Merlin (rocket engine)

Merlin (rocket engine)
Merlin 1C
SpaceX Merlin 1A (shown)
SpaceX Merlin 1A (shown)
Country of origin United States
Manufacturer SpaceX
Liquid-fueled engine
Propellant LOX / RP-1 (rocket grade kerosene)
Cycle gas-generator
Thrust (Vac.) 616 kN (138,400 lbf)
Thrust (SL) 556 kN (125,000 lbf)
Thrust-to-weight ratio 96
Chamber pressure 6.77 MPa (982 psi)[1]
Isp (Vac.) 304.8 s (3.0 km/s)[1]
Isp (SL) 275s (2.6 km/s)
Diameter 1,250 mm (49 in)[citation needed]
Dry weight 1,380 pounds (630 kg)

Merlin is a rocket engine developed by SpaceX for use on its Falcon 1 and Falcon 9 rockets. Merlin uses RP-1 and liquid oxygen as propellants in a gas-generator power cycle. The Merlin engine is designed for sea recovery and reuse.

The injector at the heart of Merlin is of the pintle type that was first used in the Apollo Program for the lunar module landing engine.

Propellants are fed via a single shaft, dual impeller turbo-pump. The turbo-pump also provides high pressure fluid for the hydraulic actuators, which then recycles into the low pressure inlet. This eliminates the need for a separate hydraulic power system and means that thrust vector control failure by running out of hydraulic fluid is not possible. A third use of the turbo-pump is to provide power to pivot the turbine exhaust nozzle for roll control purposes.



Three versions of the Merlin 1C engine are in production. The Merlin engine for Falcon 1 has a movable turbo-pump exhaust assembly which is used to provide roll control by vectoring the exhaust. The Merlin engine for the Falcon 9 first stage is nearly identical to the variant used for the Falcon 1 except that the turbo-pump exhaust assembly is not movable. Finally, the Merlin vacuum is used on the Falcon 9 second stage. This engine differs from the Falcon 9 first stage variant in that it uses a larger exhaust nozzle optimized for vacuum operation and can be throttled between 60 and 100 percent.[1]


Merlin 1A

The initial version, the Merlin 1A, used an inexpensive, expendable, ablatively cooled carbon fiber composite nozzle, and produced 340 kN (77,000 lbf) of thrust. The Merlin 1A flew only two times: First on 24 March 2006, when it caught fire and failed due to a fuel leak shortly after launch,[2][3] and the second time on 21 March 2007, where it performed successfully.[4] Both times the Merlin 1A was mounted on a Falcon 1 first stage.[5][6]

The basic design of the first engine variant was based on the abandoned FASTRAC NASA project which developed the similar Fastrac rocket engine at the end of 1990s.[7]

Merlin 1B

The Merlin 1B rocket engine was an upgraded version of the Merlin engine developed[when?] by SpaceX for its Falcon 1 launch vehicles, capable of producing 380 kN (85,000 lbf) of thrust.[citation needed] The Merlin 1B was enhanced over the 1A with a turbine upgrade (from 1490 kW to 1860 kW).[citation needed]

Initial use of the Merlin 1B was to be on the Falcon 9 launch vehicle, on whose first stage there would have been a cluster of nine of these engines. Due to experience from the Falcon 1's first flight, the 1B was never used on a flight vehicle. SpaceX moved its Merlin development to the Merlin 1C, which is regeneratively cooled. Therefore the Merlin 1B was never in use on a launch vehicle.[5][6]

Merlin 1C

Merlin 1C under construction at SpaceX factory

The Merlin 1C uses a regeneratively cooled nozzle and combustion chamber. It was fired with a full mission duty firing of 170 seconds in November, 2007,[8][9] first flew on a mission in August 2008,[10] powered the "first privately-developed liquid-fueled rocket to successfully reach orbit", Falcon 1 Flight 4, in September 2008,[10] and powered the Falcon 9 on its maiden flight in June 2010.[11]

As configured for use on Falcon 1 vehicles, the Merlin 1C had a sea level thrust of 350 kN (78,000 lbf), a vacuum thrust of 400 kN (90,000 lbf) and a vacuum specific impulse of 304 seconds. In this configuration the engine consumed 140 kg (300 lb) of propellant per second. Tests have been conducted with a single Merlin 1C engine successfully running a total of 27 minutes (counting together the duration of the various tests), which equals ten complete Falcon 1 flights.[12]

As configured for use on the newer Falcon 1e and Falcon 9 vehicles, the Merlin 1C has a sea level thrust of 560 kN (125,000 lbf), and an isp of 300.[13]

A Merlin 1C was first used as part of the unsuccessful third attempt to launch a Falcon 1. In discussing the failure, Elon Musk noted, "The flight of our first stage, with the new Merlin 1C engine that will be used in Falcon 9, was picture perfect."[14] The Merlin 1C was used in the successful fourth flight of Falcon 1 on September 28, 2008.[15]

The SpaceX turbopump was an entirely new, clean sheet design. Although the design has some similarities to the FASTRAC turbopump, it is clearly not the same design, and this can be seen in the external design and internal design.[16][17][18] The flange location, housing external shape and the lack of a separate inducer piece on the LOX end pump are among the obvious differences. Both Turbopumps were designed by the same company, Barber-Nichols, and this is the only factor that makes their designs appear similar. The NASA FASTRAC engine also has less than half of the thrust capability (60,000 vs. 138,000 lbf).

