Graphite-Epoxy Motor
The Graphite-Epoxy Motor (GEM) is a series of solid rocket boosters fueled by hydroxyl-terminated polybutadiene (HTPB) developed in the 1980s, and used from 1990 to today. GEM is currently produced by Northrop Grumman Space Systems with a carbon-fiber-reinforced polymer casing. GEM series boosters were previously used on the Delta II, Delta III, Delta IV launch vehicles, and are currently used on the Atlas V. The GEM-63XL is slated to fly on the Vulcan first-stage as early as 2022. Active variants. GEM-63. The GEM-63 was developed as a low-cost drop-in replacement  for the Aerojet Rocketdyne AJ-60A solid rocket booster used on the Atlas V. The Atlas V first flew with the GEM-63 in 2020 on the NROL-101 launch. The booster offers higher performance at about half the cost of the AJ-60A boosters formerly used on the Atlas V. GEM-63XL. The GEM-63XL is about 5 feet (152 cm) longer than the GEM-63 and will be used on the Vulcan launch vehicle beginning in 2022. Retired variants. GEM-40. The GEM-40 is a SRM used on Delta II beginning in 1990. The use of composite materials allowed for booster casings lighter than the steel  casings of the Castor 4 SRMs they replaced. The first flight of a GEM-40 occurred in 1990 on a Delta II 7925. Delta II vehicles can use three, four, or nine GEM-40s. When using three or four boosters, all GEM-40s ignite on the ground, while on Delta IIs using nine boosters six are ignited on the ground, and the remaining three are ignited in the air when the first six burn out. Failures. On August 5, 1995, an air-lit GEM-40 failed to separate from a Delta II 7925 carrying Koreasat 1. The excess mass of the booster resulted in  the satellite reaching a lower than intended transfer orbit, which it was able to compensate for using on-board propellant. On January 17, 1997, a Delta II (Delta 241) exploded due to a catastrophic failure in a GEM-40. The failure triggered the launch vehicle's self-destruct function 13 seconds after ignition. An Air Force investigation determined that the motor's casing had been damaged prior to launch, resulting in the casing splitting open soon after ignition. GEM-46. The GEM-46 was a lengthened solid motor originally developed for Delta III. This solid motor variant also included thrust vector control (TVC ) to help steer the vehicle. After the discontinuation of the Delta III, GEM-46 motors (without TVC) were used on the Delta II to create the Delta II Heavy, which could only be launched from a modified pad at Cape Canaveral. Both Delta III and Delta II-Heavy used nine GEM-46s, with six ignited on the ground and three air-lit. GEM-60. The GEM-60 was a solid motor used on the Delta IV family of launch vehicles, used with and without thrust vector control. Delta IV Medium+ launchers were built with either two or four GEM-60s. The motor was retired in  2019 after the final Delta IV Medium launch.