Regenerative cooling
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Regenerative cooling in rockets is where the propellant is passed through tubes around the combustion chamber or nozzle as the fuel is a good conductor of heat. The amount of heat is controlled by temperature difference, heat transfer coefficient and the velocity of the internal flow in the chamber or the nozzle. The fuel is then fed into a special gas generator or injected into the main combustion chamber.
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 copper tubes, although recent engines like in the Boeing Delta IV have started to use the Russian technique which is cheaper to construct.
