“AMATEURS talk strategy, but professionals talk logistics.” That military maxim’s latest consequence is the adoption by the world’s armed forces of three-dimensional (3D) printing on the front line. It will be a while before weapons robust enough for military use can be printed on demand (though civilian ones can be, see article). But if it is a question of replacing a small but crucial component that has broken—the modern equivalent of reshoeing a horse—then making what is needed to order in this way has huge potential. Moving replacement parts through a long supply chain to a far-flung ship or base can take weeks. And, if a war is on, such convoys make tempting targets. Yet it is unrealistic to keep a full range of spares near the front line. Far better to produce what is needed, when it is needed.

Having access to a printer can even encourage innovation. For example, the USS Harry S. Truman, an American aircraft-carrier, took two 3D printers on her most recent tour of duty in the eastern Mediterranean and the Persian Gulf, which began in November 2015. During the eight months she was at sea her crew devised and printed such items as better funnels for oil cans (to reduce spillage), protective covers for light switches (to stop people bumping into them and inadvertently plunging, say, the flight deck into darkness) and also a cleverly shaped widget they dubbed the TruClip. This snaps onto walkie-talkies, reinforcing a connection that is otherwise prone to break in the rough-and-tumble of naval usage. According to Commander Al Palmer, one of the Truman’s maintenance officers, TruClips alone have saved more than $40,000 in replacement parts. The printers themselves, by contrast, cost about $2,000 each. On the basis of his experience using it, Commander Palmer reckons 3D printing will become an important part of the American navy’s supply chains.

Keep your powder dry

At the moment, only plastic items can be printed at sea. Landlubbing printers can make things out of metal by building up layers of metallic powder that are then melted with a laser or electron beam and allowed to cool into a solid. But printers, like people, get seasick. A ship’s constant yawing, pitching and rolling disturbs the powder before the beam can do its work. This is why a printer of metal ship parts operated by Canada’s navy sits safely on dry land, at the Cape Scott fleet-maintenance facility in Halifax, Nova Scotia.

In time, however, metal parts may also be printed at sea. The head of engineering at the American navy’s supply command, Captain Armen Kurdian, says his organisation is looking for ways to overcome the problem of instability. Mounting printers on damping platforms that hold them steady by compensating for a ship’s motion could be one answer. Another might be to form the metal “ink” into wires instead of powders, for wire is more easily held in place than a layer of dust is. In this arrangement the laser or electron beams would melt the tips of the wires.

Nor are sailors the only servicemen who will benefit from 3D printing. China’s army prints both basic items, such as ratchets, and more sophisticated ones, including physical relief maps of local terrain that help soldiers plan operations more effectively than a paper map or screen display could. Israel’s air force prints plastic parts that are as strong as aluminium, in order to keep planes that date from the 1980s flying. And America is advising the governments of Australia, Britain and France on 3D printing, in order to speed up these allies’ supply chains, says Chris Wood, a captain of marines who works at the Pentagon and is in charge of this joint enterprise.

Captain Wood will also, within the next three months, be supervising the distribution of 3D printers to American marines in Europe, the Middle East and the Pacific. In October marines at Camp Pendleton in California finished converting a shipping container into a rugged “expeditionary-manufacturing facility” movable by lorry, ship, train or aircraft. In addition to a 3D printer, this contains conventional machining equipment. Another such mobile workshop is under construction in North Carolina. And the army, too, is involved. It has already sent some 3D printers to bases in Afghanistan.

For now, like those on board ship, “forward deployed” printers of this sort make items out of plastic only. In their case the problem with printing in metal is not constant movement but grit—for this is a much more sensitive process than printing in plastic. Even that limitation will be overcome, though, according to the United States’ Army Research Laboratory (ARL). The ARL is paying two firms to develop technologies which can turn blocks of metal into printable powder within the confines of a shipping container. The purpose of this is to recycle battlefield scrap into new equipment.

At the moment this “atomisation” process works like an old-fashioned shot tower. Molten metal poured in at the top of a chamber breaks into droplets that cool and solidify on their way down. But this requires a chamber at least six metres high, which is too tall to fit upright inside a standard shipping container. One of the firms the ARL has contracted, MolyWorks Materials of Los Gatos, California, has managed to shrink the process so that it does fit inside such a container. It does so by orienting the chamber diagonally, and employing jets of inert gas to stop the droplets touching the sides before they have cooled. If printers that make use of these solidified droplets can also be made rugged enough to withstand the battlefield, then broken parts themselves will become recyclable, supply chains may no longer need to deliver even raw materials and, the logistics taken care of, more thought can be given to the little matter of strategy.