An analysis undertaken by the United States’ Air Force Research Laboratory (‘AFRL’) has confirmed the feasibility of the Reaction Engines Synergetic Air-Breathing Rocket Engine (‘SABRE’) engine cycle concept. Reaction Engines is famous for their Skylon Spaceplane single stage to orbit design and the Mach 5 Lapcat A2 commercial hypersonic passenger plane design.
Reaction Engines Ltd (‘REL’) is a privately held company located in the United Kingdom and was formed in 1989 to develop the technologies needed for an advanced combined cycle air-breathing rocket engine class called SABRE that will enable aircraft to operate easily at speeds of up to five times the speed of sound or fly directly into Earth orbit.
They have achieved a breakthrough in aerospace engine technology by developing ultra-lightweight heat exchangers 100 times lighter than existing technologies that allow the cooling of very hot airstreams from over 1,000 °C to minus 150 °C in less than one hundredth of a second.
Reaction Engines’ technology has undergone extensive independent technical assessments, undertaken by the European Space Agency at the request of the UK Government, which have confirmed the viability of the engine technology and its vehicle applications.
The analysis was undertaken by AFRL as part of a Cooperative Research and Development Agreement (‘CRADA’) with the Air Force Research Laboratory’s Aerospace Systems Directorate (AFRL/RQ). These investigations examined the thermodynamic cycle of the SABRE concept and found no significant barrier to its theoretical viability provided the engine component and integration challenges are met.
Reaction Engines Ltd. (REL) and AFRL are now formulating plans for continued collaboration on the SABRE engine; the proposed work will include investigation of vehicle concepts based on a SABRE derived propulsion system, testing of SABRE engine components and exploration of defence applications for Reaction Engines’ heat exchanger technologies.
SABRE is an innovative class of aerospace propulsion that has the potential to provide efficient air- breathing thrust from standstill on the runway to speeds above Mach 5 (4,500mph) in the atmosphere – twice as fast as jet engines. The SABRE engine can then transition to a rocket mode of operation for flight at higher Mach numbers and space flight. Through its ability to ‘breathe’ air from the atmosphere, SABRE offers a significant reduction in propellant consumption compared to conventional rocket engines which have to carry their own oxygen – which is heavy. The weight saved by carrying less oxygen can be used to increase the capability of launch vehicles including options for high performance reusable launch vehicles with increased operational flexibility, such as horizontal take-off and landing. Additionally, the SABRE engine concept could potentially be configured to efficiently power aircraft flying at high supersonic and hypersonic speeds.
REL Heat Exchanger
REL’s ultra-lightweight heat exchangers are the key enabling components in SABRE engines for Mach 5 cruise and aircraft-like access to space.
Heat exchangers that cool the incoming air are the biggest technical challenge to the realisation of the SABRE engine. At Mach 5 (5 times the speed of sound) the heat exchangerneeds to cool air from 1,000°C to minus 150°C, in one one hundredth of a second, displacing 400 Mega-Watts of heat energy (equivalent to the power output of a typical gas-powered power station) yet weighs less than 1¼ tonnes.
To meet this challenge, REL has developed the most powerful lightweight heat exchangers in the world. The breakthrough achieved will allow heat exchangers to be used for a whole range of new applications.
The SABRE engine has two uses for the heat exchangers. The first use is to cool the incoming air (which becomes very hot at high speed) so that it can be compressed to the pressure required for it to enter the rocket combustion chamber whilst concurrently heating helium, which is used to drive the engine machinery. They call the heat exchanger on the Sabre engine a Pre-cooler.
The second use is to cool the hot helium using the cold liquid hydrogen fuel, but this uses more conventional heat exchanger technology.
The team at REL has found ways to produce heat exchangers light enough to be viable for the first time in a flying engine. REL has also pioneered solutions to the other practical problems in this application, such as the control of frost formation (where the water in the atmosphere freezes in the heat exchanger, icing it up).
Although REL has developments in other technology areas of the SABRE engine, its heat exchangers are the key to its success and also represent the biggest advance over existing technology.
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.
The UK’s Chancellor of the Exchequer, George Osborne, singled out the SABRE project that will power Skylon into space in his 2013 spending review delivered to Parliament. The rumored funding amount was £60Million ($90Million). This would not be full Phase 3 funding. It would help get private cofunding. The hybrid engine – its name stands for Synergistic Air-Breathing Rocket Engine – is currently being developed by Reaction Engines, based at Abingdon, near Oxford.
SOURCES – Reaction Engines, Vimeo
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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