Which of the following does NOT contribute to reducing thermal stress on weapons?

Maintaining a cool environment is the choice that does not directly contribute to reducing thermal stress on weapons through a physical mechanism. While having a cooler environment can help in general thermal management, it does not actively engage with the weapon system to reduce thermal stress experienced during operation.

In contrast, conduction cooling, convection cooling, and radiational cooling are all active mechanisms that help dissipate heat from the weapon system itself. Conduction cooling refers to the transfer of heat away from hot surfaces through direct contact with cooler materials. Convection cooling involves the movement of air or liquid around the weapon system, allowing heat to be carried away efficiently. Radiational cooling utilizes the emission of infrared radiation to release excess heat into the surrounding environment.

Therefore, while having a cool environment is beneficial, it does not actively work to mitigate thermal stress on the weapon itself, making it the correct selection in this context.