By David Alexander WASHINGTON (Reuters) - The U.S. Navy is planning sea trials for a weapon that can fire a low-cost, 23-pound (10-kg) projectile at seven times the speed of sound using electromagnetic energy, a "Star Wars" technology that will make enemies think twice, the Navy's research chief said. Rear Admiral Matthew Klunder, the chief of Naval Research, told a round table group recently the futuristic electromagnetic rail gun had already undergone extensive testing on land and would be mounted on the USNS Millinocket, a high-speed vessel, for sea trials beginning in 2016. It's firing," said Klunder, who planned to discuss progress on the system later on Monday with military and industry leaders at a major maritime event - the Sea-Air-Space Exposition - near Washington. "We're also talking about a gun that's going to shoot a projectile that's about one one-hundredth of the cost of an existing missile system today." The Navy research chief said that cost differential - $25,000 for a railgun projectile versus $500,000 to $1.5 million for a missile - will make potential enemies think twice about the economic viability of engaging U.S. forces.