was to be placed behind a supersonic air intake air compressed by the compression ram, a heat exchanger will cool faster than using some of the liquid hydrogen fuel storage on board. The resulting liquid air is then processed to separate the liquid oxygen to burn in the engine. The amount of hydrogen was heated too big to burn with oxygen, so the majority was to be simply dumped overboard (nevertheless giving useful thrust.)

HOTOL engine, the RB545, was similar to the original LACE system, but much simpler in detail. Like the LACE system used an air intake and a heat exchanger for hydrogen, but was able to eliminate much of the complexity of the design of the USAF, which included pumps and storage tanks along each step the separation process.

The RB545 was an "integrated" design that uses careful arrangement of the components of liquefied gases spam dump directly overboard, instead of pumping around the tank to tank. In addition, RB545 did not liquefy the oxygen or nitrogen, but then simply cooled and compressed gas in a manner similar to the turbojet engine. From that moment, the RB545, as LACE before, consisted of a fairly conventional rocket engine, but runs on a variable mix of cooling, compressed air and liquid oxygen.

In 1989, after HOTOL funding ceased, Bond and many others formed Reaction Engines Limited for further research. Liquid hydrogen Precooler The RB545 had problems with the fragility, patents and the Official Secrets Act, so Bond went on to develop SABRE in place.


Mockup of the SABRE engine, showing gray heat exchanger

Simplified thermodynamic cycle of SABRE engine

Like the RB545, SABRE design is not a conventional rocket engine or jet engine, but a precooled turborocket that burns hydrogen fuel, liquid oxygen and air. In front of the engine of a simple translation of the entry cone axisymmetric shock decreases the air at subsonic speeds using only two shock reflections.

Part of the air passes the air through a Precooler in the core, with the remainder passes directly through a ring of ramjet. The core of behind the Precooler SABRE uses two high-pressure turbo Combined Cycle / rocket engines.


 As the air enters the engine at supersonic / hypersonic speeds, it becomes very hot due to the effects of compression. High temperatures are traditionally treated in jet engines with heavy copper or nickel based materials, by reducing engine pressure ratio, and by throttling back the engine to the highest airspeeds to avoid melting . However, for a SSTO craft such heavy material is unusable, and maximum thrust is required for orbital insertion as soon as possible to minimize loss of gravity. Instead, through a loop of liquid helium coolant gas, SABRE dramatically cools the air from 1000 ° C to -140 ° C. in a heat exchanger, preventing obstruction liquefying air or water vapor from freezing.

Earlier versions of coolers as hydrogen fuel HOTOL directly through Precooler, but the insertion of the helium cooling loop between the air and cold fuel avoids problems with hydrogen embrittlement in air Precooler.

However, the dramatic cooling of the air raised a potential issue: it is necessary to prevent blockage of the Precooler frozen water vapor and other fractions. A proper Precooler, rejecting the condensed water freezes before now has been demonstrated experimentally.


Avoid liquefaction improves engine efficiency, since the liquid hydrogen is less evaporated. However, even just the air cooling needs more liquid hydrogen can be burned in the core engine. The excess is discharged into the sea through a series of "ramjet burners that are arranged in a ring around the core. They feed air passing through the Precooler.

Below Mach 1.0 a compressor is very necessary as a component of air-breathing engine. As an airplane increases its speed past Mach 1.0 air pressure created from air flow rate reduces the need for a compressor. As speeds approaching Mach 3.5 to 4.0, a compressor is still necessary. The ramjet is the most efficient engine because it has fewer components. The ramjet does not have a compressor or a turbine, and has a much higher tolerance to high temperatures. A schematic of a ramjet engine is shown below. It has an entrance, a burner, and a mouthpiece.

A ramjet has its limitations. The first is that it will not work unless the supersonic speeds, another engine must first power the aircraft at supersonic speeds. Another limitation is the combustion of fuel-air mixture in the combustion chamber. Ramjet input should decrease the air flow at supersonic speeds to subsonic speed for ignition in the burner. As the approaches Mach 6.0 ramjet air entering the burner is too hot to burn! This is caused by the friction created as the supersonic air is slower at the entrance to subsonic speed. At this speed boost not enough is being generated to follow the performance.

