Editing SpaceX Merlin (section)

==Revisions==
[[File:SpaceX Merlin engine.gif|thumb|left|upright=0.5|SpaceX Merlin 1A]]

===Merlin 1A===
The initial version, the '''Merlin 1A''', used an inexpensive, expendable, [[Ablation|ablatively cooled]] [[carbon-fiber-reinforced polymer]] [[Composite material|composite]] nozzle and produced {{cvt|340|kN|lbf}} of thrust.
The Merlin 1A flew only twice: first on March 24, 2006, when it caught fire and failed due to a fuel leak shortly after launch,<ref name="NTmuf">{{cite news |first=Brian |last=Berger |url=http://www.space.com/missionlaunches/060719_falcon1_update.html |title=Falcon 1 Failure Traced to a Busted Nut |date=July 19, 2006 |publisher=Space.com |access-date=August 2, 2008 |archive-date=June 4, 2010 |archive-url=https://web.archive.org/web/20100604062301/http://www.space.com/missionlaunches/060719_falcon1_update.html |url-status=live }}</ref><ref name="cFfqL">{{cite news |archive-url=https://web.archive.org/web/20130303034924/http://www.spacex.com/updates_archive.php?page=0606-1206#Flight1_update |archive-date=March 3, 2013 |url-status=dead |url=http://www.spacex.com/updates_archive.php?page=0606-1206#Flight1_update |title=Findings of the Falcon return to flight board |date=July 25, 2006 |publisher=SpaceX.com}}</ref> and the second time on March 21, 2007, when it performed successfully.<ref name="MkWhi">{{cite web |title=Demo Flight 2 Flight Review Update |date=June 15, 2007 |publisher=SpaceX |archive-url=https://web.archive.org/web/20120306015017/http://www.spacex.com/F1-DemoFlight2-Flight-Review.pdf |archive-date=March 6, 2012 |url-status=dead |url=http://www.spacex.com/F1-DemoFlight2-Flight-Review.pdf }}</ref> Both times the Merlin&nbsp;1A was mounted on a [[Falcon&nbsp;1]] first stage.<ref name="merlin_wired">{{cite web |first=Loretta Hidalgo |last=Whitesides |url=http://blog.wired.com/wiredscience/2007/11/spacex-complete.html |title=SpaceX Completes Development of Rocket Engine for Falcon 1 and 9 |publisher=Wired Science |date=November 12, 2007 |access-date=February 28, 2008 |archive-date=March 23, 2008 |archive-url=https://web.archive.org/web/20080323162937/http://blog.wired.com/wiredscience/2007/11/spacex-complete.html |url-status=live }}</ref><ref name="merlin_nasaspaceflight">{{cite web |first=Braddock |last=Gaskill |url=http://www.nasaspaceflight.com/2006/08/spacex-has-magical-goals-for-falcon-9/ |title=SpaceX has magical goals for Falcon 9 |publisher=Nasa Spaceflight |date=August 5, 2006 |access-date=February 28, 2008 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304024753/http://www.nasaspaceflight.com/2006/08/spacex-has-magical-goals-for-falcon-9/ |url-status=live }}</ref>

The SpaceX turbopump was an entirely new, clean-sheet design contracted to Barber-Nichols, Inc. in 2002, who performed all design, engineering analysis, and construction; the company had previously worked on turbopumps for the [[RS-88]] (Bantam) and NASA [[Fastrac (rocket engine)|Fastrac]] engine programs. The Merlin&nbsp;1A turbopump used a unique [[Friction welding|friction-welded]] main shaft, with [[Inconel]]&nbsp;718 ends and an integral aluminum RP-1 impeller in the middle. The turbopump housing was constructed using [[investment casting]]s, with Inconel at the turbine end, aluminum in the center, and 300-series stainless steel at the LOX end. The turbine was a partial-admission (i.e., working fluid is only admitted through part of the rotation of the turbine; an arc, not the whole circumference) impulse design and turned at up to 20,000&nbsp;rpm, with a total weight of {{cvt|150|lb|kg|order=flip}}.{{citation needed|date=April 2017}}

===Merlin 1B===
The '''Merlin 1B''' rocket engine was an upgraded version of the Merlin&nbsp;1A engine. The turbopump upgrades were handled by Barber-Nichols, Inc. for SpaceX.<ref name="BNI">{{cite web |url=https://www.barber-nichols.com/products/rocket-engine-turbopumps |title=Merlin LOX/RP-1 Turbopump |publisher=Barber Nichols |access-date=May 28, 2018 |archive-date=May 29, 2018 |archive-url=https://web.archive.org/web/20180529203257/https://www.barber-nichols.com/products/rocket-engine-turbopumps |url-status=live }}</ref> It was intended for [[Falcon&nbsp;1]] launch vehicles, capable of producing {{cvt|85000|lbf|kN|order=flip}} of thrust at sea level and {{cvt|95000|lbf|kN|order=flip}} in vacuum, and performing with a specific impulse of {{cvt|261|isp|km/s}} at sea level and {{cvt|303|isp|km/s}} in vacuum.

