Why Don't Rocket Engines Use Oxidisers for Regenerative Cooling?

Introduction:

The design and operation of rocket engines involve a variety of complex considerations, one of which is the method of cooling. Despite the potential benefits, oxidizers are not commonly used for regenerative cooling in rocket engines. This article will explore the reasons behind this decision, focusing on chemical compatibility, temperature control, efficiency, and the challenges associated with using oxidizers in this context.

Chemical Compatibility

A fundamental issue with using oxidizers for regenerative cooling is their chemical reactivity. Liquid Oxygen (LOX), a common oxidizer, is highly reactive and can cause corrosion or chemical reactions with engine materials. This poses a significant challenge for material selection and can limit the types of materials that can be used in the cooling channels. For instance, aluminum alloys, which are often used in rocket engine components, can form a porous oxide layer when exposed to LOX, compromising the structural integrity of the components.

Temperature Control

Regenerative cooling systems typically employ the use of propellant fuel for cooling because it can absorb heat without undergoing a significant change in state. Fuel often has a higher boiling point and remains a liquid at high temperatures, making it a more suitable coolant. In contrast, oxidizers, such as LOX, may vaporize or react unfavorably in the presence of extreme heat, which can lead to thermal stress and potential operational issues.

Efficiency

Using the fuel for cooling allows for a more efficient use of the propellant. The fuel absorbs heat, transforming it from the high-temperature combustion chamber to a lower temperature in the cooling channels. This process improves combustion efficiency and overall engine performance. By contrast, oxidizers may not absorb heat as effectively, reducing the overall efficiency of the cooling process.

Complexity and Risk

The integration of oxidizers into regenerative cooling systems would introduce additional complexity to the engine design. Any leaks or failures in the cooling system could lead to combustion instability or engine failure, posing a significant risk. The design of robust and reliable cooling systems that can handle the extreme conditions inside a rocket engine is a challenging task.

Design Traditions

Historically, many rocket engine designs have favored the use of fuel for regenerative cooling. This tradition has established a precedent in engineering practices, and it is challenging to deviate from well-established methods. Engineers often optimize designs based on proven methods, which have been extensively tested and refined over time.

Recent Advances in Liquid Oxygen Regenerative Cooling

Despite the aforementioned challenges, research into liquid oxygen regenerative cooling has shown promise. The Launcher E-1 engine, for example, employs a liquid oxygen regenerative cooling system, demonstrating the feasibility of this approach. Studies conducted in the Soviet Union and subsequently at NASA have shown that certain materials, particularly some copper alloys, do not combust with LOX until very high temperatures. C18150 Alloy, a specific copper alloy, has been tested and found to survive LOX regen cooling at substantial temperatures. Specific copper and Inconel alloys also exhibit similar properties.

Refer to the following examples for more detailed information:

Copper Alloys: C18150 Alloy and other specialized alloys have been found to withstand LOX regen cooling effectively. Inconel Alloys: These alloys have shown promising results in resisting LOX-induced degradation.

Conclusion

The decision not to use oxidizers for regenerative cooling in rocket engines is driven by a combination of chemical compatibility, temperature control, efficiency, and the risks associated with complexity. However, recent advancements in material science, particularly the development of corrosion-resistant copper alloys, offer potential solutions to these challenges. As research continues, the use of oxidizers in rocket engine cooling systems may become more feasible and practical.