There is a proposed solution to the limitation of speed ramjet (Mach 6.0). It is called supersonic combustion ramjet (scramjet). Instead of stopping the flow of air to subsonic speeds for combustion, the scramjet will light supersonic air at the same time (thus avoiding friction in the entrance). Fuel still needs to be injected into the air stream to be ignited. Unfortunately, today's fuels do not ignite quickly enough. The development of a fuel injection system for scramjet viable is still in its early.

 Below 26 km, cold air passes Precooler a reasonably conventional turbo-compressor, similar in design to those used in conventional jet engines, but it works to the unusually high pressure ratio made possible by the low air temperature admission. This feeds the compressed air at high pressure into the combustion chambers of the main engines.

Unusually for jet engines, the turbo compressor is driven by a gas turbine running in the helium loop, instead of the combustion gases as in a conventional jet engine. Thus, the turbo-compressor is driven by waste heat collected by the helium loop.


 After being released and taken to the speed of a short burst of rockets, planes have started, fed by air in the discharge cone. At this point the Precooler / turbo-compressor is not being used. As the ship rises and drops outside air pressure, air is increasingly happening in the compressor and the compression efficiency drops ram only. In this way the planes are capable of operating at a height much greater than would normally be possible.

At Mach 5.5 jets become ineffective and off, and stored liquid oxygen / liquid hydrogen is used for the remainder of the increase in rocket motors separately, the turbo pumps are powered by the loop of helium from the heat produced by cooling the engine.

Helium loop

The 'hot' helium from the air precooler, and cooling the combustion chambers is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel.
The loop forms a self starting Brayton cycle engine, and is used to both cool critical parts of the engine, but also to power turbines and numerous miscellaneous parts of the engine.
The heat passes from the air into the helium. This heat energy is not entirely wasted, it is in fact used to power the various parts of the engine, and the remainder is used to vapourise hydrogen, which is burnt in ramjets.


The idea designed / SABRE Extreme weight is high up to 14-compared with about 5 conventional jet engines, and only 2 of scramjets. This high performance is a combination of cold air more dense and therefore less compression is required, but more importantly the low air temperature alloy that can be used in much of the motor. The overall performance is much better than RB545 engine or scramjets.

The engine gives good fuel efficiency peaking at around 2800 seconds within the atmosphere. Typical systems of all-around 450 rockets that can be at most, and even "typical" nuclear thermal rockets only about 900 seconds.

Combining high fuel efficiency and low mass engines means that a single stage to orbit closer to the Skylon can be employed, with air breathing to mach 5.5 + 26 km altitude, and the vehicle reaches the orbit with more payload mass by the mass off just about any non-nuclear launch vehicle ever proposed.

As the RB545, the idea Precooler to increase the mass and complexity of the system, usually the opposite of rocket design. The Precooler is also the most aggressive and difficult part of designing a whole SABRE. The mass of this heat exchanger is an order of magnitude better than previously has been achieved, however, experimental work has shown that this can be achieved. The experimental heat exchanger has heat exchange of about 1 GW / m³, believed to be a world record. Small sections of a real Precooler that exist today.

Losses to carry around a number of engines to be turned off during part of the flight seems to be heavy, however, improving the overall efficiency more than make up for it. These losses are largely offset by the different flight plan. conventional vehicles such as the launch of the Space Shuttle usually start a boat for the expenditure of about one minute climbing almost vertically at relatively low speeds, which is inefficient, but optimal for pure rocket vehicles. By contrast, the SABRE engine permits a much slower, shallower climb, air breathing and the use of wings to support the vehicle, giving much lower fuel usage before lighting the rockets for orbital insertion.


The engine is capable of very high speed, with excellent thrust over the entire flight, from the ground to very high altitude, with high efficiency throughout.
In addition, unlike scramjets or ramjets the engine can be easily tested on the ground, which massively cuts testing costs.

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