The second test of the now-defunct HTV-2 testifies to the unforgiving realities of hypersonic flight [source: Pappalardo]. Even Concorde, which topped out at a supersonic 1,350 mph (2,172 kph), was shut down after 27 years due to safety issues and cost concerns [source: Novak].
Physics is a harsh taskmaster. As a plane speeds toward the sound barrier, air stops "getting out of the way" and compresses into a wall that a plane must punch through. Drag, lift and combustion get downright squirrelly at such speeds, and some supersonic adaptations, such as delta wings and ramjets -- simple jet engines that compress air courtesy of the craft's forward momentum -- range from inefficient to ineffective at lower speeds [sources: Darling; NASA].
Hypersonic planes entail even more specialized solutions, such as heat-shedding ablative armor and supersonic combustion ramjets, or scramjets, for propulsion [sources: Darling; NASA]. At even "low" hypersonic speeds (Mach 5-10), air molecules ionize into electrified and chemically reactive plasma, producing exothermic (heat-releasing) reactions that add to already monstrous frictional heat [sources: Fletcher; NASA].
To make it from New York to Los Angeles in 12 minutes would require flying 22 times faster than a commercial jetliner. At such speeds, air doesn't flow around you -- you tear through it, generating punishing pressures and steel-melting 3,500 F (1,900 C) surface temperatures. Supersonic planes sport sharp lines to slice through the air, but hypersonic aircraft must assume a blunter shape to better shed heat, not unlike an Apollo command capsule. Flaps struggle to overcome the vehicle's inertia, and maneuvering requires precise sensors and near-instantaneous response [sources: DARPA; Fletcher; NASA].
Adding people back into the mix ratchets up the difficulty by an order of magnitude. It's difficult to imagine a passenger-jet fuselage compatible with the aerodynamics of hypersonic flight. Moreover, any plane capable of overcoming this issue would need to saunter, not sprint, to get up to speed, lest its passengers complain of being flattened like so many pancakes during takeoffs, landings and turns.
A human body can withstand a force load of 2-3 G's (two to three times Earth's gravity) for quite a while, especially in the forward direction, but don't expect a high-paying customer to tolerate the discomfort of even 1 G for more than a few minutes. Yet, such accelerations might be unavoidable: To fly at hypersonic speeds, planes might rely on specializations that render them unwieldy pigs at lower velocities; thus, they might require rocket boosters -- and the G-forces they entail -- to reach flight altitude and speed [sources: NASA; Zuidema et al.].
The requirements of a true hypersonic plane, let alone a Mach 20 one, might not play well with the comfort and safety requirements of a passenger jet. Yet, if you believe the hype, hypersonic vehicles will soon rule the military and civilian skies.