The Merlin 1B was enhanced over the 1A with a [[turbine]] upgrade, increasing power output from {{cvt|2000|hp|kW|order=flip}} to {{cvt|2500|hp|kW|order=flip}}.<ref name="feb05update">{{cite web |archive-url=https://web.archive.org/web/20080415015952/http://www.spacex.com/updates_archive.php?page=0205-0505 |archive-date=April 15, 2008 |url-status=dead |url=http://www.spacex.com/updates_archive.php?page=0205-0505 |publisher=SpaceX |author=Elon Musk |title=Feb 2005 through May 2005 Update}}</ref> The turbine upgrade was accomplished by adding additional nozzles, turning the previously partial-admission design to full admission. Slightly enlarged impellers for both RP-1 and LOX were part of the upgrade. This model turned at a faster 22,000&nbsp;rpm and developed higher discharge pressures. Turbopump weight was unchanged at {{cvt|150|lb|kg|order=flip}}.<ref name="BNI"/>
Another notable change over the 1A was the move to [[Triethylaluminium|TEA]]–[[Triethylborane|TEB]] ([[pyrophoric]]) ignition over torch ignition.<ref name="feb05update"/>

Initial use of the Merlin 1B was to be on the [[Falcon&nbsp;9]] launch vehicle, on whose first stage there would have been a cluster of nine of these engines. Due to experience from the Falcon&nbsp;1's first flight, SpaceX moved its Merlin development to the Merlin&nbsp;1C, which is regeneratively cooled. Therefore, the Merlin&nbsp;1B was never used on a launch vehicle.<ref name="merlin_wired"/><ref name="merlin_nasaspaceflight"/>
{{clear}}

===Merlin 1C===
{{Infobox rocket engine
| name                       = Merlin 1C
| country_of_origin          = [[United States]]
| manufacturer               = [[SpaceX]]
| purpose                    = {{plainlist|
* [[Booster (rocketry)|Booster stage]] engines
* [[Upper stage]] engine
}}
| associated                 = [[Falcon 1]], [[Falcon 9]]
| status                     = Retired
| type                       = liquid
| oxidiser                   = [[LOX]]
| fuel                       = [[RP-1]]
| cycle                      = [[Gas-generator cycle|Gas-generator]]
| thrust(Vac)                = {{nowrap| {{cvt|480|kN}}<ref name="BWire_Regen"/> }}
| thrust(SL)                 = {{cvt|420|kN}}<ref name="BWire_Regen">{{cite news |url=http://home.businesswire.com/portal/site/google/index.jsp?ndmViewId=news_view&newsId=20071112005019&newsLang=en |title=SpaceX Completes Development of Merlin Regeneratively Cooled Rocket Engine |publisher=Business Wire |date=November 13, 2007 |access-date=November 12, 2007 |archive-date=January 3, 2008 |archive-url=https://web.archive.org/web/20080103164100/http://www.businesswire.com/portal/site/google/index.jsp?ndmViewId=news_view |url-status=live }}</ref>
| throttle_range             = 
| thrust_to_weight           = 96
| chamber_pressure           = {{cvt|6.77|MPa|psi}}<ref name="NSPO"/>
| specific_impulse_vacuum    = {{cvt|304.8|isp}}<ref name="NSPO">{{cite conference |url=http://www2.nspo.org.tw/ASC2008/4th%20Asian%20Space%20Conference%202008/oral/S12-11.pdf |title=Low-cost Launch Opportunities Provided by the Falcon Family of Launch Vehicles |conference=Fourth Asian Space Conference |location=Taipei |first1=Aaron |last1=Dinardi |first2=Peter |last2=Capozzoli |first3=Gwynne |last3=Shotwell |year=2008 |archive-url=https://web.archive.org/web/20120315135217/http://www2.nspo.org.tw/ASC2008/4th%20Asian%20Space%20Conference%202008/oral/S12-11.pdf |archive-date=March 15, 2012 |url-status=dead}}</ref>
| specific_impulse_sea_level = {{cvt|275|isp}}
| length                     = {{cvt|2.92|m}}<ref name="iacse.commercial-space.net">{{cite web |url=http://iacse.commercial-space.net/wp-content/uploads/2008/10/iac-08d213.pdf |title=The SpaceX Falcon 1 Launch Vehicle Flight 3 Results, Future Developments, and Falcon 9 Evolution |access-date=December 29, 2012 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304062239/http://iacse.commercial-space.net/wp-content/uploads/2008/10/iac-08d213.pdf |url-status=live }}</ref>
| diameter                   = 
| dry_weight                 = {{cvt|1380|lb|kg|order=flip}}
}}

