Merlin (rocket engine family)
Test firing at SpaceX McGregor's test stand of the Merlin 1D
|Country of origin||United States|
|Application||Falcon 9, Falcon Heavy|
|Propellant||LOX / RP-1 (rocket grade kerosene)|
|Thrust (vac.)||716 kN|
|Thrust (SL)||654 kN|
|Chamber pressure||9.7 MPa (1,410 psi)|
|Isp (vac.)||311 s (3.0 km/s)|
|Isp (SL)||282 s (2.73 km/s)|
Merlin is a family of rocket engines developed by SpaceX for use on its Falcon 1 and Falcon 9 launch vehicles, based on the TRW rocket engine family known as the NLS FASTRAC rocket assembly in the 1990s. SpaceX also plans to use Merlin engines on its Falcon Heavy. Merlin engines use RP-1 and liquid oxygen as propellants in a gas-generator power cycle. The Merlin engine was originally designed for sea recovery and reuse.
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 have been produced. The Merlin engine for Falcon 1 had a movable turbo-pump exhaust assembly which was used to provide roll control by vectoring the exhaust. The Merlin 1C 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, a Merlin 1C vacuum variant 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.
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 twice: First on 24 March 2006, when it caught fire and failed due to a fuel leak shortly after launch, and the second time on 21 March 2007, when it performed successfully. Both times the Merlin 1A was mounted on a Falcon 1 first stage.
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. Barber-Nichols Inc. applied lessons learned from the RS-88 (Bantum) and NASA Fastrac engine programs for their turbopump products. The Merlin 1A turbopump uses a unique friction welded main shaft (inconel 718 ends with an integral aluminum RP-1 impeller in the middle). The turbopump housing is constructed of investment castings (inconel at the turbine end, aluminum in the center, and 300-series stainless steel at the LOX end). The turbine is a partial-admission impulse design and turns at 20,000 rpm. Total turbopump weight was 150-lbs.
The Merlin 1B rocket engine was an upgraded version of the Merlin 1A engine. The turbopump upgrades were handled by Barber-Nichols Inc for SpaceX. It was intended for Falcon 1 launch vehicles, capable of producing 380 kN (85,000 lbf) of thrust. The Merlin 1B was enhanced over the 1A with a turbine upgrade (from 1490 kW to 1860 kW). The turbine upgrade was accomplished by adding additional nozzles (turning the previous 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 rpm and developed higher discharge pressures. Turbopump weight was unchanged at 150 lbs.
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.
Merlin 1C under construction at SpaceX factory
|Country of origin||United States|
|Propellant||LOX / RP-1 (rocket grade kerosene)|
|Thrust (vac.)||480 kN (110,000 lbf)|
|Thrust (SL)||420 kN (94,000 lbf)|
|Chamber pressure||6.77 MPa (982 psi)|
|Isp (vac.)||304.8 s (3.0 km/s)|
|Isp (SL)||275 s (2.6 km/s)|
|Length||2,920 mm (115 in)|
|Diameter||1,676 mm (66.0 in)|
|Dry weight||1,380 pounds (630 kg)|
The Merlin 1C uses a regeneratively cooled nozzle and combustion chamber. The turbopump used is a Merlin 1B model with only slight alterations. It was fired with a full mission duty firing of 170 seconds in November, 2007, first flew on a mission in August 2008, powered the "first privately-developed liquid-fueled rocket to successfully reach orbit", Falcon 1 Flight 4, in September 2008, and powered the Falcon 9 on its maiden flight in June 2010.
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. The Merlin 1C chamber and nozzle are cooled regeneratively by 45 kilograms (100 lb) per second of kerosene flow, and are able to absorb 10 megawatts (13,000 hp) of thermal heat energy.
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." The Merlin 1C was used in the successful fourth flight of Falcon 1 on September 28, 2008.
On October 7, 2012 a Merlin 1C (Engine No. 1) of the CRS-1 mission experienced an anomaly at T+00:01:20 which appears on CRS-1 launch video as a flash. 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 itself 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, but the secondary mission payload failed to achieve orbit due to safety protocols in place to prevent collisions with the ISS.
SpaceX was planning to develop a 560 kN version of Merlin 1C to be used in Falcon 9 block II and Falcon 1E boosters. This engine and these booster models were dropped in favour of more advanced Merlin 1D engine and longer Falcon 9 v1.1 booster.
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 niobium alloy 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. 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. 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.
An 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 niobium-alloy-sheet nozzle of the Merlin Vacuum engine. 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.
