Raptor (rocket Engine Family)
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Raptor Rocket Engine Family

SpaceX Raptor
SpaceX sea-level Raptor at Hawthorne - 2.jpg
Raptor at SpaceX Hawthorne facility
Country of originUnited States
Application1st and 2nd stage propulsion of the Starship vehicle
StatusUnder development
Liquid-fuel engine
PropellantLiquid oxygen / liquid methane
Mixture ratio3.55 (78% O2, 22% CH4)[1][2]
CycleFull-flow staged combustion
Pumps2 turbopumps
Nozzle ratio40 (sea-level), >150 (vacuum)
Thrust90-225 tf (0.88-2.21 MN)[3][4]
Throttle range40-100%[3]
Thrust-to-weight ratio
  • 200, sea-level, goal[5]
  • <120, vacuum[6]
Chamber pressure
  • 300 bar (4,400 psi)[7][8]
  • 330 bar (4,800 psi),[4] ~7s test
Isp (vac.)380 s (3.7 km/s), goal[7]
Isp (SL)330 s (3.2 km/s)[7]
Mass flow
  • ~650 kg/s (1,400 lb/s):[9]
    • ~510 kg/s (1,100 lb/s), O2[10]
    • ~140 kg/s (310 lb/s), CH4[10]
Length3.1 m (10 ft)[11]
Diameter1.3 m (4 ft 3 in)[12]
Dry weight1,500 kg (3,300 lb), goal[6]
Used in

Raptor is a family of full-flow staged combustion cycle rocket engines developed and manufactured by SpaceX, for use on the in-development Starship fully reusable launch vehicle. The engine is powered by cryogenic liquid methane and liquid oxygen (LOX), rather than the RP-1 kerosene and LOX used in SpaceX's prior Merlin and Kestrel rocket engines. The Raptor engine has more than twice the thrust of SpaceX's Merlin engine that powers their current Falcon 9 and Falcon Heavy launch vehicles.

Raptor will be used in both stages of the two-stage-to-orbit, super-heavy-lift Starship system[13] launch vehicle,[14] which will supersede Falcon 9 and Falcon Heavy.[15] Starship will be used in various applications, including Earth-orbit satellite delivery, deployment of a large portion of SpaceX's own Starlink megaconstellation, and the exploration and colonization of Mars.[16]

The Raptor engine first flew on the Starhopper test vehicle in July 2019 and became the first full-flow staged combustion rocket engine ever flown.[17] As of August 2020, Raptor holds the record for the highest combustion chamber pressure ever reached by an operational rocket engine, at 330 bar (33 MPa; 4,800 psi), surpassing the record held by the RD-701 rocket engine at 300 bar (30 MPa; 4,400 psi).[18][19]


Raptor engine combustion scheme

The Raptor engine is powered by subcooled liquid methane and subcooled liquid oxygen in a full-flow staged combustion cycle, a departure from the simpler "open-cycle" gas generator system and LOX/kerosene propellants that current Merlin engines use.[20] The Space Shuttle's RS-25 also used a staged combustion cycle,[21] as do several Russian rocket engines, including the RD-180[20] and the RD-191.[22]

The stated design size for the Raptor engine varied widely during 2012-2017 as detailed design continued, from a high target of 8,200 kN (1,800,000 lbf) of vacuum thrust[23] to a more recent, much lower target of 1,900 kN (430,000 lbf).[24] In its 2017 iteration, the operational engine is expected to have a vacuum Isp of 382 s (3,750 m/s) and a sea-level Isp of 334 s (3,280 m/s).[25]

The Raptor engine is designed for the use of deep cryogenic propellants--fluids cooled to near their freezing points, which is typical for cryogenic rocket engines.[26] The use of subcooled propellants increases propellant density, increasing engine performance and allowing more propellant to be stored within the vehicle's tanks. Specific impulse is increased, and the risk of cavitation at inputs to the turbopumps is reduced due to the higher mass flow rate per unit power generated.[22]

Engine ignition for all Raptor engines, both on the pad and in the air, is handled by dual-redundant spark-plug lit torch igniters,[27] eliminating the need for a dedicated, consumable igniter fluid, as used on Merlin.[22]

