SABRE
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- For other uses, see Sabre (disambiguation).
SABRE (Synergic Air BReathing Engine) is a design for a hypersonic hydrogen-fuelled air breathing combined cycle rocket engine/jet engine for propelling the Skylon launch vehicle into low earth orbit (LEO).
The SABRE design combines a jet engine with a precooler positioned just after the inlet cone. At high speeds this precooler 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.
The engine also has a rocket engine which allows the vehicle to reach low earth orbit after leaving the atmosphere. Constructing a technologically reliable and economically viable lift to orbit technolgy is one of the keys to re-opening the space frontier.
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 precooler-based LACE idea 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 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 onboard. The resulting liquid air was then processed to separate out the liquid oxygen for burning in the engine. The small amount of now-warmed hydrogen used up in this process was too difficult to feed back into the engine, so it was simply dumped overboard.
HOTOL's engine, the RB545, was very similar to the original LACE system, but much simpler in detail. Like the LACE system it used an air intake and heat exchanger system, 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 liquified gasses directly overboard, instead of pumping it around from tank to tank. From that point on the RB545, like LACE before it, consisted of a fairly conventional rocket engine.
[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 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, but a precooled turborocket that burns hydrogen fuel and air.
At the front of the engine a single moving shock cone inlet slows the air to subsonic speeds and then passes the air through a precooler. Behind the precooler, 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 combustors similar to ramjets.
[edit] The precooler
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 precooler.
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 (presumably through a suitable nozzle in afterburner mode.)
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler 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 turbocompressor, 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.
[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 precooler/turbocompressor 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 off of the helium loop from the heat produced by cooling the engine.
[edit] The 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 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, airbreathing 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 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 engines. Overall performance is much better than the RB545 engine.
The engine results in very good fuel efficiency—about 2800 seconds. 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 airbreathing to mach 5.5+, 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/m^3, believed to be a world record. Small sections of a real precooler now exist.
[edit] Resources
- A Comparison of Propulsions Concepts for SSTO Reusable launchers [1]
- An Experimental Precooler for Airbreathing Rocket Engines [2]
- The Skylon Spaceplane [3]
