I recently attended a Royal Aeronuatical Society Lecture on Reaction engines. Interesting stuff, the main aim of the cooling (helium) is so that the engines can go hypersonic as hot air entering them would be uncompressable. The options then are to fuel the engines with hydrogen and make them a hybrid.
The technology that is going into producing small but very efficient coolers I thought was amazing.
The gentleman that did the lecture was Alan Bond from Reaction Engines Ltd in Oxfordshire.
This is their website:
Reaction Engines Ltd : Space Propulsion Systems