Staged combustion cycle

The staged combustion cycle (sometimes known as topping cycle or preburner cycle) is a power cycle of a bipropellant rocket engine. In the staged combustion cycle, propellant flows through multiple combustion chambers, and is thus combusted in stages. The main advantage relative to other rocket engine power cycles is high fuel efficiency, measured through specific impulse, while its main disadvantage is engineering complexity.

Fuel-rich staged combustion cycle. Here, all of the fuel and a portion of the oxidizer are fed through the preburner, generating fuel-rich gas. After being run through a turbine to power the pumps, the gas is injected into the combustion chamber and burned with the remaining oxidizer.

Typically, propellant flows through two kinds of combustion chambers; the first called preburner and the second called main combustion chamber. In the preburner, a small portion of propellant is combusted, and the increasing volume flow is used to drive the turbopumps that feed the engine with propellant. In the main combustion chamber, the propellants are combusted completely to produce thrust.

The fuel efficiency of the staged combustion cycle is in part a result of all propellant ultimately flowing to the main combustion chamber; contributing to thrust. The staged combustion cycle is sometimes referred to as closed cycle, as opposed to the gas generator, or open cycle where a portion of propellant never reaches the main combustion chamber. The engineering complexity is partly a result of the preburner exhaust of hot and highly pressurized gas which, particularly when oxidizer-rich, produces extremely harsh conditions for turbines and plumbing.

History

Staged combustion (Замкнутая схема) was first proposed by Alexey Isaev in 1949. The first staged combustion engine was the S1.5400 (11D33) used in the Soviet planetary rocket, designed by Melnikov, a former assistant to Isaev.[1] About the same time (1959), Nikolai Kuznetsov began work on the closed cycle engine NK-9 for Korolev's orbital ICBM, GR-1. Kuznetsov later evolved that design into the NK-15 and NK-33 engines for the unsuccessful Lunar N1 rocket. The non-cryogenic N2O4/UDMH engine RD-253 using staged combustion was developed by Valentin Glushko circa 1963 for the Proton rocket.

After the abandonment of the N-1, Kuznetsov was ordered to destroy the NK-33 technology, but instead he warehoused dozens of the engines. In the 1990s, Aerojet was contacted and eventually visited Kuznetsov's plant. Upon meeting initial skepticism about the high specific impulse and other specifications, Kuznetsov shipped an engine to the US for testing. Oxidizer-rich staged combustion had been considered by American engineers, but deemed impossible.[2] The Russian RD-180 engine also employs a staged-combustion rocket engine cycle. Lockheed Martin began purchasing the RD-180 in circa 2000 for the Atlas III and later, the V, rockets. The purchase contract was subsequently taken over by United Launch Alliance (ULA--the Boeing/Lockheed-Martin joint venture) after 2006, and ULA continues to fly the Atlas V with RD-180 engines as of 2021.

The first laboratory staged-combustion test engine in the West was built in Germany in 1963, by Ludwig Boelkow.

Hydrogen peroxide/kerosene powered engines such as the British Gamma of the 1950s may use a closed-cycle process by catalytically decomposing the peroxide to drive turbines before combustion with the kerosene in the combustion chamber proper. This gives the efficiency advantages of staged combustion, while avoiding major engineering problems.

The RS-25 is another example of a staged combustion engine, and the first to use liquid oxygen and liquid hydrogen. Its counterpart in the Soviet shuttle was the RD-0120, similar in specific impulse, thrust, and chamber pressure specification to the RS-25, but with some differences that reduced complexity and cost at the expense of increased engine weight.

Variants
Oxidizer-rich turbine exhaust from a SpaceX Raptor preburner shown during a 2015 sub-system test on a test stand at Stennis Space Center. In the full-flow rocket engine, the preburner exhaust is fed into a turbine and then into the main combustion chamber.

Several variants of the staged combustion cycle exist. Preburners that burn a small portion of oxidizer with a full flow of fuel are called fuel-rich, while preburners that burn a small portion of fuel with a full flow of oxidizer are called oxidizer-rich. The RD-180 has an oxidizer-rich preburner, while the RS-25 has two fuel-rich preburners. The SpaceX Raptor has both oxidizer-rich and fuel-rich preburners, a design called full-flow staged combustion.

