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A Graphite-Epoxy Motor (GEM) is a high-performance, solid rocket motor, used for supplemental thrust on several launch vehicles, including the Boeing Delta II and Delta IV, as well as Lockheed Martin's Atlas 5. They are designed to allow launch vehicles to deliver larger payloads to orbit.
Background 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), solid motors used to produce extra thrust and boost payload capacity, such as the Thiokol Castor II and IV, used steel casings. This contained the high pressures of the burning solid fuel, but had a significant weight penalty. Designers realized that by reducing the mass of the casing, they could improve the thrust-to-weight ratio of the motor. The weight saved would allow for increased payload (since the same amount of solid fuel propelling a lighter casing allows more payload than a heavier casing propelled by the same amount of solid fuel). However, simply using thinner steel would not work, as the steel would be insufficiently strong to contain the burning fuel. This was a problem that prevented further weight reduction until the development of lightweight and strong composites. With the advent of new, lightweight composite materials in the late 1980s and early 1990s, solid motor designers finally had the material they needed to build a lighter and more powerful motor. They chose a graphite-epoxy composite, which was formed into the required casing thickness and shape using a computer-controlled filament winding machine. The resultant motor casing was significantly lighter than an equivalent steel-cased motor, as well as stronger. The first flight of a GEM occurred on 26 November 1990. Nine GEMs were used as boosters for a Delta II launch vehicle, launching a NAVSTAR GPS satellite. Variants Graphite-epoxy motors are made by different manufacturers in various sizes and capacities. Some of them include: Alliant Alliant manufactures GEMs for Boeing's Delta II, III, and IV launch vehicles in the following sizes: Aerojet Aerojet manufactures GEMs for Lockheed Martin's Atlas 5 family of launch vehicles. An Atlas 5 can carry one, two, three, four, or five GEMs, which help Atlas 5 carry heavier payloads. Reliability Graphite-Epoxy Motors have proven themselves to be very reliable. They have had a great success record; however they are not infallible. On 17 January 1997, a Delta II 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, either during manufacturing or installation - the investigation could not determine the exact cause of the damage. 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. See also Solid rocket Spacecraft propulsion | ||||||||
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