Raptor has been claimed to be able to deliver "long life ... and more benign turbine environments".[28][22] Specifically, Raptor utilizes a full-flow staged combustion cycle, where all the oxygen--with a low-fuel ratio--will power an oxygen turbopump, and all the fuel--with a low-oxygen ratio--will power a methane turbopump. Both streams--oxidizer and fuel--will be mixed completely in the gas phase before they enter the combustion chamber. Prior to 2014, only two full-flow staged-combustion rocket engines had ever progressed sufficiently to be tested on test stands: the Soviet RD-270 project in the 1960s and the Aerojet Rocketdyne Integrated Powerhead Demonstrator in the mid-2000s.[29][22][30]

Additional characteristics of the full-flow design, projected to further increase performance or reliability include:[30]

  • eliminating the fuel-oxidizer turbine interseal, which is a potential point of failure in more traditional engine designs[clarification needed];
  • lower pressures are required through the pumping system, increasing life span and further reducing risk of catastrophic failure;
  • ability to increase the combustion-chamber pressure, thereby either increasing overall performance or "by using cooler gases, providing the same performance as a standard staged combustion engine but with much less stress on materials, thus significantly reducing material fatigue or [engine] weight".

SpaceX aims at a lifetime of 1000 flights for Raptor.[31]

Many components of early Raptor prototypes were manufactured using 3D printing, including turbopumps and injectors, with the effect of increasing the speed of development and iterative testing.[26] The 2016 subscale development engine had 40% (by mass) of its parts manufactured by 3D printing.[22]

In 2019, engine manifolds were cast from SpaceX's in-house developed SX300 inconel superalloy, soon to be changed to SX500.[32]

The Raptor engine uses a large number of coaxial swirl injectors[33] to admit propellants to the combustion chamber, rather than pintle injectors used on the previous Merlin rocket engines that SpaceX mass-produced for its Falcon family of launch vehicles.[34]

In 2019 the (marginal) cost of the engine was stated to be approaching $1 million.[35] SpaceX plans to mass-produce up to 500 Raptor engines per year, each costing less than $250,000.[36][35][37]


Raptor engine development from 2009 to 2015 was funded exclusively through private investment by SpaceX, with no government funding.[28][38] In January 2016, SpaceX did agree with the US Air Force to take US$33.6 million in defense department funding in order to develop a particular Raptor model: a prototype of a new upper-stage variant of the Raptor engine designed for potential use as an upper stage on Falcon 9 and Falcon Heavy, with SpaceX agreeing to fund at least US$67.3 million on the same upper-stage development project, on a minimum 2:1 private-to-government funding basis.[39][40]

Initial concept

An advanced rocket engine design project named Raptor, burning hydrogen and oxygen propellants, was first publicly discussed by SpaceX's Max Vozoff at the American Institute of Aeronautics and Astronautics Commercial Crew/Cargo symposium in 2009.[41] As of April 2011, SpaceX had a small number of staff working on the Raptor upper-stage engine, then still a /LOX concept, at a low level of priority.[42] Further mention of the development program occurred in 2011.[43] In March 2012, news accounts asserted that the Raptor upper-stage engine development program was underway, but that details were not being publicly released.[44]

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", but declined to specify which fuel would be used.[45] They indicated that details on a new SpaceX rocket would be forthcoming in "one to three years" and that the large engine was intended for the next-generation launch vehicle using multiple of these large engines, that would be expected to launch payload masses of the order of 150 to 200 tonnes (150,000 to 200,000 kg; 330,000 to 440,000 lb) to low Earth orbit, exceeding the payload mass capability of the NASA Space Launch System.[45]

Methane and component-level development

In November 2012, Musk announced a new direction for the propulsion division of SpaceX: developing methane-fueled rocket engines.[46] He further indicated that the Raptor concept would now become a methane-based design,[46] and that methane would be the fuel of choice for SpaceX's plans for Mars colonization.[30]

Because of the presence of water underground and carbon dioxide in the atmosphere of Mars, methane, a simple hydrocarbon, can easily be synthesized on Mars using the Sabatier reaction.[47] In-situ resource production on Mars has been examined by NASA and found to be viable for oxygen, water, and methane production.[48] According to a study published by researchers from the Colorado School of Mines, in-situ resource utilization such as methane from Mars makes space missions more feasible technically and economically and enables reusability.[49]

When first mentioned by SpaceX in 2009, the term "Raptor" was applied exclusively to an upper-stage engine concept[41]--and 2012 pronouncements indicated that it was then still a concept for an upper stage engine[20]--but in early 2014 SpaceX confirmed that Raptor would be used both on a new second stage, as well as for the large (then, nominally a 10-meter-diameter) core of the Mars Colonial Transporter[30] (subsequently, in 2016, on both stages of the Interplanetary Transport System[50] and then, in 2017 on the Big Falcon Rocket).[51]

