Regenerative cooling
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Regenerative cooling in rockets is where some or all of the propellant is passed through tubes, channels or otherwise in a jacket around the combustion chamber or nozzle to cool the engine because the fuel in particular and sometimes the oxidiser are good coolants. The heated propellant is then fed into a special gas generator or injected directly into the main combustion chamber for combustion there.
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[edit] History
The concept of regenerative cooling was mentioned as early as 1928 in an article by Konstantin Tsiolkovsky. The first Russian engine to employ the technique was the ORM-50, first tested in November of 1933 by Valentin Glushko. The first German engine to be regeneratively cooled was the SR-4 tested in March of 1934 by Eugen Sänger.
The V-2 rocket engine, the most powerful of its time at 25 tons (245 kN) of thrust, was regeneratively cooled by fuel lines coiled around the outside of the combustion chamber. This was an inefficient design that required the burning of diluted alcohol at low chamber pressure to avoid melting the engine. The American Redstone engine used the same design.
A key innovation in regenerative cooling was the Russian U-1250 engine designed by Aleksei Mihailovich Isaev in 1945. Its combustion chamber was lined by a thin copper sheet supported by the corrugated steel wall of the chamber. Fuel flowed through the corrugations and absorbed heat very efficiently. This permitted more energetic fuels and higher chamber pressures, and it is the basic plan used in all Russian engines since. Modern American engines solve this problem by lining the combustion chamber with brazed copper or nickel alloy tubes, although recent engines like in the Boeing Delta IV have started to use the Russian technique which is cheaper to construct.
[edit] Heat flow and temperature
The heat flow through the chamber wall is very high indeed, 1-20 MW/m2 is not uncommon.
The amount of heat that can flow into the coolant is controlled by many factors including the temperature difference between the chamber and the coolant, the heat transfer coefficient, the thermal conductivity of the chamber wall, the velocity in the coolant channels and the velocity of the gas flow in the chamber or the nozzle.
Two boundary layers form; one in the hot gas in the chamber and the other in the coolant within the channels.
Very typically most of the temperature drop occurs in the gas boundary layer since gases are relatively poor conductors. This boundary layer can be destroyed however by combustion instabilities, and wall failure can follow very soon afterwards.
The boundary layer within the coolant channels can also be disrupted if the coolant is at subcritical pressure and film boils; the gas then forms an insulating layer and the wall temperature climbs very rapidly and soon fails. However, if the coolant engages in nucleate boiling but does not form a film, this helps disrupt the coolant boundary layer and the gas bubbles formed rapidly collapse; this can triple the maximum heat flow. However, many modern engines with turbopumps use supercritical coolants, and these techniques can be seldom used.
Regenerative cooling is seldom used in isolation, film cooling and curtain cooling are very frequently employed as well.
[edit] Mechanical considerations
With regenerative cooling, the pressure in the cooling channels is significantly above the chamber pressure hence the inner liner is under compression, while the outer wall of the engine is under significant hoop stresses.
The metal of the inner liner is greatly weakened by the high temperature, and also undergoes significant thermal expansion at the inner surface while the cold-side wall of the liner constrains the expansion. This sets up significant thermal stresses that can cause the inner surface to crack or craze after multiple firings particularly at the throat.
In addition the thin inner liner requires mechanical support to withstand the compressive loading due to the propellant's pressure, this support is usually provided by the side walls of the cooling channels and the backing plate.
The inner liner is usually constructed of relatively high temperature, high thermal conductivity materials, traditionally copper or nickel based alloys have been used.