[[File:SpaceX factory Merlin engine.jpg|thumb|left|Merlin 1C under construction at SpaceX]]
Three versions of the '''Merlin 1C''' engine were produced. The Merlin engine for Falcon&nbsp;1 had a movable turbopump exhaust assembly, which was used to provide roll control by vectoring the exhaust. The Merlin&nbsp;1C engine for the Falcon&nbsp;9 first stage is nearly identical to the variant used for the Falcon&nbsp;1, although the turbopump exhaust assembly is not movable. Finally, a Merlin&nbsp;1C [[vacuum]] variant is used on the Falcon&nbsp;9 second stage. This engine differs from the Falcon&nbsp;9 first-stage variant in that it uses a larger exhaust nozzle optimized for vacuum operation and can be throttled between 60% and 100%.<ref name="NSPO"/>

The Merlin 1C uses a [[Regenerative cooling (rocket)|regeneratively cooled]] nozzle and combustion chamber. The turbopump used is a Merlin&nbsp;1B model with only slight alterations. It was fired with a full mission duty firing of 170&nbsp;seconds in November 2007,<ref name="BWire_Regen"/> first flew on a [[List of Falcon 1 launches#Third flight|mission]] in August 2008,<ref name="sfn20080928"/> powered the "first [[private spaceflight|{{Sic|hide=y|privately|-}}developed]] liquid-fueled rocket to successfully reach orbit", [[Falcon&nbsp;1 Flight&nbsp;4]], in September 2008,<ref name="sfn20080928">{{cite web |last=Clark |first=Stephen |title=Sweet Success at Last for Falcon 1 Rocket |date=September 28, 2008 |publisher=Spaceflight Now |url=http://www.spaceflightnow.com/falcon/004/index.html |access-date=April 6, 2011 |quote=the first {{Sic|hide=y|privately|-}}developed liquid-fueled rocket to successfully reach orbit |archive-date=September 24, 2015 |archive-url=https://web.archive.org/web/20150924182644/http://www.spaceflightnow.com/falcon/004/index.html |url-status=live }}</ref> and powered the [[Falcon 9 Flight 1|Falcon&nbsp;9 on its maiden flight]] in June 2010.<ref name="msnbc20100604">{{cite news |last=Boyle |first=Alan |title=Shuttle successor succeeds in first test flight |url=http://www.nbcnews.com/id/37509776 |access-date=June 5, 2010 |newspaper=[[NBC News]] |date=June 4, 2010 |archive-date=December 21, 2019 |archive-url=https://web.archive.org/web/20191221015812/http://www.nbcnews.com/id/37509776 |url-status=dead }}</ref>

As configured for use on Falcon 1 vehicles, the Merlin&nbsp;1C had a sea-level thrust of {{cvt|78000|lbf|kN|order=flip}}, a vacuum thrust of {{cvt|90000|lbf|kN|order=flip}} and a vacuum [[specific impulse]] of {{cvt|304|isp|km/s}}. In this configuration, the engine consumed {{cvt|300|lb|kg|order=flip}} of propellant per second. Tests have been conducted with a single Merlin&nbsp;1C engine successfully running a total of 27&nbsp;minutes (counting together the duration of the various tests), which equals ten complete [[Falcon&nbsp;1]] flights.<ref name="v11rO">{{cite press release |url=http://www.spacex.com/press/2012/12/19/spacex-completes-qualification-testing-merlin-regeneratively-cooled-engine |title=SpaceX Completes Qualification Testing of Merlin Regeneratively Cooled Engine |publisher=SpaceX |date=February 25, 2008 |access-date=May 31, 2016 |archive-date=August 22, 2016 |archive-url=https://web.archive.org/web/20160822175301/http://www.spacex.com/press/2012/12/19/spacex-completes-qualification-testing-merlin-regeneratively-cooled-engine |url-status=live }}</ref> The Merlin&nbsp;1C chamber and nozzle are cooled [[Regenerative cooling (rocketry)|regeneratively]] by {{cvt|100|lb|order=flip}} per second of kerosene flow and are able to absorb {{cvt|10|MW}} of [[heat energy]].<ref name="sxua200712">{{cite web |title=Updates: December 2007 |archive-url=https://web.archive.org/web/20130405083840/http://www.spacex.com/updates_archive.php?page=121007 |archive-date=April 5, 2013 |url-status=dead |url=http://www.spacex.com/updates_archive.php?page=121007 |date=Dec 2007 |work=Updates Archive |publisher=SpaceX |access-date=December 27, 2012 |quote=(2007:) Merlin has a thrust at sea level of 95,000&nbsp;lbs, a vacuum thrust of over 108,000&nbsp;pounds, vacuum specific impulse of 304&nbsp;seconds and sea level thrust to weight ratio of 92. In generating this thrust, Merlin consumes 350&nbsp;lbs/second of propellant and the chamber and nozzle, cooled by 100&nbsp;lbs/sec of kerosene, are capable of absorbing 10&nbsp;MW of heat energy. A planned turbo pump upgrade in 2009 will improve the thrust by over 20% and the thrust to weight ratio by approximately 25%.}}</ref>

