Once upon a time, a standard of adventure movie serials, radio dramas, and pulp fiction tales was the mystery plane, a flying machine of shadowy origins and unknown capabilities. Often based on secret technologies, there was a certain style of action hero who either had one or was trying to cope with an adversary who had one. Doc Savage went through an armada of amazing flying machines during the course of his career. The more recent Sky Captain and the World of Tomorrow has a fair assortment of fabulous sky craft as well. (And airships should not be neglected either.)
Although a staple of fiction, the mystery plane has plenty of real world examples. The military likes to keep its latest aircraft under wraps, the ones with new technologies, especially those used in reconnaissance. The Air Force has had a number over the years. The U-2 was one, as was the SR-71 Blackbird. The F-117 Nighthawk was another, because of its radical new stealth technology.
All of these have now been publicly acknowledged, and only the U-2 is still in active service, mainly because nothing else does what it does so well. There are rumors of a plane still under wraps, the Aurora. It's supposed to be the next step up from the SR-71, but to date there's been no official acknowledgement that it's ever been built, much less flown.
The Boeing X-37 program qualifies as a 'mystery plane' because the United States Air Force has been very close-mouthed about what it does - but what we do know is interesting to say the least.
What It Is
Drawing from the wikipedia entry, the X-37B is a winged, unmanned reusable spacecraft launched by an expendable booster (an Atlas V) to orbit. (The X-37A was a non-orbital test version that was dropped by a carrier aircraft to test the design's aerodynamics.) Based on Space Shuttle technologies, it's smaller - 29 feet long and about 1/4 the size. It has a cargo bay that can open to space and possibly deploy payloads. It has systems to control its orientation in space (where it's pointing), and engines that allow it to change its orbit.
Power in orbit appears to be obtained by an array of solar panels that can deployed from the cargo bay and stored for re-entry. (Diagram of X-37B in orbit, with size comparison to Shuttle here, about halfway down the page.) This allows it to remain in orbit for long periods of time. When a mission is complete, it can reenter the atmosphere and fly itself to a landing like an airplane - autonomously! After refurbishment, it can be launched again.
To date, two X-37B orbiters have been built and flown. OTV-1 launched April 22, 2010 from Cape Canaveral Air Force Station. It returned to Earth at Vandenberg AFB December 3, 2010. A tire blew out on landing, but it was otherwise in good shape. OTV-2 launched March 5, 2011 from Canaveral - it stayed in orbit 469 days (exceeding the base endurance of 270 days for the design), returning June 16, 2012 to Vandenberg AFB. Both missions were to Low Earth Orbit (LEO), where the Shuttles once flew and the ISS orbits. Reports from amateur ground-based observers indicate that both vehicles changed their orbits several times during their missions.
What Is It For?
Ah, now that's where the mystery comes in. This BBC article about the upcoming mission for OTV-1 goes over the little the Air Force has released, and speculates about what else might be going on. Here's info from the Official Air Force Fact Sheet:
MissionThe first is innocuous enough - operating an experimental reusable aerospacecraft to test hardware and gain experience operating it. The Air Force has a long history of building X planes because all the theory and simulations in the world can't substitute for actually bending metal and flying something to see what works and what doesn't. Space is the ultimate "High Ground" and it behooves the Air Force to develop its capabilities to make use of it.
The X-37B Orbital Test Vehicle, or OTV, is an experimental test program to demonstrate technologies for a reliable, reusable, unmanned space test platform for the U.S. Air Force. The primary objectives of the X-37B are twofold: reusable spacecraft technologies for America's future in space and operating experiments which can be returned to, and examined, on Earth.
The second is where the fun comes in. "Operating experiments and returning them to earth" can cover anything - and speculation is rife. The BBC article discusses speculation that the X-37B is running special recon missions, testing the ability to maneuver in orbit to evade anti-satellite weapons, testing new sensors, and even spying on the Chinese orbital platform. Others have speculated about the possibility of using the cargo capacity to deliver weapons to orbit or satellite rendezvous for assorted purposes. Barring declassification of the program, speculation is what it will remain.