The earliest public hints that a staged-combustion methane engine was under consideration at SpaceX were given in May 2011 when SpaceX asked if the Air Force was interested in a methane-fueled engine as an option to compete with the mainline kerosene-fueled engine that had been requested in the USAF Reusable Booster System High Thrust Main Engine solicitation.[30]

Public information released in November 2012 indicated that SpaceX might have a family of Raptor-designated rocket engines in mind;[52] this was confirmed by SpaceX in October 2013.[16] However, in March 2014 SpaceX COO Gwynne Shotwell clarified that the focus of the new engine development program is exclusively on the full-size Raptor engine; smaller subscale methalox engines were not planned on the development path to the very large Raptor engine.[53]

In October 2013, SpaceX announced that they would be performing methane engine tests of Raptor engine components at the John C. Stennis Space Center in Hancock County, Mississippi,[54][55] and that SpaceX would add equipment to the existing test stand infrastructure in order to support liquid methane and hot gaseous methane[22] engine component testing.[56] In April 2014, SpaceX completed the requisite upgrades and maintenance to the Stennis test stand to prepare for testing of Raptor components,[57] and the engine component testing program began in earnest, focusing on the development of robust startup and shutdown procedures, something that is typically quite difficult to do for full-flow staged combustion cycle engines. Component testing at Stennis also allowed hardware characterization and verification of proprietary analytical software models that SpaceX developed to push the technology on this engine cycle that had little prior development work in the West.[22]

October 2013 was the first time SpaceX disclosed a nominal design thrust of the Raptor engine--2,900 kN (661,000 lbf)[16]--although early in 2014 they announced a Raptor engine with greater thrust, and in 2015, one with lower thrust that might better optimize thrust-to-weight.

In February 2014, Tom Mueller, the head of rocket engine development at SpaceX, revealed in a speech that Raptor was being designed for use on a vehicle where nine engines would "put over 100 tons of cargo up to Mars" and that the rocket would be more powerful than previously released publicly, producing greater than 4,400 kN (1,000,000 lbf).[30][58] A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (1,600,000 lbf) of sea-level thrust, 8,200 kN (1,800,000 lbf) of vacuum thrust, and a specific impulse (Isp) of 380 s (3,700 m/s) for a vacuum version.[59] Earlier information had estimated the design Isp under vacuum conditions as only 363 s (3,560 m/s).[30] Jeff Thornburg, who led development of the Raptor engine at SpaceX 2011-2015, noted that methane rocket engines have higher performance than kerosene/RP-1 and lower than hydrogen, with significantly fewer problems for long-term, multi-start engine designs than kerosene--methane is cleaner burning--and significantly lower cost than hydrogen, coupled with the ability to "live off the land" and produce methane directly from extraterrestrial sources.[60][61]

SpaceX successfully began development testing of injectors in 2014 and completed a full-power test of a full-scale oxygen preburner in 2015. 76 hot fire tests of the preburner, totaling some 400 seconds of test time, were executed from April-August 2015.[38] SpaceX completed its planned testing using NASA Stennis facilities in 2014 and 2015.[62]

In January 2015, Elon Musk stated that the thrust they were currently targeting was around 230 tonnes-force (2,300 kN; 510,000 lbf), much lower than older statements had mentioned.[63] By August 2015, an Elon Musk statement surfaced that indicated the oxidizer to fuel ratio of the engine would be approximately 3.8 to 1.[64]

In January 2016, the US Air Force awarded a US$33.6 million development contract to SpaceX to develop a prototype version of its methane-fueled reusable Raptor engine for use on the upper stage of the Falcon 9 and Falcon Heavy launch vehicles, which required double-matching funding by SpaceX of at least US$67.3 million. Work under the contract was expected to be completed in 2018, with engine performance testing to be done at NASA's John C. Stennis Space Center in Mississippi and Los Angeles Air Force Base, California.[39][40][needs update]

Testing and production

Testing of the Raptor's oxygen preburner at Stennis Space Center in 2015
First test firing of a Raptor development engine on 25 September 2016 in McGregor, Texas

Initial development testing[38] of Raptor methane engine components was done at the Stennis Space Center in Hancock County, Mississippi, where SpaceX added equipment to the existing infrastructure in order to support liquid methane engine testing.[16][56] Initial testing was limited to components of the Raptor engine, since the 440 kN (100,000 lbf) test stands at the E-2 complex at Stennis were not large enough to test the full Raptor engine. The development Raptor engine discussed in the October 2013 time frame relative to Stennis testing was designed to generate more than 2,900 kN (661,000 lbf) vacuum thrust.[16] A revised, higher-thrust, specification was discussed by the company in February 2014, but it was unclear whether that higher thrust was something that would be achieved with the initial development engines.[30]

