Raptor (rocket engine)

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Raptor methane rocket engine
Country of origin United States
Manufacturer SpaceX
Application multistage and deep-space propulsion
Status Development
Liquid-fuel engine
Propellant LOX / liquid methane
Mixture ratio O/F : 3.8/1
Cycle Full-flow staged combustion
Performance
Thrust (SL) 2,300 kN (230 tonnes-force)[1]
Isp (vac.) 363 s[2]
Isp (SL) 321 s
Used in
Mars Colonial Transporter, MCT launch vehicle

Raptor is the first member of a family of cryogenic[3] methane-fueled rocket engines under development by SpaceX. It is specifically intended to power high-performance lower and upper stages for SpaceX super-heavy launch vehicles. The engine will be powered by liquid methane[4] and liquid oxygen (LOX),[5] rather than the RP-1 kerosene and LOX used in all previous Falcon 9 rockets, which use Merlin 1C & D engines. Earlier concepts for Raptor would have used liquid hydrogen (LH2) fuel rather than methane.[6]

The Raptor engine will have several times the thrust of the Merlin 1D vacuum engine that powers the second stage of the current Falcon 9, the Falcon 9 Full Thrust.

The broader 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".[7] According to Elon Musk, this design will be able to achieve full reusability (all rocket stages) and, as a result, "a two order of magnitude reduction in the cost of spaceflight".[3]

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.[8][9] 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.[10]

History

Raptor was first publicly discussed by SpaceX's Max Vozoff at the American Institute of Aeronautics and Astronautics Commercial Crew/Cargo symposium in 2009.[11] 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.[12] Further mention of the development program occurred in 2011.[13] In March 2012, news accounts asserted that the Raptor upper-stage engine development program was underway, but that details were not being publicly released.[14]

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 the specific fuel to be used.[15] 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.[15]

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.[5] He further indicated that the engine concept that had been codenamed Raptor would now become a methane-based design,[5] and that methane would be the fuel of choice for SpaceX's plans for Mars colonization.[2]

Potential sources and sinks of methane (CH4) on Mars.

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.[16] In-situ resource production on Mars has been examined by NASA and found to be viable for oxygen, water, and methane production.[17] 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.[18] Methane has been discovered in meteorites from Mars.[19]

When first mentioned by SpaceX in 2009, the term "Raptor" was applied exclusively to an upper-stage engine concept[11]—and 2012 pronouncements indicate that it still was 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 (nominally, 10-meter-diameter) core of the Mars Colonial Transporter. Each booster core will utilize nine Raptor engines, similar to the use of nine Merlin 1s on each Falcon 9 booster core.[2]

Early 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.[2]

Public information released in November 2012 indicated that SpaceX may have a family of Raptor-designated rocket engines in mind;[21] this was confirmed by SpaceX in October 2013.[7] However, SpaceX COO Gwynne Shotwell clarified in March 2014 that the focus of the new engine development program is exclusively on the full-size Raptor engine; smaller subscale methalox engines are not planned on the development path to the very large Raptor engine.[22]

In October 2013, SpaceX announced that they would be performing methane engine tests of the Raptor engine at the John C. Stennis Space Center in Hancock County, Mississippi,[23][24] and that SpaceX would add equipment to the existing test stand infrastructure in order to support liquid methane engine testing.[25] In April 2014, SpaceX completed the requisite upgrades and maintenance to the Stennis test stand to prepare for testing of Raptor components, and expects to begin tests at the facility prior to the end of May 2014.[26]

October 2013 was the first time SpaceX disclosed a nominal design thrust of the Raptor engine—2,940 kN (661,000 lbf)[7]—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).[2][27] A June 2014 talk by Mueller provided more specific engine performance target specifications indicating 6,900 kN (705 tonnes-force) of sea-level thrust, 8,200 kN (840 tonnes-force) of vacuum thrust, and a specific impulse of 380 s for a vacuum version.[28] Earlier information had estimated the design Isp under vacuum conditions as only 363 s.[2] 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.[29][30][31]

