TABLE 3.3 Advantages of Oxidizer-Rich, Staged-Combustion (ORSC) Rocket Engines Over Open-Cycle Gas-Generator LO2/RP Engines

Advantages   Issues/Concerns
Higher Isp (up to 7-10%).   Because of its oxidizer-rich hot gas environment, engine components and plumbing (ducts) need to be made from compatible and flame-resistant materials or require the use of special nonburning, resilient protective coatings that do not erode or chip away during handling, testing, and operation (especially important for reusable engines).
Use of higher density fuel enables higher overall mass fraction and more favorable aerodynamics profile rocket stages/vehicle design.   Greater tendency for combustion instability because of the more difficult to burn hydrocarbon fuel operating at much higher chamber pressure in the preburner and main combustion chamber.
Because of higher chamber pressures, the engine design results in nozzles with higher sea-level area ratios and significantly higher engine thrust/weight ratios.   Preburner design and development is more difficult than fuel rich gas generators (GGs). Because of high operating pressures, there will be a greater tendency for combustion instabilities and the need for high-temperature oxidizer and flame-resistant materials in the turbines and any associated hot gas ducts and the oxidizer side of the main combustion chamber (MCC) manifolds and injectors.
Results in oxygen-rich shutdown, which minimizes carbon deposits and “coking” of injector orifices with hydrocarbon fuels—therefore, easier to restart multiple times.   Because of the high preburner operating pressures (6,000-9,000 psia), ORSC engines will require boost pumps and boost pump devices. Fuel-rich GGs typically run at much lower pressures—~1,000-1,500 psia—and are easier to design with fewer components.
Enables pumps to generate required power at much lower operating temperatures than with fuel-rich GG powered turbines, which results in increased life and durability (typically about 700°F versus ~1700°F for fuel-rich GG cycle).    
Eliminates open-cycle GG exhaust plume interactions and interference issues by running all of the turbine drive gas back into the MCC.    
Usually allows increased engine service life because of generally lower operating temperatures.    

NOTE: See for example: (1) Oscar Haidn, Advanced Rocket Engines, Lecture Series at the von Karman Institute, Belgium. RTO-EN-AVT-150, ISBN 978-92-837-0085-2, Published March 2007, Open to the public; (2) Robert Sackheim, Overview of United States Space Propulsion Technology and Associated Space Transportation Systems, AIAA Journal of Propulsion and Power 22(6), November-December 2006.

temperature of 700-800°F. The preburner exhaust gases are then run through the turbopump assembly (TPA) turbine, which typically drives both the fuel and the oxidizer pumps using a gearbox and separation seals. Note that all staged combustion engines must operate with pump discharge pressures significantly higher than the main combustion chamber (MCC) pressure because the main drive turbine operates in series with the MCC. The LO2 is fed back to the preburner and the oxygen-rich gas from the turbine exhaust is then fed into the MCC injectors, where it is mixed with the liquid RP-1 coming from the fuel pump. Both the hot oxidizer-rich gas and the RP-1 enter the MCC through the chamber injectors. Typically, the injector has hot gas/liquid RP-1 swirling elements. The number of these swirl injector elements depends on the engine thrust level and scales somewhat with engine size and thrust level.

Boost pumps are typically used to feed the high-pressure LO2 and RP-1 into the preburner. The engine start and shutdown sequences and methodology vary and tend to be somewhat complex, but they are usually established by the engine timing, calibration of the power balance, detailed operation of the flow control valves, and other fluidic elements. The oxidizer and the fuel pumps are usually mounted on the same common shaft, and dynamic seals and intercavity inert gas purges keep the two liquids well separated. An upstream start turbine is sometimes used to start TPA full operation.

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