There are four ways to cool a rocket engine. They are

• Ablative Cooling
• Film Cooling
• Regenerative Cooling
• Radiative cooling

Ablative Cooling

Ablation means erosion of materials. In this method of cooling a ablative material chamber and nozzle is used or a coating of ablative material will be given in the inside wall of combustion chamber and nozzle. The combustion gas which will be at high temperature will erode the materials by vaporization, sublimation or through pyrolysis. This process will be accompanied by a boundary layer of cold gas or low temperature gas from the vaporization process. This boundary layer will act as a thermal insulation and prevent the wall from melting

Film Cooling

Film cooling is used to reduce the convective heat transfer between the hot gases in chamber to the chamber wall. This process involves the creation of a liquid film near the chamber wall. The film will act as a thermal barrier and reduce the heat transfer rate to the wall. In rocket engine, the film cooling technique can be employed using fuel as cooling agent. The fuel (usually 5%) is injected through very fine orifices along the chamber wall (in case of sweat cooling) or through the specially designed orifices angled towards the wall in the injector plate. This high concentration fuel film will have low thermal conductivity and it will be in a low temperature. The hot gas reacts with this excess fuel layer and loses the thermal energy a bit. This reduces the heat transfer to the wall and preventing the wall from melting

Radiative Cooling

The main heat transfer process of this cooling technique is radiation. This is a passive cooling technique. The engine and nozzle wall will be made from material having high thermal melting point and has low thermal expansion coefficient with a very high thermal conductivity. The heat from hot gas will increase the wall temperature to a very high value. The nozzle will glow red hot. This material due to high temperature will radiate the heat to the surroundings. The rate of radiation depends upon the material. This type of cooling is mainly used in the upper stage engines where the nozzle will have large area of heat transfer (high expansion ratio). In vacuum there will be no air to cool the engine, thus radiation will be the only mode of heat transfer.

Regenerative Cooling

Regenerative cooling is an active cooling technique. It uses the fuel itself as coolant. The coolant is allowed to pass through small cooling passages around the combustion chamber. The heat from hot gas will be transferred to chamber wall and the chamber wall will be cooled by the fuel flow around it. This increases the internal energy of the fuel during injection thus increasing the combustion performance. Hence this method is called as regenerative cooling as there is minimum heat loss to the surroundings and the heat extracted from the gas is again driven back into the chamber. This is the most efficient way of cooling the rocket engine. The effect of cooling will depends upon the heat conductive properties of both chamber material and the coolant (Fuel). This method of cooling can be employed to any engine no matter of its size.

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