Since the founding ofSpaceX in 2002, the company has developed four families ofrocket engines —Merlin,Kestrel,Draco andSuperDraco — and since 2016developed theRaptor methane rocket engine and after 2020, a line ofmethalox thrusters.
In the first ten years of SpaceX, led by engineerTom Mueller, the company developed a variety ofliquid-propellant rocket engines, with at least one more of that type under development.As of October 2012[update], each of the engines developed to date—Kestrel,Merlin 1,Draco and Super Draco—had been developed for initial use in theSpaceX launch vehicles—Falcon 1,Falcon 9, andFalcon Heavy—or for theDragoncapsule.[1] Each main engine developed by 2012 has been Kerosene-based, usingRP-1 as the fuel withliquid oxygen (LOX) as the oxidizer, while theRCS control thruster engines have used storablehypergolicpropellants.
In November 2012, at a meeting of theRoyal Aeronautical Society in London, United Kingdom, SpaceX announced that they planned to developmethane-based engines for their future rockets. These engines would usestaged cycle combustion, for higher efficiency similar to the system used on the former Soviet Union'sNK-33 engine.[2][needs update]
By mid-2015, SpaceX had developed a total of 9 rocket engines architectures in the first 13 years of the company's existence.[3]
SpaceX has developed twokerosene-based engines through 2013, the Merlin 1 and Kestrel, and has publicly discussed a much larger concept engine high-level design namedMerlin 2. Merlin 1 powered the first stage of theFalcon 1 launch vehicle and is used both on the first and second stages of theFalcon 9 andFalcon Heavy launch vehicles. The Falcon 1 second stage was powered by a Kestrel engine.
Merlin 1 is a family ofLOX/RP-1 rocket engines developed 2003–2012.Merlin 1A andMerlin 1B utilized anablatively-cooledcarbon-fibercomposite nozzle. Merlin 1A produced 340 kilonewtons (76,000 lbf) of thrust and was used to power the first stage of the first twoFalcon 1 flights in 2006 and 2007. Merlin 1B had a somewhat more powerfulturbo-pump, and generated more thrust, but was never flown on a flight vehicle before SpaceX's move to the Merlin 1C.
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TheMerlin 1C was the first in the family to use aregeneratively-cooled nozzle and combustion chamber. It was first fired with a full mission duty firing in 2007,[4]first flew on thethird Falcon 1 mission in August 2008,[5] powered the "firstprivately-developed liquid-fueled rocket to successfully reach orbit" (Falcon 1 Flight 4) in September 2008,[5]and subsequently powered the first five Falcon 9 flights — each flown with aversion 1.0 Falcon 9 launch vehicle — from 2010 through 2013.[6]
TheMerlin 1D, developed in 2011–2012 also has a regeneratively-cooled nozzle and combustion chamber. It has 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 of 9.7 MPa (1,410 psi). A new feature for the engine is the ability to throttle from 100% to 70%.[7] The engine's 150:1 thrust-to-weight ratio is the highest ever achieved for a rocket engine.[8][9]The first flight of the Merlin 1D engine was also the maidenFalcon 9 v1.1 flight.[10] On September 29, 2013, theFalcon 9 Flight 6 mission successfully launched theCanadian Space Agency'sCASSIOPE satellite into polar orbit, and proved that the Merlin 1D could be restarted to control the first stage's re-entry back into the atmosphere—part of theSpaceX reusable launch system flight test program—a necessary step in making the rocket reusable.[11]
Kestrel was aLOX/RP-1pressure-fed rocket engine, and was developed by SpaceX as the Falcon 1 rocket's second stage main engine; it was used in 2006–2009. It was built around the samepintle architecture as SpaceX's Merlin engine but does not have aturbo-pump, and is fed only bytank pressure. Its nozzle wasablatively-cooled in the chamber andradiatively-cooled in the throat, and is fabricated from a high strengthniobium alloy. Thrust vector control is provided by electro-mechanical actuators on the engine dome for pitch and yaw. Roll control – and attitude control during the coast phase – is provided by heliumcold gas thrusters.[12][13]
In November 2012,methalox engines came on the scene when SpaceX CEOElon Musk announced a new direction for propulsion side of the company: developingmethane/LOX rocket engines.[2]SpaceX work on methane/LOX (methalox) engines is strictly to support the company's Mars technology development program. They had no plans to build an upper stage engine for the Falcon 9 or Falcon Heavy using methalox propellant.[14] However, on November 7, 2018, Elon Musk tweeted, "Falcon 9 second stage will be upgraded to be like a mini-BFR Ship," which may imply the use of a Raptor engine on this new second stage.The focus of the new engine development program is exclusively on the full-size Raptor engine for the Mars-focused mission.[14]
Raptor is a family ofmethane/liquid oxygen rocket engines under development by SpaceX since the late 2000s,[2] althoughLH2/LOXpropellant mix was originally under study when the Raptor concept development work began in 2009.[15] When first mentioned by SpaceX in 2009, the term "Raptor" was applied exclusively to an upper stage engine concept.[2] SpaceX discussed in October 2013 that they intended to build a family of methane-based Raptor rocket engines,[16] initially announcing that the engine would achieve 2.94 meganewtons (661,000 lbf) vacuum thrust.[16]In February 2014, they announced that the Raptor engine would be used on theMars Colonial Transporter. The booster would utilize multiple Raptor engines, similar to the use of nineMerlin 1s on eachFalcon 9 booster core.[17] The following month, SpaceX confirmed that as of March 2014[update], all Raptor development work is exclusively on this single very large rocket engine, and that no smaller Raptor engines were in the current development mix.[14]
The Raptor methane/LOX engine uses a highly efficient and theoretically more reliablefull-flow staged combustion cycle,[17] a departure from theopengas generator cycle system and LOX/kerosene propellants used on the current Merlin 1 engine series.[2] As of February 2014[update], preliminary designs of Raptor were looking at producing 4.4 meganewtons (1,000,000 lbf) of thrust with a vacuumspecific impulse (Isp) of 363 seconds (3.56 km/s) and a sea-level Isp of 321 seconds (3.15 km/s),[17][18]although later concept sizes being looked at were closer to 2.2 MN (500,000 lbf).