A Merlin 1C was first used as part of the unsuccessful third attempt to launch a Falcon&nbsp;1. In discussing the failure, Elon Musk noted: "The flight of our first stage, with the new Merlin&nbsp;1C engine that will be used in Falcon&nbsp;9, was picture perfect."<ref name="Mwo6O">{{cite web |url=http://www.nasaspaceflight.com/2008/08/spacex-falcon-i-fails-during-first-stage-flight/ |title=SpaceX Falcon I fails during first stage flight |first=Chris |last=Bergin |author2=Davis, Matt |date=August 2, 2008 |publisher=NASAspaceflight |access-date=February 26, 2010 |archive-date=March 3, 2016 |archive-url=https://web.archive.org/web/20160303210834/http://www.nasaspaceflight.com/2008/08/spacex-falcon-i-fails-during-first-stage-flight/ |url-status=live }}</ref> The Merlin&nbsp;1C was used in the successful [[Ratsat|fourth flight]] of Falcon&nbsp;1 on September 28, 2008.<ref name="MSC">{{cite web |url=http://www.spaceflightnow.com/falcon/004/ |title=Sweet success at last for Falcon 1 rocket |last=Clark |first=Stephen |date=September 28, 2008 |publisher=Spaceflight Now |access-date=September 28, 2008 |archive-date=March 3, 2016 |archive-url=https://web.archive.org/web/20160303211418/http://www.spaceflightnow.com/falcon/004/ |url-status=live }}</ref>

On October 7, 2012, a Merlin 1C (Engine No.&nbsp;1) of the [[SpaceX CRS-1|CRS-1]] mission experienced an [[SpaceX CRS-1#Falcon 9 engine anomaly|anomaly]] at T+00:01:20, which appears on [https://www.youtube.com/watch?v=tRTYh71D9P0 CRS-1 launch video] as a flash. The failure occurred just as the vehicle achieved max-Q (maximum aerodynamic pressure). SpaceX's internal review found that the engine was shut down after a sudden pressure loss and that only the aerodynamic shell was destroyed, generating the debris seen in the video; the engine did not explode, as SpaceX ground control continued to receive data from it throughout the flight. The primary mission was unaffected by the anomaly due to the nominal operation of the remaining eight engines and an onboard readjustment of the flight trajectory,<ref name="cQ3yS">{{cite web |last=Nelson |first=Katherine |title=SpaceX CRS-1 Mission Update |url=http://www.spacex.com/press/2012/12/19/spacex-crs-1-mission-update |publisher=SpaceX |date=October 8, 2012 |access-date=May 31, 2016 |archive-date=April 12, 2017 |archive-url=https://web.archive.org/web/20170412004145/http://www.spacex.com/press/2012/12/19/spacex-crs-1-mission-update |url-status=live }}</ref> but the secondary-mission payload failed to reach its target orbit due to safety protocols in place to prevent collisions with the ISS. These protocols prevented a second firing of the upper stage for the secondary payload.<ref name="sfn20121011">{{cite web |url=http://www.spaceflightnow.com/falcon9/004/121011orbcomm/ |title=Orbcomm craft falls to Earth, company claims total loss |publisher=Spaceflight Now |first=Stephen |last=Clark |access-date=October 11, 2012 |date=October 11, 2012 |archive-date=March 15, 2016 |archive-url=https://web.archive.org/web/20160315001856/http://spaceflightnow.com/falcon9/004/121011orbcomm/ |url-status=live }}</ref>

SpaceX was planning to develop a {{cvt|560|kN|lbf}} version of Merlin&nbsp;1C to be used in Falcon&nbsp;9 Block&nbsp;II and Falcon&nbsp;1E boosters.<ref name="ltDQy">{{cite web |archive-url=https://web.archive.org/web/20121002181416/http://www.spacex.com/Falcon1UsersGuide.pdf |archive-date=October 2, 2012 |url-status=dead |url=http://www.spacex.com/Falcon1UsersGuide.pdf |title=Falcon 1 Users Guide (Rev 7) |publisher=SpaceX |date=August 26, 2008 |page=8}}</ref> This engine and these booster models were dropped in favor of the more advanced Merlin&nbsp;1D engine and longer Falcon&nbsp;9&nbsp;v1.1 booster.[[File:Merlin 1C Vacuum engine at Hawthorne factory (2008).jpg|thumb|left|upright=0.6|Merlin 1C Vacuum engine at Hawthorne factory in 2008]]

