Graphite-Epoxy Motor


The Graphite-Epoxy Motor (GEM) is a family of solid rocket boosters first developed in the late 1980s and used from 1990 to the present day. GEM motors are manufactured with carbon-fibre-reinforced polymer casings and a fuel consisting of HTPB-bound ammonium perchlorate composite propellant. GEM is currently produced by Northrop Grumman Space Systems.[1] GEM boosters were previously used on the Delta II, Delta III, and Delta IV launch vehicles, and are currently used on the Atlas V. A new variant, the GEM 63XL, is slated to fly as part of the Vulcan Centaur launch vehicle no earlier than the 1st quarter of 2023.[2]

Graphite-Epoxy Motor
GEM 40 at SLC-2 for OCO-2.png
A GEM 40 solid rocket motor being prepared for integration with a Delta II launch vehicle
Manufacturer Hercules
Alliant Techsystems
Orbital ATK
Northrop Grumman Space Systems
Country of originUnited States
Used onGround-Based Interceptor, Delta II, Delta III, Delta IV, Atlas V, Vulcan (future)
Associated stages
ComparableCastor (rocket stage)
Launch history
First flight26 November 1990



GEM 63Edit

The GEM 63 was developed by Orbital ATK as a low-cost drop-in replacement for the Aerojet Rocketdyne AJ-60A solid rocket booster used on the Atlas V. Its overall dimensions are very similar to that of the motor it replaces. The Atlas V first flew with the GEM 63 in 2020 on the NROL-101 launch.[3] The booster offers higher performance at about half the cost of the AJ-60A boosters formerly used on the Atlas V.[4] The change to GEM motors also allows a degree of commonality between the Atlas V and Delta IV, which are both produced and marketed by the United Launch Alliance.[5]

GEM 63XLEdit

The GEM 63XL, developed by Northrop Grumman, is an extended version of the GEM 63, about 73 inches (185 cm) longer than its predecessor.[6] First fired in 2020, it will be used on the Vulcan launch vehicle beginning no earlier than 2022.[7][8] Up to 6 of the boosters will be mounted on a single Vulcan core, depending on customer needs.[9] A variant equipped with a thrust-vectoring nozzle, the GEM 63XLT, was under development for the cancelled OmegA launch vehicle.[10]


GEM 40Edit

A GEM 40 is hoisted for attachment to a Delta II

The GEM 40 was a 40.4-inch-diameter (1,030 mm) solid rocket motor developed for the 7000-series Delta II launch vehicle beginning in 1987 by Hercules.[11] Its first flight took place in 1990 on the USA-66 mission,[12] 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 1,8 meter (6 feet) compared to the Castor 4 used on 6000-series Delta II.[11][13] 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.[14] A variant with a thrust-vectoring nozzle, the GEM 40VN, was developed for the Ground-based Midcourse Defence anti-ballistic missile program.[1]


On 5 August, 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.[15]

On 17 January, 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, causing the case to split open soon after ignition.[15]

GEM 46Edit

The GEM 46 was a 45.1-inch-diameter (1,150 mm) solid rocket motor originally developed for Delta III by Alliant 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)[14] were used on the Delta II to create the Delta II Heavy, which could only be launched from a modified pad at Cape Canaveral Air Force Station, SLC-17B.[16] Both Delta III and Delta II Heavy used nine GEM 46s, with six ignited on the ground and three air-lit.[17]


On 27 August, 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.[18][19]

A GEM 60 being prepared for integration onto a Delta IV

GEM 60Edit

The GEM 60 was a 60-inch-diameter (1,500 mm) solid motor used on the Delta IV family of launch vehicles, used with and without thrust vector control.[14] Developed for the EELV program, its first flight was on 20 November 2002, boosting the first launch of the Delta IV.[20] Delta IV Medium+ launchers were built with either two or four GEM 60.[21] 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.[22] Throughout its lifetime, 64 GEM 60 boosters were flown; there were no failures.[22]

Version comparisonEdit

Name Application Length Diameter Mass Thrust Specific
Propellant First flight Final flight
Gross Propellant
GEM 40 Delta II 12.96 m (510 in) 1.03 m (40.4 in) 13,005 kg (28,671 lb) 11,767 kg (25,942 lb) 499 kN (112,200 lbf) 245 s (SL)
283 s (air-lit) [17]
63 s HTPB APCP 26 November 1990 15 September 2018
GEM 46 Delta III, Delta II 14.70 m (579 in) 1.15 m (45.1 in) 19,140 kg (42,196 lb) 16,860 kg (37,180 lb) 601 kN (135,200 lbf) 242 s (SL)
284 s (air-lit) [17]
77 s 26 August 1998 10 September 2011
GEM 60 Delta IV M+ 13.2 m (518 in) 1.5 m (60 in) 33,638 kg (74,158 lb) 29,697 kg (65,471 lb) 879 kN (197,500 lbf) 245 s (SL)[17] 91 s 20 November 2002 22 August 2019
GEM 63[23] Atlas V 20.1 m (792 in) 1.61 m (63.2 in) 49,300 kg (108,600 lb) 44,200 kg (97,500 lb) 1,663 kN (373,800 lbf) 279 s (Vacuum) [17] 94 s 13 November 2020 N/A
GEM 63XL[24] Vulcan Centaur 22.0 m (865 in) 53,400 kg (117,700 lb) 48,000 kg (105,800 lb) 2,026 kN (455,400 lbf) 280 s (Vacuum) [17] 84 s NET 2023 N/A


