Merlin is a family of
rocket engine
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
s developed by
SpaceX
Space Exploration Technologies Corp. (SpaceX) is an American spacecraft manufacturer, launcher, and a satellite communications corporation headquartered in Hawthorne, California. It was founded in 2002 by Elon Musk with the stated goal of ...
for use on its
Falcon 1,
Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and payl ...
and
Falcon Heavy launch vehicles. Merlin engines use
RP-1 and
liquid oxygen
Liquid oxygen—abbreviated LOx, LOX or Lox in the aerospace, submarine and gas industries—is the liquid form of molecular oxygen. It was used as the oxidizer in the first liquid-fueled rocket invented in 1926 by Robert H. Goddard, an applica ...
as
rocket propellant
Rocket propellant is the reaction mass of a rocket. This reaction mass is ejected at the highest achievable velocity from a rocket engine
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propuls ...
s in a
gas-generator power cycle. The Merlin engine was originally designed for sea recovery and reuse, but since 2016 the entire Falcon 9 booster is recovered for reuse by landing vertically on a landing pad using one of its nine Merlin engines.
The injector at the heart of Merlin is of the
pintle
A pintle is a pin or bolt, usually inserted into a gudgeon, which is used as part of a pivot or hinge. Other applications include pintle and lunette ring for towing, and pintle pins securing casters in furniture.
Use
Pintle/gudgeon sets have ma ...
type that was first used in the
Apollo Lunar Module
The Apollo Lunar Module (LM ), originally designated the Lunar Excursion Module (LEM), was the lunar lander spacecraft that was flown between lunar orbit and the Moon's surface during the United States' Apollo program. It was the first crewed ...
landing engine (
LMDE). Propellants are fed by a single-shaft, dual-
impeller turbopump. The turbopump also provides high-pressure fluid for the hydraulic
actuator
An actuator is a component of a machine that is responsible for moving and controlling a mechanism or system, for example by opening a valve. In simple terms, it is a "mover".
An actuator requires a control device (controlled by control signal) a ...
s, which then recycles into the low-pressure inlet. This eliminates the need for a separate
hydraulic drive system and means that
thrust vectoring
Thrust vectoring, also known as thrust vector control (TVC), is the ability of an aircraft, rocket, or other vehicle to manipulate the direction of the thrust from its engine(s) or motor(s) to control the attitude or angular velocity of the v ...
control failure by running out of
hydraulic fluid
A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoe ...
is not possible.
Revisions
Merlin 1A
The initial version, the Merlin 1A, used an inexpensive, expendable,
ablatively cooled carbon-fiber-reinforced polymer composite
Composite or compositing may refer to:
Materials
* Composite material, a material that is made from several different substances
** Metal matrix composite, composed of metal and other parts
** Cermet, a composite of ceramic and metallic materials
...
nozzle and produced 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
The RS-88 is a liquid-fueled rocket engine burning ethanol as fuel, and using liquid oxygen (LOX) as the oxidizer. It was designed and built by Rocketdyne, originally for the NASA Bantam System Technology program (1997).
In 2003, it was designa ...
(Bantam) and NASA
Fastrac
Formation Autonomy Spacecraft with Thrust, Relnav, Attitude and Crosslink (or FASTRAC) is a pair of nanosatellites (respectively named ''Sara-Lily'' and ''Emma'') developed and built by students at The University of Texas at Austin. The project ...
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 casting
Investment casting is an industrial process based on lost-wax casting, one of the oldest known metal-forming techniques. The term "lost-wax casting" can also refer to modern investment casting processes.
Investment casting has been used in vari ...
s, 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 (i.e., working fluid is only admitted through part of the rotation of the turbine; an arc, not the whole circumference) impulse design and turned at up to 20,000 rpm, with a total weight of .
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 of thrust at sea level and in vacuum, and performing with a specific impulse of at sea level and in vacuum.
The Merlin 1B was enhanced over the 1A with a
turbine
A turbine ( or ) (from the Greek , ''tyrbē'', or Latin ''turbo'', meaning vortex) is a rotary mechanical device that extracts energy from a fluid flow and converts it into useful work. The work produced by a turbine can be used for generating e ...
upgrade, increasing power output from to .
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 .
Another notable change over the 1A was the move to
TEA
Tea is an aromatic beverage prepared by pouring hot or boiling water over cured or fresh leaves of '' Camellia sinensis'', an evergreen shrub native to East Asia which probably originated in the borderlands of southwestern China and northe ...
–
TEB (
pyrophoric
A substance is pyrophoric (from grc-gre, πυροφόρος, , 'fire-bearing') if it ignites spontaneously in air at or below (for gases) or within 5 minutes after coming into contact with air (for liquids and solids). Examples are organolith ...
) ignition over torch ignition.
Initial use of the Merlin 1B was to be on the
Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and payl ...
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
![SpaceX factory Merlin engine](https://upload.wikimedia.org/wikipedia/commons/4/41/SpaceX_factory_Merlin_engine.jpg)
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
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often dis ...
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%.
