Graphite-Epoxy Motor

{{Short description|American solid rocket booster}} {{Use mdy dates|date=September 2025}} {{Infobox rocket stage  |image = GEM 40 at SLC-2 for OCO-2.png |imsize = 300 |caption = A GEM 40 solid rocket motor being prepared for integration with a Delta II launch vehicle  |name = Graphite-Epoxy Motor |manufacturer = {{Ubli | [[Hercules Inc.|Hercules]] (1990{{nbndash}}1995) | [[Alliant Techsystems]] (1995{{nbndash}}2015) | [[Orbital Sciences Corporation|Orbital ATK]] (2015{{nbndash}}2017) | [[Northrop Grumman]] (2017{{nbndash}}present) }} |country-origin = [[United States]]  |rockets = {{hlist | [[Ground-Based Interceptor]] | [[Delta II]] | [[Delta III]] | [[Delta IV]] | [[Atlas V]] | [[Vulcan Centaur]] | [[OmegA]] (Cancelled) }} |derivatives =  |comparable = {{hlist | [[AJ-60A]] | [[Castor (rocket stage)|Castor]] }}  |status = Active |launches = |success = |fail = |noburn = |partial = |first = November 26, 1990 |last =  |only =  |payloads =  }}

The '''Graphite-Epoxy Motor''' ('''GEM''') is a family of [[solid rocket booster]]s developed in the late 1980s and first flown in 1990. The motors use casings made from [[Carbon-fiber-reinforced polymers|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 Inc.|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.<ref name="NG Propulsion Products Catalog" />

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.<ref name="NG Propulsion Products Catalog" />

== Active variants ==

=== GEM 63 === The GEM 63 is a {{convert|63.2|in|mm|adj=mid|-diameter}} 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.<ref>{{Cite web |title=NROL-101 |url=https://www.nro.gov/Launch/NROL-101/ |url-status=live |archive-url=https://web.archive.org/web/20220509130303/https://www.nro.gov/Launch/NROL-101/ |archive-date=May 9, 2022 |access-date=May 9, 2022 |website=www.nro.gov}}</ref> According to ULA, the GEM 63 provides higher performance at about half the cost of the AJ-60A.<ref>{{Cite tweet|number=981336075544100865|user=torybruno|title=Higher performance. Approaching half the cost.|author=Tory Bruno|date=April 3, 2018}}</ref>

=== GEM 63XL === The GEM 63XL, developed by Northrop Grumman, is an extended version of the GEM 63, approximately {{convert|73|in|cm}} longer.<ref>{{Cite web |title=GEM 63 Updates |url=https://www.northropgrumman.com/space/gem-63-updates |url-status=live |archive-url=https://web.archive.org/web/20220509130607/https://www.northropgrumman.com/space/gem-63-updates/ |archive-date=May 9, 2022 |access-date=May 7, 2022 |website=Northrop Grumman |language=en-US}}</ref> Each booster has a mass of about {{convert|117000|lb|metric ton|abbr=off}}.<ref>{{cite web |title=Launching the Vulcan Rocket For the First Time – Smarter Every Day 297 |url=https://www.youtube.com/watch?v=Bh7Xf3Ox7K8&t=508s |website=Smarter Every Day |publisher=YouTube |access-date=May 5, 2024 |language=en}}</ref> 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.<ref name=":5">{{Cite web |last=Foust |first=Jeff |date=July 13, 2023 |title=Centaur modifications push first Vulcan launch to fourth quarter |url=https://spacenews.com/centaur-modifications-push-first-vulcan-launch-to-fourth-quarter/ |access-date=August 2, 2023 |website=SpaceNews |language=en-US}}</ref><ref name=":3">{{Cite web |date=June 22, 2022 |title=ULA Vulcan Rocket Debut Slips To 2022 |url=https://aviationweek.com/defense-space/space/ula-vulcan-rocket-debut-slips-2022 |url-status=unfit |archive-url=https://web.archive.org/web/20220509125816/https://aviationweek.com/defense-space/space/ula-vulcan-rocket-debut-slips-2022 |archive-date=May 9, 2022 |website=Aviation Week}}</ref><ref>{{cite web |last=Clark |first=Stephen |date=September 22, 2015 |title=Orbital ATK beats out Aerojet |url=http://spaceflightnow.com/2015/09/22/orbital-atk-beats-out-aerojet-in-ula-booster-selection/ |url-status=live |archive-url=https://web.archive.org/web/20220509130705/https://spaceflightnow.com/2015/09/22/orbital-atk-beats-out-aerojet-in-ula-booster-selection/ |archive-date=May 9, 2022 |access-date=September 23, 2015}}</ref> Up to six GEM 63XLs can be mounted on a Vulcan core, depending on mission requirements.<ref>{{Cite web |title=Vulcan |url=https://www.ulalaunch.com/rockets/vulcan-centaur |url-status=live |archive-url=https://web.archive.org/web/20220509131011/https://www.ulalaunch.com/rockets/vulcan-centaur |archive-date=May 9, 2022 |access-date=May 7, 2022 |website=www.ulalaunch.com}}</ref>

