The X-37B Orbital Test Vehicle (OTV) program ends the United States Air Force's long wait to finally have its own fleet of space planes. By July 2012, each of the two 8.9-meter-long1 unmanned space planes had each flown lengthy orbital missions and made autonomous landings. Although the flight to orbit via Atlas V rockets could not be hidden, once in orbit the missions of each OTV have been shrouded in secrecy, fuelling speculation. From the beginnings of the Space Age, there has been a line of thought that space is an extension of air power and many see this program as a potential means for the USAF to expand the military space to more broadly resemble the weaponized nature of air power. On the other hand, space is a different environment, and the winged X-37 vehicles may not necessarily be replicating traditional USAF warfighting roles in space.
Originally the X-37 was a NASA program to demonstrate technologies that may be useful for future spacecraft, with emphasis on recovery technologies.2 In 2004, the program was transferred to Defense Advanced Research Projects Agency (DARPA), which the completed in-atmosphere testing of the design in 2006.3 These earlier craft in the X-37 program had been airdropped to demonstrate autonomous flight control of the unpowered vehicle. Like the much larger space shuttles that were in service at the time, the X-37 program always envisioned gliding to a runway after orbital reentry. This manner of recovery has specific costs and benefits. Once the airborne testing was completed the USAF proceeded with orbit-capable spacecraft in the form of the pair of OTVs.
For early manned spaceflight and cargo recovery (such as returning film from reconnaissance) satellites, ballistic capsule reentry proved simpler to launch, but required expensive infrastructure to pick up the capsule after reentry. During the early Space Age there were parallel efforts to construct aircraft capable of reaching space. The X-15 rocket plane program boosted several test pilots up to altitudes that qualified them for astronaut wings.4 The air-launched X-15 was expected to be followed by the vertically launched X-20 Dynasoar program, which was conceived in the 1950s as a space bomber, and later became an orbit-capable reusable spacecraft useful for various military missions, in addition to the delivery of nuclear warheads. X-20 was cancelled in 1963. Prior to X-20 there were the similar BoMI (bomber missile) and Project ROBO (rocket bomber) that also investigated a U.S. rocket-powered boost-glide weapon system. Inspiring all this was Eugene Sanger's “Silverbird” concept for a suborbital “Amerika Bomber” from World War II. Another Nazi Wunderwaffe concept, the “Silverbird” was to be a horizontally launched boost-glide bomber to fulfill Hitler's desire to strike the United States. Elements of the Soviet leadership were also convinced that the U.S. Space Shuttle, which at the time was a dual NASA and DOD program, was actually a space bomber.5 It is no surprise that among the speculated roles for the pair of USAF spacecraft is attack. That said, the X-37B concept, despite its ability to maneuver would seem to be a very expensive strike option, even taking into account the experimental nature of Prompt Global Strike (PGS) work thus far. Any weapon deployed by an orbiting spacecraft, such as X-37B, would have to either survive reentry at orbital speeds, or carry propulsion to slow down—it must be noted that the size of an X-37B payload bay is about that of a pickup truck.6
Thus far, flown PGS-related hardware have all been suborbital hypersonic glide vehicles with some degree of flight control, essentially advance forms of the shapes used in existing nuclear weapon reentry vehicles. This in itself has caused some concern in that similarities between PGS and existing U.S. nuclear strike systems would lead to these two different weapon systems being confused for one another by a third-party nuclear-armed nation, causing a mistaken retaliatory strike, by actual nuclear weapons, on the United States.7 The emphasis is to extend the glide portion of a PGS weapon's flight, as currently deployed U.S. nuclear warheads lack such capabilities.8 That said, for all the complaints from near peers, and others, about potential for confusion between PGS and a nuclear strike, other nations are deploying maneuverable reentry vehicle (MaRV) warheads: Russian claims that their upgraded nuclear deterrent is able to evade U.S. missile defense, and China's antiship ballistic missile (ASBM) both point toward the use of a MaRV, especially the ASBM as it would need to seek after a moving target after its several-minutes-long flight.
