The RB545 was developed for the abandoned HOTOL space launcher. The engine was designed to deliver a high air-breathing thrust/weight ratio with moderate SFC while reverting to a high specific impulse rocket engine at transition. Since the air-breathing mode operated on a turbomachinery based cycle the engine was capable of generating static thrust (unlike ramjet cycles) and engine development could therefore take place on open test bed facilities. Optimum transition from air-breathing to rocket mode with this type of powerplant occurred at around Mach 5 and 26km after which the vehicle climbed steeply out of the atmosphere to minimise drag losses. The resulting ascent trajectory was relatively benign to both engine and airframe, leaving a reasonable choice of airframe materials capable of withstanding the ascent and reentry temperatures without active cooling. The RB545 engine was designed with state of the art technology for turbomachinery, pumps and combustion chambers etc. Current materials were specified for the engine machinery while the nacelle shell was manufactured in Sic reinforced glass and the bypass system in C/Sic.
By employing the rocket combustion chamber, nozzle and pumps in both modes the mass penalty of adding a separate air-breathing engine was reduced, while also eliminating the base drag penalty of the 'dead' rocket engine during air-breathing ascent. To generate reasonable thrust levels during air-breathing while preserving high nozzle area ratio the airflow had to be pumped up to typical rocket chamber pressures (i.e.: approx. 100-200bar). The aim of the thermodynamic cycle therefore, was to provide this high pressure airstream with minimum fuel flow, assisted by the remarkable properties of liquid hydrogen (temperature and specific heat). By treating the engine as a 'black box' it was possible to show that the minimum fuel/air ratio to achieve a 150bar air deliver y was 0.0433, at which the combined entropy rise of the two streams was zero.
However practical cycles could only achieve a fuel/air ratio approximately twice this value partly due to their thermodynamics and partly due to component inefficiencies and finite temperature differences. In order to reduce the cycle power requirement and to achieve reasonable air compressor outlet temperatures it was necessary to cool the incoming airflow, particularly at high Mach no's. However the airflow could also be viewed as a source of heat to drive a thermodynamic cycle operating between the high inlet air temperature and the low hydrogen stream 'sink' temperature. Then to allow the engine to operate over a range of speeds, the reduced incoming air enthalpy at low Mach no required 'topping up' by combustion heat release in a preburner to ensure constant turbine inlet temperature. The resulting engines ran at a nearly constant turbomachinery operating point and fuel flow over the whole air-breathing Mach no range.
The first attempt at designing an engine embodying the principles outlined above was the RB545 (HOTOL) engine. In this cycle the high pressure hydrogen flow was used to cool the airstream directly, following which the hydrogen stream split; approximately 1/3rd passing to the combustion chamber via the preburner while the remainder was expanded through the turbocompressor turbine prior to exhaust. This cycle variant had a higher fuel flow than the later Sabre engine particularly at high Mach no's due to precooler metal temperature limitations caused by hydrogen embrittlement. In addition the precooler frost control system was relatively crude, resulting in significant payload penalty.
The later Sabre engine traded simplicity for a lower fuel flow by interposing a Brayton cycle power loop in between the 'hot' airstream and the 'cold' hydrogen stream.
Thrust (sl): 1,050.700 kN (236,207 lbf). Thrust (sl): 107,143 kgf.
Status: Development ended 1985.
Thrust: 367.70 kN (82,662 lbf).
Specific impulse: 700 s.
Specific impulse sea level: 2,000 s.
Burn time: 730 s.