CAPE CANAVERAL, Fla — No sooner has the smoke and fire settled from last Wednesday’s liftoff of an Atlas V to deliver the GPS IIF-4 satellite into orbit, United Launch Alliance (ULA) is primed to rattle Cape Canaveral Air Force Station, Fla., again on Thursday, 23 May, with the roar of a Delta IV Medium booster from Space Launch Complex (SLC)-37. The mission will insert the fifth Wideband Global Satcom (WGS-5) into orbit on behalf of the U.S. Air Force. Originally scheduled to fly Wednesday, 22 May, a decision was made at the weekend to delay by 24 hours. “During acceptance testing for another mission,” explained ULA in a Sunday news release, “an avionics box that is used for controlling the Delta booster RS-68 engine experienced an anomaly.” As a result, ULA has replaced the box “with one that has been inspected and confirmed not to have the suspect condition.”
The prudence surrounding this long-awaited launch is far from misplaced. Last October, a Delta IV Medium delivered the GPS IIF-3 satellite successfully into orbit. However, during ascent, a leak developed above the narrow-throat portion of the thrust chamber in Pratt & Whitney Rocketdyne’s RL-10B2 upper stage engine. This produced a lower than expected thrust. Fortunately, the low mass of the GPS payload, together with a large fuel load and fault-tolerant guidance software, enabled the vehicle to recalculate its trajectory and fire the engine for longer than planned, thus saving the mission from failure. However, this was unacceptable to ULA and its goal of “Perfect Product Delivery,” and a joint investigation with Pratt & Whitney Rocketdyne and the U.S. Air Force began. According to the Air Force, closure reviews of the problem were due in mid-April.
The investigation identified the general characteristics of the fuel leak and a number of corrective actions, both in terms of hardware and operational procedures, were implemented. These included thorough inspections of engine and Delta IV systems for damage or the presence of foreign objects, coupled with in-flight helium purges to critical components and changes to how the restartable RL-10B2 is thermally conditioned during ascent to prepare for its first firing. The commonality between the RL-10B2 on the Delta IV and the RL-10A engine on the Atlas V—both operated by ULA—meant that both vehicles were grounded, pending a resolution of the problem. However, whilst the Delta IV-specific investigations continued, an Atlas V successfully delivered the U.S. Air Force’s X-37B spacecraft into orbit without incident on 11 December 2012.
Speaking earlier this month, Jim Sponnick, ULA’s vice president for Mission Operations, noted that “extensive analyses and engine testing, along with the mitigating actions that have been implemented,” had increased confidence that “the risks have been mitigated and that it is safe to proceed with the WGS-5 launch.” He explained that flight clearances for the mission had been granted both by ULA and by the Air Force.
Thursday’s mission will see the Delta IV in its Medium+ “5,4” configuration. This is one of five variants of the Boeing-designed booster and boasts a 5-meter-wide (16-foot) payload fairing and a modified second stage with a 5-meter liquid hydrogen tank and “stretched” liquid oxygen tank, together with four strap-on Graphite Epoxy Motors (GEM)-60, built by Alliant TechSystems. The 53-foot-tall GEM-60 is a high-performance solid-fueled motor, so numbered because its casing measures 60 inches in diameter. The Medium+ 5,4 vehicle has the capacity to loft up to 14,475 pounds of payload into 22,000-mile-high geosynchronous Earth orbits.
Both the first and second stages were fabricated by ULA at its facility in Decatur, Ala., and delivered separately to Cape Canaveral via the M/V Delta Mariner cargo vessel. Upon arrival at the launch site, final assembly of the stages was conducted inside the seven-story Horizontal Integration Facility (HIF) at SLC-37, after which the Delta will be transferred to the pad atop a diesel-powered Elevating Platform Transporter. Once it is “hard down” on the SLC-37 surface, a Fixed Pad Erector will raise the vehicle into a vertical position within the Mobile Service Tower (MST). The latter—which will be retracted early Thursday morning—contains a crane which will install the four GEM-60 motors, the WGS-5 payload, and the two-piece (“bisector”) fairing onto the vehicle.
The Delta IV was originally intended for U.S. military purposes, and low demand and high costs forced Boeing in 2003 to remove it from the commercial launcher market. First flown on 20 November 2002, with the Eutelsat 70A commercial communications satellite, it boasts an impressive record, with 21 missions to its credit. Of these missions, all but one—the first voyage of the Delta IV Heavy in December 2004, which suffered a premature engine shutdown and failed to deliver its payloads into their correct orbits—have been classified as successful. With the exception of Eutelsat 70A, the Delta IV has lofted a mixture of military and NASA payloads for communications, reconnaissance, early-warning, weather forecasting, and navigational purposes.
