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 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. 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, approximately 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.

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. The nozzle failure was later attributed to manufacturing defects.

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".

Retired variants

GEM 40

thumb|upright=0.7|A GEM 40 is hoisted for attachment to a [[Delta II]]

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.

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.

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. 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. Both Delta III and Delta II Heavy used nine GEM 46s, with six ignited on the ground and three .

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.

GEM 60

alt=A GEM 60 solid rocket motor is seen laying horizontally on a trailer at Cape Canaveral's SLC-37B.|thumb|A GEM 60 being prepared for integration onto a [[Delta IV]]The GEM 60 was a solid motor used on the Delta IV family of launch vehicles, used with and without thrust vector control. Delta IV Medium+ launchers were built with either two or four GEM 60. 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. Throughout its lifetime, 64 GEM 60 boosters were flown; there were no failures.

{| class="wikitable" style="font-size:85%"

! scope="col" rowspan=2 | Name

! scope="col" rowspan=2 | Application

! scope="col" rowspan=2 | Length

! scope="col" rowspan=2 | Diameter

! scope="col" colspan=2 | Mass

! scope="col" rowspan=2 | Thrust

! scope="col" rowspan=2 | Specific impulse

! scope="col" rowspan=2 | Burn time ()

! rowspan="2" |First flight

! rowspan="2" |Final flight

|-

! scope="col" | Gross

! scope="col" | Propellant

|-

! GEM 40

| rowspan="2" | Delta II

|

| rowspan="3" |

|

| rowspan="3" |

|

|

| 63.3

| rowspan="2" | November 26, 1990

| rowspan="2" | September 15, 2018

|-

! GEM 40 (airlit)

|

|

|

|

| 63.3

|-

! GEM 40VN

| Ground-Based Interceptor

|

|

|

|

| 64.6

|

|

|-

! GEM 46

| rowspan="2" | Delta II Heavy, Delta III

|

| rowspan="3" |

|

| rowspan="3" |

|

|

| 75.9

| rowspan="3" | August 26, 1998

| rowspan="2" | September 10, 2011

|-

! GEM 46 (airlit)

|

|

|

|

| 75.9

|-

! GEM 46 (vectorable)

| Delta III

|

|

|

|

| 76.9

| August 23, 2000

|-

! GEM 60

| rowspan="2" | Delta IV M+

| rowspan="2" |

| rowspan="2" |

|

| rowspan="2" |

|

|

| rowspan="2" | 90.8

| rowspan="2" | November 20, 2002

| rowspan="2" | August 22, 2019

|-

! GEM 60 (vectorable)

|

|

|

|-

! GEM 63

| Atlas V

|

| rowspan="2" |

|

|

|

|

| 97.6

| November 13, 2020

|

|-

! GEM 63XL

| Vulcan Centaur

|

|

|

|

|

| 87.3

| January 8, 2024

|

|}

<gallery>

File:GEM 46 Booster of an Delta II 7925H rocket.jpg|alt=GEM 46 solid rocket booster is seen in the integration facility.|A GEM 46 motor prior to mating to a Delta II 7925H

File:Rocket0722 09.jpg|Technicians prepare a GEM 46 booster

File:GEM under tow prior to OCO-2 mission (KSC-2014-2126).jpg|GEM 40 booster is towed to the integration facility

File:Delta IV Medium 4,2+ launch with GOES-N.jpg|alt=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

File:Delta II Heavy ignition with GLAST.jpg|alt=A Delta II rocket ignites its engines, including 9 GEM 46 solid rockets.|A Delta II 7920H ignites 9 GEM 46 boosters

File:STP-3 Mission (211107-X-KD758-1002).jpg|An Atlas V 541 lifts off with 4 GEM 63 boosters

</gallery>

See also

  • Solid rocket
  • Spacecraft propulsion

References