Merlin (rocket engine family)
This article's factual accuracy may be compromised due to out-of-date information. (September 2015)
Test firing of the Merlin 1D at SpaceX McGregor's test stand
|Country of origin||United States|
|Application||Booster stage engines, Upper stage engine|
|Associated L/V||Falcon 9, Falcon Heavy|
|Propellant||LOX / RP-1|
|Thrust (vac.)||914 kN (205,000 lbf)|
|Thrust (SL)||845 kN (190,000 lbf)|
|Chamber pressure||9.7 MPa (1,410 psi)|
|Isp (vac.)||311 seconds (3.05 km/s)|
|Isp (SL)||282 seconds (2.77 km/s)|
|Dry weight||1,030 pounds (470 kg)|
Merlin is a family of rocket engines developed by SpaceX for use on its Falcon 1, Falcon 9 and Falcon Heavy launch vehicles. Merlin engines use RP-1 and liquid oxygen as rocket 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 turbopump. The turbopump 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 drive system and means that thrust vectoring control failure by running out of hydraulic fluid is not possible.
The initial version, the Merlin 1A, used an inexpensive, expendable, ablatively cooled carbon-fiber-reinforced polymer composite nozzle, and produced 340 kN (76,000 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, and the second time on March 21, 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; the company had previously worked on turbopumps for the RS-88 (Bantam) and NASA Fastrac engine programs. The Merlin 1A turbopump used a unique friction-welded main shaft, with Inconel 718 ends and an integral aluminum RP-1 impeller in the middle. The turbopump housing was constructed using investment castings, 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[clarification needed] impulse design and turned at up to 20,000 rpm, with a total weight of 150 lb.
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 at sea level, and 420 kN (95,000 lbf) in vacuum, and performing with a specific impulse of 261 seconds at sea level and 303 seconds in vacuum. The Merlin 1B was enhanced over the 1A with a turbine upgrade, increasing power output from 1,500 kW (2,000 hp) to 1,900 kW (2,500 hp). 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 rpm and developed higher discharge pressures. Turbopump weight was unchanged at 150 lb. Another notable change over the 1A was the move to TEA-TEB (pyrophoric) ignition over torch ignition.
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, SpaceX moved its Merlin development to the Merlin 1C, which is regeneratively cooled. Therefore, the Merlin 1B was never used on a launch vehicle.
Merlin 1C under construction at SpaceX
|Country of origin||United States|
|Application||Main stage engine, Upper stage engine|
|Associated L/V||Falcon 9|
|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)|
|Dry weight||1,380 pounds (630 kg)|
Three versions of the Merlin 1C engine were produced. The Merlin engine for Falcon 1 had a movable turbopump 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, although the turbopump 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 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. 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, 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.
SpaceX was planning to develop a 560 kN (130,000 lbf) 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 favor of the 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). In 2011, it was 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%. Later refinements of the Merlin 1D have been operated down to 40% of full thrust.
The design goals for the new engine 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).
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 exceeded 150. As of November 2012 the Merlin section of the Falcon 9 page describes the engine 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 had 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.
The first flight of the Falcon 9 with Merlin 1D engines launched the CASSIOPE satellite for the Canadian Space Agency. CASSIOPE, an 360 kg (800 lb) weather research and communications satellite, was launched into a near-polar low Earth orbit (LEO). The second flight was the geosynchronous transfer orbit (GTO) launch of SES-8. 
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-motor-controlled butterfly valve" to provide fine control of the mixture ratio.
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 730 kN (165,000 lbf). In June 2015 Tom Mueller answered a question about the Merlin 1D thrust-weight ratios on Quora. He specified that the Merlin 1D has a mass of 470 kg (1,030 lb) including thrust actuators, a current vacuum thrust of 723 kN (162,500 lbf), and an uprated vacuum thrust of 825 kN (185,500 lbf), without an increased mass. These figures provide for a current thrust-weight ratio of ≈158 and an uprated thrust-weight ratio of ≈180. The uprated engines are currently used on Falcon 9 full thrust, an iteration of the Falcon 9 launch vehicle with multiple other changes. The vehicle launched first on Flight 20 with eleven Orbcomm OG2 satellites.
In May 2016, SpaceX announced plans to further upgrade the Merlin 1D by increasing vacuum thrust to 914 kN (205,000 lbf) and sea-level thrust to 845 kN (190,000 lbf); according to SpaceX, the additional thrust will increase Falcon 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 9 vehicle, the increase in performance is solely due to uprated engines and no other significant changes to the vehicle are publicly planned.
Merlin 1D Vacuum
In late 2012, Elon Musk tweeted an image of the Merlin 1D Vacuum firing on the test stand and stated: "Now test firing our most advanced engine, the Merlin 1D Vacuum, at 80 tons of thrust."  As of 2018[update], SpaceX's Falcon 9 product page lists the thrust of the Merlin Vacuum on the second stage of the launcher at 934 kN (210,000 lbf) and a specific impulse of 348 seconds in vacuum conditions. The increase is due to the greater expansion ratio afforded by operating in a vacuum, now 165:1 using an updated nozzle extension.
According to a SpaceX-released Payload User's Guide, the Merlin 1D Vacuum can throttle down to 39% of its maximum thrust, or 360 kN (81,000 lbf).
SpaceX uses a dual-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 the first stage, one on the second stage) used on the Falcon 9 launch vehicle.
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 maximum 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.
By June 2015, SpaceX was producing Merlin engines at the rate of four Merlin 1D engines per week, with a total production capacity in the factory of a maximum of five per week.
In February 2016, SpaceX indicated that the company will need to build hundreds of engines a year in order to support a Falcon 9/Falcon Heavy build rate of 30 rocket cores per year by the end of 2016.
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- Falcon (rocket family) - SpaceX rockets exclusively using LOX/RP-1 launch vehicle engines.
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- Comparison of orbital rocket engines
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The Merlin 1D weighs 1030 pounds, including the hydraulic steering (TVC) actuators. It makes 162,500 pounds of thrust in vacuum. that is nearly 158 thrust/weight. The new full thrust variant weighs the same and makes about 185,500 lbs force in vacuum.
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the first privately-developed liquid-fueled rocket to successfully reach orbit.
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(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%.
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– 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.
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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.'
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the Merlin engine has now successfully flown to space more than 180 times (with 130 on the Merlin 1D), reliably delivering multiple payloads for U.S, Government and commercial customers to complex orbits. Due to the engine's highly manufacturable design, SpaceX is now producing 4 Merlin 1D engines per week, with current production capacity to produce 5 engines per week, far more than any other private rocket engine producer in the world.
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