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:
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 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 (e.g. 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.
The animation below demonstrates the SABRE cycle in simplified form.
(Rollover to highlight areas)