Proponents of air-breathing space-launch propulsion have their own solution to concerns that a rocket-boosted PGS would be confused with a nuclear strike—an actual hypersonic bomber. Eugene Sanger's “Silverbird” concept was a track-launched bomber that would be initially propelled by rocket and proceed across the globe from Germany to Japan by skipping across the atmosphere. As a prelude to today's rocket versus space plane competition, there was a competing Nazi scheme to attack the U.S. homeland with ballistic missiles—the von Braun's A9/A10 multistage missiles. Both Sanger and von Braun fell into Allied hands after the war, with the latter being more successful so far at least. Hypersonic aircraft development in the United States has been ongoing since the Cold War, with the X-30 National Aero-Space Plane (NASP) being the best known in the public domain. Recent USAF planning documents do make a link between air-breathing launch vehicle technology and a hypersonic bomber, though as a means of leveraging off of each other's development as a cost-saving measure for what can only be two very expensive endeavours.9 Although orbit-to-surface strike cannot be ruled out as an X-37B mission, it does seem unlikely due to the costs versus benefits.
The other offensive mission speculated on for the pair of OTVs is satellite inspection—often used as a euphemism for anti-satellite attack. Satellite inspection and some ASAT attack profiles require on-orbit rendezvous, something that it should be noted is not listed as among the X-37's capabilities, though as mentioned earlier, there has not been much released as to specific on-orbit activities of either missions flown as of July 2012. Known autonomous on-orbit rendezvous experiments, such as the XSS-11, have generated quite a bit of controversy. Basically any time a spacecraft can operate safely in close proximity with another spacecraft there is potential for it to conduct inspection and attack.
Another potential role is for the X-37Bs to act as a satellite bus for reconnaissance and surveillance payloads. These spacecraft are able to maneuver on-orbit across missions lasting hundreds of days, meaning that they have the opportunity to surprise those on the ground through unexpected changes regarding the period during which it will be overhead. Unlike a conventional spy satellite, the winged reentry capability allows the X-37 to return the entire payload to earth for later reuse. As each vehicle has only flown once, not much is known about turnaround times, though the fact that they are launched by conventional rockets means that they are not operationally reactive in the sense that they can launch on demand. Once in orbit, however, the capacity to change orbits does give it some operational flexibility.
Finally there is the possibility that the U.S. government is being completely truthful, though vague, and the X-37B program is purely to test new space and orbital reentry technologies. It must be remembered that aside from the Soviet Buran no space plane has returned to a runway landing unmanned. By July 2012, this feat had been accomplished twice by the USAF. By definition the International Space Station (ISS) would be an inappropriate venue for DOD-specific experiments in orbit, and without the space shuttle, recoverable long-duration military experiments would have few opportunities to fly. Indeed with an X designation, the 224 and 469 day10 missions of the first and second OTVs would seem to be endurance experiments in themselves. Now of course experience gained during the X-37B program may be applicable toward other emerging military space capabilities.
Notes
1. United States Air Force, “Factsheets: X-37B Orbital Test Vehicle,” March 3, 2011, http://www.af.mil/information/factsheets/factsheet.asp?fsID=16639.
2. National Aeronautics and Space Administration. “NASA—X-37 Fact Sheet (05/03),” June 2003, http://www.nasa.gov/centers/marshall/news/background/facts/x37facts2.html.
3. Ibid.
4. National Aeronautics and Space Administration. “X-15—Hypersonic Research at the Edge of Space,” February 24, 2000, http://history.nasa.gov/x15/cover.html.
5. Bart Hendrickx and Bert Vis, Energiya-Buran: The Soviet Space Shuttle, (Chichester, UK: Springer, 2007), 53–55.
6. Stephen Clark, “Air Force Spaceplane Is an Odd Bird with a Twisted Past,” April 2, 2010, http://spaceflightnow.com/atlas/av012/100402x37update/.
7. David Axe, “New ‘ICBMs vs. Terrorists’ Plan: Now 50% Less Crazy!” Wired, March 7, 2011, http://www.wired.com/dangerroom/2011/03/new-icbms-vs-terrorists-plan-now-50-less-crazy/.
8. Ibid.
9. United States Air Force. Report on Technology Horizons: A Vision for Air Force Science & Technology During 2010–2030, Volume 1, May 15, 2010, http://www.af.mil/shared/media/document/AFD-100727–053.pdf.
10. United States Air Force, 30th Space Wing Public Affairs, “X-37B Orbital Test Vehicle lands at Vandenberg,” June 16, 2012, http://www.vandenberg.af.mil/news/story.asp?id=123306242.