Because of the large fraction of propellant rocket-powered vehicles must carry (typically about 90 percent), little mass margin remains to accommodate design features and technology needed for the level of robustness and safety required by commercial airplanes.
But low fuel efficiency limits the practical use of rockets to propel airplanes over long distances. Historically, beginning with rocket flight experiments and the X-15 rocket-plane (1959), hypersonic speed was attained with rocket propulsion, having no other options. The onset of these changes depends on several variables, including vehicle geometry.
Air passing over a vehicle traveling at hypersonic speeds becomes hot enough from shock compression and viscous dissipation to change its thermodynamic and chemical nature. Mach 5 is generally accepted as the transition point from supersonic to hypersonic flow, but it is less clearly delineated by a speed boundary than a thermal boundary. The primary benefit of hypersonic flight is extreme speed, whether to engage a time-critical or well-defended military threat, travel between global cities in a couple of hours, or achieve Earth orbit.Īlthough the definition of supersonic speed is unambiguous-flying faster than the speed of sound-the beginning of hypersonic flight has no clear boundary.
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