What is one of the requirements for the sides of jettison lockers containing incendiary ammunition?

One of the key requirements for the sides of jettison lockers containing incendiary ammunition is that they be insulated with sand. This is because sand serves as an effective fire retardant, able to absorb heat and prevent the spread of flames in the event of a fire or explosion. The use of sand helps to contain any potential ignition or fire from the incendiary ammunition, thereby enhancing safety in environments where such materials are stored.

In contrast, other materials such as cardboard, metal, or water do not provide the same level of protection or effectiveness. Cardboard could easily catch fire, while metal would not effectively absorb heat. Water might be useful in some contexts for cooling, but it is not suitable for containing incendiary devices, as it could potentially spread the fire or react poorly with certain munitions.