Air Force Mini-Shuttle Receives Green Light for 11 December Launch

An Atlas V, in its “501” configuration with a 5.4-meter-wide (18-foot) payload fairing, no strap-on boosters and a single-engine Centaur upper stage, roars aloft with the National Reconnaissance Office’s NROL-41 payload. The RL-10 engine used by the Centaur has been the primary cause of more than a month of delays. Photo Credit: ULA

After more than a month of 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 at 1:03 pm EST on Tuesday, 11 December from Pad 41 at Cape Canaveral Air Force Station. Originally scheduled for late October, the mission 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 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 also appear to have conspired against ULA.

Confirmation of the new launch date came via ULA’s website on Friday, 7 December, and it was noted that a flight clearance process had concluded that “a fuel leak occurred in a specific area of the interior of the [RL-10] thrust chamber and that this leak started during the first engine-start sequence.” The RL-10 has the capability to restart on multiple occasions in flight. ULA stressed that the investigation into the anomaly is ongoing, but that “all credible cross-over implications from the Delta anomaly for the OTV-3 Atlas vehicle and engine system have been thoroughly addressed and mitigated” to enable the decision to proceed with next week’s launch. Both the Atlas V and the Delta IV share an almost impeccable success record across their ten-year operational lives; despite a couple of premature engine shutdowns, neither booster has ever failed to deliver its primary payload into orbit. In his summary, Jim Sponnick, ULA’s vice president for Mission Operations, thanked the Air Force for its patience and paid tribute to the enormous efforts involved in clearing the Atlas V to finally fly.

Although the Delta IV launch on 4 October delivered its GPS primary payload into orbit, this success came only through a combination of robust systems and flight software, vehicle performance margins, and allowable propellant reserves. The rocket’s RL-10-fed Delta Cryogenic Second Stage (DCSS) apparently suffered a decline in performance as it began its first of three scheduled “burns,” and the autonomous systems responded by conducting a longer burn to make up for the shortfall. It would appear that the second burn was also longer than planned, but positioned the DCSS appropriately for a nominal third burn.

Exploded diagram of the Atlas V booster with its RL-10-fed Centaur upper stage at center and OTV mini-shuttle at upper right. Image Credit: ULA

An Accident Investigation Board (AIB) was set up on the direction of General William Shelton, commander of the Air Force Space Command. “While the launch was ultimately successful,” he said in a statement on 10 October, “the time-honored rigor and earnest process of an AIB will serve us well as we attempt to determine the root cause of this anomaly.” Shelton added that the implications of any launch failure “would be staggering.” The green light for Tuesday’s launch also enables ULA to reasonably stick to its previously published manifest to fly the Atlas V eight times in 2013. Key payloads include NASA’s TDRS-K communications satellite—currently scheduled to fly on 29 January—and two major science ventures, the Landsat Data Continuity Mission (LDCM) in February and the Mars Atmosphere and Volatile Evolution (MAVEN) in November.

With the clearance granted at Friday’s Launch Readiness Review, rollout of the 19-story Atlas V to Pad 41 is expected to take place on Monday morning to kick off a day-long process to undertake final preparations and fuelling of the vehicle. Air Force officials have noted that the launch window for the 11th opens at 1:03 pm and extends for five hours. The Atlas will fly in its “501” configuration, boasting a 5.4-meter-wide (18-foot) payload fairing, no strap-on solid-fuelled rocket boosters and a single-engine Centaur upper stage. At the instant of liftoff, the Atlas’ Russian-built RD-180 first-stage engine will produce a sea-level thrust in excess of 860,000 pounds, burning hot and hard for the first five minutes of the ascent. When it is exhausted and jettisoned, the turn will come of the Centaur’s RL-10, which will roar to life with a yield of 25,000 pounds in a vacuum. Although the RL-10 is a restartable engine, it will require only one burn to insert OTV-3 into orbit. Judging from two previous missions, the Air Force mini-shuttle should separate from the Centaur and commence its classified flight about 19 minutes after launch.

The OTV-3, also known as the X-37B and built by Boeing’s Phantom Works in California, will probably spend a year or more performing unspecified tasks, 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 craft 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.”

The size and wingspan of the X-37B requires the large 5.4-meter-wide (18-foot) payload fairing of the Atlas V. Photo Credit: US Air Force

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 an initial 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 will 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, other than 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 10 December and touched down on the 15,000-foot runway at Vandenberg Air Force Base. The craft sustained a burst tyre during rollout and minor damage to its underside, but the mission had proved an enormous success.

OTV-2, pictured during rollout at Vandenberg Air Force Base in June 2012, following its 469-day mission. Photo Credit: US Air Force

Although OTV-1 remains enshrouded 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 atop a Minotaur IV Lite rocket 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.

Both voyages certainly whetted many appetites for what OTV-3 might offer. It would appear that next week’s launch will be a reflight of the vehicle used for the OTV-1 mission, and in recent weeks 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.

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