More than a decade after its conception in the halls of NASA, and having snaked its way through multiple Pentagon bureaucracies, an unmanned military spaceplane is finally on the verge of launching on an unprecendented test flight.

This artist's concept of the X-37 spaceplane in orbit was produced when the project was under NASA management. Credit: Boeing Phantom Works

A stubby-winged spaceship called the Orbital Test Vehicle will fly into orbit on an Atlas 5 rocket, taking a round-trip shakedown voyage for the U.S. Air Force.

"What it offers that we have seldom had is the ability to bring back payloads and experiments to examine how well the experiments performed on-orbit," said Gary Payton, the undersecretary of the Air Force for space programs. "That's one new thing for us."

Payton, a former space shuttle flier and NASA manager, is now the Air Force's top civilian space official.

Liftoff should occur the evening of April 19 in a launch window opening at 6:49 p.m. EDT and closing at 9:12 p.m. EDT, according to an Air Force spokesperson.

Several weeks or months later, the spacecraft will drop into the atmosphere and glide to an automated landing at Vandenberg Air Force Base, Calif.

The 11,000-pound spaceplane is more popularly known as the X-37B, but its cost and ultimate objectives are top secret.

NASA guided the project through its infancy before funding dried up in 2004 and the agency handed over its X-37 assets to the Defense Advanced Research Projects Agency. The Air Force took over in 2006 after another programmatic shakeup.

The civilian space agency no longer has any formal management role in the program, but NASA officials say the upcoming test flight provides an unparalleled opportunity to study how a small spaceplane performs during re-entry.

"We stay connected with the Air Force on the thermal protection system, [and] on the guidance, navigation and autonomous re-entry and landing," said Daniel Dumbacher, the former X-37 project manager at NASA's Marshall Space Flight Center in Huntsville, Ala. "We stay in touch with them for data-sharing purposes, but that's the extent of it."

Teams have been preparing the spacecraft at Cape Canaveral since it arrived at the launch site in late February. The X-37B was built by Boeing Phantom Works in California.

United Launch Alliance is also readying the ship's Atlas 5 launch vehicle. The rocket rolled to the launch pad at Complex 41 early Thursday for a practice countdown and fueling test.

The launch team loaded propellant into the two-stage rocket Friday during a wet dress rehearsal.

The Atlas 5 rocket is fueled on the launch pad Friday.

After workers tow the rocket back inside the nearby Vertical Integration Facility, the X-37B space plane will be hoisted into place atop the booster's Centaur upper stage.

It is one-quarter the size of a space shuttle orbiter and can spend up to nine months in orbit, but the ship will be hidden inside the Atlas rocket's payload fairing for liftoff.

The Atlas 5 will use a larger version of its nose shroud to fit around the X-37B's nearly 15-foot-wide wingspan.

The Air Force isn't saying what equipment is stowed inside the spaceplane's unpressurized cargo hold, which is about the size of a pickup truck bed.

Potential OTV payloads could be as mundane as materials science experiments or low-cost technologies on the fast track for adoption into operational Air Force space missions.

"For new experimental space technologies, it gives us the ability to put them on-orbit, demonstrate whether they do or do not work, and then bring them back," Payton said in an interview with Spaceflight Now last week.

Another likely application for the spaceship could be reconnaissance of war zones and foreign strategic sites, but launches from Cape Canaveral can only reach low-inclination orbits, limiting the spacecraft's geographic coverage for such missions.

Others suggest the OTV might deploy and retrieve miniature satellites or rendezvous with another spacecraft to demonstrate orbital inspection and repair techniques.

The X-37B carries a large rocket engine mounted at the rear of the ship for significant orbit-changing and de-orbit maneuvers, but the Air Force won't divulge its specifications.

Early X-37 concepts called for a propulsion system capable of changing the spacecraft's velocity by nearly 7,000 mph. The space plane is designed to fly at altitudes between 110 and 500 nautical miles, or 126 to 575 statute miles, an Air Force spokesperson told Spaceflight Now.

The spokesperson declined to specify the orbital parameters for this month's mission.

The Air Force's plans for more X-37B missions are up in the air. A second flight is on the books for 2011, but that launch's fate is tied to the outcome of the inaugural mission, according to Payton.

"We're waiting until this first flight is over before we decide on the size of any potential X-37 fleet," Payton said.

The X-37B/OTV flight vehicle undergoes preparations for launch at a Boeing facility in California. Credit: U.S. Air Force

The OTV could eventually be partnered with the Operationally Responsive Space office, a military initiative to field new low-cost, adaptable satellite systems capable of being rapidly launched based on urgent needs.

"If we're successful with this first launch, I would love to see us proliferate the X-37 idea and marry it up with Operationally Responsive Space," Payton said.

"We could have an X-37 sitting at Vandenberg or at the Cape, and on comparatively short notice, depending on warfighter requirements, we could put a specific payload into the payload bay, launch it up on an Atlas or Delta, and then have it stay in orbit, do the job for the combatant commander, and come back home," Payton said. "And then the next flight, we could have a different payload inside, maybe even for a different combatant commander."

Since its establishment in 2007, the ORS program has flown tactical demonstration satellites and overhauled the military's space acquisition and development paradigm for small missions.

Although the X-37B is outfitted to spend up to 270 days in orbit, the first mission's length will hinge on accomplishing all of its goals.

"We'll have certain de-orbit opportunities just like the shuttle does," Payton said. "We aren't married to any particular on-orbit duration right now."

After completing its mission, the 29-foot-long spacecraft will fire its rocket engine to fall from orbit and glide to a precise touchdown on a runway at Vandenberg along California's Central Coast.

