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SABRE is a rocket engine design that improves on the liquid air cycle engine design. This allows SABRE to operate in a mode similar to a conventional jet engine using afterburners while in the lower atmosphere, whilst using more and more stored oxygen as the altitude increases until it finally transitions to pure rocket mode.
SABRE is the ultimate design of Alan Bond's series of LACE and LACE-like designs that started with HOTOL in the early/mid-1980s. After funding for that project was killed, Bond and several others formed Reaction Engines Limited (http://www.reactionengines.co.uk/) in 1989 to continue research. However his original LACE designs had been classified as top secret and could not be used in a public company, so Bond went on to develop SABRE its place.
With the LACE design the oxygen is slowed and then liquified using a liquid hydrogen heat exchanger. In order to supply enough oxygen to a rocket engine, the hydrogen flow rates must be high, and using this now-warm fuel is a difficult process and so it was dumped overboard. While this process is still arguably more efficient than carrying the oxygen up from the ground, the overall gains in the LACE design are modest.
SABRE operates in a different manner, using a precooler and a series of smaller combustion chambers that can run on smaller flow rates and are "tuned" for different performance levels. Some are essentially nothing more than a conventional afterburner, allowing the SABRE to run in a pure ramjet mode for some of its flight. Others are more like conventional rocket engines, running on either stored liquid oxygen, or air provided by the compressor/precooler system.
The precooler may very well be the most aggressive and difficult part of the whole SABRE design; the mass of this compressor is an order of magnitude better than has been achieved previously. In addition, the cooler has to avoid icing issues up to 12kms. Research suggests that building it is possible, or atleast plausible.
It is the compressor/cooler system that is key to the SABRE. Unlike LACE where the air is liquified by cooling, in SABRE it is merely precooled to slightly above the liquification point before pressurisation up to combustion chamber pressures. This reduces the quantity of hydrogen required (no heat of vapourisation need be removed) whilst easing the cooler design (it becomes a single phase cooler). The precooler for SABRE uses a closed-loop liquid helium cooling system, which in turn is cooled by the liquid hydrogen fuel. Hydrogen embrittlement of the precooler is thus avoided. The cooled, compressed air, is kept at near constant temperature and pressure over the entire flight regime and is fed into a combustion chamber.
The tradeoff in SABRE is that although the fuel is not "wasted" in liquifying the oxygen, instead the mass of the compressor system must be carried into orbit. The upside is that the compressor can be used at any speed, from sitting on a runway all the way to the edges of the atmosphere.
Resources:
A Comparison of Propulsions Concepts for SSTO Reusable launchers [1] (http://www.reactionengines.co.uk/downloads/JBIS_v56_108-117.pdf)
An Experimental Precooler for Airbreathing Rocket Engines [2] (http://www.reactionengines.co.uk/downloads/JBIS_v54_199-209.pdf)
SABRE, an acronym for "Semi-Automated Business Research Environment," is the name of the computer reservation system developed by American Airlines and IBM in 1963. Its launch revolutionized the airline industry. AA spun off SABRE's operations into a separate company, The Sabre Group, on March 1, 2000. Although AA retains a significant ownership share of TSGI, the company owns the Travelocity reservations portal and operates or has operated web site reservation systems for competitors such as US Airways.
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