Staged combustion designs can be either single-shaft or twin-shaft. In the single-shaft design, one set of preburner and turbine drives both propellant turbopumps. Examples include the Energomash RD-180 and the Blue Origin BE-4. In the twin-shaft design, the two propellant turbopumps are driven by separate turbines, which are in turn driven by the outflow of either one or separate preburners. Examples of twin-shaft designs include the Rocketdyne RS-25, the JAXA LE-7, and the Raptor. Relative to a single-shaft design, the twin-shaft design requires an additional turbine (and possibly another preburner), but allows for individual control of the two turbopumps.

In addition to the propellant turbopumps, staged combustion engines often require smaller boost pumps to prevent both preburner backflow and turbopump cavitation. For example, the RD-180 and RS-25 use boost pumps driven by tap-off and expander cycles, as well as pressurized tanks, to incrementally increase propellant pressure prior to entering the preburner.

Full-flow staged combustion cycle
Full-flow staged combustion rocket cycle

Full-flow staged combustion (FFSC) is a twin-shaft staged combustion cycle that uses both oxidizer-rich and fuel-rich preburners. The cycle allows full flow of both propellants through the turbines; hence the name.[3] The fuel turbopump is driven by the fuel-rich preburner, and the oxidizer turbopump is driven by the oxidizer-rich preburner.[4][3]

Benefits of the full-flow staged combustion cycle include turbines that run cooler and at lower pressure, due to increased mass flow, leading to a longer engine life and higher reliability. As an example, up to 25 flights were anticipated for an engine design studied by the DLR (German Aerospace Center) in the frame of the SpaceLiner project,[3] up to 1000 flights are expected for Raptor from SpaceX.[5] Further, the full-flow cycle eliminates the need for an interpropellant turbine seal normally required to separate oxidizer-rich gas from the fuel turbopump or fuel-rich gas from the oxidizer turbopump,[6] thus improving reliability.

Since the use of both fuel and oxidizer preburners results in full gasification of each propellant before entering the combustion chamber, FFSC engines belong to a broader class of rocket engines called gas-gas engines.[6] Full gasification of components leads to faster chemical reactions in the combustion chamber, allowing a smaller combustion chamber. This in turn makes it feasible to increase the chamber pressure, which increases efficiency.

Potential disadvantages of the full-flow staged combustion cycle include increased engineering complexity of two preburners, relative to a single-shaft staged combustion cycle, as well as an increased parts count.

As of 2019, only three full-flow staged combustion rocket engines had ever progressed sufficiently to be tested on test stands; the Soviet Energomash RD-270 project in the 1960s, the US government-funded Aerojet Rocketdyne Integrated powerhead demonstration project in the mid-2000s,[6] and SpaceX's flight capable Raptor engine first test-fired in February 2019.[7]

The first flight test of a full-flow staged-combustion engine occurred on 25 July 2019 when SpaceX flew their Raptor methalox FFSC engine at their South Texas Launch Site.[8]

Applications

Oxidizer-rich staged combustion

Fuel-rich staged combustion

Full-flow staged combustion
SpaceX Raptor FFSC rocket engine, sample propellant flow schematic, 2019
  • RD-270—USSR engine under development 1962–1970 for the UR-700 project; never flown.[6]
  • Integrated powerhead demonstrator—Demonstration project for the front part of a full flow engine, with no combustion chamber or other backend subsystems.[6] US project to develop a part of a new rocket engine technology in the early 2000s; no full engine ever built; never flown.
  • Raptor—SpaceX LCH4/LOX engine in development, first flown in 2019[19][20]

