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The Graphite-Epoxy Motor (GEM) 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 (ULA), as well as the Ground-Based Interceptor missile, and were previously flown on the Delta II, Delta III, and Delta IV.[1] The motor names include numerals that denote the diameter of the booster in inches.

GEM boosters are used on the Atlas V and Vulcan Centaur launch vehicles operated by United Launch Alliance (ULA), as well as the Ground-Based Interceptor missile, and were previously flown on the Delta II, Delta III, and Delta IV. Motor designations include numerals indicating booster diameter in inches.[1]

The series originated with the GEM 40 for the Delta II, followed by the larger GEM 46 for the Delta III and Delta II Heavy, which increased length and diameter and introduced optional thrust vectoring nozzles. The GEM 60 was subsequently developed for the Delta IV, providing additional liftoff thrust for Medium+ configurations with fixed or vectorable nozzles. Current variants include the GEM 63 for the Atlas V and the GEM 63XL for the Vulcan Centaur.[1]

Active variants

GEM 63

The GEM 63 is a 63.2-inch-diameter (1,610 mm) 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. 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]

GEM 63XL

The GEM 63XL, developed by Northrop Grumman, is an extended version of the GEM 63, approximately 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 the Vulcan 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 a thrust-vectoring nozzle, the GEM 63XLT, was under development for the cancelled OmegA launch vehicle.[10]

Anomalies

On October 4, 2024, a GEM 63XL experienced a partial failure about 35 seconds after liftoff during the Vulcan Centaur Cert-2 mission. change in the motor’s exhaust plume and debris falling from the vehicle were observed. Analyses indicated damage or structural failure of the nozzle. Despite the anomaly, the mission reached orbit after burnout and separation of the two 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 performance anomaly affecting one of four solid rocket motors. A ULA vice president stated that “early during flight, the team observed a significant performance anomaly on one of the four solid rocket motors”.[13]

Retired variants

GEM 40

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.[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 the Castor 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]

Failures

On August 5, 1995, an 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.[18]

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, causing the case to split open soon after ignition.[18]

GEM 40VN

The GEM 40VN, a thrust-vectoring variant of the GEM 40, was developed for the Ground-Based Midcourse Defense (GMD) anti-ballistic missile program.[19] It uses the same basic motor configuration as the GEM 40, with a modified nozzle assembly providing up to ±6-degree thrust vector control. The motor can be configured for in-line or strap-on applications, and an extended nozzle is available for air ignition. A version was qualified for use on the Boost Vehicle/Boost Vehicle Plus (BV/BV+) configurations of the GMD interceptor program.[1] The program later switched to an Orion 50-based missile.

GEM 46

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)[17] 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.[20] Both Delta III and Delta II Heavy used nine GEM 46s, with six ignited on the ground and three .[21]

Failures

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]

GEM 60

A GEM 60 solid rocket motor is seen laying horizontally on a trailer at Cape Canaveral's SLC-37B.
A GEM 60 being prepared for integration onto a Delta IV

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.[17] Developed for the EELV 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]

Version comparison

Data from Northrop Grumman catalog[1]

Name Application Length Diameter Mass Thrust Specific impulse Burn time (sec.) First flight Final flight
Gross Propellant
GEM 40 Delta II 11 m (435 in) 1.03 m (40.4 in) 12,962 kg (28,577 lb) 11,770 kg (25,940 lb) 643.8 kN (144,740 lbf) 274 s (2.69 km/s) 63.3 November 26, 1990 September 15, 2018
GEM 40 (air‑lit) 11 m (449.1 in) 13,101 kg (28,883 lb) 665.7 kN (149,660 lbf) 283.4 s (2.779 km/s) 63.3
GEM 40VN Ground-Based Interceptor 11 m (425.1 in) 13,102 kg (28,886 lb) 478.74 kN (107,625 lbf) 265.3 s (2.602 km/s) 64.6
GEM 46 Delta II Heavy, Delta III 12.59 m (495.8 in) 1.15 m (45.1 in) 18,860 kg (41,590 lb) 16,860 kg (37,180 lb) 611 kN (137,300 lbf) 277.8 s (2.724 km/s) 75.9 August 26, 1998 September 10, 2011
GEM 46 (air‑lit) 12.92 m (508.6 in) 19,069 kg (42,039 lb) 920 kN (206,000 lbf) 284 s (2.79 km/s) 75.9
GEM 46 (vectorable) Delta III 12.48 m (491.5 in) 19,140 kg (42,196 lb) 875 kN (196,600 lbf) 279.8 s (2.744 km/s) 76.9 August 23, 2000
GEM 60 Delta IV M+ 13.2 m (518 in) 1.5 m (60 in) 33,183 kg (73,156 lb) 29,698 kg (65,472 lb) 1,248.9 kN (280,767 lbf) 275 s (2.7 km/s) 90.8 November 20, 2002 August 22, 2019
GEM 60 (vectorable) 33,650 kg (74,185 lb) 1,235.9 kN (277,852 lbf) 274 s (2.69 km/s)
GEM 63 Atlas V 20.1 m (792 in) 1.62 m (63.7 in) 49,342 kg (108,781 lb) 44,087 kg (97,195 lb) 1,649.6 kN (370,835 lbf) 279.06 s (2.737 km/s) 97.6 November 13, 2020 N/a
GEM 63XL Vulcan Centaur 22 m (865.3 in) 53,030 kg (116,920 lb) 47,853 kg (105,497 lb) 2,061 kN (463,249 lbf) 280.3 s (2.749 km/s) 87.3 January 8, 2024 N/a
  • GEM 46 solid rocket booster is seen in the integration facility.
    A GEM 46 motor prior to mating to a Delta II 7925H
  • Technicians prepare a GEM 46 booster
    Technicians prepare a GEM 46 booster
  • GEM 40 booster is towed to the integration facility
    GEM 40 booster is towed to the integration facility
  • A Delta IV rocket blasts off with the help of GEM 60 solid rocket motors.
    A Delta IV M+ (4,2) lifts off with 2 GEM 60 boosters
  • A Delta II rocket ignites its engines, including 9 GEM 46 solid rockets.
    A Delta II 7920H ignites 9 GEM 46 boosters
  • An Atlas V 541 lifts off with 4 GEM 63 boosters
    An Atlas V 541 lifts off with 4 GEM 63 boosters

