Could you commute from New York to Los Angeles in 12 minutes?

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.

Hypersonic passenger planes -- and one-hour flights from New York to London -- have been touted for around 60 years. The question is not whether some military or private aircraft will achieve this goal, but when -- or if -- Joe and Jane Carryon will commute on one.

In his 1986 State of the Union address, U.S. President Ronald Reagan called for the development of an ''Orient Express,'' a plane that could jet from New York to Tokyo in two to three hours. The planned Rockwell X-30, a single-stage-to-orbit (SSTO) passenger space liner, was kiboshed before reaching the prototype stage [source: Sanger].

Supersonic flight might return, but probably not soon. In 2012, one contender under development is the Zero Emission Hypersonic Transportation (Zehst) system, the seaweed-biofuel-powered brainchild of a collaboration between EADS and Japan, who plan to roll out the craft around 2040 or 2050 [sources: Jones; Wall]. Zehst will travel at double Concorde's speed and altitude, at a ticket price of around €6,000 ($8,500) [source: Lichfield].

If successful, Zehst will carry 50-100 people between Paris and Tokyo in 2.5 hours (compared to the current 11) using three propulsion systems. Two turbofans will propel the plane on a steep climb to around Mach 0.8, after which two rocket boosters will take over, accelerating the vehicle to Mach 2.5 -- fast enough for ramjets to kick in and boost the plane to around Mach 4. Approaching its destination, the plane would glide in, with its turbofans kicking on again, and land under power [source: Wall].

Airbus' main competitor, Boeing, abandoned its supersonic Sonic Cruiser to develop the subsonic 787 Dreamliner, but you can never count the company out completely -- particularly given its military contracts, which keep it firmly in the high-speed aircraft game. Despite its dodgy test record, the technology behind Boeing's X-51A WaveRider -- which flies on its own shock wave and has broken Mach 5 multiple times -- could form the foundation for eventual space or commercial applications [sources: Bartkewicz; Boeing].

Meanwhile, European aeronautical company HyperMach has announced SonicStar, a sonic-boom-less plane designed to fly twice as fast as Concorde. According to HyperMach, SonicStar will cruise at Mach 3.6 at an altitude of 60,000 feet (18,300 meters) and carry 10-20 passengers between New York and Dubai in two hours, 20 minutes. The company believes it can get the plane flying by June 2021 [source: Jones].

Taking a suborbital approach, California-based aerospace firm XCOR is working on Lynx, a two-seat commercial aircraft designed for high-altitude, supersonic flight. If successful, Lynx will cruise at more than 2,500 mph (4,000 kph) to an altitude of 62 miles (100 kilometers), then descend, minimizing troublesome atmospheric drag, friction and turbulence [source: Waldron].

All things considered, exchanging the hypersonic dream for hyperbolic flight might make practical sense.

The problem with fast flying is that disturbances can propagate only so fast through a fluid, including air. Approach or exceed that speed, and it's the difference between gliding through a pool of water and belly flopping from the high dive. Rather than fight such a brutal battle, some opt to avoid the atmosphere entirely and make space-skimming suborbital hops.

Space planes -- fully reusable spacecraft that fly in space or atmosphere -- and high-altitude commercial hoppers have resurged with the growth of the commercial spaceflight industry. Ideally, such craft could take off and land from runways but, for now at least, they remain pipe dreams. Just as subsonic, supersonic and hypersonic designs work best in their own flight regimes, atmospheric propulsion and control systems diverge from those that work well in space. With this in mind, most designs rely on a two-stage plan, being carried aloft by a "mother ship" airplane or rocket before kicking in their onboard flight systems.

For example, Richard Branson's company, Virgin Galactic, plans to carry passengers to the edge of space (around 62 miles, or 100 kilometers) on SpaceShipTwo, a 60-foot (18-meter), six-person rocket glider slung below the airplane VirginMothership Eve. When the dual-fuselage carrier reaches 50,000 feet (15,240 meters), SpaceShipTwo will separate, fly and glide Earthward after first slowing its re-entry via a special "feathering" drag technique [source: Chang]. Branson's company has also entered into a cooperative agreement with Sierra Nevada Space Systems, possibly to act as a dealer for booking space flights aboard its planned passenger craft, Dream Chaser [source: Chang].

The Dream Chaser is a reusable mini-shuttle based on the Bor-4, the Soviet Union's defunct space shuttle design. It will launch via an Atlas V rocket and land like an airplane. Sierra Nevada plans to contract with space agencies to ferry up to seven astronauts and cargo between the International Space Station (ISS) and Earth [source: Chang]. In August 2012, the project received $212.5 million from NASA's Commercial Crew Integrated Capability (CCiCap) program to continue development [source: Sierra Nevada].

Space planes might need those commercial passengers if they cannot catch up to the competition for space deliveries. Space Exploration Technologies Corp. (SpaceX) delivered cargo to the ISS in October 2012 using a more traditional rocket-and-capsule approach. Orbital Sciences Corp., which was developing a space plane until the project lost NASA funding, has adopted a non-reusable version of this method for its planned ISS supply runs [source: Orbital].

Supersonic, hypersonic or high-hopping suborbital flights might be the wave of the future, but only time will tell if -- or when -- they get off the ground.