The Delta IV Medium+ (4,2) consists of a single Delta IV common booster core (CBC), the Delta cryogenic second stage (DCSS), and two solid rocket motors (SRM). The CBC and the DCSS are connected by a composite cylindrical interstage adapter (ISA). The SRMs, 5 ft in diameter and 53 ft long and constructed of a graphite-epoxy composite, are connected to the booster by two ball-and-socket joints and structural thrusters.
The Delta IV booster tanks are structurally rigid and constructed of isogrid aluminum barrels, spun-formed aluminum domes, machined aluminum tank skirts, and a composite center-body. Delta IV booster propulsion is provided by the RS-68 engine system. The RS-68 burns cryogenic liquid hydrogen and liquid oxygen and delivers 663,000 lb of thrust at sea level. The booster’s cryogenic tanks are insulated with a combination of spray-on and bond-on insulation and helium-purged insulation blankets.
The Delta IV booster is controlled by the DCSS avionics system, which provides guidance, flight control, and vehicle sequencing functions during CBC and DCSS phases of flight. The boost phase of flight ends 6 seconds after main engine cutoff (MECO), when the separation charge in the interstage adapter is fired and 16 pneumatic actuators push the spent Delta IV CBC stage and the DCSS apart.
The DCSS stage propellant tanks are structurally rigid and constructed of isogrid aluminum ring forgings, spun-formed aluminum domes, machined aluminum tank skirts and a composite inter-tank truss. The DCSS is also a cryogenic liquid hydrogen/liquid oxygen-fueled vehicle. It uses a single RL10B-2 engine that produces 24,750 lb of thrust. Like the CBC, the DCSS cryogenic tanks are insulated with a combination of spray-on and bond-on insulation, and helium-purged insulation blankets. An equipment shelf attached to the aft dome of the DCSS liquid oxygen tank provides the structural mountings for vehicle electronics. The structural and electronic interfaces with the satellite are provided via the payload attach fitting (PAF). The GPS missions use a 4-m diameter payload fairing (PLF). The PLF is a composite bisector (two-piece shell) fairing. The vehicle’s height, with the 38.5-ft tall PLF, is approximately 206 ft.
