SpaceX Raptor

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.[15] SpaceX had a few staff working on the Raptor upper-stage engine, then still a LH₂/LOX concept, at a low level of priority.[16] Further mention of the development program occurred in 2011.[17] In March 2012, news accounts asserted that the Raptor upper-stage engine development program was underway, but that details were not being publicly released.[18]

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.[19] 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 by a wide margin.[19]

Full flow staged combustion rocket diagram

In November 2012, Musk announced a new direction for the propulsion division of SpaceX which was towards developing methane-fueled rocket engines.[20] He further indicated that the Raptor concept would now become a methane-based design[20] and that methane would be the fuel of choice for SpaceX's plans for Mars colonization.[21] 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.[22] In-situ resource production on Mars has been examined by NASA and found to be viable for oxygen, water, and methane production.[23]

When first mentioned by SpaceX in 2009, the term "Raptor" was applied exclusively to an upper-stage engine concept[15]—and 2012 pronouncements indicated that it was then still a concept for an upper stage engine[24]—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[21] (subsequently, in 2016, on both stages of the Interplanetary Transport System[25] and then, in 2017 on the Big Falcon Rocket).[26]

Public information released in November 2012 indicated that SpaceX might have a family of Raptor-designated rocket engines in mind;[27] this was confirmed by SpaceX in October 2013.[14] 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.[28]

In October 2013, SpaceX announced that they would be performing methane engine tests of Raptor engine components at NASA's John C. Stennis Space Center and that SpaceX would add equipment to the existing test stand infrastructure to support liquid methane and hot gaseous methane engine component testing.[29][30][31] In April 2014, SpaceX completed the requisite upgrades and maintenance to the Stennis test stand to prepare for testing of Raptor components,[32] and the engine component testing program began in earnest, focusing on the development of robust startup and shutdown procedures. Component testing at Stennis also allowed hardware characterization and verification.[33]

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.[34] SpaceX completed its planned testing using NASA Stennis facilities in 2014 and 2015.[35]

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. Work under the contract was expected to be completed in 2018, with engine performance testing to be done at Stennis Space Center and at Los Angeles Air Force Base, California.[36][37]

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[34] of Raptor methane engine components was done at NASA's Stennis Space Center, where SpaceX added equipment to the existing infrastructure to support liquid methane engine testing.[14][31]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.[14] Raptor engine component testing began in May 2014 at the E-2 test complex which SpaceX modified to support methane engine tests.[14][32] SpaceX 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.

By early 2016, SpaceX had constructed a new engine test stand at their McGregor test site in central Texas for Raptor testing.[33][14] By August 2016, the first integrated Raptor rocket engine, manufactured at the SpaceX Hawthorne facility in California, was shipped to SpaceX McGregor for development testing.[38] The engine had 1 MN (220,000 lb_f) thrust, less than half the thrust of the full-scale Raptor engine used for flight tests in 2019.[39] It is the first full-flow staged-combustion methalox engine ever to reach a test stand.[33] 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 disclosed that the engine used multi-stage turbopumps. Substantial additional technical details of the ITS propulsion were summarized in an article on the Raptor engine published the next week.[33] 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. At that time, this first subscale Raptor development engine had recently been tested on a ground test stand in McGregor, but for only one brief firing.[33] The Raptor subscale development engine produced approximately 1,000 kN (220,000 lbf) thrust.[33] To eliminate flow separation problems while being tested in Earth's atmosphere, the test nozzle expansion ratio had been limited to 150. 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.[33]

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. 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.[40] By September 2017, the 200 bars (20 MPa; 2,900 psi) sub-scale test engine, with a thrust of 1 meganewton (220,000 lb_f) and "a new alloy to help its oxygen-rich turbopump resist oxidization, ... had completed 1200 seconds of firings across 42 tests."[41] This alloy is known as SX500, created by the SpaceX metallurgy team which is used to contain hot oxygen gas in the engine at up to 12000 psi.

