Although I am a big fan of Straight Shaving, I wouldn't be concerned about it in the field as a soldier.

If you must, use a cheap disposable straight shaver. They are made of stainless steel and do not require stropping. This along with a good lubricant will give you a good beard reduction shave and depending on your technique, give you a good shave, but not a BBS shave.

Shave once every three days and a pack of replacement blades will last you at a min 20+ shaves. In theory, this would last you 20+ weeks. Considering you may just do one pass to do a beard reduction, the blades will last even longer and possible up to 40 weeks.

FYI - the British Redcoats shaved every Wed. and Sunday, thus my suggestion to shave every three days.

Good Luck!

Pabster