Graphite-Epoxy Motor

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Graphite-Epoxy Motor
Delta II GEM 40 Booster.jpg
A GEM-40 is hoisted for attachment to a Delta II
Manufacturer Northrop Grumman
Country of origin United States
Used on Delta II, Delta III, Delta IV, Atlas V (Future), Vulcan (future)
Launch history
Status Active
Length 11.4 m (449 in)
Diameter 1.0 m (40 in)
Gross mass 13,005 kg (28,671 lb)
Propellant mass 11,767 kg (25,942 lb)
Thrust 499 kN (112,200 lbf)
Specific impulse

245 s (sea level)

283 s (air-lit) [1]
Burn time 63 seconds
Length 12.6 m (495 in)
Diameter 1.2 m (46 in)
Gross mass 19,140 kg (42,196 lb)
Propellant mass 16,860 kg (37,180 lb)
Thrust 601 kN (135,200 lbf)
Specific impulse

242 s (sea level)

284 s (air-lit) [2]
Burn time 77 seconds
Length 13.2 m (518 in)
Diameter 1.5 m (60 in)
Gross mass 33,638 kg (74,158 lb)
Propellant mass 29,697 kg (65,471 lb)
Thrust 879 kN (197,500 lbf)
Specific impulse 245 s (sea level)
Burn time 91 seconds

A Graphite-Epoxy Motor (GEM) is a solid-fuel rocket motor (SRM) produced by Northrop Grumman[3] (Formerly Orbital ATK) with an epoxy composite casing. GEM boosters have been used on the Delta II, Delta III, and Delta IV, and are planned for future use on the Atlas V, Vulcan and Orbital ATK’s proposed Omega.


A Boeing Delta IV launching with two GEM-60 solid motors.
The GEM-40 is a 40-inch-diameter (1,000 mm) SRM used on Delta II beginning in 1990. The use of composite materials allowed for booster casings lighter than the steel casings of the Castor 4 SRMs they replaced.[4] The first flight of a GEM-40 occurred in 1990 on a Delta II 7925.[5] Delta II vehicles can use three, four, or nine GEM-40s. When using three or four boosters, all GEM-40s ignite on the ground, while on Delta IIs using nine boosters six are ignited on the ground, and the remaining three are ignited in the air when the first six burn out.[6]
The GEM-46 was a lengthened 46-inch-diameter (1,200 mm) solid motor originally developed for Delta III. This solid motor variant also included thrust vector control (TVC) to help steer the vehicle. After the discontinuation of the Delta III, GEM-46 motors (without TVC)[6] were used on the Delta II to create the Delta II Heavy, which could only be launched from a modified pad at Cape Canaveral.[7] Both Delta III and Delta II-Heavy used nine GEM-46s, with six ignited on the ground and three air-lit.[8][9]
The GEM-60 is a 60-inch-diameter (1,500 mm) solid motor used on the Delta IV family of launch vehicles. These motors are available with and without TVC.[6] A Delta IV can use two or four GEM-60s, and is classified as a Delta IV Medium+ launch vehicle.[10]
The GEM-63 is being developed as a drop-in replacement for the Aerojet Rocketdyne AJ-60A booster used on the Atlas V. The Atlas V will begin flying with the GEM-63 in 2019.[11] An extended GEM-63, the GEM-63XL (Which is about 5 feet longer than the regular GEM-63), is planned for use on the Vulcan launch vehicle in 2020.[12][13]
ULA CEO Tory Bruno stated that the reason for choosing the GEM-63 for the Atlas V and Vulcan is because it offers higher performance and almost half the cost of the AJ-60A boosters, currently being used on the Atlas V.[14]


On August 5, 1995, an air-lit GEM-40 failed to separate from a Delta II 7925 carrying Koreasat I. The excess mass of the booster resulted in the satellite reaching a lower than intended transfer orbit, which it was able to compensate for using on-board propellant.[15]

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's self-destruct function 13 seconds after ignition. An Air Force investigation determined that the motor's casing had been damaged prior to launch, resulting in the casing splitting open soon after ignition.[15]

See also[edit]


  1. ^ "GEM 40". Archived from the original on 8 November 2017. Retrieved 24 July 2014. 
  2. ^ "GEM 46". Archived from the original on 8 November 2017. Retrieved 24 July 2014. 
  3. ^ "Northrop Grumman GEM Capabilities". Northrop Grumman. 
  4. ^ "Launch Vehicle: Solid Rocket Motors". JPL. Retrieved July 24, 2014. 
  5. ^ "GEM 40". Astronautix. Archived from the original on November 8, 2017. Retrieved July 24, 2014. 
  6. ^ a b c "ATK Product Catalog" (PDF). ATK. Retrieved July 24, 2014. 
  7. ^ "ULA Delta II successfully lofts OCO-2 to orbit". Retrieved July 22, 2014. 
  8. ^ "Delta II 7920H-10". Spaceflight 101. Archived from the original on 2014-07-14. Retrieved July 24, 2014. 
  9. ^ "Delta III Data Sheet". Space Launch Report. Retrieved July 24, 2014. 
  10. ^ "Delta IV Medium+ (4,2)". Spaceflight 101. Retrieved July 24, 2014. 
  11. ^
  12. ^ "Orbital ATK beats out Aerojet". Retrieved September 23, 2015. 
  13. ^
  14. ^
  15. ^ a b "Delta 2 Productive Years". Space Launch Report. Retrieved July 24, 2014.