In January 2015, Elon Musk made a statement that the thrust they were currently targeting was around 2,300 kN (230 tonnes-force), 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]"[1] 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.[32]

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.[9] SpaceX completed its planned testing at NASA Stennis in 2014 and 2015, although as of February 2016, the Stennis center was hopeful of establishing additional test agreements.[33]

In January 2016, the US Air Force awarded a US$33.6 million development contract to SpaceX 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, and engine performance testing will be done at NASA's John C. Stennis Space Center in Mississippi.[10][34]

Design

Scale size comparison of SpaceX first-stage launch vehicles: (from left) Falcon 9 v1.0 (2010), Falcon 9 v1.1 (2013), and a possible 10-meter diameter, 9-Raptor, first-stage booster core for the future Mars Colonial Transporter based on early-2014 information.

The Raptor engine will be powered by liquid methane and liquid oxygen using a more efficient staged combustion cycle,[20] a departure from the 'open cycle' gas generator system and lox/kerosene propellants that current Merlin engines use.[20] The Space Shuttle Main Engines (SSME) also used a staged combustion process,[35] as do several Russian rocket engines (such as the RD-180).[20]

Raptor is being explicitly designed to be able to deliver "long life ... and more benign turbine environments".[8]

More specifically, Raptor will utilize 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 demonstration project in the mid-2000s.[2]

The Raptor engine is designed to work using 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.[36]

The turbopump and many of the critical parts of the injectors will be manufactured by using 3D printing, which also increases the speed of development and iterative testing.[36]

Stated design size for the Raptor engine has varied widely as design continues, from a high target of 8,200 kN (1,800,000 lbf) of vacuum thrust[37] to a more recent, much lower target of 2,300 kN (230 tonnes-force).[1] Estimates target a vacuum Isp of 363 seconds[2] and a sea-level Isp of 321 seconds.[2][27] Final thrust and Isp specifications for the as-built engines may continue to change dramatically as SpaceX moves the engine through the multi-year development cycle.[37]

Additional characteristics of the full-flow design that are projected to further increase performance or reliability include:[2]

  • eliminating the fuel-oxidizer turbine interseal, which is a potential point of failure in more traditional engine designs
  • 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".[2]

Vacuum version

Like the SpaceX Merlin engine, a vacuum version of the Raptor rocket engine is planned which would target a specific impulse of 380s,[28] using a larger nozzle to allow more expansion by exhaust gases.

Comparison to other engine designs

Engine name Vacuum thrust
[kilonewtons (lbf)]
Vacuum specific impulse
[seconds]
Thrust-to-
weight ratio
Engine type
SpaceX Raptor (targeted,
as of January 2015[1])
2,300 (510,000)[1] 380[28] Methane/LOX full-flow staged combustion
Blue Origin BE-4 2,400 (550,000)[38] Methane/LOX oxygen-rich staged combustion
SpaceX Merlin 1D 825 (185,000)[39] 309[40] 180[41] RP-1/LOX gas generator
SpaceX Merlin 1C 610 (140,000) 304[42] 96
NK-33 1,638 (368,000)[43] 331[43] 136.66[43] RP-1/LOX staged combustion
RD-180 4,152 (933,000)[44] 338[44] 78.44[44]
RD-191 2,090 (470,000)[45] 337.5[45] 89[45]
Space Shuttle Main Engine 2,280 (510,000) 453[46] 73[47] LH/LOX staged combustion
Rocketdyne F-1 (Saturn V) 7,740 (1,740,000) 304[48] 83 RP-1/LOX gas generator
TR-107 4,900 (1,100,000)[49] [49] [49] RP-1/LOX oxygen-rich staged combustion

Engine testing

Testing of the Raptor's oxygen preburner at Stennis in 2015

Initial development testing[9] 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.[7][25] Initial testing at Stennis has been 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 2,940 kN (661,000 lbf) vacuum thrust.[7] A revised, higher-thrust, specification was discussed by the company in February 2014; but it is unclear whether that higher thrust is something that would be achieved with the initial development engines.[2]

