The successful launch to orbit from Cape Canaveral on 22 April of the US Air Force's X-37B marked the flight debut of the US military's first-ever spaceplane - and the USA's first reusable re-entry vehicle since the Space Shuttle.
At 8.9m (29.25ft)-long and with a 4.5m wingspan, the Boeing-built X-37B is much smaller than the Shuttle. The USAF has not revealed the 4,990kg (11,000lb) vehicle's orbit parameters or payload but, being unmanned, it is clearly not a direct replacement for the Shuttle.
However, on its current, 270-day stay in low-Earth orbit the X-37B will be testing technologies that US Air Force under secretary for space programmes Gary Payton describes as "one generation beyond" the Shuttle's. Those include advanced guidance, navigation and control methods, the craft's silica tile thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation and lightweight electromechanical flight-control systems.
The USAF's X-37B, the US military's first-ever spaceplane took off from Cape Canaveral on 22 April
Payton says the craft's reusability will allow the air force to inspect hardware on return "to see what's really going on in space", adding: "It's as much an experiment in low-cost operations and maintenance on the ground as it is an on-orbit experiment."
One of the Space Shuttle's weaknesses - tragically highlighted in the re-entry loss of the Columbia in 2003 - has always been the difficulty of maintaining its thermal tile heat shield. Of the X-37B, says Payton: "Our top priority is an inexpensive turnaround. If you have to do a lot of [thermal] tile changeouts, that makes it less attractive."
The goal, he says, is to be able to land the craft and fly it again less than 10-15 days later: "If we were in a surge environment, where we were putting up a whole bunch of satellites over a month or two, I would like to see the X-37B handle much more like an [Lockheed] SR-71."
Whether the air force will decide to continue the programme and build a fleet of the vehicles "depends on the success of the first two birds", says Payton. The USAF has contracted Boeing to build a second X-37B for a 2011 delivery, but has not yet manifested the vehicle on a Lockheed Martin Atlas V booster. "We don't want to launch the second until we've learned everything we can from the first," says Payton.
Technology transfer to other programmes is likely to be "piece part", or related to advanced components such as the electromechanical actuators, flight controls and tiles that might benefit other hypersonic or supersonic projects, rather than the vehicle itself. "We chose this design in the late 1990s," says Payton. "It pretty much has the same outer mould line as the Space Shuttle."
The X-37B, which features Shuttle-like payload doors, is powered by gallium arsenide solar cells that unfurl from the bay to charge lithium-ion batteries while in orbit. The orbital test vehicle has a hydrazine propulsion system for attitude control and orbital manoeuvres.
The autonomous re-entry and landing system uses gyroscopes, GPS and an altimeter system to generate control signals to electromechanical actuators for the flight controls and other systems. The Shuttle uses fuel cells for on-board power, limiting its time on orbit, and hydraulic actuators for flight controls.
Payton says the craft now on orbit will be handled "like any other satellite" via telemetry and command flow controlled by the Air Force Space Command's 3rd Space Experimentation Squadron in Colorado Springs, home of the USAF's 50th Space Wing. "We haven't established a standalone unique ground control operation in Colorado Springs," says Payton.
A date for re-entry has not been set, but the air force says the primary landing location is Vandenberg AFB in northern California, with Edwards AFB in southern California as a back-up. Payton says the vehicle has a self-destruct function but does not describe the criteria determining its use.
HISTORY OF THE NASA/DARPA/USAF X-37 PROJECT
The X-37B dates to 1999 and a NASA Marshall Space Flight Center project with Boeing Phantom Works, which led in 2002 to a full development contract to produce an X-37 Approach and Landing Test (ALT) vehicle; an orbital flight version scheduled for launch in mid-2006 was never built. In 2004 NASA gave the programme to the Defense Advanced Research Projects Agency which then transferred it to the USAF's rapid capabilities office. In September 2006 DARPA oversaw Boeing B-52 drop tests of the X-37 ALT, releasing it from 46,000ft (14,000m) to demonstrate re-entry trajectory descent and landing capabilities.
The X-37's shape was a 120%-scale derivative of the US Air Force's Boeing X-40A demonstrator and the basis for the X-37B.
Once the X-37B safely returns the USAF will celebrate its first reusable spaceplane. But in 1989 the Soviet Union accomplished much the same with its unmanned Buran spaceplane, which was launched by an expendable booster, landed autonomously and was as big as NASA's own Space Shuttle. After one successful flight the project was abandoned in the aftermath of the Soviet Union's collapse.