Hypersonic Scramjets, Spaceplanes, and Air Launched Orbitals

The Air Force's desired "High Speed Strike Weapon" would travel at five times the speed of sound or faster, theoretically launching from a stealthy F-22 Raptor jet or a future F-35 Joint Strike Fighter, and travelling so fast and at such long distances as to render an enemy's anti-aircraft systems defunct. The Air Force's Research Laboratory Munitions Directorate is gathering possible design partners later this month at Elgin Air Force Base in Florida before any solicitation. According to an Air Force notice, whatever prototype gets built will ultimately need to strike "time-critical" targets -- on the move, possibly -- from " tactically relevant standoff distances." _Wired

The hypersonic stealth missile would be powered by a scramjet air-breathing engine, traveling at speeds of Mach 5 or greater. While such a speed is not sufficient to achieve Earth orbit, it would allow jet pilots to launch from a considerable distance, in a stealthy manner.

Hypersonic Stealth Rocket
Meanwhile, the US Air Force X-37B space plane is due to land soon, after spending more than a year orbiting the Earth.

The X-37B looks much like NASA's now-retired space shuttles, only much smaller. The space plane is about 29 feet long by 15 feet wide, with a payload bay the size of a pickup truck bed. A solar array packed in the payload bay powers the spacecraft. For comparison, two entire X-37Bs could fit inside the payload bay of a space shuttle.

OTV-2's flight represents a big jump for the X-37B space plane. The vehicle has been aloft for 462 days as of June 8, more than doubling the on-orbit time of the first space-flown X-37B, known as OTV-1. _X37B Due to Land

US Air Force

The X-37B space plane is unmanned, and is launched vertically on a large booster. But the technology for air-launching larger space vehicles and space planes is improving, thanks to Microsoft billionaire Paul Allen, and aerospace engineer Burt Rutan.

The Stratolaunch system would super-size the arrangement used for the SpaceShipOne launches: Scaled Composites has been tapped to build a carrier airplane that weighs more than 1.2 million pounds, with a wingspan of more than 380 feet. That tonnage rivals the weight of the Antonov An-225, which is recognized as the world's heaviest aircraft. Stratolaunch's dual-fuselage plane would be powered by six 747 engines, and would require a 12,000-foot runway for landing.

...The plane would be capable of flying up to 1,300 nautical miles to reach its launch point. SpaceX would provide a shortened version of its Falcon 9 rocket for the next phase of Stratolaunch's route to orbit. Wentz described it as a "Falcon 4 or 5." The multistage booster would be attached to the plane using a mating and integration system developed by Dynetics, and released during the mothership's flight at 30,000 feet. After release, the 490,000-pound rocket would light up to send commercial and government payloads weighing up to 13,500 pounds into low Earth orbit. _Cosmic Log

Stratolaunch
The advantages of an air launch are several, perhaps the largest being the much wider range of orbits and launch windows achievable using much less fuel.

Combining the approaches discussed above, would give one an air-launched stealth plane which used hypersonic scramjet engines to save rocket fuel in the "second stage." The "first stage" would be the airplane that launched the spaceplane. Airplanes are highly economical and reliable, and completely reusable.

Hypersonic scramjets as a second stage should also allow complete recovery and re-usability, depending on configuration and design. Eventually, the first stage ground launched airplanes might incorporate both air breathing turbines and hypersonic scramjet engines, but such designs have a ways to go in testing. Clearly the huge Stratolaunch design could not tolerate such speeds.

Wikipedia: Scramjet and Specific Impulse
The third stage would be the spaceplane itself, which can grow larger as the first stage is safely enlarged. The spaceplane should be completely reusable, and should be able to land almost anytime and anywhere on Earth with a suitable runway.

As for stealth capability of the ground launcher and space plane, that would be a matter of design and mission requirement. If you wanted to secretly send a spaceplane and crew into orbit, the best way to do so would be using stealth air-launched spaceplane technology, unannounced, taking off from a secret runway in the Southern Hemisphere, as far away from observers as possible.

The United States is unlikely to pull something like that off without multiple leaks to the press and interested foreign powers. Russia is unlikely to be able to put all the technology and logistics together without disastrous failure. China might be able to put something similar together within 10 years, but unless China's international projection of power and coordinated activity expands significantly, it is unlikely to be able to do so secretly.

Guide to spaceplanes of the past

More 14 June: The scramjet needs to be traveling at supersonic speeds before it will work, so any second stage of an air launch system which uses scramjets will need to have some way to reach a speed fast enough for the scramjet to burn properly.

NASA: Rocket - Scramjet Hybrid Second Stage (PDF) (highly redacted)

Air Breathing Space Vehicles

Scramspace: An Australian Project