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.