McDonnell-Douglas AGM-84A Harpoon and AGM-84E SLAM

Australian Aviation, March, 1988

by Carlo Kopp
1988, 2005 Carlo Kopp

The Precision Guided Munition (PGM) is one of the most potent force multipliers available in the tactical air battle. The high kill probability of modern PGMs ensures that few rounds are required to destroy a target, which reduces the number of sorties to be flown. This in turn translates into a substantial reduction in consumed fuel, flying hours and ultimately attrition of aircraft and aircrew. While PGMs such as the laser guided bomb and TV guided bomb introduced unprecedented

accuracy, the increase in the concentration of hostile defences relative to the number of strike aircraft increased the risks inherent in overflight of the target. Glidebombs such as the GBU-15 and Walleye I/II family were a partial answer as they allowed the strike aircraft to attack from outside the target's terminal defences, often fortified with large amounts of visually aimed AAA (guns) which are understandably immune to electronic countermeasures. While the glidebomb packs a lot of punch, its limited range at low altitude and restricted terminal trajectory render its launch platform vulnerable to newer types of SAM. In particular the SA-11 Gadfly and late SA-6 Gainful have a known capability to hit low level targets at ranges of several nautical miles. The solution to this problem lies in the use of standoff PGMs with sufficient range to allow attacks from below the radar horizon of the target's area defences. This strategy has been often used in the design of antishipping missiles and the attacks on the Sheffield and Stark demonstrate its effectiveness. While there are numerous types of standoff anti-shipping missile in use today, few of these have been adapted to the land strike role. The Russian AS-4 Kitchen may have even followed the opposite development path, starting as a strategic land strike missile and eventually shifting to the anti-shipping role. Key types in the Western inventory are the AGM-109 Tomahawk family (see TE November, 1985) and the AGM-84 Harpoon family.

The McDonnell-Douglas AGM-84A Harpoon

The radar guided turbojet powered sea skimming Harpoon is the primary anti-shipping missile in the inventories of most Western powers. Harpoon was first deployed in the mid seventies and has built up a credible record over the years, including several live kills claimed by the US Navy in the Gulf of Sidra. The Harpoon is also the RAAF's primary anti-shipping missile, arming the P-3C, F-111A/C and F-18A/B. Development of the Harpoon took place during the early seventies, when the US Navy recognised the need for a low cost sea skimmer with all weather capable radar terminal homing. This missile would provide destroyers, cruisers, frigates and submarines with the ability to attack surface targets independently: this eliminated the constraints imposed by the need for naval strike aircraft to attack surface targets. The use of a sea skimming attack was particularly clever as it defeated the Russians' point defence SAM systems. The simple turbojet powerplant allowed the missile to be compact while reducinq the infra-red and visual signature of its exhaust plume.

The fuselage of the Harpoon is cylindrical with an ogival/conical nose and slightly tapered boat-tail. The cruciform wings are slightly aft of the airframe CofG, with the flush ventral low radar cross-section inlet between the lower wings. The all moving cruciform control surfaces are attached to actuators in the boat-tail section. Functionally the fuselage is divided in the Control, Sustainer, Ordnance and Guidance Sections (see cutaway). The Control Section is wrapped around the tailpipe of the engine and structurally supports the control actuators and attachment points for a booster in ship/sub launched versions. The Simmonds ATU-72 (or Singer-Kearfott) actuators contain a motor, gear train, clutches and a brake providing ~30 deg of movement for the aluminium alloy surfaces. The Sustainer Section encompasses the fuselage between the boat-tail up to the warhead section. The aftmost part of this section is occupied with the Teledyne CAE J402-CA-400 turbojet. This miniature 98lb axial/centrifugal compressor turbojet has a pressure ratio of 12.5, an SFC of 1.12 (Ib/Ib/hr) and delivers over 600lb of thrust at sea level. It draws inlet air

