SABRE
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- For other uses, see Sabre (disambiguation).
SABRE (Synergic Air Breathing Engine) is a design for a hypersonic hydrogen-fueled air breathing combined cycle rocket engine/turbojet engine for propelling the Skylon launch vehicle into low earth orbit (LEO).
The SABRE design combines a lightweight turbine-cycle jet engine with a pre-cooler positioned just behind the inlet cone. At high speeds this pre-cooler cools the hot, ram compressed air, which allows the jet engine to continue to provide high thrust even at very high speeds. In addition, the low temperature of the air allows light alloy construction to be employed which gives a very lightweight engine, which is the 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.
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.
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[edit] History
The pre-cooler concept is due to an idea originated by Robert P. Carmichael in 1955 [1]. 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. And, although the program was generally successful, changing priorities and USAF funding led to the idea being abandoned.
In an operational setting the system would be placed behind a supersonic air intake which would compress the air through ram compression, then rapidly cool it in a heat exchanger 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 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. 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 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 compressed air and liquid oxygen.
[edit] A new design
In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited to continue research. The RB545's liquid hydrogen pre-cooler 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 pre-cooled 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 and then passes the air through a pre-cooler.
Behind the pre-cooler, the SABRE system consists of a number of different engine components, each tuned to a different portion of the flight. SABRE uses two "pure" rocket engines surrounded by a ring of smaller engines similar to ramjets.
[edit] The pre-cooler
As the air enters the engine at high 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, and by throttling back the engine at the higher airspeeds to avoid melting. Instead, using a gaseous helium coolant loop, SABRE dramatically cools the air in a heat exchanger but avoids liquification.
Previous versions of SABRE 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 pre-cooler.
Avoiding liquifaction improves the efficiency of the engine since less liquid hydrogen is boiled off; even simply cooling the air needs more liquid hydrogen than can be burnt in the engine core, the excess is dumped overboard (through a ramjet.)
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the pre-cooler from frozen water vapour. A suitable precooler, which rejects condensed water before it freezes has now been experimentally demonstrated.
[edit] The compressor
The cooled air is then passed into a reasonably conventional turbo-compressor, similar in design to those used on a jet engine, but in this case 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.
[edit] The engines
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 pre-cooler/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 turbo pumps are powered off of the helium loop from the heat produced by cooling the engine.
[edit] The helium loop
The 'hot' helium from the air pre-cooler, 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 a Ramjet.
[edit] Losses
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 the optimum 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.
[edit] Performance
The designed thrust/weight ratio of SABRE ends up several times higher—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 powered engines 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 pre-cooler idea adds mass and complexity to the system, normally the antithesis of rocket design. The pre-cooler 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/m^3, believed to be a world record. Small sections of a real pre-cooler now exist.
[edit] Resources
- A Comparison of Propulsions Concepts for SSTO Reusable launchers [1]
- An Experimental Pre-cooler for Air breathing Rocket Engines [2]
- The Skylon Space plane [3]
