FRICTION burns. And the friction of the air on something travelling at five times the speed of sound burns hot. The leading edge of such an object can easily reach a temperature of 3,000℃. Inconveniently, that is above the melting point of most materials used by engineers, which makes it hard to design things like wings and nose cones for aircraft intended to achieve hypersonic velocities. The lure of hypersonic flight is such, though, that many are trying to do so. The world's air forces would love such planes. And for civilians (at least, for those with deep pockets), the idea of being able to jet in a couple of hours from Britain to Australia sounds extremely attractive.
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