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 regeneratively cooled while the 2.7 metres (9 ft)-long[19] niobium alloy[1] expansion nozzle is radiatively cooled. The engine delivers a vacuum thrust of 411 kN (92,500 lbf) and a vacuum specific impulse of 342 seconds.[20] The first production Merlin Vacuum engine underwent a full duration orbital insertion firing (329 seconds) of the integrated Falcon 9 second stage on January 2, 2010.[21] It was flown on the second stage for the inaugural Falcon 9 flight on June 4, 2010. At full power the Merlin Vacuum engine operates with the greatest efficiency ever for an American-made hydrocarbon rocket engine.[22]

An interesting unplanned test of a modified Merlin Vacuum engine was made in December 2010. Shortly before the scheduled second flight of the Falcon 9, two cracks were discovered in the 2.7 metres (9 ft)-long Merlin Vacuum niobium alloy sheet nozzle. The engineering solution was to cut off the lower 1.2 metres (4 ft) 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. Even with the shortened nozzle, the engine placed the second-stage into an orbit of 11,000 kilometres (6,800 mi) altitude.[19]

Merlin 1D
Country of origin United States
Manufacturer SpaceX
Application Falcon 9, Falcon Heavy
Liquid-fueled engine
Propellant LOX / RP-1 (rocket grade kerosene)
Cycle gas-generator
Thrust (Vac.) 690 kN (155,000 lbf)[23]
Chamber pressure 9.7 MPa (1,410 psi)[23]
Isp (Vac.) 310 s (3.0 km/s)[23]

Merlin 1D

The Merlin 1D is currently in development, and was originally (April 2011) designed for a sea level thrust of 620 kN (140,000 lbf)[24]. At the 2011 AIAA Propulsion Conference SpaceX's Tom Mueller[25] revealed that the engine would have a vacuum thrust of 690 kN (155,000 lbf), a vacuum specific impulse (Isp) of 310s, an increased expansion ratio of 16, vs. the previous 14.5 of the Merlin 1C, and chamber pressure in the "sweet spot" of 9.7 MPa (1,410 psi). A new feature to the engine would be the ability to throttle from 100% to 70%.[23]

Merlin Vacuum (1D)

A vacuum version of the Merlin 1D engine is planned for the Falcon Heavy second stage.[23]


As of August 2011, SpaceX is producing Merlin engines at the rate of eight per month, with the "eventual plan" to raise production to 400 engines per year.[23]

Conceptual future engines

Merlin 2

At the AIAA Joint Propulsion conference on July 30, 2010 SpaceX McGregor rocket development facility director Tom Markusic shared some information from the initial stages of planning for a new engine. SpaceX’s Merlin 2 LOX/rocket propellant-fueled engine, capable of a projected 7,600 kN (1,700,000 lbf) of thrust at sea level and 8,500 kN (1,920,000 lbf) in a vacuum and would provide the power for conceptual super-heavy-lift launch vehicles from SpaceX, which Markusic dubbed Falcon X and Falcon XX. Such a capability would result in an engine with more thrust than the F-1 engines used on the Saturn V.

Slated to be introduced on more capable variants of the Falcon 9 Heavy, the Merlin 2 “could be qualified in three years for $1 billion,” Markusic says.[26] By mid-August, the SpaceX CEO Elon Musk clarified that while the Merlin 2 engine architecture was a key element of any effort SpaceX would make toward their objective of "super-heavy lift" launch vehicles—and that SpaceX did indeed want to "move toward super heavy lift"—the specific potential design configurations of the particular launch vehicles shown by Markusic at the propulsion conference were merely conceptual "brainstorming ideas", just a "bunch of ideas for discussion."[27]


As of 2009, Raptor was a SpaceX rocket engine design concept for a higher performance upper stage for the Falcon 9 launch vehicle. The engine would be powered by liquid hydrogen and liquid oxygen,[28] rather than the RP-1 kerosene and liquid oxygen used in the Block 1 Falcon 9 upper stage, enabling much greater mass to be boosted into orbit.[28] If implemented, Raptor would replace the Merlin Vacuum engine in the current Falcon 9 on high performance launches.