Vandenberg's 15,000-foot-long landing strip was originally built for space shuttles returning from orbit. The OTV's debut mission will be the first time the concrete runway has been the landing point for a vehicle gliding back from space.

Edwards Air Force Base in the Mojave Desert of California is the spacecraft's backup landing site.

Despite rampant speculation on the Air Force's intentions for the X-37B, a major goal of the test flight is to verify the spaceplane will actually work.

Payton said the foremost objective of the flight will be a technical demonstration of the spacecraft's design, fulfilling its role as an experimental X-vehicle.

One of the mission's more significant technological leaps will be the first attempted autonomous atmospheric entry and runway landing in the history of the U.S. space program.

The former Soviet Union accomplished the feat with the single automated test flight of its Buran space shuttle in 1988.

The X-37B final approach will be guided by a differential Global Positioning System precision landing system to feed navigation data into the craft's flight computer, giving the vehicle cues as it flies toward Vandenberg and lines up with the runway.

Like the space shuttle, the X-37B will soar through the upper atmosphere enduring scorching temperatures with the help of a shield of heat-resistant tiles.

An artist's rendering of the X-37's re-entry into the atmosphere. Credit: NASA/MSFC

Tiles made of high-temperature ceramic material will protect the X-37B during the trip home.

"The X-37 re-entry environments were more harsh that what [the] shuttle sees," Dumbacher said Friday.

The space plane will approach the landing site and line up with the runway for a steep glide reaching speeds of nearly 300 mph.

In the last few seconds of the flight, the craft will flare its nose, deploy its tricycle landing gear and slap down on the runway traveling well above 200 mph.

Dumbacher, now the director of Marshall's engineering division, said the X-37 touches down at a higher velocity than the shuttle orbiter.

"From the outside, it looks a lot like the shuttle," Payton said. "From the outer moldline, the hypersonic drag [characteristics] and subsonic flight control system, it looks and acts a lot like the shuttle."

But on the inside, the X-37B is a different breed.

Instead of running on fuel cells like the shuttle, the OTV will unfurl a small solar array to produce electricity in orbit. And the X-37B uses electromechanical actuators instead of hydraulics to move its flight control surfaces.

The solar array gives the vehicle a renewable energy source, permitting the X-37B to loiter in orbit for much longer than the space shuttle, which is limited to missions lasting a few weeks.

Other technologies the OTV flight will test include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, and conformal reusable insulations, according to an Air Force fact sheet.

The Air Force manages the OTV project through the Rapid Capabilities Office, a top-level group reporting to a board of senior branch officials including the secretary of the Air Force, the Air Force chief of staff, and the undersecretary of defense for acquisition, technology, and logistics.

The office "expedites development and fielding of select Department of Defense combat support and weapon systems by leveraging defense-wide technology development efforts and existing operational capabilities," according to an Air Force fact sheet.

Many of the Rapid Capabilities Office's projects are implemented on accelerated timelines, the fact sheet said.

NASA selected Boeing Co. to develop the X-37 in 1999 as a testbed for reusable launch vehicle technologies. The X-37 was later described as a demonstrator for NASA's Space Launch Initiative and Orbital Space Plane projects seeking less expensive and more reliable access to low Earth orbit.

"The primary reason for selecting X-37 was the need to have a demonstrator for thermal protection systems and re-entry systems," Dumbacher said. "We can do arcjet testing and other things on the ground, but you never get the right environment until you actually fly stuff back from orbit. The reason NASA stayed [with the X-37] as long as they did was because of the thermal protection system technology demonstration aspect."

The X-40A in free flight over the Mojave Desert en route to a landing at Edwards Air Force Base. Credit: NASA/DFRC-Jim Ross

While the X-37 program was under NASA management, engineers dropped an 85-percent scale model of the X-37 named the X-40A seven times from an Army Chinook helicopter. The approach and landing tests occurred in 2001.

In 2004, after NASA cancelled the Orbital Space Plane and started work on the Constellation program in the wake of the Columbia accident, the space agency transferred the X-37 project to the DARPA.

The move instantly thrust the project into a murky middle ground somewhere between traditional white and black space programs. Unlike purely classified programs, the government acknowledges the X-37's existence and openly discusses the vehicle's design, but officials give only vague answers on the purpose of its existence.

NASA had not started manufacturing a spaceworthy X-37 when its work was transitioned to DARPA. Engineers were focusing on building a full-scale vehicle for drop tests.

DARPA oversaw the second round of glide and landing tests in 2006 using the Scaled Composites White Knight carrier aircraft.

The Air Force announced its intentions to finish developing and building the X-37B in late 2006.

Throughout its checkered history, the X-37 was supposed to launch on the space shuttle and a Delta 2 rocket. The Air Force selected the Atlas 5 rocket in 2006.

The craft is the manifestation of the Air Force's long-held, on-and-off again dream to operate its own space plane.

The first time the military flirted with the idea of a reusable spaceship was the Dyna-Soar project of the late 1950s and early 1960s. The Dyna-Soar, also named the X-20, was envisioned as a piloted spacecraft for anti-satellite, reconnaissance and even space weapons applications.

After Dyna-Soar's cancellation in 1963, the Air Force turned its attention to an ill-fated manned space station called the Manned Orbiting Laboratory. It was cancelled in 1969.

The military contributed to the earliest conceptual designs of the space shuttle beginning in 1971. The Air Force selected Vandenberg to host shuttle launches on polar orbit missions with classified national security payloads.

But the Pentagon scrubbed plans to launch the shuttle from Vandenberg after the Challenger accident. The military's last dedicated shuttle mission launched from Florida in 1992.

"There are a number of differences between our shuttle experience and this bird," Payton said. "Our top priority is getting the bird to orbit and getting it back down again."