===Merlin Vacuum (1C)===
On March 10, 2009, a SpaceX press release announced successful testing of the Merlin Vacuum engine. A variant of the 1C engine, Merlin Vacuum features a larger exhaust section and a significantly larger expansion nozzle to maximize the engine's efficiency in the vacuum of space. Its combustion chamber is [[Regenerative cooling (rocketry)|regeneratively cooled]], while the {{convert|9|ft|m|order=flip|adj=mid|sp=us|-long}}<ref name="aw20101213"/> [[niobium alloy]]<ref name="NSPO"/> expansion nozzle is [[Radiative cooling|radiatively cooled]]. The engine delivers a vacuum thrust of {{cvt|92500|lbf|kN|order=flip}} and a vacuum [[specific impulse]] of {{cvt|342|isp|km/s}}.<ref name="f0h2i">{{cite press release |url=http://www.spacex.com/press/2012/12/19/spacex-falcon-9-upper-stage-engine-successfully-completes-full-mission-duration |title=SpaceX Falcon&nbsp;9 upper stage engine successfully completes full mission duration firing |date=March 10, 2009 |publisher=SpaceX |access-date=March 12, 2009 |archive-date=December 13, 2014 |archive-url=https://web.archive.org/web/20141213023346/http://www.spacex.com/press/2012/12/19/spacex-falcon-9-upper-stage-engine-successfully-completes-full-mission-duration |url-status=dead }}</ref> The first production Merlin Vacuum engine underwent a full-duration orbital-insertion firing (329&nbsp;seconds) of the integrated Falcon&nbsp;9 second stage on January 2, 2010.<ref name="bFIrM">{{cite video |publisher=SpaceX |url=http://spacex.com/multimedia/videos.php?id=44 |title=Full Duration Orbit Insertion Firing |date=January 2, 2010 |access-date=January 5, 2010 |archive-date=April 19, 2012 |archive-url=https://web.archive.org/web/20120419062753/http://www.spacex.com/multimedia/videos.php?id=44 |url-status=dead }}</ref> It was flown on the second stage for the inaugural Falcon&nbsp;9 flight on June 4, 2010. At full power and as of March 10, 2009, the Merlin Vacuum engine operates with the greatest efficiency of any American-made hydrocarbon-fueled rocket engine.<ref name="SDotA">{{cite web |url=http://www.spacex.com/press/2012/12/19/spacex-falcon-9-upper-stage-engine-successfully-completes-full-mission-duration |title=SpaceX Falcon 9 Upper Stage Engine Successfully Completes Full Mission Duration Firing |date=March 10, 2009 |publisher=SpaceX |access-date=May 31, 2016 |archive-date=December 13, 2014 |archive-url=https://web.archive.org/web/20141213023346/http://www.spacex.com/press/2012/12/19/spacex-falcon-9-upper-stage-engine-successfully-completes-full-mission-duration |url-status=dead }}</ref>

An unplanned test of a modified Merlin Vacuum engine was made in December 2010. Shortly before the scheduled [[COTS Demo Flight 1|second flight]] of the [[Falcon&nbsp;9]], two cracks were discovered in the {{convert|9|ft|m|order=flip|adj=mid|sp=us|-long}} niobium-alloy-sheet nozzle of the Merlin Vacuum engine. The engineering solution was to cut off the lower {{cvt|4|ft|order=flip}} of the nozzle and launch two days later, as the extra performance that would have been gained from the longer nozzle was not necessary to meet the objectives of the mission. The modified engine successfully placed the second stage into an orbit of {{cvt|11000|km}} altitude.<ref name="aw20101213">{{cite news |last=Klotz |first=Irene |title=SpaceX Sees ISS Meet-up in 2011 |url=http://www.aviationweek.com/aw/generic/story_channel.jsp?channel=space&id=news/awst/2010/12/13/AW_12_13_2010_p22-275434.xml |access-date=February 8, 2011 |newspaper=Aviation Week |date=December 13, 2010 |quote=The second stage went up to 11,000 km.—and that’s with the shortie skirt }}{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

