So the latest launch of the US Air Force’s hypersonic Waverider jet failed when a faulty control fin caused it to break apart. It was a test for a missile to travel at Mach 6 (4,566 mph at sea level) and it wasn’t the first failure. What was interesting was all the discussion about how the technology could one day lead to hypersonic planes.
Hypersonic speeds are defined as Mach 5 (3,805 mph at sea level) and above. Only a few specialised craft have ever achieved them, and those rarely for any duration. Outside of military aircraft, the only commercial planes to ever exceed Mach 1 were Concorde and its Russian brethren, the Tupolev Tu-144. Most aircraft stay well below Mach 1 for a number of good reasons, fuel efficiency among them.
So why will there never be a hypersonic passenger jet? Well, for a number of reasons, both technical and practical:
- Flying Limitations
Let’s have a quick run through them. Heat is an issue both for materials and design. We think of the air around us as empty and light, but it’s actually fairly dense and at speed it causes a lot of friction, which generates heat. Part of the reason planes fly so high is because the atmosphere is thinner, meaning less friction (Concorde flew at 55,000+ feet, most commercial planes fly at 30-35,000 feet). This puts the materials used under a heavy cycle of freezing and thawing as the plane speeds up and slows down (the outside temperature at altitude can be as low as -55°C). It also means you have to cope with expansion, Concorde grew by nearly a foot in length over the course of a flight. Due to the materials of the nose, Concorde was limited to Mach 2.02 as it couldn’t cope with the higher temperatures faster speeds would generate and the whole plane had to be painted white to help reflect heat. Continue reading