Country of origin United States Application Multistage propulsion Propellant LOX / liquid methane | Manufacturer SpaceX Status In development Mixture ratio 3.8 | |
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Raptor is a family of cryogenic, methane-fueled rocket engines under development by SpaceX. The engines are specifically intended to power both high-performance lower and upper stages of the ITS launch vehicle, a part of a more extensive Interplanetary Transport System (ITS) championed by Elon Musk, that may provide technological and economic improvements in interplanetary spaceflight, particularly with respect to a long-term aim of colonizing Mars. The engine will be powered by densified liquid methane and liquid oxygen (LOX), rather than the RP-1 kerosene and LOX used in all previous Falcon 9 rockets which use Merlin 1C & D engines. The earliest concepts for Raptor considered liquid hydrogen (LH2) as fuel rather than methane. The Raptor engine will have over three times the thrust of the Merlin 1D vacuum engine that powers the current Falcon 9 launch vehicle.
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The broad Raptor concept "is a highly reusable methane staged-combustion engine that will power the next generation of SpaceX launch vehicles designed for the exploration and colonization of Mars". According to Elon Musk, this design will be able to achieve full reusability of all rocket stages and, as a result, "a two order of magnitude reduction in the cost of spaceflight". A variety of Raptor engines will be used on both stages of the ITS launch vehicle, but also on the long-duration spacecraft used to support all aspects of the Interplanetary Transport System on-orbit. Both the ITS tanker—a transport carrier of propellant cargo to Earth orbit—and also the Interplanetary Spaceship—a very long-duration carrier of both passengers and space cargo to interplanetary destinations, and which will also serve as both a descent and ascent vehicle at Mars—will be powered by six vacuum-optimized Raptor rocket engines with three additional sea-level-nozzle Raptor engines to be used for maneuvering. The ITS booster stage will be powered by 42 Raptors. Unlike nearly all other launch vehicles or spacecraft, on all Earth-away launches, the long-duration spacecraft (tanker or spaceship) will also provide second-stage acceleration to orbital velocity; all propulsion is provided by Raptor engines.
The engine development from 2009 to 2015 was funded exclusively by private investment by SpaceX, and not as a result of any funding from the US government. 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 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.
In August 2016, a development Raptor engine was shipped to their McGregor testing facility in Texas, where it is undergoing development testing.
History
An advanced rocket engine design project named Raptor—then a hydrolox engine—was first publicly discussed by SpaceX's Max Vozoff at the American Institute of Aeronautics and Astronautics Commercial Crew/Cargo symposium in 2009. As of April 2011, SpaceX had a small number of staff working on the Raptor upper-stage engine, then still a LH2/LOX concept, at a low level of priority. Further mention of the development program occurred in 2011. In March 2012, news accounts asserted that the Raptor upper-stage engine development program was underway, but that details were not being publicly released.
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. They indicated that details would be forthcoming in "one to three years" and that the large engine was intended for a new SpaceX rocket, using multiple of these large engines, that would notionally launch payload masses of the order of 150 to 200 tonnes (150,000 to 200,000 kg) to low Earth orbit, exceeding the payload mass capability of the NASA Space Launch System.
This was cleared up the next month when, in November 2012, CEO Elon Musk announced a new direction for the propulsion division of SpaceX: developing methane-fueled rocket engines. He further indicated that the engine concept that had been codenamed Raptor would now become a methane-based design, and that methane would be the fuel of choice for SpaceX's plans for Mars colonization.
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. In-situ resource production on Mars has been examined by NASA and found to be viable for oxygen, water, and methane production. 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.
When first mentioned by SpaceX in 2009, the term "Raptor" was applied exclusively to an upper-stage engine concept—and 2012 pronouncements indicate that it still was a concept for an upper stage engine—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 then-named Mars Colonial Transporter (later, after 2016, Interplanetary Transport System). At the time, each booster core was thought to utilize nine Raptor engines, similar to the use of nine Merlin 1s on each Falcon 9 booster core.
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.
Public information released in November 2012 indicated that SpaceX might have a family of Raptor-designated rocket engines in mind; this was confirmed by SpaceX in October 2013. 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.
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, and that SpaceX would add equipment to the existing test stand infrastructure in order to support liquid methane and hot gaseous methane engine component testing. In April 2014, SpaceX completed the requisite upgrades and maintenance to the Stennis test stand to prepare for testing of Raptor components, and the testing program began in earnest, enabling the development of robust startup and shutdown procedures, something that is ordinarily 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.
October 2013 was the first time SpaceX disclosed a nominal design thrust of the Raptor engine—2,900 kN (661,000 lbf)—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). 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 for a vacuum version. Earlier information had estimated the design Isp under vacuum conditions as only 363 s. 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.
In January 2015, Elon Musk made a statement that the thrust they were currently targeting was around 2,300 kN (510,000 lbf), much lower than older comments mentioned. This brought into question much of the speculation surrounding a 9-engine booster, as he stated "there will be a lot of [engines]". By August 2015, an Elon Musk statement surfaced that indicated the oxidizer to fuel ratio of the Mars-bound engine would be approximately 3.8 to 1.