Initial component-level testing of Raptor technology began in May 2014, with an injector element test.[16][19]The first complete Raptor development engine, approximately one-third the size of the full-scale engines planned for the use on various parts of the Starship, with approximately 1,000 kN (220,000 lbf) thrust, began testing on aground test stand in September 2016. The test nozzle has an expansion ratio of only 150, in order to eliminateflow separation problems while tested in Earth's atmosphere.[20]
Raptor's full-flow staged combustion cycle will pass 100 percent of the oxidizer (with a low-fuel ratio) to power the oxygen turbine pump, and 100 percent of the fuel (with a low-oxygen ratio) to power the methane turbine pump. Both streams—oxidizer and fuel—will be completely in thegas phase before they enter thecombustion chamber. Prior to 2016, only two full-flow staged combustion rocket engines had ever progressed sufficiently to be tested on test stands: theSovietRD-270 project in the 1960s and theAerojet RocketdyneIntegrated powerhead demonstration project in the mid-2000s, which did not test a complete engine but rather only the powerhead.[17][20]
Other characteristics of the full-flow design are projected to further increase performance or reliability, with the possibility to do design trade offs of one against the other:[17]
SpaceX is developing gaseous Methox thrusters that will utilize gas that needs to be vented from the propellant tanks to control attitude.
In his announcement of theInterplanetary Transport System (ITS) at the 67thInternational Astronautical Congress on September 27, 2016, Elon Musk indicated that all of the reaction control system thrusters for the ITS (subsequently renamed toStarship) would operate from thegaseous methane and oxygen supply in each of those vehicles, and that new thrusters would be developed for the purpose.[21][22][23]
By 2020, one set of high‑thrust methoxRCS thrusters were planned to be located mid‑body on theStarship HLS lunar-landing Starship variant and will be used during the final "tens of meters" of any terminal lunar descent and landing,[24] as well as used for departing from the lunar surface.[24]: 50:30 The mid-body design is specifically to address the problem of lunar surface erosion and the creation of Moon-wide dust from use of Raptor engines that are located at the base of Starship.
In 2021, hot gas thrusters were seen on the body of Starship prototypes,[25] however it is unclear whether these will be used on future Starships.
Draco arehypergolicliquid-propellant rocket engines that utilize a mixture ofmonomethyl hydrazinefuel andnitrogen tetroxideoxidizer. Each Dracothruster generates 400 newtons (90 lbf) of thrust.[26] They are used asReaction Control System (RCS) thrusters on both theDragon spacecraft, and on theFalcon 9 launch vehicle second-stage.[27]
SuperDracostorable-propellanthypergolic engines generate 67,000 newtons (15,000 lbf) of thrust, making the SuperDraco the third most powerful engine developed by SpaceX, more than 200 times[28] more powerful than the regular Draco RCS thruster engines. By comparison, it is more than two times as powerful as theKestrel engine used in SpaceX's Falcon 1 launch vehicle second stage, and about 1/9 the thrust of aMerlin 1D engine. They are used asLaunch Abort System engines on theSpaceX Dragon 2 for crew transport tolow Earth orbit.[29]
[SpaceX develops] all of our engines in-house and in the United States. The company is currently on its fourth generation of booster engines, which have included the Merlin 1A, the Merlin 1B, the Merlin 1C, and the Merlin 1D. In addition, we have developed the Kestrel vacuum engine, the Merlin 1C vacuum engine, and the Merlin 1D vacuum engine for our second stages on Falcon 1, Falcon 9 and Falcon Heavy. SpaceX has also developed Draco and SuperDraco engines which provide in-space and abort propulsion capability for Dragon ... We are also moving forward with significant R&D on a next generation rocket engine Raptor. ... SpaceX has successfully developed the 9 rocket engines mentioned above in the past 13 years.
our focus is the full Raptor size
for the terminal descent of Starship, a few tens of meters before we touch down on the lunar surface, we actually use a high-thrust RCS system, so that we don't impinge on the surface of the Moon with the high-thrust Raptor engines. ... uses the same methane and oxygen propellants as Raptor.
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