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.
If the rocket for space is not used then the US air force could use Skylon and SABRE engine technology to develop a 4000 mph hypersonic fighter plane or spy plane.
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 kilometers 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
In 2012, Reaction Engines built the core new technology (pre-cooler) for their hypersonic engine.
Ultra-Lightweight Heat Exchangers:
These cool the incoming airstream very quickly and effectively, from over 1,000 ° C to minus 150 ° C in less than 1 / 100th of a second (six times faster than the blink of an eye) They are extremely lightweight -. Approximately 100 times lighter than current technology - allowing them to be used for aerospace applications for the first time.
The impact that these miniaturised heat exchangers will have on aerospace propulsion systems is comparable to the impact of the silicon chip on computing: new products, new markets, new capabilities.
SABRE Pre-cooler Demonstration Facts:
- Over 50 km of heat exchanger tubing for a weight penalty of less than 50kg
- Heat exchanger tube wall thickness less than 30 microns (less than the diameter of a human hair)
- Incoming airstream to be cooled to -150 ° C in less than 20 milliseconds (faster than the blink of an eye)
- No frost formation during low temperature operation
The next stage of the SABRE programme includes the construction of a full engine demonstrator.
"ESA are confident that a ground test of a sub-scale engine can be successfully performed to demonstrate the flight regime and cycle and will be a critical milestone in the development of this program and a major breakthrough in propulsion worldwide."