Merlin (rocket engine family)

Variants

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.

Merlin 1A

The initial version, the Merlin 1A, used an inexpensive, expendable, ablatively cooled carbon-fiber-reinforced polymer composite nozzle, and produced 340 kN (77,000 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 impulse design and turned at up to 20,000 rpm, with a total weight of 150 lb.

Merlin 1B

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

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 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 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.

Merlin 1D

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 (I_sp) 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 (I_sp) of 282 s and a vacuum specific impulse (I_sp) 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.

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 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 165,000 pounds-force (730 kN). 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 weight of 1,030 lb (470 kg) including thrust actuators, a current vacuum thrust of 162,500 pounds-force (723 kN), and an uprated vacuum thrust of 185,500 pounds-force (825 kN), which still weighs the same. 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 uprate the Merlin 1D by increasing vacuum thrust to 914 kN and sea-level thrust to 845 kN; 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

A vacuum version of the Merlin 1D engine was developed for the Falcon 9 v1.1 and the Falcon Heavy second stage.

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." Currently the official 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 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).

Engine control

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 first stage, one on second stage) used on a Falcon 9 launch.

Turbopump

The Merlin LOX/RP-1 turbopump used on Merlin engines 1A–1C was designed and developed by Barber-Nichols.

Production

As of August 2011, 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.