A Graphite-Epoxy Motor (GEM) is a high-performance, solid rocket motor with an epoxy composite casing, used for supplemental thrust on several launch vehicles, including the Boeing Delta II and Delta IV. They are designed to allow launch vehicles to deliver larger payloads to orbit. The name "Graphite-Epoxy Motor" refers specifically to solid motors produced by Alliant Techsystems, although boosters of similar construction are used on other launch vehicles. They are generally considered to be mechanically less complex than liquid fuelled motors, but also allow less control following ignition.
A solid rocket motor consists primarily of a casing that is packed with propellant grain (a mixture of a solid fuel, such as a rubber or aluminum, and an oxidizer, such as ammonium perchlorate), and a nozzle at the aft end of the motor. The casing is crucial for the solid motor because it contains the pressure of the burning solid fuel; if the casing was not strong enough, the motor would rupture and explode.
Before the development of Graphite-Epoxy Motors (GEMs), the company's (first Thiokol, now Alliant Techsystems) Castor boosters used steel solid motors used to produce extra thrust and boost payload capacity. By using a lighter material, the motor could be made larger to contain more propellant, increasing thrust and payload capacity, without increasing weight excessively. However, simply using thinner steel would not work, as the steel would be insufficiently strong to contain the burning fuel.
Stronger composites eventually enabled the construction of motors that were lighter than the older steel-case motors while still retaining the strength necessary to contain the pressure. The graphite-epoxy composite is lighter than steel, allowing the composite motor to be larger, improving thrust and performance. For example, the GEM-40 motors used on the Delta II are 6 feet longer than the Castor IVA motors they replace, allowing them to produce over 6,000 pounds more thrust and burn seven seconds longer, while still weighing over 200 pounds less. 
Alliant Techsystems manufactures GEMs for Boeing's Delta II, III, and IV launch vehicles in the following sizes:
- GEM-40, 40-inch-diameter (1,000 mm) solid motor used on Delta II beginning in 1990. Delta II vehicles can use three, four, or nine GEM-40s. A special version of the GEM-40, the GEM-40VN (Vectorable Nozzle), is used as the first stage in the United States' Missile Defense System interceptor vehicle, for destroying nuclear warheads. 
- GEM-46 (GEM-LDXL), lengthened 46-inch-diameter (1,200 mm) solid motor developed for Delta III. This solid motor variant also includes Thrust Vector Control (TVC), which helps 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) are also used on Delta II vehicles to boost payload capacity further. A Delta II with GEM-46 motors is considered a "Heavy" variant. Both Delta III and Delta II-Heavy use nine GEM-46s. 
- GEM-60, 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. 
Graphite-Epoxy Motors have proven themselves to be reliable; however they are not infallible. On 17 January 1997, a Delta II (Delta 241) exploded 13 seconds after launch due to a rupture in a graphite-epoxy casing. The failure was a result of the casing having been damaged at some point in the transportation of the vehicle. The investigation team found the damage to the motor case to be caused by the stress induced by the supports which held the GEM during storage and transportation. When the motor ignited, the pressure inside the casing built up until the damaged casing could not hold in the pressure of the burning fuel and exploded, destroying the launch vehicle. 
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