The Sabre engine is by no means a new concept. It is described in the 1964 book, "Jet Propulsion for Aerospace Applications", 2nd ed., which references Astronautics, December, 1961 and Aviation Week, November 6, 1961. It has taken a long time to get to this point and it remains to be seen how well the concept works in practice. Originally, the concept was called LACE (liquid air cycle engine) and called for using liquid hydrogen to liquefy air, which is the method being done now. Tests have been done in England with the pre-cooler and more tests are scheduled to be performed in Colorado as shown in post #7. One earlier report talked about the importance of removing water condensation in the pre-cooler.