The U.S. Navy is again looking at designing and building a new type of "destroyer." The original destroyer was developed a century ago as a small ship capable of destroying even smaller boats carrying the (then) new torpedo. After several decades of work, the modern torpedo loomed as a cheap threat to larger warships. So the "torpedo boat destroyer" (soon shortened to just "destroyer") was developed. Modern destroyers have grown to the size of cruisers (9,000 tons and a draft of 31 feet) and are now too expensive to go after smaller ships equipped with torpedoes or, as is more common, anti-ship missiles or just lots of explosives for suicide attacks. The navy has specified that the new ship must be between 1,000 and 4,000 tons, be capable of up to 90 kilometers an hour. The new ship must have a draft of less than 20 feet, so it can operate close to shore. The design must also allow for modular weapons systems, so that missile, electronic, gun or other systems can be quickly swapped in or out as the ships mission changes. Six design firms have each been given half a million dollars to come up with proposals.