According to Space.com
Within the next year, the U.S. Air Force plans to unveil novel spacecraft concepts that would be powered by a potentially revolutionary reusable engine designed for a private space plane. Since January 2014, the Air Force Research Laboratory (AFRL) has been developing hypersonic vehicle concepts that use the Synergetic Air-Breathing Rocket Engine (SABRE), which was invented by England-based Reaction Engines Ltd. and would propel the company’s Skylon space plane. In April 2015, Reaction Engines announced that an AFRL study had concluded that SABRE is feasible. And AFRL is bullish on the technology; the lab will reveal two-stage-to-orbit SABRE-based concepts either this September, at the American Institute of Aeronautics and Astronautics’ (AIAA) SPACE 2016 conference in Long Beach, California, or in March 2017, at the 21st AIAA International Space Planes and Hypersonic Systems and Technologies Conference in China, said AFRL Aerospace Systems Directorate Aerospace Engineer Barry Hellman.
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Proposed Skylon spaceplane @pix.avaxnews.com
SABRE (Synergistic Air-Breathing Rocket Engine) is a concept under development by Reaction Engines Limited for a hypersonic precooled hybrid air breathing rocket engine. The engine is being designed to achieve single-stage-to-orbit capability, propelling the proposed Skylon spaceplane to low Earth orbit. SABRE is an evolution 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.
SABRE is at heart a rocket engine designed to power aircraft directly into space (single-stage to orbit) to allow reliable, responsive and cost effective space access, and in a different configuration to allow aircraft to cruise at high speeds (five times the speed of sound) within the atmosphere.
In the past, attempts to design single stage to orbit propulsion systems have been unsuccessful largely due to the weight of an on-board oxidiser such as liquid oxygen, needed by conventional rocket engines. One possible solution to reduce the quantity of on-board oxidizer required is by using oxygen already present in the atmosphere in the combustion process just like an ordinary jet engine. This weight saving would enable the transition from single-use multi-stage launch vehicles to multi-use single stage launch vehicles.
SABRE is the first engine to achieve this goal by operating in two rocket modes: initially in air-breathing mode and subsequently in conventional rocket mode:
- Air breathing mode – the rocket engine sucks in atmospheric air as a source of oxygen (as in a typical jet engine) to burn with its liquid hydrogen fuel in the rocket combustion chamber
- Conventional rocket mode – the engine is above the atmosphere and transitions to using conventional on-board liquid oxygen.
In both modes the thrust is generated using the rocket combustion chamber and nozzles. This is made possible through a synthesis of elements from rocket and gas turbine technology.
This approach enables SABRE-powered vehicles to save carrying over 250 tons of on-board oxidant on their way to orbit, and removes the necessity for massive throw-away first stages that are jettisoned once the oxidant they contain has been used up, allowing the development of the first fully re-usable space access vehicles such as SKYLON.
While this sounds simple, the problem is that in air-breathing mode, the air must be compressed to around 140 atmospheres before injection into the combustion chambers which raises its temperature so high that it would melt any known material. SABRE avoids this by first cooling the air using a Pre-cooler heat exchanger until it is almost a liquid. Then a relatively conventional turbo compressor using jet engine technology can be used to compress the air to the required pressure.
This means when SABRE is in the Earth’s atmosphere the engine can use air to burn with the hydrogen fuel rather than the liquid oxygen used when in rocket mode, which gives an 8 fold improvement in propellant consumption. The air-breathing mode can be used until the engine has reached over 5 times the speed of sound and an altitude of 25 kilometres which is 20% of the speed and 20% of the altitude needed to reach orbit. The remaining 80% can be achieved using the SABRE engines in rocket mode.
For space access, the thrust during air-breathing ascent is variable but around 200 tonnes per engine. During rocket ascent this rises to 300 tonnes but is then throttled down towards the end of the ascent to limit the longitudinal acceleration to 3.0g. @reactionengines.co.uk
The design comprises a single combined cycle rocket engine with two modes of operation. 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 a very high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited along with stored liquid hydrogen. The high pressure ratio allows the engine to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed and allow a very lightweight engine—essential for reaching orbit. In addition, unlike the LACE concept, SABRE’s precooler does not liquefy the air, letting it run more efficiently.
SABRE’s precooler @mfcrabs.kinja.com
After shutting the inlet cone off at Mach 5.14, 28.5 km altitude, the system continues as a closed cycle high-performance rocket engine burning liquid oxygen and liquid hydrogen from on-board fuel tanks, potentially allowing a hybrid spaceplane concept like Skylon to reach orbital velocity after leaving the atmosphere on a steep climb.
LAPCAT A2 hypersonic passenger jet @jetlinemarvel.net
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.
Comparing size of A2 hypersonic passenger jet to A380 @email@example.com
Simplified flow diagram of SABRE engine
Artist’s rendering of Skylon spacecraft Credit: reactionengines.co.uk @3.bp.blogspot.com
Air-breathing SABRE rocket engine @reactionengines.co.uk
This advanced combined cycle air-breathing SABRE rocket engine enables aircraft to operate easily at speeds of up to five times the speed of sound or fly directly into Earth orbit.