through a cast aluminium alloy inlet duct (note the stiffening ribs) which runs though the cylindrical fuselage cavity. This cavity is sealed and forms an integral fuel tank containing over 100lb of JP-10 fuel. The engine is started by redundant cartridges and ignitors and will reach full thrust in about 7 seconds. Fuel pressure is initially provided by spring loaded bellows with bleed air providing pressure once nominal thrust is attained. The Sustainer Section provides structural support for the missile wings and the fuel tank cavity is used to house two oneshot silver zinc batteries, once activated these provide power for the avionics. The Ordnance (warhead) Section is a separate structural assembly, with the warhead penetration casing used as a load bearing structure. The casing is designed to penetrate into a ship's structure before the delayed fuse fires the high explosive in the warhead. The warhead assembly weighs in at 488lb which places it in the class of a 500lb demolition bomb. The Guidance Section of the missile occupies the forward fuselage and is unique to specific versions of the missile. The aftmost assembly is the Midcourse Guidance Unit (MGU), built up with an IBM 4PiSP-OA general purpose digital computer and a Northrop or Lear Siegler 3-axis strapdown inertial Attitude Reference Assembly (ARA). A Honeywell AN/APN-194 short pulse radar altimeter is mounted fore of the MGU, with a receiver antenna mounted under the warhead section. The MGU functions as an autopilot, while also interfacing to the launch aircraft for downloading of mission parameters. Terminal guidance is provided by a Texas Instruments PR-53/DSQ-28 active radar seeker, which is a solid state, two-axis frequency agile design. Frequency agility can defeat some forms of jamming and chaff (ie: the threat must use more jam power,or image jamming, and broadband chaff) and is a useful form of ECCM, while preventing interference between multiple rounds in a salvoed Harpoon attack. Harpoon also employs azimuth proportional terminal homing to directly hit manoeuvring targets, while also offering various pop-up and sea skimming terminal trajectories.

Harpoon Mission Profile

With four basic operating modes, a range of terminal trajectories, launch platforms and targeting data sources, it is impossible to tag any particular mission profile as typical. However, a P-3C pop-up targeting/launch manoeuvre is a common RAAF tactic and will be covered. Newer aircraft such as the F18A have fully integrated digital fire control and stores management systems in which the software handles Harpoon initialisation and launch sequencing; the P-3 is the opposite extreme with a dedicated Harpoon Aircraft Command and Launch Set (HACLS). The HACLS is comprised of a launch control panel at the TACCO's station, a digital computer and several black boxes which interface the aircraft's reference signal outputs and power to the HACLS and Harpoon. In a typical scenario, a RAAF P-3 tasked with maritime strike would sweep an area using its Electronic Support Measures to detect hostile shipping without being detected. Upon detecting a hostile signature, the P-3 would drop to low level and approach below the radar horizon to within Harpoon range, which is about 60 nm. At this stage, the Harpoon is prepared for launch. The HACLS will power up identification circuits in the missile which will reply with a missile present signal. Selection of the missile is the next step, and this results in the application of heater power to the missile to bring the battery, seeker and MGU up to nominal operating temperature. Once this occurs, the seeker and MGU heaters are turned off and

electrical power from the aircraft applied.

The gyros in the ARA spin up to speed. At this stage targeting is required. While ESM bearing and range data are adequate, better discrimination can be gained with radar. The P-3 will therefore pull up steeply until it crosses the radar horizon of the target, after which it sweeps

the target several times with its radar to get a fix and then dives down back below the radar horizon. The threat has had only several seconds to detect the P-3, during which time the HACLS has reformatted the target coordinates and passed them serially to the Harpoon, together with selected mode, launch sequencing information and attitude/position. The Harpoon will return a GO signal upon accepting the data. From this instant the missile is ready to launch and depressing the firing button initiates the launch. The HACLS sends a launch signal to the Harpoon which activates its battery and returns a ready signal. The missile is now committed and the HACLS removes aircraft power and does a final check on missile status. If the missile returns a GO, it is released. If airspeed/altitude are below a set limit the engine is fired up on the pylon otherwise it starts during the missile's dive. The umbilical detaches, unshorting all of the electroexplosive devices and an arming lanyard enables the warhead arming system. The missile descends in a 33 degree dive commanded by the MGU, until cruise altitude is attained and the missile pulls out, entering its midcourse cruise toward the target. The four basic modes are Bearing Only Launch (BOL) and Range and Bearing Launch (RBL) with small, medium and large search patterns. At a range given by the set mode, the seeker is activated and searches for the target. Once a target is acquired lock-on occurs and Harpoon descends for its low level run-in. Subject to preloaded parameters, the missile enters its terminal trajectory, punching through the skin of the target and exploding inside. Harpoon has to date demonstrated a good kill probability, the probability of a failed launch is low as the launch platform fire control system verifies that the target is within the missile envelope and that the launch aircraft is within the missile launch envelope prior to committing to a launch. The operator can also initiate a Built-In-Test prior to committing to a launch, BIT verifies actuator speed, power system and MGU function. Combined with a high performance launch platform equipped with a digital fire control system and capable radar and ESM, Harpoon is a potent and flexible anti-shipping weapon. Its better attributes are common to its land strike capable derivative, SLAM.