The Merlin 1D engine was developed by SpaceX in 2011–2012, with first flight in 2013. The Merlin 1D was originally (April 2011) designed for a sea level thrust of 620 kN (140,000 lbf). At the 2011 AIAA Propulsion Conference, SpaceX's Tom Mueller revealed that the engine would have a vacuum thrust of 690 kN (155,000 lbf), a vacuum specific impulse (Isp) of 310 s, an increased expansion ratio of 16 (as opposed to 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 for the engine is the ability to throttle from 100% to 70%.
The design goals for the new engine design included increased reliability (increased fatigue life and increased chamber and nozzle thermal margins), improved performance (thrust design objective 140,000 pounds-force (620 kN) and 70-100 percent throttle capability), and improved manufacturability (lower parts count and fewer labor hours). The engine's 150:1 thrust-to-weight ratio would be the highest ever achieved for a rocket engine.
When engine testing was completed in June 2012, SpaceX stated that the engine had completed a full mission duration test firing of 185 seconds delivering 650 kN (147,000 lbf) of thrust and also confirming the expected thrust-to-weight ratio exceeding 150.[dead link] As of November 2012 the Falcon 9's page Merlin section describes the characteristics as having a sea level thrust of 650 kN (147,000 lbf), a vacuum thrust of 720 kN (161,000 lbf), a sea level specific impulse (Isp) of 282 s and a vacuum specific impulse (Isp) of 311 s. The engine has the highest specific impulse ever achieved for a gas-generator cycle kerosene rocket engine. On March 20, 2013 SpaceX announced the Merlin 1D engine has achieved flight qualification. In June 2013, the first orbital flight vehicle to use the Merlin 1D, the Falcon 9 1.1 first stage, completed development testing. On November 24, 2013, during a joint teleconference of SES and SpaceX regarding the SES-8 launch, 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 165,000 pounds-force (730 kN).
The first flight of the Falcon 9 with Merlin 1D engines launched the CASSIOPE satellite for the Canadian Space Agency. CASSIOPE, an 800 pounds (360 kg) weather research and communications satellite, was launched into a highly elliptical low Earth orbit (LEO). The second flight was the geosynchronous transfer orbit (GTO) launch of SES-8. 
Merlin Vacuum (1D)
In late 2012, Elon Musk tweeted an image of the Merlin 1D-Vac firing on the test stand and stated "Now test firing our most advanced engine, the Merlin 1D-Vac, at 80 tons of thrust."  An August 2013 update to SpaceX's official Falcon 9 product page lists the thrust of Merlin Vacuum at 801 kN (180,000 lbf) in vacuum conditions. Specific Impulse numbers for Merlin 1D-Vac is rated as 340 seconds. The increase is due to the greater expansion ratio afforded by operating in a vacuum.
SpaceX uses a triple-redundant design in the Merlin flight computers. The system uses three computers in each processing unit, each constantly checking on the others, to instantiate a fault-tolerant design. One processing unit is part of each of the ten Merlin engines (nine on first stage, one on second stage) used on a Falcon 9 launch.
As of August 2011[update], SpaceX was producing Merlin engines at the rate of eight per month, planning eventually to raise production to about 33 engines per month (or 400 per year). By September 2013, SpaceX total manufacturing space had increased to nearly 93,000 square meters (1,000,000 sq ft) and the factory had been configured to achieve a production rate of up to 40 rocket cores per year, enough to use the 400 annual engines envisioned by the earlier engine plan. By October 2014, SpaceX announced it had manufactured the 100th Merlin 1D engine and that engines were now being produced at a rate of 4 per week, soon to be increased to 5. 
SpaceX has other main-engine development programs underway and they have also released details on future engine concepts. The concepts have included liquid hydrogen (LH2) fueled engines in addition to SpaceX's Merlin family of RP1-fueled engines currently in production. However, in November 2012, SpaceX CEO Elon Musk announced a new direction for propulsion side of the company: developing methane/LOX rocket engines, which have cost advantages and a slight Isp advantage over Kerosene while avoiding adverse aspects of liquid hydrogen technology.
Merlin 2 concept
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/RP-1-fueled engine on a gas generator cycle, 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. 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."
Since its announcement, the status of the Merlin 2 engine has become unclear. At the 2011 Joint Propulsion Conference, Elon Musk stated that SpaceX were instead working towards a potential staged cycle engine. In October 2012, SpaceX publicly announced concept work on a rocket engine that would be "several times as powerful as the Merlin 1 series of engines, and won't use Merlin's RP-1 fuel". They indicated that the large engine was intended for a new SpaceX rocket, using multiple of these large engines could notionally launch payload masses of the order of 150 to 200 tonnes (150,000 to 200,000 kg) to low-Earth orbit. The forthcoming engine currently under development by SpaceX has been named "Raptor". Raptor will use liquid methane as a fuel, and will have a sea-level thrust of 1,000,000 pounds-force (4,400 kN).