Raptor engine component testing began in May 2014[57] at the E-2 test complex which SpaceX modified to support methane engine tests.[16] The first items tested were single Raptor injector elements,[65] various designs of high-volume gas injectors.[66] The modifications to the test stands made by SpaceX are now a part of the Stennis test infrastructure and are available to other users of the test facility after the SpaceX facility lease was completed.[16] SpaceX successfully completed a "round of main injector testing in late 2014" and a "full-power test of the oxygen preburner component" for Raptor by June 2015. Tests continued at least into September 2015.[38]

By early 2016, SpaceX had constructed a new engine test stand at their McGregor test site in central Texas for Raptor testing.[22][16]

By August 2016, the first integrated Raptor rocket engine, manufactured at the SpaceX Hawthorne facility in California, was shipped to the McGregor rocket engine test facility in Texas for development testing.[67] The engine had 1 MN (220,000 lbf) thrust, which makes it approximately one-third the size of the full-scale Raptor engine planned for flight tests in 2019/2020 timeframe. It is the first full-flow staged-combustion methalox engine ever to reach a test stand.[22] This 2016 development engine had "an expansion ratio of just 150, the maximum possible within Earth's atmosphere" to prevent flow separation problems.[22] It performed an initial 9-second firing test on 26 September 2016, the day before Musk's talk at the International Aeronautical Congress.

On 26 September 2016, Elon Musk tweeted two images of the first test firing of an integrated Raptor in SpaceX's McGregor test complex.[68][69] On the same day Musk revealed that their target performance for Raptor was a vacuum specific impulse of 382 s (3,750 m/s), with a thrust of 3 MN (670,000 lbf), a chamber pressure of 300 bar (30 MPa; 4,400 psi), and an expansion ratio of 150 for the vacuum-optimized variant.[70][71][72] When asked if the nozzle diameter for such version was 14 ft (4.3 m), he stated that it was pretty close to that dimension. He also disclosed that it used multi-stage turbopumps.[73][74] On the 27th he clarified that 150 expansion ratio was for the development version, that the production vacuum version would have an expansion ratio of 200.[75] Substantial additional technical details of the ITS propulsion were summarized in a technical article on the Raptor engine published the next week.[22]

By September 2017, the development Raptor engine--with 200 bars (20 MPa; 2,900 psi) chamber pressure--had undergone 1200 seconds of test fire testing in ground-test stands across 42 main engine tests, with the longest test being 100 seconds (which is limited by the capacity of the ground-test propellant tanks). As of September 2017, the first version of the flight engine is intended to operate at a chamber pressure of 250 bars (25 MPa; 3,600 psi), with the intent to raise it to 300 bars (30 MPa; 4,400 psi) at a later time.[76]

By September 2017, the 200 bars (20 MPa; 2,900 psi) sub-scale test engine, with a thrust of 1 meganewton (220,000 lbf) and "a new alloy to help its oxygen-rich turbopump resist oxidization, ... had completed 1200 seconds of firings across 42 tests."[77] This alloy is known as SX500 which is used to contain hot oxygen gas in the engine at up to 12000 psi. SX500 was created by the SpaceX metallurgy team.[78]

While plans for Raptor flight testing have consistently been on the new-generation fiber-composite-material construction flight vehicles since 2016, the specific vehicle was not clarified until October 2017, when it was indicated that initial suborbital test flights would occur with a Big Falcon Ship.[79] In November 2016, the first flight tests of the Raptor engine were projected to be on the Interplanetary Transport System, no earlier than the early 2020s.[22] By July 2017, the plan had been modified to do flight testing on a much smaller launch vehicle and spacecraft, and the new system architecture had "evolved quite a bit" since the ITS concept from 2016. A key driver of the 2017 architecture was to make the new system useful for substantial Earth-orbit and cislunar launches so that the new system might pay for itself, in part, through economic spaceflight activities in the near-Earth space zone.[80][14]

