For the second time in 2014, United Launch Alliance (ULA) is primed to fly its venerable Delta IV on a mission to deliver another member of the Global Positioning System (GPS) IIF “constellation” of satellites into a medium orbit, some 11,047 nautical miles (20,460 km) above Earth. Liftoff of the Delta IV, which will fly in its Medium+ 4,2 configuration—numerically designated to identify a 13-foot-diameter (4-meter) payload fairing and the presence of two solid-fueled Graphite Epoxy Motors (GEM)-60—is scheduled to occur at 8:08 p.m. EDT Thursday, 15 May, from Space Launch Complex (SLC)-37 at Cape Canaveral Air Force Station, Fla. The “window” extends for 18 minutes, but according to Air Force meteorologists there exists only a 30 percent likelihood of acceptable weather conditions for an on-time launch.
According to the 45th Weather Squadron, two systems are currently threatening a Thursday launch. “The first is a frontal system moving in the south-east U.S. that will move into Central Florida on Thursday and clear the area by Friday morning,” it was explained. “The second is a weak tropical wave moving through the Bahamas.” The frontal system is anticipated to violate Launch Commit Criteria with regard to the anvil and cumulus cloud rules, as well as the lightning rule. Solar activity is a further concern, with several active sunspots carrying the potential to produce high proton-flux levels. Conditions appear more benign for a backup launch attempt on Friday, where the weather is expected to be about 80-90 percent favorable for GPS IIF-6 to fly. In spite of the risk of a delay, ULA completed its Launch Readiness Review yesterday (Wednesday) and will commence the lengthy process of fueling the Delta IV on Thursday afternoon. The “terminal countdown” will commence at T-5 hours and 15 minutes, after which the cryogenic “tanking” of liquid oxygen and hydrogen propellants will get underway.
The central component of the Delta IV Medium+ 4,2 is a single Common Booster Core (CBC), which stands 134 feet (41 meters) tall and is powered by a single RS-68 liquid oxygen and hydrogen engine, which produces 663,000 pounds (300,000 kg) of propulsive yield at liftoff. This will ignite at T-5 seconds and undergo a period of computer-controlled health monitoring, ahead of the command to fire the GEM-60 boosters at T-0.01 seconds and release the hold-down clamps for launch at T-0. The Medium+ 4,2 is capable of delivering a payload weighing up to 12,890 pounds (5,850 kg) into a 22,300-mile (35,000-km) geostationary transfer orbit.
Since its maiden voyage on 20 November 2002, the Delta IV has proven to be an exceptionally reliable launcher. Although its first flight—which also flew in the Medium+ 4,2 configuration—despatched the Eutelsat W5 commercial payload into orbit, it has been almost exclusively dedicated to military customers, carrying satellites for communications, weather monitoring, early warning, and reconnaissance/intelligence. These have included members of the Global Positioning System (GPS), the Defense Satellite Communications System (DSCS), the Defense Support Program (DSP), the Geostationary Operational Environmental System (GOES), members of the Wideband Global Satcom (WGS) series, the Defense Meteorological Satellite Program (DMSP), and a number of classified National Reconnaissance Office Launches (NROL). Only the first flight of the Delta IV Heavy in December 2004 suffered a premature shutdown of its CBCs, which caused one of its satellite cargoes to enter an incorrect orbit and the other to fail to achieve orbit entirely. Thursday’s scheduled launch of GPS IIF-6 will be the 11th occasion on which the Medium+ 4,2 variant has been employed in the Delta IV’s 26-flight history, and, with the exception of a mishap in October 2012, the configuration has performed admirably.
Following the liftoff command, the 206-foot-tall (62-meter) Delta IV will be released from the pad surface and begin a fast climb away from SLC-37. About eight seconds after liftoff, the vehicle will execute an automated pitch, yaw, and roll program maneuver to establish itself on the proper 44.55-degree flight azimuth to deliver GPS IIF-6 into orbit. Powered by its RS-68 engine and twin boosters, it will burst through the sound barrier at about T+47 seconds. At one minute into the flight, it will encounter a period of maximum aerodynamic turbulence on the airframe, known colloquially as “Max Q.”
The twin boosters will be jettisoned about 100 seconds after launch, and the RS-68 will continue to propel the stack toward orbit for a further three minutes. It will shut down and be discarded about 4 minutes and 14 seconds into the flight, after which the Delta Cryogenic Second Stage (DCSS)—powered by Pratt & Whitney Rocketdyne’s RL-10B2 engine—will roar to life at T+4 minutes and 30 seconds and take over with 24,750 pounds (11,220 kg) of thrust. Fifteen minutes and 30 seconds after leaving SLC-37, the DCSS will achieve an initial, elliptical “parking orbit” and shut down, then coast for almost three hours, before reorienting itself for a second burn. According to ULA, the DCSS will ignite for its second burn, lasting a little under 2 minutes, at T+3 hours and 3 minutes into the mission. This will position GPS IIF-6 correctly for separation from its 38.5-foot-long (11.7-meter) Payload Attach Fitting (PAF) at about 3 hours, 15 minutes, and 50 seconds into the flight. The satellite will be injected into orbit at an altitude of 11,047 nautical miles (20,460 km), inclined 55 degrees to the equator.
Weighing 3,600 pounds (1,630 kg), GPS IIF-6 is the latest in an “interim” class of Global Positioning System satellites to keep a critical worldwide positioning, velocity, and timing asset operational until the next-generation GPS Block IIIA comes online later in 2014. They continuously transmit digital radio signals pertaining to the exact time, using atomic clocks, and exact location of the satellites. Each GPS IIF satellite is designed to remain operational for 12 years. The spacecraft boasts improved-accuracy positioning systems, a reprogrammable processor, capable of future upgrades, an interference-free civilian signal for commercial aviation search-and-rescue, and better resistance to electronic jamming through the new “M-code” military GPS signal. The first member of the planned 12-strong constellation of Block IIF satellites was launched in May 2010. It was followed by GPS IIF-2 in July 2011, GPS IIF-3 in October 2012, then GPS IIF-4 (aboard an Atlas V booster) in May 2013, and, most recently, GPS IIF-5 in February 2014.
“As each IIF satellite becomes operational, we continue the seamless transformation of the GPS constellation into an even more accurate, reliable, and durable navigation resource for the U.S. military and the global civilian user community,” said Craig Cooning, vice president and general manager of GPS Block IIF prime contractor Boeing Space & Intelligence Systems. “Our efficient pulse-line manufacturing process, adapted from Boeing’s commercial airplane production lines, also ensures that we deliver each spacecraft on time and on cost.”
In the meantime, the U.S. Air Force awarded Lockheed Martin a $1.4 billion contract in May 2008 to develop the Block IIIA network, which may eventually consist of as many as 32 satellites. At present, the Air Force has formally contracted for four Block IIIAs. With 500 times the transmitter power of current systems, Block IIIA will also benefit from new navigational warfare capabilities, enabling them to shut off GPS service to limited geographical locations whilst maintaining service to U.S. and allied forces. The system is operated and controlled by the 50th Space Wing, located at Schriever Air Force Base, Colo.
Assuming a successful launch of GPS IIF-6, this will be followed by GPS IIF-7 in late July and by GPS IIF-8 in October. Both will be lofted by ULA’s Atlas V 401 boosters from Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station.