{{clear}}

===Merlin 1D===
The Merlin 1D engine was developed by SpaceX between 2011 and 2012, with first flight in 2013. The design goals for the new engine included increased reliability, improved performance, and improved manufacturability.<ref name="i9MbN">{{cite web |url=https://www.nasaspaceflight.com/2012/01/spacex-testing-reusable-falcon-9-technology-this-year/ |title=SpaceX to begin testing on Reusable Falcon 9 technology this year |date=January 11, 2012 |website=NASASpaceFlight.com |language=en-US |access-date=January 11, 2020 |archive-date=January 9, 2020 |archive-url=https://web.archive.org/web/20200109110231/https://www.nasaspaceflight.com/2012/01/spacex-testing-reusable-falcon-9-technology-this-year/ |url-status=live }}</ref> In 2011, performance goals for the engine were a vacuum thrust of {{cvt|155000|lbf|kN|order=flip}}, a vacuum specific impulse (''I''<sub>sp</sub>) of {{cvt|310|isp|km/s}}, an expansion ratio of 16 (as opposed to the previous 14.5 of the Merlin&nbsp;1C) and chamber pressure in the "sweet spot" of {{cvt|1410|psi|MPa|order=flip}}. Merlin&nbsp;1D was originally designed to throttle between 100% and 70% of maximal thrust; however, further refinements since 2013 now allow the engine to throttle to 40%.<ref name="crin4">{{cite web |url=https://twitter.com/elonmusk/status/728753234811060224 |title=@lukealization Max is just 3X Merlin thrust and min is ~40% of 1 Merlin. Two outer engines shut off before the center does. |last=Mage |first=Buff |date=May 6, 2016 |website=@elonmusk |language=en |access-date=January 11, 2020 |archive-date=February 5, 2017 |archive-url=https://web.archive.org/web/20170205184532/https://twitter.com/elonmusk/status/728753234811060224 |url-status=live }}</ref>

The basic Merlin fuel/oxidizer mixture ratio is controlled by the sizing of the propellant supply tubes to each engine, with only a small amount of the total flow trimmed out by a "[[Servo control|servo-motor-controlled]] [[butterfly valve]]" to provide [[Vernier throttle|fine control]] of the mixture ratio.<ref name="micromo201508">{{cite web |title=Servo Motors Survive Space X Launch Conditions |url=http://www.micromo.com/applications/aerospace-defense/space-x-shuttle-launch |access-date=August 14, 2015 |publisher=MICROMO/Faulhabler |date=2015 |quote=the fuel-trim valve adjusts the mixture in real time. The fuel-trim device consists of a servo-motor-controlled butterfly valve. To achieve the proper speed and torque, the design incorporates a planetary gearbox for a roughly 151:1 reduction ratio, gearing internal to the unit. The shaft of the motor interfaces with the valve directly to make fine adjustments. 'The basic mixture ratio is given by the sizing of the tubes, and a small amount of the flow of each one gets trimmed out', explains Frefel. 'We only adjust a fraction of the whole fuel flow.' |archive-date=February 20, 2017 |archive-url=https://web.archive.org/web/20170220212836/http://www.micromo.com/applications/aerospace-defense/space-x-shuttle-launch |url-status=live }}</ref>

On November 24, 2013, Elon Musk stated that the engine was actually operating at 85% of its potential, and they anticipated to be able to increase the sea-level thrust to about {{cvt|165000|lbf|kN|order=flip}} and a thrust-to-weight ratio of 180.<ref name="ses8teleconference">{{cite web |last=Elon |first=Musk |title=SES-8 Prelaunch Teleconference |url=http://forum.nasaspaceflight.com/index.php?topic=33335.msg1125118#msg1125118 |access-date=November 28, 2013 |archive-url=https://web.archive.org/web/20131203033340/http://forum.nasaspaceflight.com/index.php?topic=33335.msg1125118#msg1125118 |archive-date=December 3, 2013 |date=November 24, 2013 }} Also [https://soundcloud.com/matthew-clarke-30/spacex-ses-8-pre-launch at SoundCloud] {{Webarchive|url=https://web.archive.org/web/20131207132151/http://soundcloud.com/matthew-clarke-30/spacex-ses-8-pre-launch |date=December 7, 2013 }}.</ref> This version of the Merlin&nbsp;1D was used on [[Falcon&nbsp;9 Full Thrust]] and first flew on [[Falcon 9 flight 20|Flight&nbsp;20]].

In May 2016, SpaceX announced plans to further upgrade the Merlin&nbsp;1D by increasing vacuum thrust to {{cvt|914|kN|lbf}} and sea-level thrust to {{cvt|845|kN|lbf}}; according to SpaceX, the additional thrust will increase the Falcon&nbsp;9 LEO payload capability to about 22 metric tons on a fully expendable mission. SpaceX also noted that unlike the previous Full Thrust iteration of the Falcon&nbsp;9 vehicle, the increase in performance is solely due to upgraded engines, and no other significant changes to the vehicle are publicly planned.

