For the second time this year, United Launch Alliance (ULA) will boost a workhorse Delta IV aloft—this time, to deliver the sixth Wideband Global Satcom satellite (WGS-6) into geostationary orbit for the U.S. Air Force. Liftoff of the Delta IV, which is flying in its Medium+ “5,4” configuration—with a 5-meter (16-foot) payload fairing, a modified second stage with 5-meter liquid hydrogen tank, and “stretched” liquid oxygen tank, together with four strap-on Graphite Epoxy Motors (GEM)-60—is currently scheduled for 8:29 p.m. EDT on 7 August, at the start of a 49-minute “window.” The mission will be staged from Space Launch Complex (SLC)-37 at Cape Canaveral Air Force Station, Fla.
According to Jim Sponnick, ULA’s vice president for Atlas and Delta Programs, the launch represents the 23rd Delta IV flight and the fourth by the vehicle in its Medium+ 5,4 configuration. This comes in the wake of last May’s WGS-5, together with the WGS-4 mission in January 2012, and the configuration’s first launch, with WGS-3, in December 2009. (The WGS-1 and WGS-2 satellites were both lofted aboard ULA Atlas V vehicles.) “The ULA team is focused on attaining Perfect Product Delivery for the WGS-6 mission,” explained Sponnick in the online launch kit, “which includes a relentless focus on mission success … and also excellence and continuous improvement in meeting all of the needs of our customers.”
That Perfect Product Delivery is a boast about which ULA can be well proud. Although the Delta IV (in its Medium+ “4,2” configuration) suffered a problematic flight last October, both it and ULA’s Atlas V have chalked up an impressive success record in over ten years of operations. The Delta IV vehicle has flown 22 times since November 2002, only one of which—the maiden voyage of its “Heavy” configuration—was classified as a partial failure, when a premature engine shutdown delivered its payloads into improper orbits. Most recently, last October’s Delta IV flight (though successful) experienced a leak in its upper stage engine, which produced lower than expected thrust. Fortunately, the rocket benefited from a large fuel load and fault-tolerant guidance software; this enabled it to recalculate its ascent trajectory and fire the engine for longer to achieve mission success.
Processing of the Delta IV Medium+ 5,4 for Wednesday’s mission has been ongoing at Cape Canaveral Air Force Station for several months, since the arrival of the first stage of the mammoth booster in February. Both its first and second stages were fabricated by ULA at its Decatur, Ala., facility, after which they were transferred to Florida aboard the M/C Delta Mariner cargo vessel. Upon arrival at the launch site, the stages underwent final assembly inside the seven-story Horizontal Integration Facility (HIF) at SLC-37, and rollout of the 217-foot-tall Delta took place on Monday, 5 August atop a diesel-powered Elevating Platform Transporter. Once the stack is “hard down” on the SLC-37 surface, a Fixed Pad Erector will raise it into a vertical orientation within the Mobile Service Tower. The latter contains a crane which will install the four GEM-60 boosters, as well as the WGS-6 satellite in its two-piece (“bisector”) payload fairing.
The Delta IV was designed by Boeing, although the four GEM-60 high-performance solid-fueled boosters—each of which stand 53 feet tall—have been developed by Alliant TechSystems. The boosters are so numbered because each casing measures 60 inches in diameter. The Delta IV Medium+ 5,4 has the potential to transport payloads weighing up to 14,475 pounds into a 22,000-mile geosynchronous orbit. As a 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 market. First flown on 20 November 2002, with the Eutelsat-70 communications satellite, it has since lofted a mixture of military and NASA spacecraft for communications, reconnaissance, early-warning, weather forecasting, and navigation.
Its first stage consists of 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 Wednesday’s opening launch attempt of WGS-6 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. After telemetry data confirms that the engine-start sequence is satisfactory, the four GEM-60s will roar to life at T-0.01 seconds, and the Delta will depart the launch pad at precisely T-zero.
Shortly after clearing the SLC-37 tower, it will execute a computer-commanded pitch, yaw, and roll program maneuver. This will actively guide the vehicle onto an easterly heading and establish it onto the proper 100.97-degree flight azimuth to deliver WGS-6 into orbit. Fifty 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 the solid boosters. A minute and a half after launch, the GEM-60s will exhaust their propellant and are scheduled to 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 conclude its portion of the boost to orbit and separate from the rocket.
The turn will then come for the Delta Cryogenic Second Stage (DCSS) and its single hydrogen/oxygen-fueled RL-10B2 engine, capable of 24,750 pounds of thrust. This engine features an extendable carbon-carbon nozzle to enhance its specific impulse and will conduct two critical “burns” to inject WGS-6 into orbit. The first burn will begin at T+268 seconds, about 13 seconds after the separation of the CBC. The engine will fire for 16 minutes and 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 is scheduled to be much shorter—about three minutes in duration—and WGS-6 will separate from the DCSS at T+41 minutes. By this stage, the satellite will be in “supersynchronous transfer orbit,” with a perigee of 238.5 nautical miles and an apogee of 36,145.5 nautical miles, inclined 24 degrees to the equator.
Weighing 13,200 pounds, WGS-6 is the latest in a series of high-capacity communications satellites operated by the 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 Department of Defense 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 DoD and the Australian Defence Force. Canada is also a partner in the project.
Three “Block 1” satellites—WGS-1, 2, and 3—were launched into orbit between October 2007 and December 2009, providing complete strategic coverage of the Pacific and Atlantic Oceans and supporting U.S. Central Command in Afghanistan, Iraq, and western Asia. The first member of the upgraded “Block 2” network, WGS-4, flew in January 2012, followed by WGS-5 in May 2013. The WGS-6 satellite was transported to Florida from Boeing’s production facility in El Segundo, Calif., on 17 May 2013 to undergo testing, fueling, and integration ahead of launch. Boeing is currently working on the development of WGS-7, having received a $182 million contract from the U.S. Air Force in August 2010 to build follow-on satellites.
Current projections show an 80-percent chance of favorable weather conditions for an on-time launch of the Delta IV with WGS-6 on Wednesday night.
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