Merlin 1C engine specifications
- Sea level thrust: 556 kN (125,000 lbf)
- Vacuum thrust: 616 kN (138,400 lbf)
- Sea level specific impulse: 275 s (2.6 kN·s/kg)
- Vacuum specific impulse: 304 s (3.0 kN·s/kg)
- Fuel: RP-1 (rocket grade kerosene)
- Oxidizer: liquid oxygen
- Chamber pressure: 6.77 MPa (982 psi)
- Thrust-to-weight ratio (fully accounted): 96
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- Merlin Engine, 2013-03-26, acc 2014-11-01.
- Chaikin, Andrew (January 2012). "1 visionary + 3 launchers + 1,500 employees = ? : Is SpaceX changing the rocket equation?". Air & Space Smithsonian. Retrieved 2012-11-13.
- "SpaceX Unveils Plans To Be World’s Top Rocket Maker". AviationWeek. 2011-08-11. Retrieved 2014-06-28.
- "Falcon 9's Page Merlin section". SpaceX. Retrieved 2012-10-16.[not in citation given]
- 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.
- Berger, Brian (2006-07-19). "Falcon 1 Failure Traced to a Busted Nut". Space.com.
- "Findings of the Falcon return to flight board". SpaceX.com. July 25, 2006.
- "Demo Flight 2 Flight Review Update" (PDF). SpaceX. June 15, 2007.
- Whitesides, Loretta Hidalgo (2007-11-12). "SpaceX Completes Development of Rocket Engine for Falcon 1 and 9". Wired Science. Retrieved 2008-02-28.
- Gaskill, Braddock (2006-08-05). "SpaceX has magical goals for Falcon 9". Nasa Spaceflight. Retrieved 2008-02-28.
- "SpaceX Completes Development of Regeneratively Cooled Rocket Engine". SpaceX. Retrieved 2012-12-25.
- "The SpaceX Falcon 1 Launch Vehicle Flight 3 Results, Future Developments, and Falcon 9 Evolution". Retrieved 29 December 2012.
- "SpaceX Completes Development of Merlin Regeneratively Cooled Rocket Engine". Business Wire. November 13, 2007.
- "SpaceX Completes Development of Merlin Regeneratively Cooled Rocket Engine". SpaceX. November 13, 2007. Retrieved 2009-03-12.
- 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."
- Boyle, Alan (June 4, 2010). "Shuttle successor succeeds in first test flight". MSNBC. Retrieved 2010-06-05.
- "SpaceX Completes Qualification Testing of Merlin Regeneratively Cooled Engine for Falcon 1 Rocket" (Press release). SpaceX. 2008-02-25.
- "Updates: December 2007". Updates Archive. SpaceX. Dec 2007. Retrieved 2012-12-27. "(2007:) Merlin has a thrust at sea level of 95,000 lbs, a vacuum thrust of over 108,000 pounds, vacuum specific impulse of 304 seconds and sea level thrust to weight ratio of 92. In generating this thrust, Merlin consumes 350 lbs/second of propellant and the chamber and nozzle, cooled by 100 lbs/sec of kerosene, are capable of absorbing 10 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%. "
- Bergin, Chris; Davis, Matt. "SpaceX Falcon I fails during first stage flight". NASAspaceflight.
- Clark, Stephen (2008-09-28). "Sweet success at last for Falcon 1 rocket". Spaceflight Now. Retrieved 2008-09-28.
- Nelson, Katherine. "SpaceX CRS-1 Mission Update". SpaceX. Retrieved 9 October 2012.
- Clark, Stephen (11 October 2012). "Orbcomm craft falls to Earth, company claims total loss". Spaceflight Now. Retrieved 11 October 2012.
- "Falcon 1 Users Guide (Rev 7)" (PDF). SpaceX. 2008-08-26. p. 8.
- 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"
- "SpaceX Falcon 9 upper stage engine successfully completes full mission duration firing." (Press release). SpaceX. 2009-03-10. Retrieved 2009-03-12.
- Full Duration Orbit Insertion Firing. SpaceX. 2010-01-02.
- "SpaceX Falcon 9 Upper Stage Engine Successfully Completes Full Mission Duration Firing". SpaceX. 2009-03-10. Retrieved 31 March 2011.
