Northrop Grumman Innovation Systems
|Country of origin||United States|
|Used on||Delta II, Delta III, Delta IV, Atlas V, Vulcan (future)|
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 Innovation 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.
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-40||Delta II||12.96 m (510 in)||1.016 m (40.0 in)||13,005 kg (28,671 lb)||11,767 kg (25,942 lb)||499 kN (112,200 lbf)||245 s (SL)
283 s (air-lit) 
|63 s||HTPB, Al / AP|
|GEM-46||Delta III, Delta II||14.70 m (579 in)||1.1684 m (46.00 in)||19,140 kg (42,196 lb)||16,860 kg (37,180 lb)||601 kN (135,200 lbf)||242 s (SL)
284 s (air-lit) 
|77 s||HTPB, Al / AP|
|GEM-60||Delta IV M+||13.2 m (518 in)||1.5 m (60 in)||33,638 kg (74,158 lb)||29,697 kg (65,471 lb)||879 kN (197,500 lbf)||245 s (SL)||91 s||HTPB, Al / AP|
|GEM-63||Atlas V||20.1 m (792 in)||1.6 m (63 in)||49,300 kg (108,600 lb)||44,200 kg (97,500 lb)||1,663 kN (373,800 lbf)||279 s (SL) ||94 s||HTPB, Al / AP|
|GEM-63XL||Vulcan Centaur||22.0 m (865 in)||1.6 m (63 in)||53,400 kg (117,700 lb)||48,000 kg (105,800 lb)||2,026 kN (455,400 lbf)||280 s (SL) ||84 s||HTPB, Al / AP|
The GEM-40 is a 40-inch-diameter (1,000 mm) 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.
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.
The GEM-46 was a lengthened 46-inch-diameter (1,200 mm) 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.
The GEM-60 was a 60-inch-diameter (1,500 mm) 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.