Well, this is their secret sauce, which they claim will make the SABRE engine work where other scramjets have failed. The tests they're currently running are o

Well, this is their secret sauce, which they claim will make the SABRE engine work where other scramjets have failed. The tests they're currently running are of the pre-coolers, which have a proprietary design to prevent icing. If it works, they will have overcome the biggest hurdle to building a hybrid scramjet/rocket engine.