From Martin Savage
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.
Then there is fuel consumption. Doubling the speed means you roughly quadruple the drag, so you burn through the fuel in a quarter of the time. The SR-72’s predecessor, the SR-71, was already a flying fuel tank. There’s really no point in charging around at Mach 6 if you have to drop to subsonic speeds every hour or so to refuel.
Jomtien, Thailand