A variant equipped with a [[Thrust vectoring|thrust-vectoring]] nozzle, the GEM 63XLT, was under development for the cancelled [[OmegA]] launch vehicle.<ref>{{Cite tweet |author=Northrop Grumman |user=northropgrumman |number=1197536657798520833 |date=November 21, 2019 |title=We've started winding our first GEM 63XLT! |access-date=May 9, 2022 |link=https://twitter.com/northropgrumman/status/1197536657798520833 }}</ref>

==== 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.<ref>{{Cite news |last=Foust |first=Jeff |date=October 4, 2024 |title=Vulcan competes second flight despite SRB anomaly |url=https://spacenews.com/vulcan-competes-second-flight-despite-srb-anomaly/ |access-date=September 4, 2025 |work=SpaceNews}}</ref> The nozzle failure was later attributed to manufacturing defects.<ref>{{Cite web |last=Foust |first=Jeff |date=March 12, 2025 |title=Manufacturing defect blamed for Vulcan solid rocket motor anomaly |url=https://spacenews.com/manufacturing-defect-blamed-for-vulcan-solid-rocket-motor-anomaly}}</ref>

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".<ref>{{Cite web |last=Clark |first=Stephen |date=2026-02-12 |title=ULA's Vulcan rocket suffers another booster problem on the way to orbit |url=https://arstechnica.com/space/2026/02/ulas-vulcan-launcher-still-has-a-solid-rocket-booster-problem/ |access-date=2026-04-17 |website=Ars Technica |language=en}}</ref>

== Retired variants ==

=== GEM 40 === [[File:Delta II GEM 40 Booster.jpg|thumb|upright=0.7|A GEM 40 is hoisted for attachment to a [[Delta II]]]] The GEM 40 was a {{convert|40.4|in|mm|adj=mid|-diameter}} solid rocket motor developed for the 7000-series [[Delta II]] launch vehicle beginning in 1987 by [[Hercules Inc.|Hercules]].<ref name=":2">{{Citation |last1=Vlahakis |first1=Nick |title=Graphite epoxy motors (GEM) for the Delta II launch vehicle |url=https://arc.aiaa.org/doi/abs/10.2514/6.1989-2313 |work=25th Joint Propulsion Conference |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.1989-2313 |access-date=May 7, 2022 |last2=Va |first2=Darryl|date=1989 |url-access=subscription }}</ref> Its first flight took place in 1990 on the USA-66 mission,<ref>{{Cite web |last=McDowell |first=Jonathan |date=May 7, 2022 |title=Launch Log |url=https://planet4589.org/space/gcat/data/derived/launchlog.html |url-status=live |archive-url=https://web.archive.org/web/20220507204819/https://planet4589.org/space/gcat/data/derived/launchlog.html |archive-date=May 7, 2022 |access-date=May 7, 2022 |website=Jonathan's Space Report}}</ref> 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 (rocket stage)|Castor 4]] SRMs they replaced. The reduction in weight was used to extend the GEM 40 by {{convert|1.8|m|ft|order=flip}} compared to the Castor 4 used on 6000-series Delta II.<ref name=":2" /><ref>{{cite web |title=Launch Vehicle: Solid Rocket Motors |url=http://mars.jpl.nasa.gov/mer/mission/launch_srm.html |url-status=live |archive-url=https://web.archive.org/web/20220509131137/https://mars.nasa.gov/mer/mission/launch_srm.html |archive-date=May 9, 2022 |access-date=July 24, 2014 |website=JPL}}</ref> 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.<ref name="atkp">{{cite web |title=ATK Product Catalog |url=http://www.ltas-vis.ulg.ac.be/cmsms/uploads/File/DataSheetSolidATK.pdf |url-status=dead |archive-url=https://web.archive.org/web/20180730082316/http://www.ltas-vis.ulg.ac.be/cmsms/uploads/File/DataSheetSolidATK.pdf |archive-date=July 30, 2018 |access-date=July 24, 2014 |website=ATK}}</ref>

==== 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.<ref name="slrd2p">{{cite web |last=Kyle |first=Ed |date=December 2, 2012 |title=Delta 2 Productive Years |url=http://www.spacelaunchreport.com/thorh14.html |url-status=dead |archive-url=https://web.archive.org/web/20220321061510/http://www.spacelaunchreport.com/thorh14.html |archive-date=March 21, 2022 |access-date=July 24, 2014 |website=Space Launch Report}}</ref>

