Editing SpaceX Merlin (section)

===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>