A model of SABRE |
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| Country of origin | United Kingdom |
|---|---|
| Designer | Reaction Engines Limited |
| Application | Single-stage-to-orbit |
| Associated L/V | Skylon |
| Predecessor | RB545 |
| Status | Research and development |
| Liquid-fuel engine | |
| Propellant | Air/LO2 / Liquid hydrogen [1] |
| Cycle | combined cycle precooled jet engine + closed cycle rocket engine |
| Performance | |
| Thrust (Vac.) | ~ 1,800 kN [2] |
| Thrust (SL) | ~ 1,350 kN [2] |
| Thrust-to-weight ratio | up to 14 (atmospheric) [3] |
| Isp (Vac.) | 460 s [2] |
| Isp (SL) | 3600 s [2] |
SABRE (Synergistic Air-Breathing Rocket Engine) [1] is a concept by Reaction Engines Limited for a hypersonic precooled hybrid air breathing rocket engine [4] for propelling the proposed 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 design comprises a single combined cycle rocket engine with two modes of operation.[2] The air breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram compressed air leading to an unusually high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited with stored liquid hydrogen. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit. In addition, unlike the LACE concept that preceded it, SABRE’s precooler does not liquefy the air letting it run more efficiently.[3]
After shutting the inlet cone off at Mach 5.14, 28.5 km altitude,[2] the system continues as a closed cycle high performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks allowing Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.
An engine derived from the SABRE concept called Scimitar has been designed for the company’s A2 hypersonic passenger jet proposal for the European Union-funded LAPCAT study.[5]
Alan Bond says that the technology readiness level of the SABRE engine is, as of May 2009, 2-3.[6] A project called the Technology Demonstration Programme, funded through private finance as well as a European Space Agency grant, began in February 2009 with the aim of validating key aspects of the engine by the end of 2011. The company anticipates this will bring the system to a technology readiness level of 4-5.[7][8]
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The precooler concept is due to an idea originated by Robert P. Carmichael in 1955.[9] 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.[3]
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.)
In 1989, after funding for HOTOL ceased, Bond and several others formed Reaction Engines Limited to continue research. The RB545's precooler had issues with embrittlement, relatively high liquid hydrogen consumption, patents and Official Secrets Act, so Bond went on to develop SABRE in its place. [10]
Like the RB545, the SABRE design is neither a conventional rocket engine nor jet engine, but a Precooled Hybrid Air Breathing Rocket Engine that burns liquid hydrogen fuel combined with an oxidant of either compressor-fed gaseous air or stored liquid oxygen fed using a turbopump.
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 through a precooler into the central core, with the remainder passing directly through a ring of bypass ramjets. The central core of SABRE behind the precooler uses a turbo-compressor run off the same gaseous helium loop Brayton cycle which compresses the air and feeds it into four high pressure combined cycle rocket engine combustion chambers.
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 [11] in a heat exchanger while avoiding liquefaction 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.[11]
Below 28.5 km, 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 turbo-compressor is powered by a gas turbine running on a 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.
The 'hot' helium from the air precooler 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 vaporise hydrogen, which is burnt in ramjets.
Due to the static thrust capability of the hybrid rocket engines, the vehicle can take off under air breathing mode without assistance much like conventional turbojets.[2] 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 the preburner.
Unusually the combustion chambers in the SABRE engine are to be cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen [8] to further reduce the systems overuse of liquid hydrogen compared to stoichiometric.
The most efficient atmospheric pressure at which a conventional propelling nozzle works at is set by the geometry of the bell. While the geometry of the conventional bell remains static the atmospheric pressure changes with altitude and therefore nozzles designed for high performance in the lower atmosphere will significantly drop in efficiency as they reach higher altitudes. This is overcome in traditional rockets through the multistage process so engines designed for different atmospheric pressures can be used during the most appropriate stages of flight. An SSTO engine must use the same nozzles however. As part of the work on the Technology Development Programme tests were carried out on an expansion deflection nozzle called STERN to overcome the problem of non-dynamic exhaust expansion.[8] The successful experiment means that this technology is likely to be incorporated into the final SABRE design.[12]
Avoiding liquefaction improves the efficiency of the engine since less entropy is generated and therefore 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 – "spill duct ramjet burners"[2][13] which are arranged in a ring around the central core. These are fed air that bypasses the precooler. This bypass ramjet system is designed to reduce the negative effects of drag resulting from air that passes into the intakes but doesn’t get fed into the main rocket engine, rather than generating appreciable thrust of their own. At low speeds the ratio of the volume of air entering the intake to the volume that the compressor can feed to the combustion chamber is at its highest, requiring the bypassed air to be accelerated to maintain efficiency at these low speeds. This distinguishes the system from a turboramjet where a turbine-cycle’s exhaust is used to increase air-flow for the ramjet to become efficient enough to take over the role of primary propulsion.[14]
As of August 2010[update], hardware testing of the "heat exchanger technology crucial to [the] hybrid air- and liquid oxygen-breathing [Sabre] rocket motor" is under way. This is an important step in the Sabre development process, to demonstrate to investors that the technology is viable. Tests are scheduled to continue until the end of December 2011.[15]
The Sabre engine "relies on a heat exchanger capable of cooling incoming air to −140 °C (−220 °F), to provide liquid oxygen (LOX) for mixing with hydrogen to provide jet thrust during atmospheric flight before switching to tanked LOX when in space." The autumn 2011 test program will validate that the critical heat exchanger technology can perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.[15]
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.[4] 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 3500 seconds within the atmosphere.[2] 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 5.14+ at 28.5 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, referred to as modules, now exist.[11]
The losses from carrying the added weight of systems shut down during the closed cycle mode (namely the precooler and turbo-compressor) as well as the added weight of Skylon’s wings 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.
Unlike traditional rocket engines, and like other types of air breathing jet engine, a hybrid jet engine can utilise air to create combustion saving on propellant weight and therefore increasing payload fraction.
Ramjets and Scramjets must spend a significant amount of time within the lower atmosphere to build speed to reach orbital velocity creating issues with extremely high drag leading to intense heating and the subsequent weight and complexity of required thermal protection. A hybrid jet like SABRE needs only reach low hypersonic speeds inside the lower atmosphere before engaging its closed cycle mode, whilst climbing, to build speed.
Unlike ramjet or scramjet engines the design is able to provide high thrust from zero speed up to Mach 5.5,[16] with excellent thrust over the entire flight, from the ground to very high altitude, with high efficiency throughout.
In addition this static thrust capability means the engine can be easily tested on the ground, which drastically cuts testing costs.[4]
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