Details of the proprietary frost-control technology behind the SABRE airbreathing rocket propulsion system SABRE were publicly revealed for the first time by the developer Reaction Engines. The SABRE engine is designed to power an aerospace vehicle from a standing start to Mach 5.5 in airbreathing mode, and then onwards to low Earth orbit in pure rocket mode. A crucial component of the engine is the precooler, a complex heat-exchanger made of miles of fine tubing, which allows oxygen to be taken for rocket combustion directly from the ambient air.
The precooler chills incoming air from over 1000C to -150C in less than 0.01 seconds, before passing the cooled air through a turbo-compressor and into the rocket combustion chamber, where it is cooled with subcooled liquid hydrogen. The critical technical hurdle has been preventing the rapid buildup of ice on the precooler as the vehicle flies through moist air.
Speaking at at the American Institute of Aeronautics and Astronautics International Space Planes and Hypersonics conference in Glasgow, Reaction Engines technical director and chief designer Richard Varvill described a system of methanol injection which functions as antifreeze. The methanol is not simply injected into the airflow, as the air temperature drops below the point where methanol itself would freeze.
To make the methanol system work, Reaction Engines has “borrowed a trick from the chemical process industry,” Varvill says. “We inject the methanol at one of the coldest points and we effectively get the mix of water and methanol to flow forward in the matrix – against the direction of the airflow.” While conceding this could seem counterintuitive, Varvill says the system achieves this by catching the water-methane mix and re-injecting it farther upstream. “We have multiple injection and extraction points in the matrix, but the overall effect is the mix of methanol and water is actually flowing forward in the matrix against the airflow direction.”
Reaction Engines has decided to publish the frost control technology because of pending patent applications. “The trigger for patenting was the awareness that to execute this program we are going to have to involve other companies,” says Mark Thomas, former chief engineer for technology and future programs at Rolls-Royce, who recently took the reins as managing director of Reaction Engines. “You can’t keep trade secrets very long in that situation, so it is better to be protected formally and legally on the clever stuff.” Thomas adds that Reaction is close to “having those approved.”