By April 2014, Raptor engine component testing was expected to initiate testing the next month,[26] at the E-2 test complex which SpaceX modified to support methane engine tests.[7] The first items tested were single Raptor injector elements,[50] various designs of high-volume gas injectors.[51]

The modifications to the test stands made by SpaceX are now a part of the Stennis test infrastructure and will be available to other users of the test facility after the SpaceX facility lease is completed.[7]

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 are continuing on Raptor preburner components as of September 2015.[9]

SpaceX will need to construct a new engine test stand or reconstruct an existing one to handle the larger thrust of the full Raptor engine.[7] The B-2 test stand at Stennis Space Center is already being upgraded to accommodate testing of NASA's 7,440 kN SLS core stage.[52]

See also

References

  1. 1.0 1.1 1.2 1.3 1.4 Musk, E. (January 6, 2015) "Thrust to weight is optimizing for a surprisingly low thrust level, even when accounting for the added mass of plumbing and structure for many engines. Looks like a little over 230 metric tons (~500 klbf) of thrust per engine, but we will have a lot of them :)" Reddit.com
  2. 2.00 2.01 2.02 2.03 2.04 2.05 2.06 2.07 2.08 2.09 2.10 2.11 Belluscio, Alejandro G. (2014-03-07). "SpaceX advances drive for Mars rocket via Raptor power". NASAspaceflight.com. Retrieved 2014-03-07.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  3. 3.0 3.1 https://www.youtube.com/watch?v=hJD0MMP4nkM
  4. http://spaceref.com/news/viewsr.html?pid=47400
  5. 5.0 5.1 5.2 Todd, David (2012-11-20). "Musk goes for methane-burning reusable rockets as step to colonise Mars". FlightGlobal Hyperbola. Retrieved 2015-11-04. "We are going to do methane." Musk announced as he described his future plans for reusable launch vehicles including those designed to take astronauts to Mars within 15 years, "The energy cost of methane is the lowest and it has a slight Isp (Specific Impulse) advantage over Kerosene," said Musk adding, "And it does not have the pain in the ass factor that hydrogen has".<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  6. Markusic, Tom (2010-07-28). SpaceX Propulsion (PDF). 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference. pp. 12–15. Retrieved 2015-10-28.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  7. 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 Leone, Dan (2013-10-25). "SpaceX Could Begin Testing Methane-fueled Engine at Stennis Next Year". Space News. Retrieved 2013-10-26.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  8. 8.0 8.1 Gwynne Shotwell (17 March 2015). "STATEMENT OF GWYNNE SHOTWELL, PRESIDENT & CHIEF OPERATING OFFICER, SPACE EXPLORATION TECHNOLOGIES CORP. (SPACEX)" (PDF). Congressional testimony. US House of Representatives, COMMITTEE ON ARMED SERVICES SUBCOMMITTEE ON STRATEGIC FORCES. pp. 14–15. Retrieved 2016-01-11. SpaceX has already begun self-funded development and testing on our next-generation Raptor engine. ... Raptor development ... will not require external development funds related to this engine.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  9. 9.0 9.1 9.2 9.3 "NASA-SpaceX testing partnership going strong" (PDF). Lagniappe, John C. Stennis Space Center. NASA. September 2015. Retrieved 2016-01-10. this project is strictly private industry development for commercial use<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  10. 10.0 10.1 "Contracts: Air Force". U.S. Department of Defense Contracts press release. 13 January 2016. Retrieved 2016-01-15.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  11. 11.0 11.1 "Long term SpaceX vehicle plans". HobbySpace.com. 2009-07-07. Archived from the original on 2010-02-14. Retrieved 2009-07-13.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  12. "Notes: Space Access'11: Thurs. - Afternoon session - Part 2: SpaceX". RLV and Space Transport News. 2011-04-07. Archived from the original on 2012-03-20. Retrieved 2011-04-08.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  13. "SpaceX Raptor LH2/LOX engine". RLV and Space Transport News. 2011-08-08. Archived from the original on 2011-11-02. Retrieved 2011-08-09.