Comparisons & Larger Implications
At the present time, the only other reusable spacecraft being tested and flown is the Dragon Capsule from SpaceX. While also launched by an expendable booster (and SpaceX is working on that!), it's more akin to Apollo style technology, using a heat shield for reentry and parachuting down to a landing in the ocean. No wings, in other words.
Its design duration in orbit does not match the X-37B, but it has already delivered cargo to and from the International Space Station. The Air Force fact sheet does not list payload capacity for the X-37B, so comparing them on that basis will have to wait. The X-37B is not equipped to dock with the I.S.S. in any case, unless it could carry a special payload module for that purpose, and it would be cargo only. The Dragon capsule is intended to be refurbished and reused, like the X-37 - but again lacking information it's not possible to compare how quickly that can be done, or how easily.
SpaceX is planning to develop a man-rated version of Dragon; a crew of up to 7 is planned. The X-37B is not designed to carry people - but Boeing has proposed a scaled up version, the X-37C which could carry up to 6 astronauts in several possible configurations. (The illustrations at the link suggest in one version they'd be strictly cargo, while another shows a cockpit.)
A manned version would not have anywhere near the endurance of the X-37B because of the added burdens of maintaining life support sytems and consumables, at least while astronauts were on board. Standby mode while parked at the I.S.S. might be several months. At a guess, it would be a taxi to and from LEO at best, and there are no signs the Air Force or Boeing is going to pursue it at this time. Further, Boeing is also developing their own answer to the Dragon Capsule, the CST-100, which would be designed to parachute down on land instead of water.
That being said, a taxi to orbit and back just for people would be a cool thing to have. One of the problems with the Space Shuttle was that it had to do so many things - support a crew for days, haul payloads up and down, serve as a flying research platform, and so on. All of that made it more complicated to service, and more expensive to operate. The completion of the I.S.S. (International Space Station) means we now have a place in orbit for long term stuff with semi-permanent infrastructure - we don't have to haul all of it back and forth on every mission. Specialized vehicles for people or cargo are practical now - and they can be optimized for those tasks.
And the I.S.S. isn't the only place that could use some taxi service. There's a market for space tourism waiting for more routine access to LEO. A Russian proposal has come up with some nifty pictures for a hotel that would be serviced by Soyuz capsules. Bigelow Aerospace is promoting an inflatable design offering a lot of room. Did somebody call a cab?
With regard to the X-37 design and known results to date, there are some interesting possibilities to explore. Originally intended to fit in the cargo bay of the Shuttle to reach LEO, the program has survived a transfer from NASA to the USAF, and redesign for launch by an expendable booster. The design and associated hardware seem to be working out well in practice so far. If nothing else, it's giving Boeing and the Air Force practical experience in operating a reusable space transport system that doesn't seem to require the massive infrastructure the Shuttle needed.
IF Boeing pursues a commercial version, the capabilities demonstrated so far are exciting. Getting payloads to orbit AND returning them is currently not possible unless they can fit on a Soyuz Capsule or Dragon Capsule up to the I.S.S. and back down. A commercial X-37B would allow companies to fly packages to microgravity for months at a time, and return them to anyplace with a runway that could handle the X-37B.
Companies building satellites for example would be able to fly components, test them in space, and bring them back down for examination before assembling everything into a complete satellite and launching it. (What the Air Force is doing, apparently.) Validating sensor designs, finding out why something failed before locking in a design - it's got to be a capability that should find a lot of customers.
Materials experiments in microgravity uncomplicated by the presence of humans doing all kinds of vibration generating things, bio-reactors trying new methods of protein growth and structural analysis, academic institutions flying experiment packages... Routine access to and from orbit has long been the Holy Grail of spaceflight. The X-37B could bring that a lot closer.
This, of course, means at some point the Air Force is going to have to see how well the X-37B can pull off an orbital rendezvous. Progress space freighters dock automatically at the I.S.S. as do the European larger ATV spacecraft - but that's with a crew on board monitoring the process. It's another question if the X-37B could manage it with an orbital target without a crew on board to assist. It would be surprising if the Air Force didn't have some missions planned down the road to see what it could do. The X-37B could be launched with its own test payload in the cargo bay to practice on - release it in orbit, then try to recapture it. Or, there's plenty of space junk already in orbit; a mission to rendezvous with something and possibly bring it back would be a good experiment. If this works out...
