The engine has been made possible with the development of ultra-lightweight heat exchangers needed by aircraft to cool hot air entering their engines, as well as frost prevention technology.
The SABRE engine is able to cool the incoming airstream very quickly and effectively, from over 1000C to -150C in less than 0.01s. They are extremely lightweight — approximately 100 times lighter than current technology — allowing them to be used for aerospace applications for the first time.
Dr Prab Mistry, chair of IChemE’s Thames Valley Members Group, said: “The SABRE engine and Reaction Engine’s work to develop SKYLON – an unpiloted, reusable spaceplane intended to provide reliable, responsive and cost effective access to space – has received major world-wide attention. Reaction Engine’s work has the potential to revolutionise space travel and their work has wider applications of great interest to chemical engineers, especially heat transfer.”
SKYLON uses SABRE engines in air-breathing mode to accelerate from take-off to Mach 5.5 which allows 1250 tonnes of atmospheric air to be captured and used in the engines, of which 250 tonnes is oxygen which therefore does not have to be carried in propellant tanks. At Mach 5.5 and 25 kilometres altitude the SABRE engine transitions to its rocket engine mode, using liquid oxygen stored on board SKYLON, to complete its ascent to orbit at a speed of Mach 25. In this space access application, SABRE engines need an operational life of only 55 hours to achieve 200 flights, significantly less than the 10,000s of hours needed for conventional jet engines.
SKYLON's fuselage and wing load bearing structure is made from carbon fibre
reinforced plastic and consists of stringers, frames, ribs and spars built as warren girder structures. The aluminium
propellant tankage is suspended within this, free to move under thermal and pressurisation displacements.
The external shell (the aeroshell) is made from a fibre reinforced ceramic and carries only aerodynamic pressure loads which are transmitted to the fuselage structure through flexible suspension points. This shell is thin (0.5mm) and corrugated for stiffness. It is free to move under thermal expansion especially during the latter stages of the aerodynamic ascent and re-entry.
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. As with any heat exchanger, such as a car radiator, the weight and space/thermal efficiency depends on thin sections of thermally conductive material arranged alternately with minimal air gaps.
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 eight-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 25km, 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 design of SABRE evolved from Liquid-Air Cycle Engines (LACE) which have a single rocket combustion chamber with associated pumps, pre-burner and nozzle which are utilised in both modes. LACE engines employ the cooling capacity of the cryogenic liquid hydrogen fuel to liquefy incoming air prior to pumping. Unfortunately, liquefying air in this type of cycle necessitates very high fuel flow.
These faults are avoided in the SABRE engine, which only cools down the air to the vapour boundary and avoids liquefaction, thus reducing the cooling requirement and the liquid hydrogen fuel flow. It also allows the use of a relatively conventional turbo compressor and avoids the requirement for an air condenser.
The SABRE engine is essentially a closed cycle rocket engine with an additional pre-cooled turbo-compressor to provide a high pressure air supply to the combustion chamber. This allows operation from zero forward speed on the runway and up to Mach 5.5 in air-breathing mode during ascent. As the air density falls with altitude the engine eventually switches to a pure rocket propelling its vehicle (such as SKYLON) to orbital velocity (around Mach 25).
The diagram shows in simplified form the complete SABRE cycle. The air from the intake (blue) is shown going though the Pre-cooler and into the compressor. The cooling is achieved with helium (green) that has been itself cooled by HX4 using the liquid hydrogen fuel (purple). Once it has left the Pre-cooler the helium is further heated in HX3 by the products of the Pre-burner to give it enough energy to drive the turbine and the liquid hydrogen (LH2) pump.
In rocket mode HX3 provides all the energy to drive the LH2 pump and the liquid oxygen (LOX) pump within the engine. Re-using the heat in this way increased engine efficiency.
As the cycle diagram illustrates, the use of lightweight heat exchangers is required in SABRE engines and is the key technological innovation to enable SABRE engines to be developed.