In the Medium+ 5,4 configuration—which has flown only twice—the rocket’s first stage is a single Common Booster Core (CBC), propelled by Pratt & Whitney Rocketdyne’s RS-68 engine. When this was introduced in 2002, it became the first large liquid-fueled powerplant to be developed by the United States since the Space Shuttle Main Engine. Loading of liquid oxygen and hydrogen propellants into the CBC for Thursday’s opening launch attempt will require a complex, 4.5-hour procedure, terminating a little over two hours before liftoff. Ignition of the RS-68 will occur at T-5 seconds, enabling the engine to ramp up to its full 663,000 pounds of thrust. When telemetry confirms that the engine-start sequence is healthy, the four GEM-60 motors will ignite at T-0.01 seconds and the 217-foot-tall Delta will depart the launch pad at precisely T-zero.
Liftoff is scheduled to occur at 8:28 p.m. EDT Thursday, right on the opening of a 32-minute “window.” Shortly after the vehicle clears the SLC-37 tower, it will execute a computer-commanded pitch, yaw, and roll program maneuver to establish itself onto the proper 100.97-degree flight azimuth for delivery of the WGS-5 satellite into orbit. About 50 seconds into the ascent, the Delta will burst through the sound barrier and press on toward orbit under the combined impulse of the RS-68 and solids. A minute and a half after launch, the GEM-60s will exhaust their propellant and will be jettisoned in pairs at T+100 seconds and T+102 seconds. A little over two minutes later, the payload fairing will be jettisoned and the CBC will complete its portion of the boost to orbit and will separate from the vehicle.
The turn will then come for the Delta’s cryogenic second stage and its single hydrogen/oxygen-fueled RL-10B2, capable of 24,750 pounds of thrust. This engine features an extendable carbon-carbon nozzle to enhance its specific impulse and is tasked with two critical “burns” to inject WGS-5 into its operational orbit. The first of these firings will begin at T+267 seconds, about 13 seconds after the separation of the CBC. It is scheduled to burn for about 16 minutes and will establish the satellite into its “parking” orbit, after which it will coast for eight minutes, ahead of a second burn which will get underway at T+28 minutes. The second firing will be shorter—about three minutes in duration—and WGS-5 will separate at T+41 minutes. By this stage, the satellite will be in a geosynchronous transfer orbit, with a perigee of 238 nautical miles and an apogee of 36,134 nautical miles, inclined 24 degrees to the equator.
Weighing 13,200 pounds, WGS-5 is the latest in a series of high-capacity communications satellites, operated by the U.S. Air Force Space Command’s Space and Missile Systems Center. It supports communications links in the 500 MHz range of the X-band and 1 GHz range of the Ka-band, and has the capacity to filter and route up to 4.875 GHz of instantaneous bandwidth. The new satellite includes a high-bandwidth radio frequency bypass capability and can support data-transmission rates between 2.4 and 3.6 Gbps, some three times faster than previous DoD systems. It is also equipped with a xenon-ion propulsion system, which offers a tenfold efficiency improvement over conventional bipropellants, and carries triple-junction gallium arsenide solar cells and deployable radiators with flexible heat pipes. A single WGS is believed to possess as much bandwidth as the entire Defense Satellite Communications System (DSCS) network currently in service. Following agreements signed in November 2007, the WGS system will be operated in partnership between the Department of Defense and the Australian Defence Force. Canada is also a partner in the project.
The system’s high-bandwidth has been described as a “quantum leap” in communications capability and will provide deployed forces with unprecedented access to bandwidth-intensive applications, such as video streaming, teleconferencing, real-time data transmission, and high-resolution imaging. Additionally, WGS will support the new generation of unmanned aerial vehicles, such as Global Hawks and MQ-9 Reapers.
Three “Block 1” satellites—WGS-1, 2, and 3—were launched into orbit between October 2007 and the end of 2009, providing complete strategic coverage of the Pacific and Atlantic Oceans and supporting U.S. Central Command in Afghanistan, Iraq, and western Asia. The final member of the Block 1 network, WGS-3, was launched on the very first flight of the Delta IV Medium+ 5,4 in December 2009. The inaugural component of the Block 2 system, WGS-4, also rode the same Delta IV configuration in January 2012, making Thursday’s launch of WGS-5 only the third time that a Medium+ 5,4 has been used. A sixth satellite (WGS-6) was transported to Florida from Boeing’s production facility in El Segundo, Calif., on 17 May 2013, ahead of its scheduled launch from SLC-37 atop another Delta IV Medium+ 5,4 in August. Boeing is currently working on the development of a seventh WGS, having received a $182 million contract from the Air Force for the construction of follow-on satellites in August 2010.
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