A GEM-40 Solid rocket booster is hoisted for attachment to a Delta II
|Country of origin||United States|
|Used on||Delta II, Delta III, Delta IV|
|Length||449 in (11.4 m)|
|Diameter||40 in (1.0 m)|
|Gross mass||28,671 lb (13,005 kg)|
|Propellant mass||25,942 lb (11,767 kg)|
|Thrust||112,200 lbf (499 kN)|
|Specific impulse||245 s (sea level)
283 s (air-lit) 
|Burn time||63 seconds|
|Length||495 in (12.6 m)|
|Diameter||46 in (1.2 m)|
|Gross mass||42,196 lb (19,140 kg)|
|Propellant mass||37,180 lb (16,860 kg)|
|Thrust||135,200 lbf (601 kN)|
|Specific impulse||242 s (sea level)
284 s (air-lit) 
|Burn time||77 seconds|
|Length||518 in (13.2 m)|
|Diameter||60 in (1.5 m)|
|Gross mass||74,158 lb (33,638 kg)|
|Propellant mass||65,471 lb (29,697 kg)|
|Thrust||197,500 lbf (879 kN)|
|Specific impulse||245 s (sea level)|
|Burn time||91 seconds|
A Graphite-Epoxy Motor (GEM) is a solid rocket motor (SRM) produced by Alliant Techsystems with an epoxy composite casing, used as boosters for the Delta II, Delta III, and Delta IV launch vehicles. The use of composite materials allowed for booster casings several times 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.
- The GEM-40 is a 40-inch-diameter (1,000 mm) SRM used on Delta II beginning in 1990. 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.
- 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), which helped to steer the vehicle by changing (or vectoring) the direction of the thrust. With the discontinuation of the Delta III, the GEM-46 motors (without TVC) were also used on the Delta II Heavy variant, where they 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 is a 60-inch-diameter (1,500 mm) solid motor used on the Delta IV family of launch vehicles. These motors are available with and without TVC. A Delta IV can have two or four GEM-60s, and a Delta IV with these motors is classified as a Delta IV Medium+ launch vehicle.
On August 5, 1995, an air-lit GEM-40 failed to separate from a Delta II 7925 carrying Koreasat I. 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.
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