Technology Explained

Another class of weapons that deserves attention at this stage are laser-guided bombs (see TE, Sept. 1981). The Tl GBU-10, 12, 16 Paveway family represents the most widespread weapon currently in service. LGBs are difficult to use, where the opponent has decent SAMs and/or one lacks extensive ECM; however, if the vessel under attack has to rely on guns or short-range/low-altitude SAMs, the LGB may be quite effective (the limits of its launch envelope lie at 30,000 ft/ 7. 5 nm). A direct hit by a 2000 lb demolition bomb will sink most modern, soft-skinned vessels. In this sense, the LGB is an effective means of taking out defenceless tankers, freighters, troopships or anything else devoid of effective air cover. The F-111C/Pave Tack combination offers a very effective means of delivering this class of weapons (one should never underrate the free fall demolition bomb, where it can be used, it is devastating; consider the Battle of Midway or count RN losses in the Falklands, as a recent reminder).

Another newer member of the Paveway family is the Rockwell/ Hughes GBU-15 Cruciform Wing Weapon, a modular glide bomb built around the Mk 84 2000 lb demolition bomb. Currently, in the process of being integrated with the RAAF F-111C, the GBU-15 allows for low altitude release (toss bombing) between 200 and 1000 ft at ranges up to 5 nm. After release, the bomb is steered via command link, the F-111C navigator viewing the scene from the bomb’s nose camera on a cockpit CRT. Once in visual range, the seeker may be locked onto the target, as the AGM-65, and the bomb glides into its victim. The GBU-15 does have range limitations, but it packs a lot of punch and has no IR signature worth mentioning. Later versions of the CWW may carry the IIR seeker developed for the AGM-65D, enabling night/ adverse weather operation.

Having exhausted our options among general-purpose weapons, we can now turn to dedicated, sea-skimming antishipping missiles. Up to recently, this class of weapon was used only by the more advanced powers, but the late seventies saw the proliferation of sea skimmers among Third World countries. The author declines to categorise the SS-N-2 Styx, and its air-launched cousins, among true sea skimmers, as these weapons seem to follow shallow, low altitude, diving trajectories, obliterating the target with massive 2000 lb warheads. The various members of the Aerospatiale Exocet family are becoming very popular with developing countries, as they are very simple to operate; delivered as packaged rounds, they require little support. This places further incentive on the major powers, to deploy naval air power, as air power is the most effective and failsafe means of stopping Exocet, by destroying the missile’s launch platform before it may launch the weapon.

The AM. 39 Exocet is a 4. 69 m missile, weighing in at 660 kg (165 kg blast/fragmentation warhead). Powered by a solid propellant rocket, the missile cruises at Mach 0. 93 to a range of up to 40 nm. Released at medium to low altitude, with range/ bearing information supplied by the launch vehicle (generally the Super Etendard fitted with the Thomson-CSF Agave X-band monopulse radar; this aircraft is a light strike fighter, which would easily fall prey to any modern fighter), the missile descends to a cruise altitude of 10-15 m, under the control of its twin gyro inertial navigator coupled with a radar altimeter. The missile activates its active radar seeker when 6-8 nm from the target; once locked on, it descends to 5 m, just prior to impact, Exocet drops to 2 m. Fusing is impact and backup proximity.

Though Exocet is an earlier generation sea skimmer, with limited range and a large IR signature, it does have home-on-jam modes and is a quite lethal weapon, as evidenced by the fate of the Sheffield and Atlantic Conveyor. Even an unsophisticated (i. e. incapable of correctly fusing iron bombs) service as the Fuerza Aerea Argentina could operate the weapon, which proved to be a real pain in the neck for the RN, with no AEW and limited fighter cover.

Another weapon in the class of Exocet is the slightly smaller Messerschmitt-Bolkow-Blohm Kormoran, carried by F-104Gs and Tornadoes of the West German Navy.

A superior class of sea skimmers are the newer, turbojet-powered weapons, offering greater range, lower IR signature, and smokeless operation, at the cost of slightly lower speed. The dominant weapon in this class is, currently, the McDonnell Douglas AGM-84A Harpoon, used by most advanced Western nations. Harpoon is carried by the RAN’s Oberon class attack subs and FFG frigates, the air-launched version by the P-3C Orion. The weapon is being integrated with the fire control of the F-111C and will be a standard fit on the F/A-18.