With the Pre-cooler heat exchanger and other SABRE engine advanced technology development programmes nearing completion, the next stage of the SABRE programme is the construction of a full engine demonstrator.. @reactionengines.co.uk
In 2011, hardware testing of the heat exchanger technology “crucial to [the] hybrid air- and liquid oxygen-breathing [SABRE] rocket motor” was completed, demonstrating that the technology is viable. The tests validated that the heat exchanger could perform as needed for the engine to obtain adequate oxygen from the atmosphere to support the low-altitude, high-performance operation.
In November 2012, Reaction Engines announced it had successfully concluded a series of tests that prove the cooling technology of the engine, one of the main obstacles towards the completion of the project. The European Space Agency (ESA) evaluated the SABRE engine’s pre-cooler heat exchanger, and accepted claims that the technologies required to proceed with the engine’s development had been fully demonstrated.
In June 2013 the United Kingdom government announced further support for the development of a full-scale prototype of the SABRE engine, providing £60M of funding between 2014-2016 with the ESA providing an additional £7M. The total cost of developing a test rig is estimated at £200M.
The ground test rig for the engine pre-cooler @theengineer.co.uk
“The SABRE engine offers an attractive development path because it is suitable for ground based test bed development. This is because the flow downstream of the SABRE engine intake is always subsonic irrespective of the speed at which the engine is flying. As a consequence it is possible to simulate the air-breathing flight conditions of the core engine up to Mach 5 on the ground prior to flight testing by appropriately heating the incoming airstream to represent high Mach flight conditions. This provides a cost advantage because it avoids costly flight tests and enables substantial development time on the engine’s core components on the test rig. The pre-cooler testing is an example of the benefits of ground based demonstrations with over 200 tests undertaken to date.”
Robert Bond, Corporate Programmes Director (Partial quote) @theengineer.co.uk
Ground test concept @neowin.ne
By June 2015, SABRE’s development continued with The Advanced Nozzle Project in Westcott, UK. The test engine, operated by Airborne Engineering Ltd., is being used to analyze the aerodynamics and performance of the advanced nozzles that the SABRE engine will use, in addition to new manufacturing technologies such as the 3D-printed propellant injection system.
In April 2015, the SABRE engine concept passed a theoretical feasibility review conducted by the U.S. Air Force Research Laboratory. In August 2015 the European Commission competition authority approved UK government funding of £50 million for further development of the SABRE project. This was approved on the grounds that money raised from private equity had been insufficient to bring the project to completion. Then in October 2015, British company BAE Systems agreed to buy a 20% stake in the company for £20.6 million as part of an agreement to help develop the SABRE hypersonic engine
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 a single stage to orbit (SSTO) spaceplane, 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 −150 °C in a heat exchanger while avoiding liquefaction of the air or blockage from freezing water vapour.
The European Space Agency (ESA) has evaluated the SABRE engine’s pre-cooler heat exchanger on behalf of the UK Space Agency, and has given official validation to the test results:
“The pre-cooler test objectives have all been successfully met and ESA are satisfied that the tests demonstrate the technology required for the SABRE engine development.” @rocketeers.co.uk
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler. SABRE inserts a helium cooling loop between the air and the cold fuel to avoid problems with hydrogen embrittlement in the precooler.
The dramatic cooling of the air created a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other air fractions. On October 2012, the cooling solution was demonstrated for 6 minutes using freezing air. The cooler consists of a fine pipework heat exchanger and cools the hot in-rushing atmospheric air down to the required −150 °C in 0.01s. The ice prevention system had been a closely guarded secret, but REL disclosed a methanol-injecting 3D-printed de-icer in 2015 through patents, as they needed partner companies and could not keep the secret while working closely with outsiders.
Below 5 times the speed of sound and 25 kilometres of altitude, which is 20% of the speed and 20% of the altitude needed to reach orbit, the cooled air from the precooler passes into a modified turbo-compressor, similar in design to those used on conventional jet engines but running at an unusually high pressure ratio made possible by the low temperature of the inlet air. The compressor feeds the compressed air at 140 atmospheres into the combustion chambers of the main engines.
Artist’s rendering of Skylon spacecraft powering into orbit at this stage still running conventional jet engines but running at an unusually high pressure ratio made possible by the low temperature of the precooler inlet air Credit: reactionengines.co.uk
The turbo-compressor is powered by a gas turbine running on a helium loop, rather than by combustion gases as in a conventional jet engine. 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, cooling critical parts of the engine and powering turbines. The heat passes from the air into the helium. This heat energy is used to power various parts of the engine and to vaporise hydrogen, which is then burnt in ramjets.
Diagram showing ‘hot’ helium from the air precooler is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel @ggsoku.com
Due to the static thrust capability of the hybrid rocket engine, the vehicle can takeoff under air breathing mode, much like a conventional turbojet. 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 drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.