- Harwood, William (2011-04-05). "World's biggest private space rocket planned". CBS. Retrieved 2011-04-05.
- Lindsey, Clark S. (2011-08-01). "SpaceX Merlin 1D". Archived from the original on 2012-10-11. Retrieved 2011-08-06.
- Bergin, Chris (2012-01-11). "SpaceX to begin testing on Reusable Falcon 9 technology this year". NASA Spaceflight.com. Retrieved 2012-12-28. "– Increased reliability: Simplified design by eliminating components and sub-assemblies. Increased fatigue life. Increased chamber and nozzle thermal margins,” noted SpaceX in listing the improvements in work. – Improved Performance: Thrust increased from 95,000 lbf (sea level) to 140,000 lbf (sea level). Added throttle capability for range from 70-100 percent. Currently, it is necessary to shut off two engines during ascent. The Merlin 1D will make it possible to throttle all engines. Structure was removed from the engine to make it lighter. – Improved Manufacturability: Simplified design to use lower cost manufacturing techniques. Reduced touch labor and parts count. Increased in-house production at SpaceX."
- Bergin, Chris (20 June 2013). "Reducing risk via ground testing is a recipe for SpaceX success". NASASpaceFlight (not affiliated with NASA). Retrieved 21 June 2013.
- Elon, Musk (2013-11-24). "SES-8 Prelaunch Teleconference". Archived from the original on 2013-11-28. Retrieved 2013-11-28.
- Rosenberg, Zach (2012-03-16). "SpaceX readies upgraded engines". Flightglobal. Retrieved 2012-03-17.
- Graham, William (2013-12-03). "Falcon 9 v1.1 successfully lofts SES-8 in milestone launch". NASASpaceflight.com. Retrieved 2013-12-03.
- "SpaceX Falcon 9 product page". Retrieved 2013-08-08.
- Elon, Musk (2012-09-30). "Now test firing our most advanced engine, the Merlin 1D-Vac, at 80 tons of thrust.". Retrieved 2012-10-16.
- Svitak, Amy (2012-11-18). "Dragon's "Radiation-Tolerant" Design". Aviation Week. Retrieved 2013-11-06. ""We've got computers in the Falcon 9, we've got three computers in one unit on each engine in the Falcon 9, so that's 30 computers right there.""
- "Merlin LOX/RP-1 Turbopump". website "Products" page: Rocket Engine Turbopumps. Barber-Nichols. Retrieved 2012-11-22.
- "Production at SpaceX". SpaceX. 2013-09-24. Retrieved 2013-09-30.
- "Production at SpaceX". SpaceX. 2014-10-22. Retrieved 2014-10-16.
- Todd, David (2012-11-22). "SpaceX’s Mars rocket to be methane-fuelled". Flightglobal. Retrieved 2012-12-05. "Musk said Lox and methane would be SpaceX’s propellants of choice on a mission to Mars, which has long been his stated goal. SpaceX’s initial work will be to build a Lox/methane rocket for a future upper stage, codenamed Raptor. The design of this engine would be a departure from the “open cycle” gas generator system that the current Merlin 1 engine series uses. Instead, the new rocket engine would use a much more efficient “staged combustion” cycle that many Russian rocket engines use."
- Todd, David (2012-11-20). "Musk goes for methane-burning reusable rockets as step to colonise Mars". FlightGlobal Hyperbola. Retrieved 2012-11-22. ""We are going to do methane." Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, "The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene," said Musk adding, "And it does not have the pain in the ass factor that hydrogen has"."
- "SpaceX Merlin 2 engine, heavy lift designs". hobbyspace. 2010-07-30.
- SpaceX Unveils Heavy-Lift Vehicle Plan, Aviation Week, 2010-08-05, accessed 2010-08-16.
- "Exploration Musk Clarifies SpaceX Position On Exploration]". Aviation & Space Week. 2010-08-11. Retrieved 2010-08-16. (subscription required)
- "Webcasts of Elon Musk & Gwynne Shotwell at AIAA mtg". hobbyspace. 2011-08-01.
- Rosenberg, Zach (2012-10-15). "SpaceX aims big with massive new rocket". Flightglobal. Retrieved 2012-10-17.
- "SpaceX propulsion chief elevates crowd in Santa Barbara". Pacific Business Times. 19 February 2014. Retrieved 22 February 2014.
- "SpaceX Merlin Engine". SpaceX. Retrieved 17 July 2014.
- Belfiore, Michael (January 18, 2005). "Race for Next Space Prize Ignites". Wired.