Elon Musk announced in September 2017 that the initial flight platform for any Raptor engine would be some part of the Big Falcon Rocket. BFR was a 9 m (30 ft)-diameter launch vehicle.[76] In October 2017, Musk clarified that "[initial flight testing will be with] a full-scale 9-meter-diameter ship doing short hops of a few hundred kilometers altitude and lateral distance ... [projected to be] fairly easy on the vehicle, as no heat shield is needed, we can have a large amount of reserve propellant and don't need the high area ratio, deep-space Raptor engines."[79]

Notably, Musk also announced that the new Raptor-powered BFR launch vehicle was planned to entirely replace both Falcon 9 and Falcon Heavy in the early 2020s, initially aiming at the Earth-orbit market, but SpaceX is explicitly designing in substantial capability to the spacecraft vehicles to support long-duration spaceflight in the cislunar and Mars mission environment as well. SpaceX intends this approach to bring significant cost savings which will help the company justify the development expense of designing and building the new launch vehicle design.[76] In addition to orbital spaceflight missions, BFR is being considered for the point-to-point Earth transportation market,[79] with ~30-60-minute flights being available between any two points on the globe.[76][81]

Flight tests

The first flight version of the Raptor engine arrived in McGregor, Texas in late January 2019.[82]

On 3 February 2019, SpaceX performed the first test of a flight version engine. The test lasted two seconds with the engine operating at 60 percent of rated thrust at a chamber pressure of 170 bars (17,000 kPa).[83] Just four days later, the test engine achieved the power levels needed for use in SpaceX Starship.[84] The engine reached 172 tonnes-force (1,690 kN; 380,000 lbf) thrust with a chamber pressure of 257 bars (25.7 MPa). The test was conducted using warm propellant, with expectations of a 10% to 20% increase in performance when switching to deep cryogenic temperatures for the propellant.[85] On 10 February 2019, Musk announced on Twitter that the flight version engine had attained a chamber pressure of 268.9 bars (26.89 MPa) on a test stand.[86] On 19 June 2020, Musk announced that the Raptor engine tests achieved the expected chamber combustion pressure of 300 bars (30 MPa) on a test stand.[7][8]

By March, serial number 2 (SN2) of the flight version Raptor engine had been delivered to the SpaceX South Texas Launch Site east of Brownsville, Texas for system integration testing on Starhopper, the first test article of Starship,[87] approximately one year ahead of schedule.[88] On 3 April 2019, SpaceX conducted a successful static fire test, firing the engine for a few seconds while the vehicle remained tethered to the ground.[89][90] A second tethered test followed just two days later, on 5 April 2019.[91]

Starship SN5 takes flight, powered by Raptor SN27

The first flight test of a Raptor engine, SN6, occurred on 25 July 2019, carrying Starhopper to an altitude of 20 metres (66 ft).[17] Starhopper, with SN6, was reflown on 27 August 2019, reaching an altitude of 150 metres (490 ft). A horizontal translation and descent onto a nearby landing pad concluded the roughly 1 minute flight.[92] On August 4, 2020 a single Raptor engine (SN27) propelled Starship prototype SN5 to an altitude of 150 metres (490 ft) in a replication of the earlier Starhopper flight; this was the first flight of a full-size Starship prototype vehicle. The Raptor engine was mounted off-center and controlled the Starship during lift off, horizontal traverse, and landing. The total flight time was approximately 50 seconds.[93]

In August 2020, a ground stand test of a Raptor achieved 330 bar (33,000 kPa) chamber pressure, producing ~225 tf (2,210 kN; 500,000 lbf) of thrust.[4] This achievement surpassed the 300 bar (30,000 kPa) record set by the RD-701 engine and set a new record for the highest pressure ever reached in a rocket engine combustion chamber.[94][19] Tests have also shown that the engine--designed to be throttleable from the outset[13]:3--was rated for down to 40 percent of maximum thrust. Elon Musk stated that Raptor was capable of much lower throttle.[3]

On September 3, 2020, Raptor SN29 propelled Starship SN6 to an altitude of 150 metres (490 ft) at the Boca Chica test facility; as with Starship SN5, the engine was mounted off center and controlled the prototype during the entire flight, which lasted for approximately 45 seconds. Unlike the Raptor engine (SN27) mounted on the SN5 Starship prototype, which suffered a small fire during the flight,[95] Raptor SN29 did not seem to have any issues.[96]

On December 9, 2020, three Raptor engines propelled Starship SN8 to an altitude of about 12.5km (41,000 ft) at the Boca Chica test facility. After shutting down all engines at apogee, the vehicle transitioned into a horizontal flight profile and descended back down to the launch site. An abnormally low methane feed pressure caused SN8 to fail its landing maneuver and be destroyed.[97]

On 2 February 2021, Starship prototype SN9, basically similar vehicle as the SN8, was launched to an altitude of about 10 km (33,000ft). The SN9 vehicle was destroyed in landing.

