Why does a flameless ration heater in the MRE NOT come with water already in it?

Answer 1

Because adding water to it activates it. It would be pointless to pack a previously activated ration heater in MREs.

That, and the one thing service members can pretty well count on, even in combat situations is water for their canteen or Camel Back hydration system.

Additionally, the MRE pouch is a tough, sealed, plastic bag, not easily opened without a knife. The FRH is essentially a battery when water is added (see the chemical reaction between magnesium and iron powder, salt, and water). One of the byproducts of activation of the FRH is hydrogen gas, which anyone who has seen the video footage of the Hindenburg Disaster knows is extremely volatile. Imagine the service member sitting on a rock in the field, preparing to enjoy his nutritious Meal, Ready-to-eat, puffing away on his Lucky Strike, when he drives his M7 bayonet into the bloated pouch containing his FRH with the water conveniently added by the manufacturer. Now imagine the expression on the faces of the group of insurgents a klick or so away when they hear a sharp crack, look up to see a small white mushroom cloud over the next hill, and get a whiff of burnt GI crewcut.

That, kids, is why the flameless reaction heater doesn't come with water already inside.