Staged combustion cycle

Staged combustion rocket cycle. Usually, all of the fuel and a portion of the oxidizer are fed through the pre-burner (fuel rich) to power the pumps. An oxygen rich circuit is possible also, but less common because of the metallurgy required.

The staged combustion cycle, also called topping cycle or pre-burner cycle,^[1] 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, or "closed", combustion cycle is that all of the engine cycles' gases and heat go through the combustion chamber. An alternative design, called a gas-generator cycle, exhausts the turbopump driving gases separately from the main combustion chamber, which leads to a few percent of loss of efficiency in thrust.

Another advantage that staged combustion gives is an abundance of power which permits very high chamber pressures that allow high expansion ratio nozzles. These nozzles give better efficiencies at low altitude.

The disadvantages of this cycle include harsh turbine conditions, exotic plumbing to carry the hot gases, and complicated feedback and control. In particular, running the full oxidizer stream through both a pre-combustor and main-combustor chamber (oxidizer-rich staged combustion) produces extremely corrosive gases.

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.^[2] 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. 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.^[3] 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 specific impulse, thrust, and chamber pressure specification to the SSME, but with some differences that reduced complexity and cost at the expense of increased engine weight.

Full-flow staged combustion cycle

Full-flow staged combustion rocket cycle

Full-flow staged combustion (FFSC) 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.^[4]

In a FFSC engine, a fuel-rich preburner gas turbine drives the fuel turbopump, while an oxidizer-rich preburner gas turbine drives the oxidizer (typically LOX) turbopump.^[4]

Both 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 interpropellant turbine seal is also eliminated.^[5] Full gasification of components leads to faster chemical reactions in the combustion chamber and, as compared to the partial staged combustion cycle, it results in an increase of specific impulse up to 10–20 seconds (e.g., RD-270 and RD-0244).

Because FFSC engine designs typically limit the nominal characteristic conditions in the engine, engine life is generally longer in FFSC designs. Up to 25 flights are anticipated for one particular engine design studied by the ESA.^[4]

Early demonstration tests

Prior to 2014, only two full-flow staged combustion rocket engines have ever progressed sufficiently to be tested on test stands: the Soviet Energomash RD-270 project in the 1960s and the US government-funded Aerojet Rocketdyne Integrated powerhead demonstration project in the mid-2000s.^[5]

Raptor

The Raptor engine currently under development by SpaceX is a full-flow staged combustion engine that will be powered by liquid methane and liquid oxygen^[5][6] This is a distinct departure from the 'open cycle' gas generator system and LOX/kerosene propellants used by the existing set of SpaceX Merlin engines used for orbital launches.^[6]

As in other full-flow designs, Raptor will flow 100 percent of the oxidizer (with a low-fuel ratio) to power the oxygen turbine pump, and 100 percent of the methane fuel (with a low-oxygen ratio) to power the methane turbine pump. Both streams—oxidizer and fuel—will be completely in the gas phase before they enter the combustion chamber, making the engine gas-gas, full-flow staged combustion engine.^[5]

Raptor is being designed to produce about 230 tf (510,000 lbf) of thrust^[7] with a vacuum I_sp of 363 seconds and a sea-level I_sp of 321 seconds.^[5][8]

Additional characteristics of the gas-gas full-flow design are projected to further increase performance, reliability, or both:^[5]

• eliminating the fuel-oxidizer turbine interseal which is traditionally a point of failure in modern chemical rocket engines
• 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."^[5]

Applications

Staged combustion engines include the following:

• Space Shuttle Main Engine—US developed 1970–1980s, flown on the Space Shuttle through 2011, and planned for further use on the Space Launch System after 2018.
• NK-33 (used on the Soviet N-1 launch vehicle 1969–1972) and was later sold by Russia and refurbished/remarketed by US company Aerojet Rocketdyne as the AJ-26 (used on Antares block 1 launch vehicles in 2013–2014). Use was halted after 2014.
• RD-170
• RD-180—1990s Russian engine currently used on several Russian and American launch vehicles.
• RD-253—Soviet developed engine in the 1960s, and used on Proton first stages for many years.
• RD-270 (full flow)^[5]—Soviet engine under development 1962–1970; never flown.
• Integrated powerhead demonstrator (demonstration project for the front part of a full flow engine, with no combustion chamber or other backend subsystems)^[5]—US project to develop a part of a new rocket engine technology in the early 2000s; no full engine ever built; never flown.
• YF-100—Chinese engine developed in the 2000s; currently used on the Long March 5 launch vehicle.
• CE-7.5
• Raptor — methane/LOX engine (full flow) in development since the 2010s, being developed with private funding rather than under government contract as has been usual with most large rocket engines.^[5][6]
• LE-7/7A
• BE-4 — an oxygen-rich 'methalox' (methane/liquid oxygen) engine under development by Blue Origin; it will serve as an American-made replacement for the Atlas V's RD-180 engines.^[9][10]

Staged combustion engines have been used in:

• Space Shuttle
• Atlas III
• Atlas V
• Antares (rocket)
• N1 (rocket)
• H-II
• H-IIA
• H-IIB
• GSLV
• Proton (rocket family)
• Zenit (rocket family)
• Energia

Staged combustion engines are planned for:

• Mars Colonial Transporter^[5]
• Vulcan^[11]

See also

• Expander cycle
• Gas-generator cycle
• Pressure-fed engine

References

External links

• Rocket power cycles
• Nasa's full flow stages combustion cycle demonstrator
• Design Tool for Liquid Rocket Engine Thermodynamic Analysis