See also

References

  1. ^ a b c d e Propulsion Products Catalog (PDF). Northrop Grumman. pp. 29–39. Retrieved October 24, 2024.
  2. ^ “NROL-101”. www.nro.gov. Archived from the original on May 9, 2022. Retrieved May 9, 2022.
  3. ^ Tory Bruno [@torybruno] (April 3, 2018). “Higher performance. Approaching half the cost” (Tweet) – via Twitter.
  4. ^ “GEM 63 Updates”. Northrop Grumman. Archived from the original on May 9, 2022. Retrieved May 7, 2022.
  5. ^ “Launching the Vulcan Rocket For the First Time – Smarter Every Day 297”. Smarter Every Day. YouTube. Retrieved May 5, 2024.
  6. ^ Foust, Jeff (July 13, 2023). “Centaur modifications push first Vulcan launch to fourth quarter”. SpaceNews. Retrieved August 2, 2023.
  7. ^ “ULA Vulcan Rocket Debut Slips To 2022”. Aviation Week. June 22, 2022. Archived from the original on May 9, 2022.
  8. ^ Clark, Stephen (September 22, 2015). “Orbital ATK beats out Aerojet”. Archived from the original on May 9, 2022. Retrieved September 23, 2015.
  9. ^ “Vulcan”. www.ulalaunch.com. Archived from the original on May 9, 2022. Retrieved May 7, 2022.
  10. ^ Northrop Grumman [@northropgrumman] (November 21, 2019). “We’ve started winding our first GEM 63XLT!” (Tweet). Retrieved May 9, 2022 – via Twitter.
  11. ^ Foust, Jeff (October 4, 2024). “Vulcan competes second flight despite SRB anomaly”. SpaceNews. Retrieved September 4, 2025.
  12. ^ Foust, Jeff (March 12, 2025). “Manufacturing defect blamed for Vulcan solid rocket motor anomaly”.
  13. ^ Clark, Stephen (February 12, 2026). “ULA’s Vulcan rocket suffers another booster problem on the way to orbit”. Ars Technica. Retrieved April 17, 2026.
  14. ^ a b Vlahakis, Nick; Va, Darryl (1989), “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 May 7, 2022
  15. ^ McDowell, Jonathan (May 7, 2022). “Launch Log”. Jonathan’s Space Report. Archived from the original on May 7, 2022. Retrieved May 7, 2022.
  16. ^ “Launch Vehicle: Solid Rocket Motors”. JPL. Archived from the original on May 9, 2022. Retrieved July 24, 2014.
  17. ^ a b c “ATK Product Catalog” (PDF). ATK. Archived from the original (PDF) on July 30, 2018. Retrieved July 24, 2014.
  18. ^ a b Kyle, Ed (December 2, 2012). “Delta 2 Productive Years”. Space Launch Report. Archived from the original on March 21, 2022. Retrieved July 24, 2014.
  19. ^ “Northrop Grumman GEM Capabilities”. Northrop Grumman. Archived from the original on February 3, 2019.
  20. ^ Graham, William (July 2, 2014). “ULA Delta II successfully lofts OCO-2 to orbit”. NASASpaceflight.com. Archived from the original on May 9, 2022. Retrieved July 22, 2014.
  21. ^ “Propulsion Products Catalog GEM MOTOR SERIES pdf” (PDF).
  22. ^ “Boeing Pinpoints Cause of Delta III Failure, Predicts Timely Return to Flight”. MediaRoom. Retrieved May 7, 2022.
  23. ^ Furniss, Tim (September 1, 1998). “Boeing Delta III explodes on maiden flight”. FlightGlobal. Archived from the original on May 7, 2022. Retrieved May 7, 2022.
  24. ^ Ray, Justin (November 20, 2002). “Spaceflight Now | Delta Launch Report | Boeing’s Delta 4 rocket debuts successfully”. spaceflightnow.com. Archived from the original on May 9, 2022. Retrieved May 7, 2022.
  25. ^ “Delta IV Medium+ (4,2)”. Spaceflight 101. Retrieved July 24, 2014.
  26. ^ a b Siegel, Jim (August 25, 2019). “Delta IV Medium ends 17-Year run with 100% success”. SpaceFlight Insider. Archived from the original on May 9, 2022. Retrieved May 9, 2022.