Water vapour in the air forms frost when it passes over the sub-zero Pre-cooler heat exchanger used to cool the air entering the SABRE engine. If uncontrolled, this frost would completely block Pre-cooler operation in a matter of seconds and prevent the engine from working.
REL has invented and proven the technology that prevents the formation of this frost; the details of this frost control system are highly confidential to REL.
However, an independent review undertaken by ESA on behalf of the UK government has confirmed the viability of the frost control system that REL has developed stating that:
"ESA can confirm that the frost control mechanism of the SABRE engine, (at laboratory scale), works and is repeatable. In addition ESA expects these positive results to be repeated on the planned tests of the heat exchanger when it is tested on a VIPER jet engine."
"In particular, based on REL's flight like heat exchanger technology and their successful demonstration of the frost control mechanism at laboratory scale (a major milestone that has so far eluded other international developments), 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 both a critical milestone in the development of this program and a major breakthrough in propulsion worldwide".