The staged combustion cycle is a thermodynamic cycle of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to power the engine's turbines and pumps. The exhausted gas is then injected into the main combustion chamber, along with the rest of the propellant, and combustion is completed.
The advantage of the staged combustion cycle is that all of the engine cycles' gases and heat go through the combustion chamber, and overall efficiency essentially suffers no pumping losses at all. Thus this combustion cycle is often called 'closed cycle' since the cycle is closed as all propellant products go through the chamber; as opposed to open cycle which dumps the turbopump driving gases, representing a few percent of loss.
Another very significant advantage that staged combustion gives is an abundance of power which permits very high chamber pressures. Very high chamber pressures mean high expansion ratio nozzles can be used, whilst still giving ambient pressures at takeoff. These nozzles give far better efficiencies at low altitude.
The disadvantages of this cycle are harsh turbine conditions, more exotic plumbing is required to carry the hot gases, and a very complicated feedback and control design is necessary.
Staged combustion engines are the most difficult types of rocket engines to design. A simplified version is called the Gas-generator cycle.
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. 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 were developed by Valentin Glushko around 1963 for the Proton rocket.
After the failure of the N-1, Kuznetsov had been 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. Initially sceptical about the high specific impulse and other specifications, Kuznetsov shipped an engine to the US for testing. Staged combustion had been considered by American engineers, but deemed impossible. The Russian RD-180 engine, purchased by Lockheed Martin (subsequently by United Launch Alliance) for the Atlas III and V rockets, also employs this technique.
In the West, the first laboratory staged-combustion test engine was built in Germany in 1963, by Ludwig Boelkow.
Hydrogen peroxide / kerosene fuelled engines such as the British Gamma of the 1950s may use a closed-cycle process (arguably not staged combustion, but that's mostly a question of semantics) 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, whilst avoiding the major engineering problems.
The Space shuttle main engine 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 specification to the SSME but with a number of technological improvements.
Full flow staged combustion cycle
Full flow staged combustion (FFSCC) is a variation on the staged combustion cycle where all of the fuel and all of the oxidizer pass through their respective power turbines. A small amount of fuel and oxidizer is swapped and combusted to supply power for the turbines.
The turbines run cooler in this design since more mass passes through them, leading to a longer engine life and higher reliability. The design can provide higher chamber pressures and therefore greater efficiency. An intropropellant turbine seal is also eliminated. A current version under development is the Integrated powerhead demonstrator. Also this scheme was applied in russian experimental RD-270 engine in late sixties of twentieth century, which was designed for several soviet projects of lunar rockets.
Published - July 2009
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