In May 2018, ahead of the first flight of [[Falcon&nbsp;9 Block&nbsp;5]], SpaceX announced that the {{cvt|845|kN|lbf}} goal had been achieved.<ref name="1RmZI">{{cite tweet |first=Eric |last=Berger |user=SciGuySpace |number=994649495861432321 |date=May 10, 2018 |title=Musk: Merlin rocket engine thrust increased by 8 percent, to 190,000 lbf.}}</ref> The Merlin 1D is now close to the sea-level thrust of the retired [[Rocketdyne H-1]] / [[RS-27]] engines used on [[Saturn I]], [[Saturn IB]], and [[Delta II]].

On February 23, 2024, one of the nine Merlin engines powering that launch flew its 22nd mission, which was at the time the flight leading engine. It is already the most flown rocket engine to date, surpassing [[Space Shuttle Main Engine]] no. 2019's record of 19 flights.<ref name="RB242">{{cite tweet |user=SpaceX | number=1760882381148623202 |date=February 23, 2024 |title=Main engine cutoff and stage separation. One of the nine Merlin engines powering tonight's first stage is our flight leader, powering its 22nd mission to Earth orbit}}</ref>

==== Anomalies ====
The March 18, 2020, launch of [[Starlink]] satellites on board a [[Falcon&nbsp;9]] experienced an early engine shutdown on ascent. The shutdown occurred 2&nbsp;minutes 22&nbsp;seconds into the flight and was accompanied with an "event" seen on camera. The rest of the Falcon&nbsp;9 engines burned longer and did deliver the payload to orbit. However, the first stage was not successfully recovered. In a subsequent investigation SpaceX found that [[isopropyl alcohol]], used as cleaning fluid, was trapped and ignited, causing the engine to be shut down. To address the issue, in a following launch SpaceX indicated that the cleaning process was not done.<ref name="BJbif">{{Cite web |last=Clark |first=Stephen |title=Falcon 9 rocket overcomes engine failure to deploy Starlink satellites |website=Spaceflight Now |url=https://spaceflightnow.com/2020/03/18/falcon-9-rocket-overcomes-engine-failure-to-deploy-starlink-satellites/ |access-date=November 1, 2020 |language=en-US |archive-date=October 26, 2020 |archive-url=https://web.archive.org/web/20201026101330/https://spaceflightnow.com/2020/03/18/falcon-9-rocket-overcomes-engine-failure-to-deploy-starlink-satellites/ |url-status=live }}</ref><ref name="Vnfs2">{{Cite web |last=Clark |first=Stephen |title=SpaceX's Starlink network surpasses 400-satellite mark after successful launch |website=Spaceflight Now |url=https://spaceflightnow.com/2020/04/22/spacexs-starlink-network-surpasses-400-satellite-mark-after-successful-launch/ |access-date=November 1, 2020 |language=en-US |archive-date=April 30, 2020 |archive-url=https://web.archive.org/web/20200430031030/https://spaceflightnow.com/2020/04/22/spacexs-starlink-network-surpasses-400-satellite-mark-after-successful-launch/ |url-status=live }}</ref><ref name="GjzNf">{{Cite web |date=April 23, 2020 |title=Safety panel concludes May launch of commercial crew test flight is feasible |url=https://spacenews.com/safety-panel-concludes-may-launch-of-commercial-crew-test-flight-is-feasible/ |access-date=November 1, 2020 |website=SpaceNews |language=en-US}}</ref>

On October 2, 2020, the launch of a GPS-III satellite was aborted at T-2 seconds due to a detected early startup on 2 of the 9 engines on the first stage. The engines were removed for further testing and it was found that a port in the gas generator was blocked. After removing the blockage the engines started as intended. After this, SpaceX inspected other engines across its fleet and found that two of the engines on the Falcon 9 rocket intended for the Crew-1 launch also had this problem. Those engines were replaced with new M1D engines.<ref>{{Cite web|last=Berger|first=Eric|date=October 28, 2020|title=How a tiny bit of lacquer grounded new Falcon 9 rockets for a month|url=https://arstechnica.com/science/2020/10/nasa-and-spacex-set-new-date-for-crew-launch-explain-merlin-engine-issue/|access-date=October 24, 2021|website=Ars Technica|language=en-us}}</ref>

On February 16, 2021, on Falcon 9 flight 108 launching [[Starlink]] satellites, an engine shut down early due to hot exhaust gasses passing through a damaged heat-shielding cover. The mission was a success, but the booster could not be recovered.<ref>{{cite web |last=Cao |first=Sissi |date=February 16, 2021 |title=SpaceX Fails Falcon 9 Rocket Landing in Rare Miss During Latest Starlink Mission |url=https://observer.com/2021/02/spacex-fail-landing-starlink-mission-falcon9/ |access-date=February 26, 2021 |website=Observer }}</ref>