Big Falcon Rocket with its Super Heavy booster firing (artist's concept)

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.[42] 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.[33] 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 in 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.[43]

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.[40] 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."[42]

For the flight vehicles, Elon Musk discussed two engines: a sea-level variant (expansion ratio 40:1) 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 the vacuum.[33] In addition, three gimbaled sea-level Raptor engines would be used for performing landings of the ITS second stage on Earth and 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 were envisioned then to target a specific impulse of 382s, using a much larger nozzle giving an expansion ratio of 200.[44] Six of these non-gimbaled engines were planned to provide primary propulsion for the 2016 designs of the ITS second stage.[33]

A year later, at the 68th IAC in September 2017, and following a year of 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.[45][33] 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 a specific impulse of 330 s (3,200 m/s) increasing to a specific impulse of 356 s (3,490 m/s) in vacuum.[41] 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 a specific impulse of 375 s (3,680 m/s).[41] The earliest versions of the flight engine are designed to operate at 250 bars (25,000 kPa; 3,600 psi) chamber pressure; but SpaceX is expected to increase this to 300 bar (30,000 kPa; 4,400 psi) in later iterations.[41]

In the BFR update given in September 2018, Musk showed a video of a 71-second 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 (metric) tons 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."[6]

In July 2021, SpaceX announced that they would be building a second production facility for Raptor engines, this one in south Texas near the existing rocket engine test facility. Dallas Morning News reported in July that SpaceX will "break ground soon" and that the facility will concentrate on the serial production of Raptor 2, while the California facility will produce Raptor Vacuum and new/experimental Raptor designs. The new facility is expected to eventually produce 800 to 1000 rocket engines each year.[46][47] SpaceX aims at a lifetime of 1000 flights for Raptor.[48] In 2019 the (marginal) cost of the engine was stated to be approaching $1 million. SpaceX plans to mass-produce up to 500 Raptor engines per year, each costing less than $250,000. Each Starship booster will use 33 sea-level Raptors, while each Starship spacecraft will use 3 sea-level Raptors plus 3 Vacuum-optimized Raptors.

By late 2021, SpaceX said that scaling Raptor production to support the frequent Starship test program planned for 2022 was "currently the biggest constraint on how many vehicles we can make" and that failing to achieve a flight rate of at least once every two weeks by late 2022 would open up the possibility of bankruptcy for SpaceX. The reason given was that Starship orbital launch capability is necessary to deliver the next generation Starlink satellites needed to operationalize the massively capital-intensive Starlink broadband internet constellation.[49]

Raptor Vacuum[50] (also RVac) is a variant of Raptor with an extended, regeneratively-cooled nozzle for higher specific impulse in the vacuum of space. While the optimized Raptor vacuum engine is aiming for a specific impulse of ~380 s (3,700 m/s),[51] the v1.0 Raptor vac design to support early Starship development has been made more conservative and is projecting a specific impulse of only 365–370 s (3,580–3,630 m/s), intentionally decreasing engine performance to obtain test engines sooner. In addition, Raptor Vacuum v1 will have a smaller engine nozzle to avoid flow separation when the engine is fired at sea-level atmospheric pressure. 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.[50] The first three operational Raptor Vacuum engines were slated to fly on SpaceX Starship prototype S20 and were installed on 4 August 2021.[52] However, SN20 was retired before it flew.

Raptor 2 is the newest version of Raptor[53] and is a complete redesign of the version 1 Raptor engine. The turbomachinery, chamber, nozzle, and electronics were all redesigned, as well as deleting some parts and converting many flanges to welds.[54]

In September 2019, SpaceX stated their "current plan" was to use Raptor 2 for the three "sea-level" engines on the Starship second-stage and also for all booster engines — both those that gimbal and those that do not — on the Super Heavy first stage.[55] As of February 2022, no Raptor 2 engines have flown on the various Starship prototype vehicles.[54] These engines will eventually be produced at SpaceX's new engine development facility near McGregor, Texas; no date has yet been made public as to when the facility might be operational for production.[53] On 18 December 2021, Elon Musk announced on his Twitter account that Raptor 2 started production, and it will have over 230 tons of force. [56] In a Starship update on 10 February 2022, Musk showed the capabilities of Raptor 2 and how it is simplified but more powerful than the original Raptor.[54] [57]

Raptor 2 engines were achieving 230 t_f (510,000 lbf) of thrust consistently by February 2022, although SpaceX expects to be able to tune engine parameters and design over time to achieve at least 250 t_f (550,000 lbf). Moreover, Musk indicated that the engine production cost was approximately half that of the Raptor 1 version SpaceX had been using in 2018–2021.[54] In June 2022, Musk tweeted that 250 tons was achievable.[58]

Production rate for Raptor 2 had reached five per week by February 2022 and was expected to achieve 7 per week by March.[54]