Can I ask a quick question here whilst on the subject?
Do you need to have water in the water jacket if you are still using the fire / jacket to cook over.
It boils water really quickly but I find when cooking over it I am concerned that if it boils dry will it ruin it?
You're right to worry. You can easily get 700-800 degrees Celsius from a wood fire, and aluminium alloys melt at much less than that. So an aluminium kettle will melt if there's no water in it keeping it cool, and that's the end of the kettle. Steels melt at well over 1000 degrees, so a steel one will fare a lot better when empty, but it's still a bad idea to heat it when dry. The sheet metal is thin and will eventually burn through, but well before that the stainless properties will be degraded, and it will also distort and probably start to leak at the seams. You're right, the water boils very quickly, and it's so quick that the idea of cooking over the same flame has always seemed nonsense to me. The water boils so fast that you wouldn't have enough time to cook anything, and when it boils you'd be frantically looking for somewhere to put the food while the boiling water was spraying all around the camp. At least that's my take on it.