The U.S. Air Force's secretive X-37B space plane may eventually get some company in low-Earth orbit as other countries such as Italy and Russia push forward with plans for their own reusable winged spaceships.
Italy's prototype space plane, named Pollux, successfully carried out high-speed maneuvers that slowed it down from a falling speed of Mach 1.2 during a test flight in April. More recently, Russia has begun considering whether to revive a Cold War era, air-launched mini-shuttle in response to the U.S. X-37B space plane debut.
Such efforts may not immediately lead to full-fledged operational flights. But in the case of the Italian Center for Aerospace Research (CIRA) in Capua, Italy, aerospace engineers hope to provide crucial lessons for future space planes, such as how to pull off autonomous re-entry and survive the return trip through Earth's atmosphere.
IN PICTURES: The X-37 space plane
"Everybody knows about the unmanned X-37B flight, but nobody knows if it will re-enter with an autonomous modality," said Gennaro Russo, CIRA's Space Programs lead and USV (Unmanned Space Vehicles) program manager. [Video: X-37B space plane spotted.]
The Russian MAKS system represents a more complete space plane system that's not unlike the X-37B. But there's no word as to how or when a resurrected MAKS mini-shuttle might fly.
Italy aims for hypersonic
Pollux might lack an engine and all the other advanced technology riding aboard X-37B, but it has helped push understanding of the physical stress and control requirements of space planes that want to land in one piece.
The prototype space plane rode up to a drop altitude of about 15 miles (24 km) aboard a stratospheric balloon on April 11. The drop allowed it to reach a speed of just over Mach 1.2 and simulate the final part of a reentry trajectory.
A series of complex, pre-programmed maneuvers over the course of 140 seconds tested the vehicle's limits and slowed its speed down to about Mach 0.2, so that a parachute could deploy for the eventual splashdown in the Mediterranean Sea.
That success built on an earlier test flight by a less-advanced twin to Pollux, the engineless prototype space vehicle named Castor, which reached a top speed of Mach 1.08 back in March 2007. All the results will help in designing future space planes and plan for additional Pollux tests, Russo said.
"My feeling is that we will look for deepening the understanding of the vehicle behaviors within its operating envelope, and this means going towards its possible maximum Mach number (1.8) as well as implementing further autonomy in the control laws with eventually more complex maneuvers," Russo told SPACE.com.
A dropped supersonic test flight that pushes Mach 1.8 is currently scheduled for the last quarter of 2011, but final confirmation depends on the availability of Italian government funding.
Still, Italy's CIRA is already looking beyond even that test to a future air-breathing vehicle that could push hypersonic speeds around Mach 8. CIRA has already signed on with the University of Queensland and Australia's Department of Defense to collaborate on the effort.
Such a test would use a rocket launcher similar to the one used by the Hypersonic International Flight Research Experimentation (HiFIRE) project, a joint program between the U.S. and Australia. It would also involve a smaller vehicle just less than 4 feet (1.2 meters) long, compared to the 30-foot (9.1-meter) length of Pollux.
Reviving a Russian mini-shuttle
Meanwhile, Russia may be dusting off plans for its reusable multipurpose aerospace system (MAKS), according to a report by the Russian state news agency ITAR-TASS.
"An airplane carrier is to be used as the first stage," said Vladimir Skorodelov, a designer for NPO Molniya, during an ITAR-TASS interview. "So, the system is more capable and powerful than the US one."
The first stage of the MAKS system involves an An-225 MRIA aircraft that carries a reusable orbital vehicle and a fuel tank on its back, as detailed on the NPO Molniya website. The reusable orbital vehicle could carry two people.
By contrast, the U.S. Air Force's X-37B space plane launched vertically aboard an Atlas 5 rocket.
Besides the MAKS-OS configuration, a MAKS-T configuration would deliver an orbital payload of up to 18 tons and a fuel tank, rather than launch a reusable orbital vehicle. A MAKS-M configuration would do away with an external fuel tank and simply launch an advanced version of an unmanned reusable orbital vehicle.
A MAKS system could deliver 1 kilogram of cargo into orbit for $1,000 or $2,000, compared to the U.S. space shuttle's delivery cost of $20,000 for the same weight, according to ITAR-TASS. The Russian news agency added that Russian experts want to see the "national 'mini-shuttle'" launch.
Making a true space plane
Just how quickly more space planes get aloft remains uncertain. There has not been any lack of interest in such vehicles, which coincides in part with the final space shuttle missions taking place over the next half-year or so.
In 2009, the European Space Agency and British government kicked off their own futuristic space plane effort in by awarding $1 million euros ($1.28 million dollars) to Reaction Engines Limited (REL), a British aerospace company, to develop an air-breathing rocket engine. The engine is intended to power the unpiloted, reusable Skylon space plane.
The Skylon has the ambitious goal of combining an air-breathing and rocket engine into a hybrid engine, so that it could theoretically take off and land from the same airstrip. Neither the X-37B nor the Russian MAKS system will fulfill that futuristic dream of space planes taking off and landing by themselves just yet.
Still, hypersonic tests being carried out by the U.S. and Australia, as well as future tests planned by Italy, do perhaps point to the day when space planes won't need to hitch a ride aboard rockets or carrier planes.
IN PICTURES: The X-37 space plane