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), 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.
The first flight of a GEM occurred on 26 November 1990. Nine GEMs were used as boosters for a Delta II launch vehicle (Delta 201), launching a NAVSTAR GPS satellite.
Read more about this topic: Graphite-Epoxy Motor
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