Add orbital service calls to the mix. The articles linked above already discuss the idea of using the X-37B equipped with mechanical arms (Waldos) to rendezvous with satellites to refuel them, make simple repairs, etc. Space junk could be brought down - or collected for orbital re-use; satellites stuck in the wrong orbit might be moved, and so on. Give the X-37B some kind of ion thrusters, and it might be able to expand its orbital reach quite a bit - possibly up to GEO.
Or, consider the idea of orbiting factory modules. An X-37 type space craft could rendezvous with them to bring down finished products that can only be made in microgravity. It would also be resupplying the modules, given some kind of standardized systems for docking and hardware for cargo transfer. It might be something as simple as coming up with a generic cargo management module to fit in the payload bay.
The winged design of the X-37 suggests another experiment. One of the more complex challenges in orbital mechanics is changing the plane of an orbit. (This affects how far above and below the equator the ground track of a body in orbit passes. It's a geometric plane, not an airplane plane.) Ideally, a spacecraft launched from the equator due east would be able to carry its maximum payload to orbit because it would be getting the full advantage of the speed of the earth's rotation. The more a launch path ends up at an angle to the equator, the less of that advantage is available. That's one reason the U.S. uses Florida for launches - its lower latitude. The Russians launch from Baikonur which is farther away from the equator - spacecraft launched from there end up crossing the equator at a steeper angle.
Sometimes that's good - satellites that need to look down at the ground from LEO will see more of it if their orbit carries them north and south of the equator. But, getting there involves a trade off in payload and fuel - Delta V is the term usually used to cover this. Theoretically, the X-37B could be launched to orbit from the equator or near it, and then change orbital plane after reaching a preliminary orbit. This would involve changing that initial orbit to an ellipse where the part that passed closest to the earth would just skim the atmosphere. The X-37B's wings would allow it to use the air to change its direction - make a turn - and a short burn would allow it to climb back out of the atmosphere with a new orbital plane.
Whether it could be done or not is purely speculative at this point, but I'd bet someone in the X-37 program is thinking about this...
The economics of the X-37 bear some thinking about. We only have two. What would happen if we set up a production line to build a dozen? A hundred? Cost per copy would come down - economies of scale. Availability would go up. At some point, the ability to launch and recover payloads with increasing frequency should trigger a rise in demand for launches, and it should accelerate. Consider the difference in having to wait, say 5 years, to fly a payload versus being able to book a flight next month - or next week. At some point customers would start lining up. That was one of the original hopes for the Shuttle - the X-37 program might make it a reality.
It gets better. If SpaceX does succeed in building a reusable booster, there's nothing that would keep them from launching an X-37 with it. All of a sudden, we wouldn't have to budget for throwing away most of the hardware just to get something to LEO and back. For that matter, there's no reason Boeing or any other aerospace company couldn't decide to come up with a reusable launching system to get the X-37 to orbit. That would really be a game changer. We're still a ways from building something like a Delta Clipper Single Stage To Orbit (SSTO) kind of mystery plane but this would be progress in the right direction.
Down to Earth
All of this is fun to consider of course, but the painful reality is that this is just an experimental program with only two prototypes to date, with only the third launch coming up. It's entirely possible the Air Force will run out the program and then close it down. There's only so much money for funding experimental work, and the Air Force is pretty strapped right now keeping its regular missions going. Without dedicated patrons willing to fight for the program, it can't be expected to go anywhere in our current fiscal miasma without some major mojo behind the scenes. There have been many lost opportunities in our space program over the years because of serious efforts by anti-spending fanatics to ensure we only go so far, but not one step farther.
The X-37 may end up as just one more X plane that racked up some air time, proved a few concepts, and tested some ideas only to be packed up and put away with seemingly nothing to show afterwards. But damn! What little we've seen so far could lead to so much. Here's hoping.
UPDATE Checking some of the links led me to a pdf file by Brian Weeden of the Secure World Foundationwhich examined the X-37B back in 2010 and discussed various possible military applications for it. Given how much interest that has generated in comments, I thought it worth pointing out. It also looks like it rules out some other ambitious hopes for the X-37B. Oh well - it's still a promising start. Now if only Boeing decides to build a larger C model, and maybe a comparable D....