After several delays, United Launch Alliance’s venerable Atlas V rocket is scheduled to loft the third Orbital Test Vehicle (OTV-3)—a secretive and highly controversial Air Force mini-shuttle, also known as the X-37B—into space early next month from Pad 41 at Cape Canaveral Air Force Station. Launch was originally scheduled for late October, but was repeatedly postponed when a ULA Delta IV experienced abnormally low thrust in its RL-10 upper stage engine during an otherwise successful effort to insert a Global Positioning System (GPS) satellite into orbit. Both the Atlas and Delta rockets use slightly different versions of Pratt & Whitney Rocketdyne’s RL-10, and it was considered prudent to push back the OTV-3 launch until a solution could be found. Weather and range constraints appear to have also conspired against ULA and speculation exists that even the current no-earlier-than launch date of 11 December may itself slip to the right.
According to General William Shelton, head of the Air Force’s Space Command, the problems may impose a ripple effect on ULA’s current manifest, which calls for the Atlas V to fly eight times in 2013. Key payloads include NASA’s TDRS-K communications satellite in January, the Landsat Data Continuity Mission in February, and the Mars Atmosphere and Volatile Evolution (MAVEN) to Mars in November. It has been noted that only the Delta IV’s robust systems, flight software, vehicle margins, and propellant reserves saved the GPS mission, and Shelton stressed that “the cost of a launch failure would be staggering”. The Atlas V has enjoyed a virtually unblemished flight history since its maiden voyage in August 2002; with the exception of an upper-stage problem which left one classified satellite in a low orbit, it has successfully completed all of its 33 missions.
For the OTV-3 launch, the 19-story Atlas V will fly in its ‘501’ configuration, boasting a 5.4-metre-wide (18-foot) payload fairing, no-strap on solid-fuelled rocket boosters and a single-engine Centaur upper stage. Housed within the payload fairing, the OTV—also known as the X-37B and built at Boeing’s Phantom Works in California—will be boosted into low-Earth orbit, where it will probably spend a year or more performing unspecified objectives prior to re-entry and landing. Originally developed as a NASA project with Air Force participation in early 1999, it was transferred to the Defense Advanced Research Projects Agency (DARPA) as a classified project in September 2004 and underwent an initial drop test over Edwards Air Force Base in California in April 2006. The vehicle overran the runway and suffered minor damage, but two further drop tests in August and September proved successful.
Physically, the 11,000-pound OTV bears a striking resemblance to the Space Shuttle, albeit a quarter of the size and with the capability to remain aloft for approximately nine months. It measures 29 feet long with a wingspan of 15 feet and is powered by a single liquid hydrazine engine, together with a deployable array of gallium arsenide solar cells and lithium-ion batteries. The OTV’s payload bay is seven feet long and four feet wide, tailored for cargoes weighing between 500-660 pounds. An advanced avionics suite and airframe, electromechanical actuators, and autonomous guidance controls focused the X-37B’s mandate onto “risk reduction, experimentation, and operational concept development for reusable vehicle technologies in support of long-term developmental space objectives”.
Its thermal protection system is impressive…and so are their acronyms: Toughened Uni-piece Fibrous Refractory Oxidation-resistant Ceramic (TUFROC) tiles line the leading edges of the wings, instead of the reinforced-carbon-carbon used on the Shuttle, whilst highly-durable Toughened Uni-piece Fibrous Insulation (TUFI) impregnated silica tiles and Advanced Conformal Reusable Insulation (CRI) blankets cover the airframe. Originally scheduled for launch in the Shuttle’s payload bay, the X-37B was transferred to the Delta II and later to the Atlas V. Like the Soviet-era Buran vehicle, it has the capability to land autonomously on a runway at either Edwards or Vandenberg Air Force Bases after a hypersonic re-entry.
The 2006 approach and landing tests were all conducted using the initial NASA version of the spacecraft, dubbed the X-37A, but the Air Force later developed the modified X-37B for its own purposes. More recently, in October 2011, a Boeing presentation to the American Institute of Aeronautics and Astronautics revealed plans for an X-37C, which will be 165-180 percent larger than the existing vehicle and have the capacity to transport up to six astronauts into orbit. This could also be used to carry equipment to and from the International Space Station, allowing sensitive microgravity experiments—including protein crystals and biological specimens—to be returned to Earth in a timely manner and at relatively gentle deceleration rates of around 1.5 G.
