A railgun is an electrically powered electromagnetic projectile launcher based on similar principles to the homopolar motor. A railgun comprises a pair of parallel conducting rails, along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail.[2] Railguns have long existed as experimental technology but the mass, size and cost of the required power supplies have prevented railguns from becoming practical military weapons. However, in recent years, significant efforts have been made towards their development as feasible military technology. For example, in the late 2000s, the U.S. Navy tested a railgun that accelerates a 3.2 kg (7 pound) projectile to hypersonic velocities of approximately 2.4 kilometres per second (5,400 mph), about Mach 7 .[3] They gave the project the Latin motto "Velocitas Eradico", which is Latin for "I, [who am] speed, eradicate", but may have been intended as "Speed kills" or similar. In addition to military applications, railguns have been proposed to launch spacecraft into orbit; however, unless the launching track was particularly long, and the acceleration required spread over a much longer time, such launches would necessarily be restricted to unmanned spacecraft. The United States Armed Forces[N 1] are the military forces of the United States of America. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.[6] The U.S. has a strong tradition of civilian control of the military. The President of the United States is the military's overall head, and helps form military policy with the U.S. Department of Defense (DoD), a federal executive department, acting as the principal organ by which military policy is carried out. The DoD is headed by the Secretary of Defense, who is a civilian and Cabinet member. The Defense Secretary is second in the military's chain of command, just below the President, and serves as the principal assistant to the President in all DoD-related matters.[7] To coordinate military action with diplomacy, the President has an advisory National Security Council headed by a National Security Advisor. Both the President and Secretary of Defense are advised by a seven-member Joint Chiefs of Staff, which includes the head of each of the Defense Department's service branches as well as the chief of the National Guard Bureau. Leadership is provided by the Chairman of the Joint Chiefs of Staff and the Vice Chairman of the Joint Chiefs of Staff.[8] The Commandant of the Coast Guard is not a member of the Joint Chiefs of Staff. All of the branches work together during operations and joint missions, under the Unified Combatant Commands, under the authority of the Secretary of Defense with the exception of the Coast Guard. The Coast Guard falls under the administration of the Department of Homeland Security and receives its operational orders from the Secretary of Homeland Security. The Coast Guard may be transferred to the Department of the Navy by the President or Congress during a time of war.[9] All five armed services are among the seven uniformed services of the United States; the others are the U.S. Public Health Service Commissioned Corps and the National Oceanic and Atmospheric Administration Commissioned Corps. The United States Navy (USN) is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. It is larger than the next 13 largest navies combined in terms of battle fleet tonnage, according to one estimate.[5][6] The U.S. Navy also has the world's largest carrier fleet, with 10 in service, one under construction (two planned), and two in reserve. The service has 317,054 personnel on active duty and 109,671 in the Navy Reserve. It operates 286 ships in active service and more than 3,700 aircraft.[3] The navy traces its origins to the Continental Navy, which was established during the American Revolutionary War and was essentially disbanded as a separate entity shortly thereafter. It played a major role in the American Civil War by blockading the Confederacy and seizing control of its rivers. It played the central role in the World War II defeat of Japan. The 21st century United States Navy maintains a sizable global presence, deploying in such areas as East Asia, the Mediterranean, and the Middle East. It is a blue-water navy with the ability to project force onto the littoral regions of the world, engage in forward areas during peacetime, and rapidly respond to regional crises, making it an active player in U.S. foreign and defense policy. The Navy is administratively managed by the Department of the Navy, which is headed by the civilian Secretary of the Navy. The Department of the Navy is itself a division of the Department of Defense, which is headed by the Secretary of Defense.

• published: 29 Oct 2013
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