On 3 March 2021, Starship prototype SN10, again a vehicle basically the same as SN8 and SN9, was launched to an altitude of about 10 km (again). The SN10 vehicle did land somewhat successfully but was destroyed minutes after landing in a fire that started in the landing process.

On 30 March 2021, Starship prototype SN11, a vehicle similar to SN8, SN9 and SN10, was yet again launched to an altitude of about 10 km. The SN11 vehicle was destroyed midair at the beginning of the landing sequence due to a hard start of the CH4 turbopump in one of its engines.

On 5 May 2021, Starship prototype SN15 using 3 Raptor engines, was launched to an altitude of 10 km. SN15 successfully performed the bellyflop maneuver, lit 2 engines and successfully landed on the pad.[98]


The Raptor engine has gone through a number of design concepts for engine thrust, specific impulse, and sea-level-nozzle/vacuum-nozzle sizings, depending on the vehicle design concept SpaceX was working on at the time. Subscale versions of Raptor engines were also built for early testing on ground test stands. After 2013, all engine design concepts were methalox using the full-flow staged combustion (FFSC) cycle. In addition, in 2016-2018, a custom prototype upper-stage methalox FFSC Raptor engine was designed and tested for the Falcon 9 and Falcon Heavy launch vehicles, strictly for the US Air Force to meet US military space readiness objectives. SpaceX never implemented plans to switch the Falcon 9/Falcon Heavy upper stage to methalox propellants.

SpaceX Starship

In September 2016, at the 67th International Astronautical Congress, Musk mentioned several Raptor engine designs that were planned for the Interplanetary Transport System. In addition, a much smaller subscale development engine had already been built for design validation purposes. At that time, this first subscale Raptor development engine had recently been tested on a ground test stand, but for only one brief firing.[22]

The Raptor subscale development engine produced approximately 1,000 kN (220,000 lbf) thrust.[22] In order to eliminate flow separation problems while being tested in Earth's atmosphere, the test nozzle expansion ratio had been limited to 150. The engine began testing in September 2016 on the Raptor test stand at McGregor. Sources differed on the performance of the test engine. In reporting during the two weeks following the Musk ITS launch vehicle reveal on 27 September, NASASpaceFlight reported that the development engine was only one-third the size of any of the several larger engine designs that were discussed for the later flight vehicles.[22]

For the flight vehicles, Elon Musk discussed two engines: both a sea-level variant (expansion ratio 40) for the first stage, or ITS booster and a vacuum variant (expansion ratio 200:1) to obtain higher performance with the second stage. 42 of these sea-level engines were envisioned in the high-level design of the first stage, with a thrust per engine of 3,050 kN (690,000 lbf) at sea level and 3,285 kN (738,000 lbf) in vacuum.[22] In addition, three gimbaled sea-level Raptor engines would be used for performing landings of the ITS second stage on Earth and on Mars. Six vacuum-optimized Raptor engines, providing 3,500 kN (790,000 lbf) of thrust each, would also be used on the ITS second stage, for a total of nine engines. The higher-efficiency Raptor Vacuum engines for in-space conditions was envisioned then to target a specific impulse of 382s, using a much larger nozzle giving an expansion ratio of 200.[25] Six of these non-gimbaled engines were planned to provide primary propulsion for the 2016 designs of the ITS second stage.[22]

A year later, at the 68th IAC in September 2017, and following a year of testing and iterative development by the propulsion team, Musk said that a smaller Raptor engine--with slightly over half as much thrust as the previous concept designs for the ITS -- would be used on the next-generation rocket, now a 9 m (30 ft)-diameter launch vehicle and publicly referred to as Big Falcon Rocket (BFR). With the much smaller launch vehicle, fewer Raptor engines would be used on each stage. BFR was then slated to have 31 Raptors on the first stage and 6 on the second stage.[99][22] By mid-2018, SpaceX was publicly stating that the sea-level flight version Raptor engine design, with a nozzle exit diameter of 1.3 m (4.3 ft), was expected to have 1,700 kN (380,000 lbf) thrust at sea level with an Isp of 330 s (3,200 m/s) increasing to an Isp of 356 s (3,490 m/s) in vacuum.[77] The vacuum flight version, with a nozzle exit diameter of 2.4 m (7.9 ft), was expected to exert 1,900 kN (430,000 lbf) force with an Isp of 375 s (3,680 m/s).[77] The earliest versions of the flight engine is designed to operate at 250 bars (25,000 kPa; 3,600 psi) chamber pressure; but SpaceX expected to increase this to 300 bar (30,000 kPa; 4,400 psi) in later iterations.[77] The flight engine is designed for extreme reliability, aiming to support the airline-level of safety required by the point-to-point Earth transportation market.[79]