Merlin 1C Engine specifications

Current published ratings for Merlin 1C variant:[29]

  • Sea level thrust: 556 kN (125,000 lbf)
  • Vacuum thrust: 616 kN (138,400 lbf)
  • Chamber pressure: 6.77 MPa (890 psi)
  • Sea level specific impulse: 275 s (2.6 kN·s/kg)
  • Vacuum specific impulse: 304 s (3.0 kN·s/kg)
  • Thrust-to-weight ratio (fully accounted): 96
  • Fuel: RP-1 (rocket grade kerosene)
  • Oxidizer: Liquid oxygen

See also


  1. ^ a b c d Dinardi, Aaron; Capozzoli, Peter; Shotwell, Gwynne (2008). "Low-cost Launch Opportunities Provided by the Falcon Family of Launch Vehicles" (PDF). Fourth Asian Space Conference. Taipei. 
  2. ^ Berger, Brian (2006-07-19). "Falcon 1 Failure Traced to a Busted Nut". 
  3. ^ "Findings of the Falcon return to flight board". July 25, 2006. 
  4. ^ "Demo Flight 2 Flight Review Update" (PDF). SpaceX. June 15, 2007. 
  5. ^ a b Whitesides, Loretta Hidalgo (2007-11-12). "SpaceX Completes Development of Rocket Engine for Falcon 1 and 9". Wired Science. Retrieved 2008-02-28. 
  6. ^ a b Gaskill, Braddock (2006-08-05). "SpaceX has magical goals for Falcon 9". Nasa Spaceflight. Retrieved 2008-02-28. 
  7. ^ "FASTRACT background facts". Nasa. 199-02-02. 
  8. ^ "SpaceX Completes Development of Merlin Regeneratively Cooled Rocket Engine". Business Wire. November 13, 2007. 
  9. ^ "SpaceX Completes Development of Merlin Regeneratively Cooled Rocket Engine". SpaceX. November 13, 2007. Retrieved 2009-03-12. 
  10. ^ a b Clark, Stephen (2008-09-28). "Sweet Success at Last for Falcon 1 Rocket". Spaceflight Now. Retrieved 2011-04-06. "the first privately-developed liquid-fueled rocket to successfully reach orbit." 
  11. ^ Boyle, Alan (June 4, 2010). "Shuttle successor succeeds in first test flight". MSNBC. Retrieved 2010-06-05. 
  12. ^ "SpaceX Completes Qualification Testing of Merlin Regeneratively Cooled Engine for Falcon 1 Rocket" (Press release). SpaceX. 2008-02-25. 
  13. ^ "Falcon 1 Users Guide (Rev 7)" (PDF). SpaceX. 2008-08-26. p. 8. 
  14. ^ Bergin, Chris; Davis, Matt. "SpaceX Falcon I fails during first stage flight". NASAspaceflight. 
  15. ^ Clark, Stephen (2008-09-28). "Sweet success at last for Falcon 1 rocket". Spaceflight Now. Retrieved 2008-09-28. 
  16. ^ Page 15
  17. ^ Image
  18. ^ Image
  19. ^ a b Klotz, Irene (2010-12-13). "SpaceX Sees ISS Meet-up in 2011". Aviation Week. Retrieved 2011-02-08. "The second stage went up to 11,000 km.—and that’s with the shortie skirt" 
  20. ^ "SpaceX Falcon 9 upper stage engine successfully completes full mission duration firing." (Press release). SpaceX. 2009-03-10. Retrieved 2009-03-12. 
  21. ^ Full Duration Orbit Insertion Firing. SpaceX. 2010-01-02. 
  22. ^ "SpaceX Falcon 9 Upper Stage Engine Successfully Completes Full Mission Duration Firing". SpaceX. 2009-03-10. Retrieved 31 March 2011. 
  23. ^ a b c d e f "SpaceX Unveils Plans To Be World’s Top Rocket Maker". Aviation Week and Space Technology. 2011-08-11. Retrieved 2011-08-07 (print edition is out prior to publication date). "Revealing several new details of the 1D, Tom Mueller, propulsion engineering vice president, says the engine is designed to produce 155,000 lb. vacuum thrust and have a chamber pressure at “the sweet spot” of roughly 1,410 psia. “We’ve also increased the nozzle expansion ratio to 16 [compared with 14.5 on the Merlin 1C],” says Mueller, who adds that the initial engine “is doing better than we hoped.” The engine is designed for an Isp (specific impulse) of 310 sec. and has a thrust-to-weight ratio of 160:1. “We took structure off the engine to make it lighter. The engine we shipped [for test] to Texas was a development engine and hopefully the production engines will be even better.”" 
  24. ^ Harwood, William (2011-04-05). "World's biggest private space rocket planned". CBS. Retrieved 2011-04-05. 
  25. ^ "SpaceX Merlin 1D". Aviation Week. 2011-08-01. Retrieved 2011-08-06. 
  26. ^ SpaceX Unveils Heavy-Lift Vehicle Plan, Aviation Week, 2010-08-05, accessed 2010-08-16.
  27. ^ Musk Clarifies SpaceX Position On Exploration, Aviation Week, 2010-08-11, accessed 2010-08-16.
  28. ^ a b "Long term SpaceX vehicle plans". Retrieved 2009-07-13. 
  29. ^ "SpaceX Merlin Engine". SpaceX. 

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