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. SpaceX completed its planned testing at NASA Stennis in 2014 and 2015, although as recently as February 2016, the Stennis center indicated publicly that it was hopeful of establishing additional test agreements.
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 is expected to be completed in 2018, with engine performance testing to be done at NASA's John C. Stennis Space Center in Mississippi.
By August 2016, the first integrated Raptor rocket engine, manufactured at the SpaceX Hawthorne facility in California, shipped to the McGregor rocket engine test facility in Texas for development testing. The engine had 1 MN 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 to ever reach a test stand.
On September 26, 2016, Elon Musk tweeted two images of the first test firing of an integrated Raptor in SpaceX's McGregor test complex. On the same day Musk revealed that their target performance for Raptor was a vacuum specific impulse of 382 seconds, with a thrust of 3 MN (670,000 lbf) with a chamber pressure of 30 MPa (4,400 psi) and an expansion ratio of 150 for the altitude optimized version. 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. 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. Substantial additional technical details of the ITS propulsion were summarized in a technical article on the Raptor engine published the next week.
Versions
Design
The Raptor engine is powered by subcooled liquid methane and subcooled liquid oxygen using a more efficient staged combustion cycle, a departure from the 'open cycle' gas generator system and lox/kerosene propellants that current Merlin engines use. The Space Shuttle Main Engines (SSME, with hydrolox propellant) also used a staged combustion process, as do several Russian rocket engines (such as the RD-180 and the very-high chamber pressure (25.74 MPa) RD-191).
Raptor has been explicitly designed to be able to deliver "long life ... and more benign turbine environments".
More specifically, Raptor utilizes a full-flow staged combustion cycle, where 100 percent of the oxidizer—with a low-fuel ratio—will power the oxygen turbine pump, and 100 percent of the fuel—with a low-oxygen ratio—will power the methane turbine pump. Both streams—oxidizer and fuel—will be completely in the gas phase before they enter the combustion chamber. Prior to 2014, only two full-flow staged combustion rocket engines have 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.
The Raptor engine is designed for the use of deep cryogenic methalox propellants—fluids cooled to near their freezing points, rather than nearer their boiling points which is more typical for cryogenic rocket engines. The use of subcooled propellants—in addition to densification and therefore allowing more propellant to be carried by a given tank volume and has two benefits for the engine as well: specific impulse is increased, and the risk of cavitation at inputs to the turbopumps is reduced. Engine ignition for all Raptor engines, both on the pad and in the air, will be by spark ignition, which will totally eliminate the pyrophoric mixture of triethylaluminum-triethylborane (TEA-TEB) used for engine ignition on the Falcon 9 and Falcon Heavy.
The turbopump and many of the critical parts of the injectors are manufactured by using 3D printing, which also increases the speed of development and iterative testing. Forty percent of the 1 MN engine (by mass) on the test stand in 2016 was manufactured by 3D printing.
Stated design size for the Raptor engine varied widely during 2012–2016 as detailed design continued, from a high target of 8,200 kN (1,800,000 lbf) of vacuum thrust to a more recent, much lower target of 3,500 kN (790,000 lbf). The engine targets a vacuum Isp of 361 seconds and a sea-level Isp of 334 seconds. and an expansion ratio of 40.
Additional characteristics of the full-flow design, projected to further increase performance or reliability include:
Vacuum version
Like the SpaceX Merlin engine, a vacuum version of the Raptor rocket engine is planned which would target a specific impulse of 382s, using a larger nozzle giving an expansion ratio of 200.
Engine testing
Initial development testing 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. 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. 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. Raptor engine component testing began in May 2014 at the E-2 test complex which SpaceX modified to support methane engine tests. The first items tested were single Raptor injector elements, various designs of high-volume gas injectors. 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. 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.
SpaceX constructed a new engine test stand at their site of McGregor in central Texas that can handle the larger thrust of the full Raptor engine. The B-2 test stand at Stennis Space Center was upgraded in 2014 to accommodate testing of NASA's 7,440 kN (1,700,000 lbf) SLS core stage.
In August 2016, SpaceX confirmed a Raptor engine was shipped to the testing site in McGregor for development tests.
The 1,000 kN (220,000 lbf) development Raptor did an initial 9-second firing test on 26 September 2016, the day before Musk's talk at the International Aeronautical Congress. The development engine has "an expansion ratio of just 150, the maximum possible within Earth’s atmosphere" to prevent flow separation problems.
Applications
As of September 2016, the Raptor engine is slated to be used in, broadly, three spaceflight vehicles, which although capable of independent flight, also make up the two launch stages of an ITS launch vehicle stack. The first stage is always an Interplanetary booster while the second stage may be either an Interplanetary Spaceship (for beyond-Earth-orbit missions) or an ITS tanker (for on-orbit propellant transfer operations nearer to Earth).
The SpaceX Interplanetary booster will use 42 sea-level optimized Raptors in the first stage of the ITS launch vehicle. The SpaceX Interplanetary Spaceship—which makes up the second stage of the ICT launch vehicle on Earth launches is also an interplanetary spacecraft carrying cargo and passengers to beyond-Earth-orbit destinations after on-orbit refueling—will use six vacuum-optimized Raptors for primary propulsion plus three Raptors with sea-level nozzles for maneuvering.