Thanks for the thorough reply. > it will be done in parallel by a large number of soldiers personally grinding small amounts of grain each, not by a mill I'd

Thanks for the thorough reply. > it will be done in parallel by a large number of soldiers personally grinding small amounts of grain each, not by a mill I'd had it in my head for some reason that a mill stone was the only reasonable way to turn grain into flour. This was the main factor I wasn't considering. The weight/effort to handle a portable mill certainly wouldn't beat hand tools. Even if to just help keep soldiers busy. EDIT: I expect that if one were to analyze an army's flour requirement as some form of equipment weight + work-hours / grain-throughput * army size there would be a point at which it made sense to haul a portable mill. But it definitely would not make sense to power it with steam engine compared to having soldiers turn it by hand. I would also expect that a force large enough to utilize that efficiency would be unheard of in roman times.