The first X-37B mission—designated OTV-1—was launched atop an Atlas V from Cape Canaveral Air Force Station on 22 April 2010. The Air Force revealed nothing of its classified payload, save for a bland statement that it would “demonstrate various experiments and allow satellite sensors, subsystems, components, and associated technology to be transported into space and back”. It would seem, however, that advanced guidance, navigation, and control systems are being tested, advanced thermal protection materials, structures, and seals are being evaluated, and lightweight electromechanical flight hardware is being wrung out. The X-37B has also been described as “a rapid-turnaround technology demonstrator”, perhaps indicative of the Air Force’s need for a vehicle which NASA’s Shuttle could never fully provide: regular and routine access to low-Earth orbit.
Control of the OTV-1 mission was undertaken by the 3d Space Experimentation Squadron and 21st Space Wing of the Air Force Space Command, based in Colorado Springs, and observations by amateur astronomers seemed to imply an orbit which “repeated” every three or four days, perhaps indicative of its nature as an imaging reconnaissance platform. Several orbit-changing manoeuvres were executed by OTV-1, reaching a maximum apogee of 276 miles and a minimum perigee of 175 miles. Its 224-day mission completed, the craft re-entered the atmosphere on 3 December and touched down on the 15,000-foot runway at Vandenberg Air Force Base. The craft sustained a burst tire during rollout and minor damage to its underside, but the mission had proved an enormous success.
Although OTV-1 remains shrouded in secrecy, much speculation has been given to its possible objectives. The Air Force’s Hypersonic Technology Vehicle (HTV-2), an unmanned rocket-propelled glider, was launched from Vandenberg on 22 April 2010 less than an hour before the X-37B, and space analyst James Oberg suggested that the two missions were related. Unfortunately, the HTV-2 began to suffer violent oscillations, prompting the craft’s on-board computer to issue a flight termination command. Contact was lost just a few minutes into the planned 25-minute flight. Other, more chilling, possibilities include the use of the X-37B as a testbed for new satellite components or even as a delivery mechanism for advanced weapons systems. The Pentagon has denied claims that the craft is any way related to weapons proliferation.
The second vehicle, OTV-2, was launched on 5 March 2011 from Cape Canaveral and spent far longer than its nine-month design life in orbit. This would appear to be due to the presence of the large gallium arsenide solar array, which unfurled from the X-37B’s payload bay provided a renewable energy source and enabled it to “loiter” for 15 months. Certainly, it would appear that the extended mission became possible as late as November 2011, when Major Tracy Bunko, a spokesperson for the Secretary of the Air Force, announced that “we are learning new things about the vehicle every day” and that “we initially planned for a nine-month mission, but will continue to extent it as circumstances allow”. OTV-2 landed at Vandenberg on 16 June 2012 after a 469-day flight.
Until the early part of this week, T-0 for the Atlas V with OTV-3 was scheduled for sometime in the midst of a four-and-a-half-hour “window” running from 11:15 am until 3:45 pm EST on 27 November, but it was already widely known that this date was untenable, pending the outcome of the RL-10 investigation. ULA’s website has released an updated launch date of 11 December, although some observers doubt that the mission will fly until early in the new year. The Atlas’ first stage, powered by a Russian RD-180 engine, will provide the impetus for the first five minutes of ascent. Its propellant mix of rocket-grade kerosene (RP-1) and liquid oxygen produces a sea-level yield in excess of 860,000 pounds. When it is exhausted and jettisoned, the Centaur upper stage—propelled by the trouble-plagued RL-10—will roar to life with a thrust of 25,000 pounds in a vacuum. Although the RL-10 is designed to be restartable, it will require only a single burn to insert OTV-3 into orbit. Judging from the two previous X-37B missions, the spacecraft should separate from the Centaur to commence its mission about 19 minutes after launch.
Both voyages certainly whetted many appetites for what OTV-3 might offer. It would appear that next week’s launch will be a re-flight of the vehicle used for the OTV-1 mission, and in recent days it has been speculated that a touchdown might be attempted in Florida at the Shuttle Landing Facility. This will be the first occasion on which an X-37B has been reused for a second mission. Major Bunko revealed that the Air Force was actively “looking at Space Shuttle infrastructure for possible cost-saving measures”, including the potential for “consolidating landing, refurbishment, and launch operations at Kennedy Space Center or Cape Canaveral Air Force Station”. According to Bunko, the exact duration of the OTV-3 mission has yet to be determined.