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  14. Rosenberg, Zach (2012-03-16). "SpaceX readies upgraded engines". Flightglobal. Retrieved 2012-03-17. SpaceX is in the midst of a variety of ambitious engine programmes, including the Merlin 2, a significant modification of the Merlin 1 series, and the Raptor upper stage engine. Details of both projects are tightly held.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  15. 15.0 15.1 Rosenberg, Zach (2012-10-15). "SpaceX aims big with massive new rocket". Flightglobal. Retrieved 2012-10-17.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  16. GPUs to Mars: Full-Scale Simulation of SpaceX's Mars Rocket Engine. YouTube. 5 May 2015. Retrieved 4 June 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  17. mmooney (8 November 2015). "In-Situ Resource Utilization - Mars Atmosphere/Gas Chemical Processing". NASA SBIR/STTR. NASA. Retrieved 2 June 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  18. "Comparative study of ISRU-based transportation architectures for the Moon and Mars: LOX/LH2 vs. LOX/Methane" (PDF). Lunar and Planetary Institute. Retrieved 2 June 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  19. http://phys.org/news/2015-06-scientists-methane-mars-meteorites.html
  20. 20.0 20.1 20.2 20.3 Todd, David (2012-11-22). "SpaceX's Mars rocket to be methane-fuelled". Flightglobal. Retrieved 2012-12-05. Musk said Lox and methane would be SpaceX’s propellants of choice on a mission to Mars, which has long been his stated goal. SpaceX’s initial work will be to build a Lox/methane rocket for a future upper stage, codenamed Raptor. The design of this engine would be a departure from the “open cycle” gas generator system that the current Merlin 1 engine series uses. Instead, the new rocket engine would use a much more efficient “staged combustion” cycle that many Russian rocket engines use.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  21. Todd, David (2012-11-20). "Musk goes for methane-burning reusable rockets as step to colonise Mars". FlightGlobal Hyperbola. Retrieved 2012-11-22. The new Raptor upper stage engine is likely to be only the first engine in a series of lox/methane engines.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  22. Gwynne Shotwell (2014-03-21). Broadcast 2212: Special Edition, interview with Gwynne Shotwell (audio file). The Space Show. Event occurs at 21:25–22:10. 2212. Archived from the original (mp3) on 2014-03-22. Retrieved 2014-03-22. our focus is the full Raptor size<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  23. "NASA Stennis Space Center to Test SpaceX Next Generation Rocket Engines Systems". Mississippi Development Authority. October 23, 2013. Retrieved 27 October 2013.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  24. "Cochran: Space-X Decision Bodes Well for Job Growth in South Mississippi". Senator Cochran. October 23, 2013. Retrieved 27 October 2013.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  25. 25.0 25.1 Messier, Doug (2013-10-23). "SpaceX to Conduct Raptor Engine Testing in Mississippi". Parabolic Arc. Retrieved 2013-10-23.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  26. 26.0 26.1 Guess, Natalie (2014-04-21). "NASA, SpaceX Cut Ribbon To Launch Testing Partnership". MS EIGS. Retrieved 2014-04-22.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  27. 27.0 27.1 "SpaceX propulsion chief elevates crowd in Santa Barbara". Pacific Business Times. 19 February 2014. Retrieved 22 February 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  28. 28.0 28.1 28.2 Butler, Amy; Svitak, Amy. "AR1 vs. Raptor: New rocket program will likely pit kerosene against methane" (2014-06–09). Aviation Week & Space Technology. SpaceX is developing the Raptor as a reusable engine for a heavy-lift Mars vehicle, the first stage of which will feature 705 metric tons of thrust, making it 'slightly larger than the Apollo F-1 engine,' Tom Mueller, SpaceX vice president of propulsion development, said during a space propulsion conference last month in Cologne, Germany. The vacuum version is targeting 840 metric tons of thrust with 380 sec. of specific impulse. The company is testing subscale components using the E-2 test stand at NASA's Stennis Space Center in Mississippi, says Stennis spokeswoman Rebecca Strecker. ... Mueller said many people ask why the company switch to methane for its Mars rocket. With reusability in mind, SpaceX's cost studies revealed that 'by far the most cost-effective propellant to use is methane,' he said, which would be easier than hydrogen to manufacture on Mars. |access-date= requires |url= (help)<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  29. "The Wind Rises at SpaceX". SpaceNews. 24 December 2015. Retrieved 2015-12-26.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  30. House testimony bio, HHRG-114-AS29, June 2015]
