Northrop Grumman dicit:
Northrop Grumman is developing the next generation of the GEM family of strap-on boosters to support intermediate- and large-class space launch vehicles. These boosters build on the company’s extensive history of the successful GEM 40, GEM 46, and GEM 60 motors. The GEM 63 and GEM 63XL motors will be capable of supporting national security, science and commercial payloads. The company is leveraging its industry-leading expertise in composite large solid rocket motor development and manufacturing to provide customers with a cost-effective, reliable, flexible, and on-time system, starting in 2019 for United Launch Alliance’s Atlas and Vulcan launch vehicles.
Video Credit: Northrop Grumman