See alsoEdit


  1. ^ a b "Northrop Grumman GEM Capabilities". Northrop Grumman. Archived from the original on 2019-02-03.
  2. ^ "ULA Sets Path Forward for Inaugural Vulcan Flight Test". Retrieved 2022-12-21.
  3. ^ "NROL-101". Archived from the original on 2022-05-09. Retrieved 2022-05-09.
  4. ^ Tory Bruno [@torybruno] (3 April 2018). "Higher performance. Approaching half the cost" (Tweet) – via Twitter.
  5. ^ "First GEM 63 solid motors delivered to Cape Canaveral". SpaceFlight Insider. 2020-07-24. Archived from the original on 2022-05-09. Retrieved 2022-05-09.
  6. ^ "GEM 63 Updates". Northrop Grumman. Archived from the original on 2022-05-09. Retrieved 2022-05-07.
  7. ^ "ULA Vulcan Rocket Debut Slips To 2022". Aviation Week. 2022-06-22. Archived from the original on 2022-05-09.{{cite web}}: CS1 maint: unfit URL (link)
  8. ^ Clark, Stephen (2015-09-22). "Orbital ATK beats out Aerojet". Archived from the original on 2022-05-09. Retrieved September 23, 2015.
  9. ^ "Vulcan". Archived from the original on 2022-05-09. Retrieved 2022-05-07.
  10. ^ Northrop Grumman [@northropgrumman] (2019-11-21). "We've started winding our first GEM 63XLT!" (Tweet). Retrieved 2022-05-09 – via Twitter.{{cite web}}: CS1 maint: url-status (link)
  11. ^ a b Vlahakis, Nick; Va, Darryl, "Graphite epoxy motors (GEM) for the Delta II launch vehicle", 25th Joint Propulsion Conference, American Institute of Aeronautics and Astronautics, doi:10.2514/6.1989-2313, retrieved 2022-05-07
  12. ^ McDowell, Jonathan (2022-05-07). "Launch Log". Jonathan's Space Report. Archived from the original on 2022-05-07. Retrieved 2022-05-07.
  13. ^ "Launch Vehicle: Solid Rocket Motors". JPL. Archived from the original on 2022-05-09. Retrieved July 24, 2014.
  14. ^ a b c "ATK Product Catalog" (PDF). ATK. Archived from the original (PDF) on July 30, 2018. Retrieved July 24, 2014.
  15. ^ a b Kyle, Ed (2012-12-02). "Delta 2 Productive Years". Space Launch Report. Archived from the original on 2022-03-21. Retrieved July 24, 2014.
  16. ^ Graham, William (2014-07-02). "ULA Delta II successfully lofts OCO-2 to orbit". Archived from the original on 2022-05-09. Retrieved July 22, 2014.
  17. ^ a b c d e f "Propulsion Products Catalog GEM MOTOR SERIES pdf" (PDF).{{cite web}}: CS1 maint: url-status (link)
  18. ^ "Boeing Pinpoints Cause of Delta III Failure, Predicts Timely Return to Flight". MediaRoom. Retrieved 2022-05-07.
  19. ^ Furniss, Tim (1998-09-01). "Boeing Delta III explodes on maiden flight". FlightGlobal. Archived from the original on 2022-05-07. Retrieved 2022-05-07.
  20. ^ Ray, Justin (2002-11-20). "Spaceflight Now | Delta Launch Report | Boeing's Delta 4 rocket debuts successfully". Archived from the original on 2022-05-09. Retrieved 2022-05-07.
  21. ^ "Delta IV Medium+ (4,2)". Spaceflight 101. Retrieved July 24, 2014.
  22. ^ a b Siegel, Jim (2019-08-25). "Delta IV Medium ends 17-Year run with 100% success". SpaceFlight Insider. Archived from the original on 2022-05-09. Retrieved 2022-05-09.
  23. ^ "GEM 63/GEM 63XL Fact Sheet" (PDF). Northrop Grumman. 5 April 2016. Archived from the original (PDF) on 18 September 2018. Retrieved 18 September 2018.
  24. ^ "Developing Vulcan Centaur" (PDF). 8 April 2019. Archived from the original (PDF) on 25 August 2019. Retrieved 24 August 2019.