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.<ref name="slrd2p" />

=== 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.<ref name=":1">{{Cite web |title=Northrop Grumman GEM Capabilities |url=https://www.northropgrumman.com/Capabilities/GEM/Pages/default.aspx |url-status=dead |archive-url=https://web.archive.org/web/20190203113001/https://www.northropgrumman.com/Capabilities/GEM/Pages/default.aspx |archive-date=February 3, 2019 |website=Northrop Grumman}}</ref> 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.<ref name="NG Propulsion Products Catalog" /> The program later switched to an [[Orion (rocket stage)|Orion 50]]-based missile.

=== GEM 46 === The GEM 46 was a {{convert|45.1|in|mm|adj=mid|-diameter}} 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)<ref name="atkp" /> were used on the Delta II to create the Delta II Heavy, which could only be launched from a modified pad at [[Cape Canaveral Space Force Station|Cape Canaveral Air Force Station]], [[Cape Canaveral Space Launch Complex 17|SLC-17B]].<ref>{{cite web |last=Graham |first=William |date=July 2, 2014 |title=ULA Delta II successfully lofts OCO-2 to orbit |url=http://www.nasaspaceflight.com/2014/07/delta-ii-oco-2-launch-vandenberg/ |url-status=live |archive-url=https://web.archive.org/web/20220509131516/https://www.nasaspaceflight.com/2014/07/delta-ii-oco-2-launch-vandenberg/ |archive-date=May 9, 2022 |access-date=July 22, 2014 |website=NASASpaceflight.com}}</ref> Both Delta III and Delta II Heavy used nine GEM 46s, with six ignited on the ground and three .<ref name=":0">{{Cite web|title=Propulsion Products Catalog GEM MOTOR SERIES pdf|url=https://www.northropgrumman.com/wp-content/uploads/GEM-Motor-Series.pdf}}</ref>

==== 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.<ref>{{Cite web |title=Boeing Pinpoints Cause of Delta III Failure, Predicts Timely Return to Flight |url=https://boeing.mediaroom.com/1998-09-05-Boeing-Pinpoints-Cause-of-Delta-III-Failure-Predicts-Timely-Return-to-Flight |access-date=May 7, 2022 |website=MediaRoom}}</ref><ref>{{Cite web |last=Furniss |first=Tim |date=September 1, 1998 |title=Boeing Delta III explodes on maiden flight |url=https://www.flightglobal.com/boeing-delta-iii-explodes-on-maiden-flight/22700.article |url-status=live |archive-url=https://web.archive.org/web/20220507213018/https://www.flightglobal.com/boeing-delta-iii-explodes-on-maiden-flight/22700.article |archive-date=May 7, 2022 |access-date=May 7, 2022 |website=FlightGlobal}}</ref> === GEM 60 === [[File:GEM-60 solid booster of Delta IV rocket.jpg|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 {{convert|60|in|mm|adj=mid|-diameter}} solid motor used on the [[Delta IV]] family of launch vehicles, used with and without thrust vector control.<ref name="atkp" /> Developed for the [[Evolved Expendable Launch Vehicle|EELV]] program, its first flight was on November 20, 2002, boosting the first launch of the Delta IV.<ref>{{Cite web |last=Ray |first=Justin |date=November 20, 2002 |title=Spaceflight Now {{!}} Delta Launch Report {{!}} Boeing's Delta 4 rocket debuts successfully |url=https://spaceflightnow.com/delta/delta4/021120launch/ |url-status=live |archive-url=https://web.archive.org/web/20220509131904/https://spaceflightnow.com/delta/delta4/021120launch/ |archive-date=May 9, 2022 |access-date=May 7, 2022 |website=spaceflightnow.com}}</ref> Delta IV Medium+ launchers were built with either two or four GEM 60.<ref>{{cite web |title=Delta IV Medium+ (4,2) |url=http://www.spaceflight101.com/delta-iv-medium-42.html |access-date=July 24, 2014 |website=Spaceflight 101}}</ref> 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.<ref name=":4">{{Cite web |last=Siegel |first=Jim |date=August 25, 2019 |title=Delta IV Medium ends 17-Year run with 100% success |url=https://www.spaceflightinsider.com/organizations/ula/delta-iv-medium-ends-17-year-run-with-100-success/ |url-status=live |archive-url=https://web.archive.org/web/20220509132208/https://www.spaceflightinsider.com/organizations/ula/delta-iv-medium-ends-17-year-run-with-100-success/ |archive-date=May 9, 2022 |access-date=May 9, 2022 |website=SpaceFlight Insider |language=en-US}}</ref> Throughout its lifetime, 64 GEM 60 boosters were flown; there were no failures.<ref name=":4" /> {{clear}}

