Graphite-Epoxy Motor
The Graphite-Epoxy Motor is a family of solid rocket boosters developed in the late 1980s and first flown in 1990. The motors use casings made from carbon-fiber-reinforced polymer and a propellant consisting of ammonium perchlorate composite propellant, formulated with hydroxyl-terminated polybutadiene as a binder, ammonium perchlorate as an oxidizer, and aluminum powder as a fuel.
Production of GEM motors has passed through several companies due to mergers and acquisitions. They were manufactured by Hercules from 1990 to 1995, Alliant Techsystems from 1995 to 2015, and Orbital ATK from 2015 to 2017, before being taken over by Northrop Grumman in 2017.
GEM boosters are currently used on the Atlas V and Vulcan Centaur launch vehicles operated by United Launch Alliance, as well as the Ground-Based Interceptor missile, and were previously flown on the Delta II, Delta III, and Delta IV. The motor names include numerals that denote the diameter of the booster in inches.
Active variants
GEM 63
The GEM 63 is a solid motor used on the Atlas V. It was developed by Orbital ATK as a "drop-in" replacement for the AJ-60A solid rocket booster built by Aerojet Rocketdyne and previously used with the Atlas V. Its dimensions are similar to those of the AJ-60A. The Atlas V first flew with the GEM 63 on the NROL-101 mission in 2020. According to ULA, the GEM 63 provides higher performance at about half the cost of the AJ-60A.GEM 63XL
The GEM 63XL, developed by Northrop Grumman, is an extended version of the GEM 63, about longer. Each booster has a mass of about. Static test firings began in 2020, and the booster entered service with the Vulcan Centaur launch vehicle on its first flight on January 8, 2024. Up to six GEM 63XLs can be mounted on a Vulcan core, depending on mission requirements.A variant equipped with a thrust-vectoring nozzle, the GEM 63XLT, was under development for the cancelled OmegA launch vehicle.
Anomaly
On October 4, 2024, a GEM 63XL experienced a partial failure 35 seconds after liftoff during the Vulcan Centaur Cert-2 mission. A change in the motor's exhaust plume and falling debris were observed. Analyses suggested that the nozzle was damaged or suffered a structural failure. Despite the anomaly, Vulcan Centaur Cert-2 reached orbit after burnout and separation of its two GEM 63XL boosters at about 2 minutes 10 seconds into flight. The nozzle failure was later attributed to manufacturing defects.Retired variants
GEM 40
The GEM 40 was a solid rocket motor developed for the 7000-series Delta II launch vehicle beginning in 1987 by Hercules. Its first flight took place in 1990 on the USA-66 mission, when 9 boosters were used on a Delta II 7925 launcher. The use of composite materials allowed for casings lighter than the steel casings of the Castor 4 SRMs they replaced. The reduction in weight was used to extend the GEM 40 by compared to the Castor 4 used on 6000-series Delta II. Delta II vehicles could be configured with three, four, or nine GEM 40 boosters. When using three or four boosters, all GEM 40s were ignited on the ground. On nine-booster Delta II, six were ignited on the ground; the remaining three were ignited in flight after burnout of the first six. A variant with a thrust-vectoring nozzle, the GEM 40VN, was developed for the Ground-based Midcourse Defense anti-ballistic missile program, but GMD later switched to Orion-50-based missile.Failures
On August 5, 1995, an air-lit GEM 40 failed to separate from a Delta II 7925 carrying Koreasat 1. The excess mass of the booster resulted in the satellite reaching a lower orbit than intended. The satellite was able to correct for the error using on-board propellant.On January 17, 1997, a Delta II exploded due to a catastrophic failure in a GEM 40. The failure triggered the launch vehicle's self-destruct function 13 seconds after ignition. An Air Force investigation determined that the motor's casing had been damaged prior to launch, causing the case to split open soon after ignition.