The McDonnell Douglas Harpoon, now entering service, is the U. S. Navy’s first antiship missile intended to arm surface vessels, submarines, and aircraft. The ability to use a variety of ship and airborne fire control systems is achieved by varying the printed-circuit boards in an interface unit between the missile’s electronics and those in the launch vehicle. The weapon itself is standard, apart from the deletion of the boost motor in the AGM-84A air-launched variant and the use of folding wings and fins when the round is fired from submarines, canister launchers, or Tartar equipment. Harpoon is treated as a round of ammunition requiring no on-board maintenance.
Target information, derived from existing sensors, is fed to the missile before launch along with data giving launch-vehicle parameters such as attitude and speed. In its airborne role the round is mounted on a normal weapon pylon and, if launched at medium or high altitudes, drops clear before the sustainer motor is ignited. If launching is to take place at low level the power-plant can be run up before release. In surface ships Harpoon uses existing ASROC, Tartar, or multi-purpose launchers, and for smaller vessels a specialized four-tube canister launcher/container has been developed. When operated from submarines the weapon is expelled from the torpedo tube in a buoyant capsule. This capsule is ejected from a normal 21-inch tube at about 30-feet-per-second, steering fins unfold to trim the capsule for the correct ascent angle through the water, and the nose and tail caps are blown off as the surface is broached. The 15,000-pound thrust Aerojet solid-propellant boost motor ignites and powers the missile from the remains of the capsule in a 35° climb with an acceleration of about ten Gs.
After reaching an apogee of some 2,500 feet following a boosted launch, or descending from altitude in the case of an airborne firing, the missile flies at low level under the control of a mid-course guidance unit (MGU) and a radar altimeter. It can turn through 90° if the launch angle is offset from the target bearing. The MGU combines a strap-down three-axis reference assembly with a digital computer/autopilot. Cruise propulsion at Mach 0.85 is provided by a 660-pound thrust Teledyne CAE J402-CA-400 single-spool turbojet burning JP5 fuel, of which just over 100 pounds is carried. This fuel capacity is sufficient for 15 minutes endurance—more than enough time for the weapon to achieve its maximum effective range of 60 nautical miles.
The attack phase is under the control of a Texas Instruments frequency-agile, two-axis active radar seeker in the weapon’s nose. The search scan, initiated by the computer at a preset distance from the expected position of the enemy vessel, can be one of three patterns depending on target characteristics, such as range, size, and speed. Steering commands are fed through the MGU, which acts as an auto-pilot, to the four tail-mounted fins. If the target’s range is not known before firing, the weapon is used in the bearing-only launch (BOL) mode. The operator feeds in the time at which the seeker is to begin its search, and the azimuth sweep and depth of ranging thereafter both automatically increase gradually until the target is acquired or the missile reaches maximum range and destroys itself.
Harpoon flies a terminal pop-up maneuver to confuse rate-aided tracking radars and enhance destructive effect by diving at some 7° and penetrating the ship’s lightly armored upperworks. The 500-pound cylindrical blast/fragmentation warhead is detonated only after it has entered the hull; a secondary proximity fuze is also fitted. Missile length is 151 inches, and weight is 1,100 pounds; in the airborne version, the booster adds 29 inches and 300 pounds.
The exact number of U. S. Navy vessels to be armed with Harpoon has not yet been established. Two lightweight (8,500 pounds complete with four rounds) canister launchers are likely to be installed on the PHM hydrofoils, FFG-7 patrol frigates, Spruance-class destroyers, and guided-missile cruisers. Other ships will use existing launchers. All nuclear-powered submarines are expected to carry the missile for defense against ASW vessels. Aircraft slated for Harpoon include the A-6E Intruder, A-7E Corsair, P-3C Orion, and S-3A Viking. The carrier-based attackers will use the missile to engage enemy forces before they can launch their own antiship weapons. The Department of Defense points out that an Intruder armed with four Harpoons has a range greater than 1,000 miles, although the Soviet Navy’s new aircraft carriers carry SS-N-12 missiles capable of flying twice this distance at reduced speeds. Plans to mount the weapon on F-4s and B-52S have been dropped; the bombers will now carry GBU-15 glide bombs, which are substantially cheaper. There is also some dispute as to whether the maritime patrol/antisubmarine types should carry Harpoon.
Overseas customers for Harpoon are Turkey, Holland, Denmark, Iran, Australia, Israel, South Korea, Saudi Arabia, and Britain. Harpoon systems are being installed on a variety of aircraft and ships, including destroyers, frigates, fast patrol boats, and hovercraft. The Royal Navy has ordered Harpoon to equip its nuclear-powered submarines operated in an offensive role against Soviet surface forces. The RN’s boats have several differences compared with those of the U. S. Navy, and one of the modifications necessary is the design of a new launch capsule. An extensive co-production arrangement is being set up in a number of European countries.