The SABRE engine (Synergic Air BReathing Engine) is a design by Reaction Engines Limited for a hypersonic hydrogen-fueled air breathing combined cycle rocket engine/turbojet engine/ramjet engine for propelling the Skylon launch vehicle into low earth orbit (LEO). SABRE is the logical continuation 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 SABRE design combines a lightweight turbine-cycle jet engine with an air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram compressed air, and allows the jet engine to continue to provide high thrust even at very high speeds and altitudes. In addition, the low temperature of the air allows light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit.
The engine also includes rocket engine features which allow the vehicle to reach low earth orbit after leaving the atmosphere after shutting the inlet cone off at Mach 5.5, 26 km altitude.
Alan Bond says that the technology readiness level of the engine is, as of May 2009, 2-3.
The precooler concept is due to 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 Marquardt and General Dynamics in the 1960s as part of the US Air Force's aerospaceplane efforts. This work eventually culminated in medium-thrust engines that ran for several minutes at a time. Although the program was generally successful, changing priorities and loss of USAF funding led to the idea being abandoned.
In an operational setting with LACE, the 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 out the liquid oxygen for burning in the engine. The amount of warmed hydrogen was too great to burn with the oxygen, so most was to be simply dumped overboard (nevertheless giving useful thrust.)
HOTOL's engine, the Reaction Engines RB545, was similar to the original LACE system, but much simpler in detail. Like the LACE system it used an air intake and a hydrogen heat exchanger, but was able to do away with much of the complexity of the USAF design, which included pumps and storage tanks along each step of the separation process.
The RB545 was a "straight-through" design that used careful arrangement of the components to dump the unwanted liquefied gasses directly overboard, instead of pumping it around from tank to tank. In addition, the RB545 did not liquefy the oxygen or nitrogen, but cooled and then simply compressed the gas in a similar way to turbojet engines. From that point on, the RB545, like LACE before it, consisted of a fairly conventional rocket engine, but running on a variable mixture of cooled, compressed air and liquid oxygen.
In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited to continue research. The RB545's liquid hydrogen precooler had issues with embrittlement, patents and The Official Secrets Act, so Bond went on to develop SABRE in its place.
Like the RB545, the SABRE design is not a conventional rocket engine nor jet engine, but a precooled turborocket that burns hydrogen fuel, liquid oxygen and air.
At the front of the engine a simple translating axisymmetric shock cone inlet slows the air to subsonic speeds using just two shock reflections.
Part of the air then passes the air through a precooler into the central core, with the remainder passing directly through a ring of ramjets. The central core of SABRE behind the precooler uses two high pressure combined cycle turbojet/rocket engines.
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 an SSTO craft, 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 -140 °C. in a heat exchanger while avoiding liquification of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler, but inserting a helium cooling loop between the air and the cold fuel avoids problems with hydrogen embrittlement in the air precooler.
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other fractions. A suitable precooler, which rejects condensed water before it freezes has now been experimentally demonstrated.
Avoiding liquification improves the efficiency of the engine since less liquid hydrogen is boiled off. However, even simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is dumped overboard through a series of burners- ramjets which are arranged in a ring around the central core. These are fed from air that bypasses the precooler.
Below 26km, the cooled air from the precooler passes into a reasonably conventional turbo-compressor, similar in design to those used on conventional jet engines but running at unusually high pressure ratio made possible by the low temperature of the inlet air. This feeds the compressed air at very high pressure into the combustion chambers of the main engines.
Unusually for jet engines, the turbocompressor is powered by a gas turbine running on the helium loop, rather than off combustion gases as in a conventional jet engine. Thus, the turbo-compressor is powered by waste heat collected by the helium loop.
After being launched and brought to speed by a short burst of the rockets, the jets are started, fed by air bled from the shock cone. At this point the precooler/turbo-compressor is not being used. 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 alone 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 jets become inefficient and are powered down, and stored liquid oxygen/liquid hydrogen is used for the rest of the ascent in the separate rocket engines; the turbopumps are powered by the helium loop from the heat produced by cooling the engine.
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 designed thrust/weight ratio of SABRE ends is high—up to 14—compared to about 5 for conventional jet engines, and just 2 for scramjets. This high performance is a combination of the cooled air being denser and hence requiring less compression, but more importantly, of the low air temperatures permitting lighter alloy to be used in much of the engine. Overall performance is much better than the RB545 engine or scramjets.
The engine gives good fuel efficiency peaking at about 2800 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds.
The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with air breathing to mach 5.5+ at 26 km 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.
Like the RB545, the precooler idea adds mass and complexity to the system, normally the antithesis of rocket design. The precooler is also the most aggressive and difficult part of the whole SABRE design. The mass of this heat exchanger is an order of magnitude better than has been achieved previously; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler now exist.
The losses from carrying around a number of engines that will be turned off for some portion of the flight would appear to be heavy, yet the gains in overall efficiency more than make up for this. These losses are greatly offset by the different flight plan. Conventional launch vehicles such as the Space Shuttle usually start a launch by spending around a 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, air breathing and using wings to support the vehicle, giving far lower fuel usage before lighting the rockets to do the 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.
Published - July 2009
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