Current/past applications of staged-combustion engines

Future applications of staged-combustion engines

See also

References
  1. George Sutton, "History of Liquid Propellant Rocket Engines", 2006
  2. Cosmodrome History Channel, interviews with Aerojet and Kuznetsov engineers about the history of staged combustion
  3. Sippel, Martin; Yamashiro, Ryoma; Cremaschi, Francesco (10 May 2012). Staged Combustion Cycle Rocket Engine Design Trade-offs for Future Advanced Passenger Transport (PDF). Space Propulsion 2012. ST28-5. Bordeaux: DLR-SART. Retrieved 19 March 2014.
  4. Emdee, Jeff (2004). "Launch Vehicle Propulsion" (PDF). Crosslink. Vol. 5 no. 1 (2004 Winter ed.). Aerospace Corp. pp. 12–19. Archived from the original on 8 March 2016. Retrieved 30 September 2016.
  5. O'Callaghan, Jonathan (31 July 2019). "The wild physics of Elon Musk's methane-guzzling super-rocket". Wired UK. ISSN 1357-0978. Retrieved 1 January 2021.
  6. Belluscio, Alejandro G. (7 March 2014). "SpaceX advances drive for Mars rocket via Raptor power". NASAspaceflight.com. Retrieved 9 March 2014.
  7. Wall, Mike (4 February 2019). "Elon Musk Unveils SpaceX's 1st Rocket Engine Test for Starship Rocket (Video)". Space.com. Retrieved 27 July 2019.
  8. Burghardt, Thomas (25 July 2019). "Starhopper successfully conducts debut Boca Chica Hop". NASASpaceFlight.com. Retrieved 26 July 2019.
  9. Haeseler, Dietrich; Maeding, Chris; Preclik, Dieter; Rubinski, Vitali; Kosmatechva, Valentina (9 July 2006). "LOX-Kerosene Oxidizer-Rich Gas-Generator and Main Combustion Chamber Subscale Testing". 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit. doi:10.2514/6.2006-5197. ISBN 9781624100383.
  10. Rui C. Barbosa (25 June 2016). "China successfully debuts Long March 7 – Recovers capsule". NASASpaceFlight.com. Retrieved 28 September 2016.
  11. "AR1 Booster Engine | Aerojet Rocketdyne". Rocket.com. Retrieved 28 September 2016.
  12. "ULA now planning first launch of Vulcan in 2021". SpaceNews.com. 25 October 2018. Retrieved 27 July 2019.
  13. Blue Origin. "BE-4 Rocket Engine" (PDF). ULA Website 2014. Retrieved 19 March 2014.
  14. Berger, Eric (9 March 2016). "Behind the curtain: Ars goes inside Blue Origin's secretive rocket factory". Ars Technica. Retrieved 12 March 2016.
  15. "Home". Ursa Major Technologies. Retrieved 20 May 2017.
  16. "Instagram post by Ursa Major Technologies • May 16, 2017 at 11:07pm UTC". Instagram. Retrieved 20 May 2017.
  17. "Berthoud Business News".
  18. "GSLV MkIII, the next milestone". Frontline. 7 February 2014.
  19. Todd, David (22 November 2012). "SpaceX's Mars rocket to be methane-fuelled". Flightglobal. Retrieved 5 December 2012. 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.
  20. Grush, Loren (26 July 2019). "SpaceX's new test rocket briefly hovers during first free flight". The Verge. Retrieved 27 July 2019.
  21. Chris B.-NSF [@NASASpaceflight] (25 July 2019). "One thing some of us missed last night, but some noticed. "Starhopper is a test vehicle that SpaceX is using to help develop its Starship Launch System." So they are calling it SLS? A bit cheeky! pic.twitter.com/rs8mQguuAN" (Tweet). Retrieved 27 July 2019 – via Twitter.
  22. e^ 👁 🥧 (22 July 2019). "For 37 engine config, 6 are mounted outboard under landing leg fairings". @elonmusk. Retrieved 27 July 2019.
  23. Williams, Matt (24 January 2019). "Blue Origin has Shown off a New Video of its New Glenn Rocket Design". Universe Today. Retrieved 27 July 2019.
  24. "ULA selects Blue Origin to provide Vulcan main engine". SpaceNews.com. 27 September 2018. Retrieved 27 July 2019.
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