  31. "SpaceX Prepared Testimony by Jeffrey Thornburg". SpaceRef.com. 26 June 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  32. How (and Why) SpaceX Will Colonize Mars, accessed 19 Augus7 2015. Musk: "The critical elements of the solution are rocket reusability and low cost propellant (CH4 and O2 at an O/F ratio of ~3.8). And, of course, making the return propellant on Mars, which has a handy CO2 atmosphere and lots of H2O frozen in the soil."
  33. "Stennis set for busy 2016 test schedule" (PDF). Lagniappe. NASA-John C. Stennis Space Center. February 2016. p. 3. Retrieved 2016-03-02. After completing successful test series in 2014 and 2015 on components for the new Raptor rocket engine being developed by SpaceX, there also is hope for additional test agreements with the company.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  34. "Orbital ATK, SpaceX Win Air Force Propulsion Contracts". SpaceNews. 13 January 2016. Retrieved 2016-01-15.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  35. "Space Shuttle Main Engines". NASA. Retrieved 6 March 2013.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  36. 36.0 36.1 Elon Musk, Mike Suffradini (7 July 2015). Elon Musk comments on Falcon 9 explosion - Huge Blow for SpaceX (2015.7.7) (video). Event occurs at 39:25–40:45. Retrieved 2015-12-30.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  37. 37.0 37.1 Bergin, Chris (2014-08-29). "Battle of the Heavyweight Rockets -- SLS could face Exploration Class rival". NASAspaceflight.com. Retrieved 2014-08-30.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  38. Ferster, Warren (2014-09-17). "ULA To Invest in Blue Origin Engine as RD-180 Replacement". Space News. Retrieved 2014-09-19.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  39. "SpaceX Falcon 9 product page". Retrieved 2013-08-08.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  40. "Falcon Heavy Merlin Engines". SpaceX. 2013-04-12. Retrieved 2014-07-12.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  41. Mueller, Thomas (2015-06-08). "Is SpaceX's Merlin 1D's thrust-to-weight ratio of 150+ believable?". Retrieved 2015-07-09.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  42. "Merlin 1C". Astronautix.com. Retrieved 2013-11-02.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  43. 43.0 43.1 43.2 "NK-33". Astronautix.com. Retrieved 1 April 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  44. 44.0 44.1 44.2 "RD-180". NPO Energomash. Retrieved 30 June 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  45. 45.0 45.1 45.2 "RD-191". NPO Energomash. Retrieved 7 April 2016.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  46. "SSME". Astronautix.com. Retrieved 2013-11-02.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  47. "Encyclopedia Astronautica: SSME". Retrieved 2014-07-07.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  48. "F-1". Astronautix.com. Retrieved 2013-11-02.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  49. 49.0 49.1 49.2 "TR-107" (PDF). NASA Marshall. Retrieved 17 August 2015.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  50. "SpaceX to test methane rocket engine in Miss". The Sacramento Bee. 22 April 2014. Archived from the original on 29 April 2014.<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>
  51. SpaceX Commercial Spaceflight, Garrett Reisman, Future in Space Operations (FISO) Colloquium, 2014-08-27, Retrieved 2014-08-28.
  52. Rebecca Strecker (May 15, 2014). "NASA Moving Forward on Test Stand Upgrades for SLS Core Stage Testing".<templatestyles src="Module:Citation/CS1/styles.css"></templatestyles>

External links