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
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 , a vacuum thrust of and a vacuum
specific impulse of . In this configuration, the engine consumed 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 per second of kerosene flow and are able to absorb of
heat energy
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is al ...
.
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 o
CRS-1 launch videoas 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 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
niobium alloy A niobium alloy is one in which the most common element is niobium.
Alloys used for the production of other alloys
The most common commercial niobium alloys are ferroniobium and nickel-niobium, produced by thermite reduction of appropriate mixtures ...
expansion nozzle is
radiatively cooled. The engine delivers a vacuum thrust of and a vacuum
specific impulse of .
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 and as of March 10, 2009, the Merlin Vacuum engine operates with the greatest efficiency of any American-made hydrocarbon-fueled rocket engine.
An unplanned test of a modified Merlin Vacuum engine was made in December 2010. Shortly before the scheduled
second flight
This single contains the opening and ending themes for the anime series ''Onegai Twins''. Second Flight by Kotoko and Hiromi Satō is the opening theme while Asu e no Namida by Mami Kawada is the ending theme.
Track listing
#Second Flight ( K ...
of the
Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and payl ...
, two cracks were discovered in the niobium-alloy-sheet nozzle of the Merlin Vacuum engine. The engineering solution was to cut off the lower 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. The modified engine successfully placed the second stage into an orbit of altitude.
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.
In 2011, performance goals for the engine were a vacuum thrust of , a vacuum specific impulse (''I''
sp) of , an expansion ratio of 16 (as opposed to the previous 14.5 of the Merlin 1C) and chamber pressure in the "sweet spot" of . Merlin 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%.
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, 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 and a thrust-to-weight ratio of 180.
[ Als]
at SoundCloud
. This version of the Merlin 1D was used on
Falcon 9 Full Thrust and first flew on
Flight 20.
In May 2016, SpaceX announced plans to further upgrade the Merlin 1D by increasing vacuum thrust to and sea-level thrust to ; according to SpaceX, the additional thrust will increase the 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 upgraded engines, and no other significant changes to the vehicle are publicly planned.
In May 2018, ahead of the first flight of
Falcon 9 Block 5, SpaceX announced that the goal had been achieved.
The Merlin 1D is now close to the sea-level thrust of the retired
Rocketdyne H-1
The Rocketdyne H-1 was a thrust liquid-propellant rocket engine burning LOX and RP-1. The H-1 was developed for use in the S-I and S-IB first stages of the Saturn I and Saturn IB rockets, respectively, where it was used in clusters of eight e ...
/
RS-27
The RS-27 was a liquid-propellant rocket engine developed in 1974 by Rocketdyne to replace the aging Rocketdyne_S-3D, MB-3 in the Delta. Incorporating components of the venerable MB-3 and the H-1 designs, the RS-27 was a modernized version of t ...
engines used on
Saturn I
The Saturn I was a rocket designed as the United States' first medium lift launch vehicle for up to low Earth orbit payloads.Terminology has changed since the 1960s; back then, 20,000 pounds was considered "heavy lift". The rocket's first stag ...
,
Saturn IB
The Saturn IB (also known as the uprated Saturn I) was an American launch vehicle commissioned by the NASA, National Aeronautics and Space Administration (NASA) for the Apollo program. It uprated the Saturn I by replacing the S-IV second stage (, ...
, and
Delta II
Delta II was an expendable launch system, originally designed and built by McDonnell Douglas. Delta II was part of the Delta rocket family and entered service in 1989. Delta II vehicles included the Delta 6000, and the two later Delta 7000 va ...
.
Anomalies
The March 18, 2020, launch of
Starlink satellites on board a
Falcon 9
Falcon 9 is a partially reusable medium lift launch vehicle that can carry cargo and crew into Earth orbit, produced by American aerospace company SpaceX.
The rocket has two stages. The first (booster) stage carries the second stage and payl ...
experienced an early engine shutdown on ascent. The shutdown occurred 2 minutes 22 seconds into the flight and was accompanied with an "event" seen on camera. The rest of the Falcon 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
Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a colorless, flammable organic compound with a pungent alcoholic odor. As an isopropyl group linked to a hydroxyl group (chemical formula ) it is the simple ...
, 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.
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.
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.
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.
As of 2020, the thrust of the Merlin 1D Vacuum is
with a specific impulse of 348 seconds,
the highest specific impulse ever for a U.S. hydrocarbon rocket engine.
The increase is due to the greater
expansion ratio The expansion ratio of a liquefied and cryogenic substance is the volume of a given amount of that substance in liquid form compared to the volume of the same amount of substance in gaseous form, at room temperature and normal atmospheric pressur ...
afforded by operating in vacuum, now 165:1 using an updated nozzle extension.
The engine can throttle down to 39% of its maximum thrust, or .
Design
Engine control
SpaceX uses a triple-redundant design in the Merlin engine 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.
Turbopump
The Merlin LOX/RP-1
turbopump used on Merlin engines 1A–1C was designed and developed by Barber-Nichols.
It spins at 36,000
revolutions per minute
Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines.
Standards
ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...
, delivering .
