Regenerative cooling (rocketry)
The heated propellant is then fed into a special gas-generator or injected directly into the main combustion chamber.
[3] Robert Goddard built the first regeneratively cooled engine in 1923, but rejected the scheme as too complex.
[4] A regeneratively cooled engine was built by the Italian researcher, Gaetano Arturo Crocco in 1930.
The first German engine of this type was also tested in March 1933 by Klaus Riedel in the VfR.
This inefficient design required the burning of diluted alcohol at low chamber pressure to avoid melting the engine.
This permitted more energetic fuels and higher chamber pressures, and is the basic plan used in all Russian engines since.
American engines usually solved this problem by lining the combustion chamber with brazed copper or nickel alloy tubes.
The American style of lining the engine with copper tubes is called the "spaghetti construction", and the concept is credited to Edward A. Neu at Reaction Motors Inc. in 1947.
Regenerative cooling remains the predominant method for managing the thermal loads in thrust chambers.
Typically the rocket fuel acts as a coolant as it enters the engine through passages at the nozzle exit.
The cross-sections of these passages are smaller, increasing the coolant velocity and maximizing cooling efficiency in high-heat areas.
[7]: 98 A common method for estimating the heat flux flowing out from the hot combustion gases is to use the Bartz equation:[8]
Two boundary layers form: one in the hot gas in the chamber (which is modeled with the Bartz equation above) and the other in the coolant within the channels.
[7]: 104–105 Very typically most of the temperature drop occurs in the gas boundary layer since gases are relatively poor conductors.
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.
The inner liner is under compression, while the outer wall of the engine is under significant hoop stresses.
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.
Several different manufacturing techniques can be used to create the complex geometry necessary for regenerative cooling.
[9] The geometry can also be created through direct metal 3D printing, as seen on some newer designs such as the SpaceX SuperDraco rocket engine.