The air-launched AGM-84A Harpoon is slightly smaller and lighter than Exocet, weighing 527 kg; the missile is 3. 84 m long. Harpoon may be physically divided into four sections. The nose of the weapon, covered by a dielectric radome, contains the guidance section. This is comprised of a Texas Instruments PR- 53/DSQ-28 two-axis, frequency agile, active radar seeker for terminal homing, and a midcourse guidance unit consisting of a Lear-Siegler or Northrop three-axis strap-down inertial reference system, an IBM 4rrSP-OA general purpose digital computer and a Honeywell AN/APN-194 radar altimeter. The ordnance section follows, being comprised of a Naval Weapons Center 227 kg penetration blast warhead. The missile is fitted with a delayed impact fuse and proximity fuses, to ensure detonation. The sustainer section begins just at the fore of the main wings and runs to the tail of the missile. A one-shot Silver-Zinc battery, the power source for onboard systems, is situated between the wings, inside the sealed, non-vented fuel tank which carries the JP-type fuel. The semi flush, fixed inlet for the engine lies between the ventral pair of wings.

The engine is a Teledyne CAE J402-CA-400 single-spool turbojet, delivering 600 lb of thrust at sea level. The final section of the missile is wrapped around the tailpipe of the engine; it contains four electromechanical control actuators, which move the four control surfaces on the tail of the missile. Harpoon is supplied as a complete round, the fitting of its wings and fins is the only major operation required to upload the weapon onto the launch aircraft.

Harpoon may be launched with the aircraft at various attitudes and speeds. Immediately after release attitude control activates, stabilising in roll and maintaining a constant 33-degree dive angle, until the radar altimeter commands pullout. During the dive, the weapon begins to steer toward the target and the turbojet is activated. Harpoon then levels off at its cruise altitude, under the control of the midcourse guidance unit. Once a specified distance is reached from the target, the radar seeker activates and commences its search pattern. The missile has two main modes, Range and Bearing Launch, where the target range and bearing are roughly known, or Bearing bearing-only launch, though the missile may also be launched without initial data, by pointing the aircraft in the direction of the target for 40secondsprior to launch or casualty (Line-Of-Sight mode). Initial target information is derived from sensors on the launch aircraft, say radar or electronic surveillance equipment, and may be entered automatically or manually.

RBL mode has three search patterns, Large, Medium and Small. The Large pattern is used at maximum range; it searches a small area and then expands the pattern until a target is detected. The Small pattern offers good discrimination between closely spaced targets, but need not work at maximum range. BOL mode will attack the first target detected, activating the seeker during the initial dive.

The active radar seeker is frequency agile, this allows it to circumvent jamming and avoid interfering with other Harpoons, aimed at close or identical targets. After the target has been detected, the Harpoon locks on and commences its low-level run-in, the altitude depending on the wave size. At. a specified range, the terminal phase manoeuvre commences. Harpoon may be preprogrammed to a number of trajectories, ranging from an ultra-low sea-skimming attack, for vessels with defensive SAMs or guns, in order to confuse the fire control with clutter from the surface, to medium and high apogee pop-up attacks, ideal for small, fast manoeuvring targets as missile/torpedo boats, the weapon diving down and punching through the deck. The missile employs proportional guidance in the azimuthal plane.

Harpoon represents a flexible and effective all-weather antishipping missile. One of its greatest assets is flexibility in targeting, as the launch aircraft may gain bearing information on its target via the use of radar warning receivers and launch Harpoon without even being detected. Alternately, it may provoke a vessel into an ECM battle, to ascertain its bearing, and then launch. Finally, it may use information supplied by other aircraft or vessels. Combined with high-performance launch platforms such as the F-111C or F /A-18A, Harpoon becomes a serious threat to any navy not equipped with the best in look-down shoot-down air cover, capable of stopping these aircraft at least 60 nm from the defended vessels.

Having entered service in the late seventies, Harpoon is, by no means, the last word in standoff anti-shipping weapons. The General Dynamics MRASM (Medium Range Anti-Ship Missile) is a derivative of the AGM-109 Tomahawk cruise missile airframe, with a conventional warhead and Harpoon guidance system, offering a range of several hundred miles. This weapon would equip USN aircraft and vessels; it may also be fitted to long-range maritime bombers (a possible pre-retirement role for the B-52).

A supersonic successor to the Exocet is under development and the British Aerospace Sea Eagle is likely to enter the scene very shortly.

What one could expect to see in future anti-shipping missiles is a separation into two major classes. Short-range weapons, in the 30 nm class, will become smaller and faster, being launched by agile, high-performance fighter bombers. Long-range weapons will remain slow but are likely to increase in range, and acquire much more devious guidance, combining say radar with IR systems possessing the ·capability to recognise and select specific targets from their shape/thermal signature. These weapons would employ stealth measures and follow complicated terminal homing trajectories to confuse the target’s point defence SAMs and guns. Either way, unhealthy systems to tangle with. The question arises, what are the defensive measures and how effective are they, in the face of such weapons?

That will become the subject of Part 2