Skylon spaceplane developers reveal the antifreeze method for the sabre hypersonic engine

Reaction Engines of the UK is developing the hypersonic Synergetic Air-Breathing Rocket Engine (Sabre). It is designed to power a vehicle from a standing start to Mach 5.5 in air-breathing mode, and from the edge of the atmosphere to low Earth orbit in pure rocket mode. A fundamental enabler of the concept is a complex heat-exchanger system made up of miles of fine tubing that allows oxygen to be taken straight from the atmosphere for use as fuel.

The system chills incoming air from more than 1,000C to minus 150C in less than 1/100th of a second before passing the pre-cooled air through a turbo-compressor and into the rocket combustion chamber, where it is burned with sub-cooled liquid hydrogen. But until now the means by which the system does this without clogging up the pre-cooler with ice has remained a closely guarded company secret.

Reaction Engines uses methanol as an antifreeze. The methanol is used with the objective of minimizing the amount that is needed.

They use chemical process industry tricks.

* inject the methanol at one of the coldest points
* get the mix of water and methanol to flow forward in the matrix – against the direction of the airflow
* use multiple injection and extraction points in the matrix
* Eventually you end up with a situation where you have extracted all the water vapor as liquid from the airflow and that leaves you essentially with dry air below 215 Kelvin. The partial pressure of the water vapor at this point is so low that you can allow it to pass through the heat exchanger and it does not freeze

Skylon Spaceplane

3D Printed injector


Sabre engine

Engine test rig

Rocket nozzle

Reaction Engines decided to go public with 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.”

The company is developing the Sabre engine principally for the Skylon single-stage-to-orbit spaceplane. But the propulsion system and its pre-cooler technology are attracting wider interest for potential aircraft and two-stage launch vehicle applications.