Gas generator
The LOX/RP-1 turbopump on each Merlin engine is powered by a fuel-rich open-cycle
gas generator
A gas generator is a device for generating gas. A gas generator may create gas by a chemical reaction or from a solid or liquid source, when storing a pressurized gas is undesirable or impractical.
The term often refers to a device that uses a ...
similar to that used in the Apollo-era
Rocketdyne F-1 engine.
[Sutton, “History of Liquid Propellant Rocket Engines”, AIAA 006]
Production
, 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 , 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 that it had manufactured the 100th Merlin 1D engine and that engines were now being produced at a rate of four per week, soon to be increased to five.
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.
Each Falcon 9 booster uses nine Merlin engines, and the second stage uses one Merlin vacuum engine. The second stage is expended, so each launch consumes one Merlin Vacuum engine. SpaceX designed the booster with its engines to be recovered for reuse by propulsive landing, and the first recovered booster was reused in March 2017. By 2020, only five of the 26 Falcon 9 launches used new boosters. These 26 launches used 25 new Merlin Vacuum engines and 45 new sea-level Merlin engines. By 2021, only two of the 31 Falcon 9 launches used new boosters. In the first half of 2022, there were 27 Falcon 9 launches, two of which used a new booster.
See also
*
SpaceX Draco
The SpaceX Draco is a hypergolic liquid rocket engine designed and built by SpaceX for use in their space capsules. Two engine types have been built to date: Draco and SuperDraco.
The original Draco thruster is a small rocket engine for use on t ...
– SpaceX
RCS
RCS may refer to:
Organisations
*Racing Club de Strasbourg Alsace
* Radio Corporation of Singapore
*Radcliffe Choral Society
* Rawmarsh Community School
*Red Crescent Society
*Red Cross Society
* Representation of Czechs and Slovaks, a football t ...
thruster for
SpaceX Dragon
*
SpaceX Kestrel
The SpaceX Kestrel was an LOX/RP-1 pressure-fed rocket engine. The Kestrel engine was developed in the 2000s by SpaceX for upper stage use on the Falcon 1 rocket. Kestrel is no longer being manufactured; the last flight of Falcon 1 was in 2009 ...
– SpaceX small upper stage engine for Falcon 1
*
SpaceX Raptor – SpaceX methane/LOX engine for the
Starship
A starship, starcraft, or interstellar spacecraft is a theoretical spacecraft designed for interstellar travel, traveling between planetary systems.
The term is mostly found in science fiction. Reference to a "star-ship" appears as early as 188 ...
*
Falcon 1 – First rocket powered by
Merlin 1A
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 cycle, gas-generator power cy ...
*
Comparison of orbital rocket engines
This page is an incomplete list of orbital rocket engine data and specifications.
Current, Upcoming, and In-Development rocket engines
Retired and canceled rocket engines
See also
* Comparison of orbital launch systems
* Comparison of o ...
*
Rocket engine
A rocket engine uses stored rocket propellants as the reaction mass for forming a high-speed propulsive jet of fluid, usually high-temperature gas. Rocket engines are reaction engines, producing thrust by ejecting mass rearward, in accordanc ...
*
Pintle injector
The pintle injector is a type of propellant injector for a bipropellant rocket engine. Like any other injector, its purpose is to ensure appropriate flow rate and intermixing of the propellants as they are forcibly injected under high pressure into ...
*
TR-106
The TR-106 or low-cost pintle engine (LCPE) was a developmental rocket engine designed by TRW Inc., TRW under the Space Launch Initiative to reduce the cost of Launch service provider, launch services and space flight. Operating on LOX/LH2 the en ...
– Low Cost Pintle Engine (LCPE) using LOX/LH2 developed by TRW in 2000
*
TR-107
The TR-107 was a developmental rocket engine designed in 2002 by Northrop Grumman for NASA and DoD funded Space Launch Initiative. Operating on LOX/RP-1 the engine was throttleable and had a thrust of at a chamber pressure of , making it one of ...
– RP-1 engine developed under SLI for future reusable launch vehicles
*
RS-27A
The RS-27A is a liquid-fuel rocket engine developed in 1980s by Rocketdyne for use on the first stage of the Delta II and Delta III launch vehicles. It provides of thrust burning RP-1 and LOX in a gas-generator cycle. The engine is a modified ver ...
– RP-1 engine used in the US
Delta II
Delta II was an expendable launch system, originally designed and built by McDonnell Douglas. Delta II was part of the Delta rocket family and entered service in 1989. Delta II vehicles included the Delta 6000, and the two later Delta 7000 va ...
launcher; Saturn 1B
H-1 heritage
*
Rocketdyne F-1 – LOX/RP-1 main engine of the
Saturn V
Saturn V is a retired American super heavy-lift launch vehicle developed by NASA under the Apollo program for human exploration of the Moon. The rocket was human-rated, with multistage rocket, three stages, and powered with liquid-propellant r ...
moon rocket
References
Sources
*
External links
SpaceX official website
{{DEFAULTSORT:Merlin (Rocket Engine)
SpaceX rocket engines
Rocket engines using kerosene propellant
Rocket engines using the gas-generator cycle
Rocket engines of the United States