Artist’s rendering of Skylon spacecraft in orbit @bisbos.com
At Mach 5.5 the air-breathing system becomes inefficient and is powered down, replaced by the onboard stored oxygen which allows the engine to accelerate to orbital velocities (around Mach 25).
The combustion chambers in the SABRE engine are cooled by the oxidant (air/liquid oxygen) rather than by liquid hydrogen to further reduce the systems use of liquid hydrogen compared to stoichiometric systems.
The most efficient atmospheric pressure at which a conventional propelling nozzle works is set by the geometry of the nozzle 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 lose efficiency as they reach higher altitudes. In traditional rockets this is overcome by using multiple stages designed for the atmospheric pressures they encounter. An SSTO engine must use a single set of nozzles. Successful tests were carried out in 2010 on an expansion deflection nozzle called STERN that varies the nozzle output to overcome the problem of non-dynamic exhaust expansion.
A hypersonic cruise missile engine used in the first-ever ground test of a full-scale, fully integrated hypersonic cruise missile using conventional liquid hydrocarbon fuel. @room.eu.com
The maturation of hypersonics technology is inspiring countries and organisations to explore its application to hypersonic aircraft. A good example of this is a joint European–Japanese research effort in key high-speed technologies for future air transport, dubbed Hikari1, which comprises 16 partners, 12 of them in Europe and four in Japan. This research combines Japan’s long-standing efforts to develop the technology for supersonic and hypersonic airliners with Europe’s high-speed technology research programmes, including Lapcat (propulsion and aircraft concepts), Atllas (materials and structures), and Zehst (zero emissions highspeed technologies and aircraft concepts).
Its engine is called SCIMITAR, a precooler and turbocompressor fed (sort of) bypass ramjet. This is the engine the USAF probably testing Mach 5+. @neowin.s3.amazonaws.com
Another example is the development of an advanced Mach 5 air-breathing rocket engine that cools incoming air with hydrogen fuel run through a heat exchanger. The engine, called Sabre, is being developed by UK-based Reactions Engines. The US Air Force Research Lab (AFRL), under a co-operative research and development agreement with Reaction Engines, is exploring technical details of the engine and whether it offers unique performance and vehicle integration advantages compared to other highspeed engines applied to hypersonic aircraft or two-stage reusable launch vehicles. @room.eu.com
Avoiding liquefaction improves the efficiency of the engine since less entropy is generated and therefore less liquid hydrogen is boiled off. However, simply cooling the air needs more liquid hydrogen than can be burnt in the engine core. The excess is expelled through a series of burners called “spill duct ramjet burners”, that are arranged in a ring around the central core. These are fed air that bypasses the precooler.
Diagram shows “spill duct ramjet burners” or Bypass duct at lower part of engine @reactionengines.co.uk
This bypass ramjet system is designed to reduce the negative effects of drag resulting from air that passes into the intakes but is not fed into the main rocket engine, rather than generating thrust. 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.
The designed thrust-to-weight ratio of SABRE is 14 compared to about 5 for conventional jet engines, and 2 for scramjets. This high performance is a combination of the denser, cooled air, requiring less compression, and, more importantly, the low air temperatures permitting lighter alloys to be used in much of the engine. Overall performance is much better than the RB545 engine or scramjets.
Fuel efficiency (known as specific impulse in rocket engines) peaks at about 3500 seconds within the atmosphere. Typical all-rocket systems peak around 450 seconds and even “typical” nuclear thermal rockets at about 900 seconds.
The combination of high fuel efficiency and low mass engines permits a single-stage-to-orbit (SSTO) approach, with air-breathing to Mach 5.14+ at 28.5 km (17.7 mi) 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.
The precooler adds mass and complexity to the system, and is the most aggressive and difficult part of the design, but the mass of this heat exchanger is an order of magnitude lower than has been achieved previously. The experimental device achieved heat exchange of almost 1 GW/m3. 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 are offset by the gains in overall efficiency and the proposed flight plan. Conventional launch vehicles such as the Space Shuttle spend about one 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, breathing air and using its wings to support the vehicle therefore increasing payload fraction.
A hybrid jet engine like SABRE needs only reach low hypersonic speeds inside the lower atmosphere before engaging its closed cycle mode, whilst climbing, to build speed.
The SR-72 isn’t the first attempt to crack hypersonic flight, too. Boeing has been working on the X-51 scramjet tech demo for the last decade, and in 2013 it finally completed a successful hypersonic (Mach 5.1, 3,400 mph, 5,400 kph) test flight. The scramjet within the X-51 may eventually find its way into the US military’s High Speed Strike Weapon, an air-launched missile that travels fast enough to evade early warning systems and countermeasures. Hybrid engines, such as the SR-72’s, may eventually find their way into long-range missiles that can travel great distances to strike almost anywhere on Earth. @.extremetech.com
Unlike ramjet or scramjet engines, the design is able to provide high thrust from zero speed up to Mach 5.5, 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 realistically tested on the ground, which drastically cuts testing costs.
In 2012, REL expected test flights by 2020, and operational flights by 2030.
Main article source: wikipedia.org