I don't know exactly how this relates to this but I remember reading something Paul Czysz mentioned about a pre-cooling system which entailed cooling the airflow into the engine above Mach 1.8 to avoid an excessive rise in temperature and optimize the benefits of ram compression or something and simultaneously increasing the engine RPM while doing this and likened it to Skylon. Maybe I got the exact explanation wrong, but do you know what I'm talking about?