In the BFR update given in September 2018, Musk showed video of a 71-second hot fire test of a Raptor engine, and stated that "this is the Raptor engine that will power BFR, both the ship and the booster; it's the same engine. ... approximately a 200 tonne engine aiming for roughly 300 bar chamber pressure. ... If you had it at a high expansion ratio, has the potential to have a specific impulse of 380."[7]

Raptor Vacuum

Raptor Vacuum[100] (also RVac) is a variant of Raptor with an extended, regeneratively-cooled nozzle for higher specific impulse in vacuum conditions. While the optimized Raptor vacuum engine is aiming for an Isp of ~380 s (3,700 m/s),[101] the v1.0 Raptor vac design to support early Starship development has been made more conservative and is projecting an Isp of only 365-370 s (3,580-3,630 m/s), intentionally decreasing engine performance to obtain having test engines sooner.[102] In addition, Raptor Vacuum v1 will have a smaller engine nozzle in order to avoid flow separation when the engine is fired at sea-level atmospheric pressure.[103] A full-duration test of version 1 of the Raptor Vacuum engine was completed in September 2020 at the SpaceX development facility in McGregor, Texas.[100]

Falcon 9 upper stage

In January 2016, the United States Air Force (USAF) awarded a US$33.6 million development contract to SpaceX to develop a prototype version of its methane-fueled reusable Raptor engine for use on the upper stage of the Falcon 9 and Falcon Heavy launch vehicles. The contract required double-matching funding by SpaceX of at least US$67.3 million.[39][104] Work under the contract was expected to be completed no later than December 2018, and engine performance testing was planned to be completed at NASA's Stennis Space Center in Mississippi under US Air Force supervision.[39][40] The USAF contract called only for the development and build of a single prototype engine with a series of ground tests, with no upper stage launch vehicle design funded by the contract.[39] The Air Force was working with the US Congress in February 2016 to pursue new launch systems.."[105]

In October 2017 the US Air Force (USAF) awarded a US$40.8 million modification for the development of the Raptor engine prototype for the Evolved Expendable Launch Vehicle program, with work under that contract expected to be completed by April 2018.[106]

Little technical detail was ever publicly released about the USAF second stage engine, as is typical for defense contracts. The prototype however was to be designed:[39]

The USAF contract called only for the development and build of a prototype, to be demonstrated in a USAF-supervised set of tests. No upper stage vehicle design/redesign was funded by the contract.[39] Neither the Air Force nor SpaceX subsequently published any results of this non-Starship oriented rocket engine contract.

Comparison to other engines

Engine Rockets Thrust Specific impulse,
weight ratio
Propellant Cycle
Raptor sea-level Starship 2,200 kN (500,000 lbf)[107] ~350 s (3.4 km/s)[101] 200 (goal)[5] LCH4 / LOX Full-flow staged combustion
Raptor vacuum ~380 s (3,700 m/s)[101] <120[6]
Merlin 1D sea-level Falcon booster stage 914 kN (205,000 lbf) 311 s (3,050 m/s)[108] 176[109] RP-1 / LOX
Gas generator
Merlin 1D vacuum Falcon upper stage 934 kN (210,000 lbf)[110] 348 s (3,410 m/s)[110] 180[109]
Blue Origin BE-4 New Glenn, Vulcan 2,400 kN (550,000 lbf)[111] LNG / LOX Oxidizer-rich staged combustion
Energomash RD-170/171M Energia, Zenit, 7,904 kN (1,777,000 lbf)[112] 337.2 s (3,307 m/s)[112] 79.57[112] RP-1 / LOX
Energomash RD-180 Atlas III, Atlas V 4,152 kN (933,000 lbf)[113] 338 s (3,310 m/s)[113] 78.44[113]
Energomash RD-191/181 Angara, Antares 2,090 kN (470,000 lbf)[114] 337.5 s (3,310 m/s)[114] 89[114]
Kuznetsov NK-33 N1, 1,638 kN (368,000 lbf)[115] 331 s (3,250 m/s)[115] 136.66[115]
Energomash RD-275M Proton-M 1,832 kN (412,000 lbf) 315.8 s (3,097 m/s) 174.5 / UDMH
Rocketdyne RS-25 Space Shuttle, SLS 2,280 kN (510,000 lbf) 453 s (4,440 m/s)[116] 73[117] LH2 / LOX Fuel-rich staged combustion
Rocketdyne F-1 Saturn V 7,740 kN (1,740,000 lbf) 304 s (2,980 m/s)[118] 83 RP-1 / LOX Gas generator

