The Graphite-Epoxy Motor (GEM) is a series of solid rocket boosters fueled by HTPB and 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, and Delta IV. They will fly on the Atlas V and Vulcan.
The GEM-63XL is about 5 feet longer than the regular GEM-63 and will be used on the Vulcan launch vehicle starting in 2021. GEM-63XL offers higher performance at almost half the cost of the AJ-60A boosters currently being used on the Atlas V.
Video credit: Northrop Grumman