The SABRE engine, currently being developed by a company in Oxfordshire, England, is a single stage to orbit, experimental hybrid air-breathing/LOX rocket engine with a rather fascinating heat exchanger that pre-cools the air before entering the combustor. What fascinated me the most about this heat exchanger is that it will employ something absurd like 1400 kilometers of millimeter diameter inconel tubing that's with a wall thickness on the order of tens of microns. This precooler apparently cools well over a hundred kilograms of air every second up to Mach 5. The papers I have read are deliberately vague on exactly how they were able to mitigate the frost accumulation during this cooling. I'm sure it's a company secret. But, like much proprietary information, the answer is not necessarily something that hasn't been speculated before. For more information, go to http://www.reactionengines.co.uk/pdf_documents.html [Broken] and specifically read the article titled "Heat exchanger development at REL." I have a general idea of how this engine is put together, and what the different heat exchangers do. However, I do not have nearly as firm of a grasp on thermodynamics, in general, as some other people on these forums might have. If you were to try come up with a way of mitigating frost for cooling 100 kilograms of air to subzero temperatures every second, what methods would you try to keep the frost from forming?