== Version comparison == ''Data from'' Northrop Grumman catalog<ref name="NG Propulsion Products Catalog">{{Cite book |url=https://www.prd.ngc.agencyq.site/wp-content/uploads/NG-Propulsion-Products-Catalog.pdf |title=Propulsion Products Catalog |publisher=[[Northrop Grumman]] |pages=29-39 |access-date=October 24, 2024}}</ref>

{| 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 ({{abbr|sec.|in seconds}}) ! rowspan="2" |First flight ! rowspan="2" |Final flight |- ! scope="col" | Gross ! scope="col" | Propellant |- ! GEM 40 | rowspan="2" | [[Delta II]] | {{cvt|435|in|m|order=flip|0}} | rowspan="3" | {{cvt|40.4|in|m|order=flip}} | {{cvt|28,577|lb|order=flip}} | rowspan="3" | {{cvt|25,940|lb|order=flip}} | {{cvt|144,740|lb-f|kN|order=flip}} | {{cvt|274|isp}} | 63.3 | rowspan="2" | November 26, 1990 | rowspan="2" | September 15, 2018 |- ! GEM 40 (air{{nbhyph}}lit) | {{cvt|449.1|in|m|order=flip|0}} | {{cvt|28,883|lb|order=flip}} | {{cvt|149,660|lb-f|kN|order=flip}} | {{cvt|283.4|isp}} | 63.3 |- ! GEM 40VN | [[Ground-Based Interceptor]] | {{cvt|425.1|in|m|order=flip|0}} | {{cvt|28,886|lb|order=flip}} | {{cvt|107,625|lb-f|kN|order=flip}} | {{cvt|265.3|isp}} | 64.6 | | |- ! GEM 46 | rowspan="2" | [[Delta II Heavy]], [[Delta III]] | {{cvt|495.8|in|m|abbr=on|order=flip}} | rowspan="3" | {{cvt|45.1|in|m|order=flip}} | {{cvt|41,590|lb|order=flip}} | rowspan="3" | {{cvt|37,180|lb|order=flip}} | {{cvt|137,300|lb-f|kN|order=flip}} | {{cvt|277.8|isp}} | 75.9 | rowspan="3" | August 26, 1998 | rowspan="2" | September 10, 2011 |- ! GEM 46 (air{{nbhyph}}lit) | {{cvt|508.6|in|m|abbr=on|order=flip}} | {{cvt|42,039|lb|order=flip}} | {{cvt|206,000|lb-f|kN|order=flip}} | {{cvt|284|isp}} | 75.9 |- ! GEM 46 (vectorable) | [[Delta III]] | {{cvt|491.5|in|m|abbr=on|order=flip}} | {{cvt|42,196|lb|order=flip}} | {{cvt|196,600|lb-f|kN|order=flip}} | {{cvt|279.8|isp}} | 76.9 | August 23, 2000 |- ! GEM 60 | rowspan="2" | [[Delta IV|Delta IV M+]] | rowspan="2" | {{cvt|518|in|m|order=flip}} | rowspan="2" | {{cvt|60|in|m|order=flip}} | {{cvt|73,156|lb|order=flip}} | rowspan="2" | {{cvt|65,472|lb|order=flip}} | {{cvt|280,767|lb-f|kN|1|order=flip}} | {{cvt|275|isp|4=1}} | rowspan="2" | 90.8 | rowspan="2" | November 20, 2002 | rowspan="2" | August 22, 2019 |- ! GEM 60 (vectorable) | {{cvt|74,185|lb|order=flip}} | {{cvt|277,852|lb-f|kN|1|order=flip}} | {{cvt|274|isp|4=}} |- ! GEM 63 | [[Atlas V]] | {{cvt|792|in|m|order=flip}} | rowspan="2" | {{cvt|63.7|in|m|order=flip}} | {{cvt|108,781|lb|order=flip}} | {{cvt|97,195|lb|order=flip}} | {{cvt|370,835|lb-f|kN|1|order=flip}} | {{cvt|279.06|isp|4=3}} | 97.6 | November 13, 2020 | {{n/a}} |- ! GEM 63XL | [[Vulcan Centaur]] | {{cvt|865.3|in|m|order=flip|0}} | {{cvt|116,920|lb|order=flip}} | {{cvt|105,497|lb|order=flip}} | {{cvt|463,249|lb-f|kN|order=flip|0}} | {{cvt|280.3|isp}} | 87.3 | January 8, 2024 | {{n/a}} |}

== Gallery == <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 == {{Reflist}}

[[Category:Solid-fuel rockets]]