When the wheels of the second Air Force X-37B Orbital Test Vehicle came to a stop on the tarmac at Vandenberg Air Force Base in California a few weeks ago, the true military utility of this enigmatic, robotic spacecraft remained as hush-hush as the classified payload it carried.
After circuiting Earth for 469 days and then winging its way down from space over the Pacific Ocean, the petite auto-piloted craft glided back into a cocoon of secrecy.
Built by Boeing Government Space Systems, the second flight of the X-37B – labeled Orbital Test Vehicle-2 (OTV-2) — "tested additional capabilities," said Paul Rusnock, Boeing vice president of Government Space Systems.
OTV-2 eclipsed the nearly 225-day mission duration of the first vehicle, as well as the vehicle's 270-day baseline design specifications. [Photos: Air Force's 2nd Secret X-37B Mission]
Later this year, the same craft that flew the OTV-1 mission is to roar into orbit atop an Atlas 5 booster on a refurbished, repeat voyage. Once again, its workload is classified.
So what do we know about the rationale for the X-37B project?
"We look forward to the second launch of OTV-1 later this year and the opportunity to demonstrate that the X-37B is an affordable space vehicle that can be repeatedly reused," said Rusnock in a Boeing press statement.
Just as tight-lipped about the program as its military customer, Boeing does view the X-37B as innovative and combining "the best of an aircraft and a spacecraft into an affordable, responsive unmanned vehicle."
Last year at Space 2011, a conference organized by the American Institute of Aeronautics and Astronautics (AIAA), an X-37B derivatives plan was sketched out by Arthur Grantz, chief engineer in the Experimental Systems Group at Boeing Space and Intelligence Systems in Seal Beach, Calif.
Grantz detailed a vision for the spacecraft to support space station cargo deliveries and for a scaled-up version to carry astronauts into orbit.
The X-37B program is managed by the U.S. Air Force Rapid Capabilities Office and the vehicle is built to showcase risk reduction, experimentation and concept of operations development for reusable space vehicle technologies.
"With the retirement of the space shuttle fleet, the X-37B OTV program brings a singular capability to space technology development," said Lt. Col. Tom McIntyre, X-37B program manager. "The return capability allows the Air Force to test new technologies without the same risk commitment faced by other programs."
McIntyre has stated that the X-37B's heat-thwarting advanced thermal protection and on-orbit solar power systems, as well as environmental modeling and range safety technologies are just some of the things being tested. "Each mission helps us continue to advance the state-of-the-art in these areas," he said in an Air Force press statement.
Implications for the future
So how did this last flight of the X-37B fare on its 15-month stealthy sojourn?
Mum's the word from Boeing due to it being a classified program.
Pentagon spokesperson for the X-37 project, Air Force Major Tracy Bunko, said post-mission assessments on the second orbital test vehicle are progressing.
"But it's premature to discuss any results or findings at this time. Our next, third, mission is on track for October 2012, pending final launch assignment and range scheduling," Bunko said.
While inquiring minds may want to know what's next for X-37B, military and space analysts contacted by SPACE.com remain perplexed. [US Air Force's Secretive X-37B Space Plane (Infographic)]
"I haven't a clue," said John Pike, director of Globalsecurity.org. This program has been around for over a decade, he said, and has had multiple sponsors and multiple nominal missions as well as multiple suspected/attributed missions.
"Those who say don't know. And those who know don't say," Pike said. "Part of the story is just blind institutional momentum, the imperative to get the program funded and the jobs saved."
Pike's email calling card carries the tag line: "Vision without funding is called hallucinating."
Regarding the X-37B, Pike said, at this point, he thinks the main "purpose" of the effort is to keep Chinese military intelligence officials guessing "since they will have to respond to everything it might be."
Somewhat similar in view is Yousaf Butt, a scientific consultant to the Federation of American Scientists.
"I am not aware of anything this spacecraft can do that couldn't be done cheaper with a disposable [spacecraft] bus. I believe the ongoing project is indicative of institutional inertia," Butt said. "Just like the space shuttle never made economic sense, neither does X-37B. However, it is merely an experimental program so may not need to justify itself on economics."
Similarly in the dark about the purpose of the X-37 initiative is Steven Aftergood, director of the Project on Government Secrecy at the Federation of American Scientists in Washington, D.C.
"Since I really don't know what the program is all about, I am reluctant to speculate," Aftergood said. "I would like to believe that it represents a thrilling technological breakthrough, brilliantly executed with profound national security benefits. But who knows?"