OPINION LETTERS

摘要

You reported on the proposal for a US hypersonic uncrewed aircraft - Lockheed Martin's SR-72 equipped with a Mach-3 busting scramjet with built-in jet engines (9 November, p 4). It's easy to make an aircraft go faster by putting bigger engines on it. What wasn't mentioned in the article are the real obstacles to practical hypersonic flight. First is heating of the aircraft's outer skin. If you upgrade from Mach 3 to Mach 6, there is more skin friction, and to cruise at that speed you need to use special alloys and cooling systems to stop the aircraft melting.