Graphite-Epoxy Motor

A GEM 40 solid rocket motor being prepared for integration with a Delta II launch vehicle
Manufacturer	Hercules (1990‍–‍1995) Alliant Techsystems (1995‍–‍2015) Orbital ATK (2015‍–‍2017) Northrop Grumman (2017‍–‍present)
Country of origin	United States
Used on	Ground-Based Interceptor Delta II Delta III Delta IV Atlas V Vulcan Centaur OmegA (Cancelled)
Associated stages
Comparable	AJ-60A Castor
Launch history
Status	Active
First flight	November 26, 1990

TheGraphite-Epoxy Motor (GEM) is a family ofsolid rocket boosters developed in the late 1980s and first flown in 1990. The motors use casings made fromcarbon-fiber-reinforced polymer and a propellant consisting ofammonium perchlorate composite propellant, formulated withhydroxyl-terminated polybutadiene as a binder,ammonium perchlorate as an oxidizer, andaluminum powder as a fuel.

Production of GEM motors has passed through several companies due tomergers and acquisitions. They were manufactured byHercules from 1990 to 1995,Alliant Techsystems from 1995 to 2015, andOrbital ATK from 2015 to 2017, before being taken over byNorthrop Grumman in 2017.

GEM boosters are currently used on theAtlas V andVulcan Centaur launch vehicles operated byUnited Launch Alliance (ULA), as well as theGround-Based Interceptor missile, and were previously flown on theDelta II,Delta III, andDelta IV.^[1] The motor names include numerals that denote the diameter of the booster in inches.

The GEM 63 is a 63.2-inch-diameter (1,610 mm) solid motor used on the Atlas V. It was developed byOrbital ATK as a "drop-in" replacement for theAJ-60A solid rocket booster built byAerojet 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.^[2] According to ULA, the GEM 63 provides higher performance at about half the cost of the AJ-60A.^[3]

The GEM 63XL, developed by Northrop Grumman, is an extended version of the GEM 63, about 73 inches (190 cm) longer.^[4] Each booster has a mass of about 117,000 pounds (53 metric tons).^[5] Static test firings began in 2020, and the booster entered service with theVulcan Centaur launch vehicle on its first flight on January 8, 2024.^[6][7][8] Up to six GEM 63XLs can be mounted on a Vulcan core, depending on mission requirements.^[9]

A variant equipped with athrust-vectoring nozzle, the GEM 63XLT, was under development for the cancelledOmegA launch vehicle.^[10]

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.^[11] The nozzle failure was later attributed to manufacturing defects.^[12]

In February 2026 another Vulcan mission experienced a GEM 63XL partial failure. A ULA vice president said, “Early during flight, the team observed a significant performance anomaly on one of the four solid rocket motors.”^[13]

The GEM 40 was a 40.4-inch-diameter (1,030 mm) solid rocket motor developed for the 7000-seriesDelta II launch vehicle beginning in 1987 byHercules.^[14] Its first flight took place in 1990 on the USA-66 mission,^[15] 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 theCastor 4 SRMs they replaced. The reduction in weight was used to extend the GEM 40 by 5.9 feet (1.8 m) compared to the Castor 4 used on 6000-series Delta II.^[14][16] 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.^[17] A variant with athrust-vectoring nozzle, the GEM 40VN, was developed for theGround-based Midcourse Defenseanti-ballistic missile program,^[18] but GMD later switched toOrion-50-based missile.

On August 5, 1995, an air-lit GEM 40 failed to separate from a Delta II 7925 carryingKoreasat 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.^[19]

On January 17, 1997, a Delta II (Delta 241) exploded due to a catastrophic failure in a GEM 40. The failure triggered the launch vehicle'sself-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.^[19]

The GEM 46 was a 45.1-inch-diameter (1,150 mm) solid rocket motor originally developed forDelta III byAlliant Techsystems. This solid motor variant included thrust vector control (TVC) to help steer the vehicle. After the discontinuation of the Delta III, GEM 46 motors (without TVC)^[17] were used on the Delta II to create the Delta II Heavy, which could only be launched from a modified pad atCape Canaveral Air Force Station,SLC-17B.^[20] Both Delta III and Delta II Heavy used nine GEM 46s, with six ignited on the ground and three air-lit.^[21]

On August 27, 1998, the GEM 46 boosters on the first Delta III, carrying the Galaxy 10 satellite, depleted their hydraulic fluid used to control the thrust-vectoring nozzle. This was due to guidance issues with the rest of the rocket, which forced the solid rocket motors to make rapid adjustments to compensate, using up the supply of hydraulic fluid before burnout. The nozzles were then stuck in a position that turned the rocket over, triggering the vehicle's self-destruct function 70 seconds after ignition.^[22][23]

The GEM 60 was a 60-inch-diameter (1,500 mm) solid motor used on theDelta IV family of launch vehicles, used with and without thrust vector control.^[17] Developed for theEELV program, its first flight was on November 20, 2002, boosting the first launch of the Delta IV.^[24] Delta IV Medium+ launchers were built with either two or four GEM 60.^[25] The added performance from the solid rocket motors allowed variants of the Delta IV Medium+ to accommodate a larger second stage. The motor was retired in 2019 after the final Delta IV Medium launch.^[26] Throughout its lifetime, 64 GEM 60 boosters were flown; there were no failures.^[26]

Data from Northrop Grumman catalog^[1]

• 
  A GEM 46 motor prior to mating to a Delta II 7925H
• 
  Technicians prepare a GEM 46 booster
• 
  GEM 40 booster is towed to the integration facility
• 
  A Delta IV M+ (4,2) lifts off with 2 GEM 60 boosters
• 
  A Delta II 7920H ignites 9 GEM 46 boosters
• 
  An Atlas V 541 lifts off with 4 GEM 63 boosters

• Solid rocket
• Spacecraft propulsion
• Composite overwrapped pressure vessel

• Solid-fuel rockets

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