===Merlin 1D Vacuum===
A vacuum version of the Merlin 1D engine was developed for the [[Falcon&nbsp;9&nbsp;v1.1]] and the [[Falcon Heavy]] second&nbsp;stage.<ref name="awst20110811">{{cite news |url=http://aviationweek.com/awin/spacex-unveils-plans-be-world-s-top-rocket-maker |title=SpaceX Unveils Plans To Be World's Top Rocket Maker |date=August 11, 2011 |newspaper=AviationWeek |access-date=June 28, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20150621134912/http://aviationweek.com/awin/spacex-unveils-plans-be-world-s-top-rocket-maker |archive-date=June 21, 2015}}</ref> As of 2020, the thrust of the Merlin&nbsp;1D Vacuum is {{cvt|220500|lbf|kN|abbr=}}<ref name="7BFCT">{{Cite web |title=SpaceX |url=http://www.spacex.com/ |access-date=September 24, 2020 |website=SpaceX |language=en |archive-date=March 7, 2011 |archive-url=https://web.archive.org/web/20110307010135/http://www.spacex.com/ |url-status=live }}</ref> with a specific impulse of 348&nbsp;seconds,<ref name="sxf9ProductPage20130808">{{cite web |url=http://www.spacex.com/falcon9 |title=Falcon 9 |date=2017 |publisher=SpaceX |archive-url=https://web.archive.org/web/20180208031148/http://www.spacex.com/falcon9 |archive-date=February 8, 2018}}</ref> the highest specific impulse ever for a U.S. hydrocarbon rocket engine.<ref name="TDZWY">{{cite web |title=SpaceX Falcon 9 Data Sheet |url=https://www.spacelaunchreport.com/falcon9.html |website=Space Launch Report |access-date=September 21, 2019 |archive-date=December 4, 2019 |archive-url=https://web.archive.org/web/20191204173318/http://www.spacelaunchreport.com/falcon9.html |url-status=usurped}}</ref> The increase is due to the greater [[Rocket engine expansion|expansion ratio]] afforded by operating in vacuum, now 165:1 using an updated nozzle extension.<ref name="sxf9ProductPage20130808"/><ref name="f9userguide2015">{{cite web |url=http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf |title=Falcon 9 Launch Vehicle Payload User's Guide |publisher=SpaceX |version=Revision 2 |date=October 21, 2015 |access-date=November 29, 2015 |archive-date=March 14, 2017 |archive-url=https://web.archive.org/web/20170314002928/http://www.spacex.com/sites/spacex/files/falcon_9_users_guide_rev_2.0.pdf |url-status=dead }}</ref>

The engine can throttle down to 39% of its maximum thrust, or {{cvt|360|kN|lbf}}.<ref name="f9userguide2015"/>
====Merlin 1D Vacuum improvements and variants====
Transporter-7 mission launch debuted a new Merlin Vacuum engine (MVac for short) nozzle extension design or variant aimed at increasing cadence and reducing costs. This new nozzle extension is shorter and, as a result, decreases both performance and material usage. This nozzle is only used on lower-performance missions, as with this nozzle, the MVac engine produces 10% less thrust in space. The nozzle decreases the amount of material needed by 75%; this means that SpaceX can launch over three times as many missions with the same amount of rare [[niobium]] metal as with the longer design.<ref name=TR7>{{cite web |url=https://nextspaceflight.com/launches/details/6808 |title=Transporter 7 |access-date=March 17, 2023}}</ref><ref name=":0">{{Cite web |last=Sesnic |first=Trevor |date=July 22, 2023 |title=EchoStar 24 {{!}} Falcon Heavy |url=https://everydayastronaut.com/echostar-24-falcon-heavy/ |access-date=July 29, 2023 |website=Everyday Astronaut |language=en-US}}</ref>

==== Anomalies ====

On July 11, 2024, Falcon 9 flight 354 launching [[Starlink]] group 9-3 from Vandenberg AFB in California experienced an anomaly with its MVac during an engine relight attempt to raise the [[perigee]] of the 22 Starlink satellites for deployment. On X, Elon Musk and SpaceX both confirmed the engine failed explosively during a second attempted relight, albeit in a manner that did not appear to damage the second stage of the vehicle as the stage went on to deploy the satellites on board.<ref>{{cite web |last=Musk |first=Elon |date=June 11, 2024 |title=SpaceX X Post regarding Starlink 9-3 anomoly. |url=https://x.com/SpaceX/status/1811635860481454487 |access-date=June 11, 2024 |website=X.com }}</ref>