Probably the one, first, and most requested recipie, is for hardtack (also known as 'tack, ironplate biscuits, army bread, and other colorful names). From the 1862 US Army book of recipes, is one that is guaranteed to keep your dentist happy with bridge and upper plate work, and not to satisfy your culinary hunger. But these actually work and stay fresh for eons.
In a bowl, combine the ingredients to form a stiff, but not dry dough. The dough should be pliable, but not stick a lot to your hands.

Take this mound of dough, and flatten it out onto a greased cookee sheet (the ones with a small lip around the edge...like a real shallow pan...), and roll the dough into a flat sheet aprx. 1/2 inch thick.

Using a breadknife, divide the dough into 3x3 squares. taking a 10-penny nail, put a 3x3 matrix of holes into the surface of the dough, all the way thru, at even intervals (Village tinsmithing works sells a cutter that does all of this...works great!).

Bake in the oven for aprx 20 Min., till lightly browned. Take out and let cool.

Do this the day before your go on the field, and your will have enough tack to fill your haversack. It will be somewhat soft on Saturday morning, but, by Sunday, you should soak it in your coffee before eating, else you will have a hard time chewing.