See also


  1. ^ @elonmusk (7 May 2020). "Starship + Super Heavy propellant mass is 4800 tons (78% O2 & 22% CH4). I think we can get propellant cost down to ~$100/ton in volume, so ~$500k/flight. With high flight rate, probably below at at about $1.5M fully burdened cost for 150 tons to orbit or ~$10/kg" (Tweet) – via Twitter.
  2. ^ "SpaceX Starship Super Heavy and Raptor Engine Evolution, Starship SN-6 Hop, Starlink, SAOCOM 1B" on YouTube "Also mentioned was that the propellant for this was intended to be 78% liquid oxygen and 22% liquid methane from this point on."
  3. ^ a b c @elonmusk (17 August 2020). "Max demonstrated Raptor thrust is ~225 tons & min is ~90 tons, so they're actually quite similar. Both Merlin & Raptor could throttle way lower with added design complexity" (Tweet) – via Twitter.
  4. ^ a b c @elonmusk (17 August 2020). "Raptor engine just reached 330 bar chamber pressure without exploding! ... SN40 ... has several upgrades over 330 bar engine. For reference, 330 bar on Raptor produces ~225 tons (half a million pounds) of force" (Tweet) – via Twitter.
  5. ^ a b Michael Sheetz on Twitter: Musk goes into a long explanation of the potential capabilities of the SpaceX Raptor rocket engine (which will power Starship), saying that the company thinks Raptor could achieve a thrust-to-weight ratio of 200.
  6. ^ a b c Elon Musk on Twitter: Max thrust version of Raptor should achieve true T/W > 170. Target is 1.5 ton engine with >260 t-F. Max Isp version should achieve ~380 sec, but T/W probably <120 due to big nozzle. These are just guesses for now.
  7. ^ a b c d e Musk, Elon (17 September 2018). "First Lunar BFR Mission". YouTube. Event occurs at 45:30. And this is the Raptor engine that will power BFR both the ship and the booster, it's the same engine. And this is approximately a 200-ton thrust engine that's aiming for roughly a 300-bar or 300-atmosphere chamber pressure. And if you have it at a high expansion ratio it has the potential to have a specific impulse of 380.
  8. ^ a b Elon Musk on Twitter:Reaching chamber pressure of 300 atmospheres
  9. ^ At 2.2MN thrust and 350s specific impulse
  10. ^ a b 78% O2, 22% CH4 mixture ratio
  11. ^ https://www.spacex.com/starship
  12. ^ Musk, Elon (29 September 2017). "Making Life Multiplanetary". youtube.com. SpaceX. Retrieved 2017.
  13. ^ a b "Starship Users Guide, Revision 1.0, March 2020" (PDF). SpaceX/files. SpaceX. March 2020. Archived from the original (PDF) on 2 April 2020. Retrieved 2020. SpaceX's Starship system represents a fully reusable transportation system designed to service Earth orbit needs as well as missions to the Moon and Mars. This two-stage vehicle -- composed of the Super Heavy rocket (booster) and Starship (spacecraft)
  14. ^ a b Grush, Loren (28 September 2017). "What to expect from Elon Musk's Mars colonization update this week". The Verge.
  15. ^ Gebhardt, Chris (29 September 2017). "The Moon, Mars, & around the Earth - Musk updates BFR architecture, plans". NASASpaceflight.com. Archived from the original on 1 October 2017. Retrieved 2017. In a move that would have seemed crazy a few years ago, Mr. Musk stated that the goal of BFR is to make the Falcon 9 and the Falcon Heavy rockets and their crew/uncrewed Dragon spacecrafts redundant, thereby allowing the company to shift all resources and funding allocations from those vehicles to BFR. Making the Falcon 9, Falcon Heavy, and Dragon redundant would also allow BFR to perform the same Low Earth Orbit (LEO) and Beyond LEO satellite deployment missions as Falcon 9 and Falcon Heavy - just on a more economical scale as multiple satellites would be able to launch at the same time and on the same rocket thanks to BFR's immense size.
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