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[5.0] Modern Smart Bombs (1)

v4.0.0 / chapter 5 of 10 / 01 may 14 / greg goebel / public domain

* Although work on guided glide bombs went on the fade in the 1950s, technological advances and the US war in Vietnam resurrected them. Confronted with tough targets and strong air defenses in the bombing campaign against North Vietnam, the US Navy and Air Force developed glide bombs guided by television and laser beams, and such weapons have been refined ever since. This chapter discusses the development of modern television-guided and laser-guided bombs.

JDAM


[5.1] WALLEYE GUIDED BOMB
[5.2] HOBOS & THE EO GUIDED BOMBS
[5.3] THE PAVEWAY LASER GUIDED BOMBS
[5.4] LASER-GUIDED ROCKETS

[5.1] WALLEYE GUIDED BOMB

* One of the problems with early glide weapons was the crudity of electronic technology available during World War II. TV and infrared sensors were still primitive, and suffered from poor performance and reliability. By the 1960s, TV was well established, and solid-state electronics were available to make electronics systems much more compact and reliable. The technology was finally ready for combat.

The start of the air war over Southeast Asia in 1964 gave the US military a strong motivation to come up with new and powerful guided weapons. Strike pilots reported that North Vietnamese bridges, important targets in the effort to constrict the flow of supplies to Communist insurgents in South Vietnam, were proving very difficult to destroy. The bridges were very hard to hit with unguided weapons, and though attempts were made to destroy them with Bullpup missiles, discussed in a later chapter, the Bullpup's warhead was too small, and the pilot had to give his attention to guiding the missile into the bridge using a hand controller, which left him unable to take evasive action against ground fire.

Both the US Navy and the US Air Force wanted better solutions. Research into new guided bombs in the 1960s followed two main paths: weapons guided by television or other imaging sensors, and weapons that locked onto laser light reflected by a target. This section discusses the TV-guided bombs, while the next discusses the laser-guided bombs.

* The Navy effort to build an electro-optic guided bomb (EOGB) resulted in the "Walleye". This weapon was the product of Navy research begun in 1963, leading to the award of a production contract to Martin's Orlando Division in early 1966. The weapon was originally designated the "AGM-62", where "AGM" meant "Air to Ground Missile" -- but then the Navy decided that a glide bomb wasn't really a "missile" and dropped that designation.

The "Walleye I" was a purpose-built weapon, not a modified dumb bomb. It had four long-chord delta wings arranged in a cruciform pattern; a vidicon-tube TV camera in the nose; a spinner in the tail to drive an electrical generator and hydraulic pump; and a 375 kilogram (825 pound) warhead. The weapon was 3.44 meters (11 feet 4 inches) long, had a diameter of 32 centimeters (12.5 inches), a wingspan of 1.16 meters (3 feet 10 inches), and a weight of 500 kilograms (1,100 pounds). The wings were fixed but had control fins at the trailing edge.

The Walleye I went into operation with Navy Douglas A-4 Skyhawks in early 1967, with impressive results. In the first strike, a Skyhawk pilot put the bomb directly into a window of a barracks building. Of the 68 Walleyes expended over a period of seven months, 65 hit their targets accurately.

AGM-65 Walleye

In operation, the image provided by the Walleye's seeker was displayed on a TV screen in the controlling aircraft. The operator locked the crosshairs on the display onto a high-contrast scene element like a window or a door, set the fuzing option as needed, and then released the weapon. The weapon would then glide accurately into the target, guided by the target's contrast pattern.

Details of the seeker system's operation are unclear, but it seemed to use an analog pattern-matching scheme. One plausible scheme for seeker operation would be for it to convert the image into two analog electrical waveforms, one for the vertical axis of the image and one for the horizontal axis, with the amplitude of the waveforms reflecting scene brightness. The seeker control electronics would then detect the transitions in the waveforms corresponding to abrupt changes in scene brightness, and use them as "goalposts" to guide the missile to target. What is clear is that the seeker needed to have a stable high-contrast pattern to work properly, and so it sometimes would break lock if the day were gray and hazy, or there were shifting clouds or sources of transient sun glare. In such cases, "fire and forget" actually turned out to mean that the weapon was fired and then forgot where it was going. However, when the weapon worked, it worked very well, with accuracies of a few meters.

The Navy was enthusiastic enough about the Walleye to configure A-6 Intruder and A-7 Corsair II strike aircraft to carry the weapon, and develop a 900 kilogram (2,000 pound) "Walleye II", known as a "Fat Albert". The Walleye II was built by Hughes under subcontract to Martin Marietta. It looked like a scaled-up Walleye I, being 4.04 meters (13 feet 3 inches) long, with a diameter of 46 centimeters (18 inches), and a wingspan of 1.3 meters (4 feet 3 inches). The Walleye II went into service in 1974.

The Walleye II was followed in 1975 by the "Extended Range / Data Link (ERDL)" Walleye, which featured bigger wings for longer glide range and a more sophisticated guidance and control system. Most of the Walleye I and II weapons were upgraded to "Walleye I ERDL" and "Walleye II ERDL" configuration.

The ERDL system allowed an operator to monitor the Walleye's seeker over an "AN/AWW-9" (later "AN/AWW-13") datalink pod, and change the target lock after launch if necessary. The controlling aircraft wasn't necessarily the same as the launch aircraft. As AZON drops in Burma had shown over two decades before, it was better to have one aircraft stand off from the target and guide the bombs, while others released them and got out of the target area. That not only enhanced effectiveness and increased crew safety, but meant that not all the strike aircraft needed to be fitted with the datalink pod. An updated "digital phase-shift keying (DPSK)" guidance system with a more reliable communications channel was retrofitted in the 1980s, resulting in the "Walleye I ERDL DPSK" and "Walleye II ERDL DPSK".

The Walleye was also obtained in small quantities by the USAF, as well as the Israeli Air Force. The Israelis used Walleye IIs in the Yom Kippur War in 1973 with mixed results, and then came up with a modified version, the "Tadmit", with an improved seeker and a booster rocket, that was used successfully in the 1982 war in southern Lebanon. The US Navy used Walleyes to good effect in the Gulf War, dropping 124 of them on Iraqi targets. The weapon was finally withdrawn from service in 1995. Inert Walleye training rounds were built, and even a nuclear-tipped Walleye, though it is unclear if the nuclear variant ever reached operational status. About 5,000 Walleyes of all types were built.

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[5.2] HOBOS & THE EO GUIDED BOMBS

* In 1967, the US Air Force, working with Rockwell International, began development of their own EOGB. The first weapon in this series materialized in combat over Southeast Asia in 1969 as the "GBU-8/B HOBOS (Glide Bomb Unit 8, Homing Bomb System)".

HOBOS consisted of a kit, designated "KMU-353/B", that could be fitted to a Mark 84 900 kilogram (2,000 pound) or Mark 118 1,350 kilogram (3,000 pound) bomb. The kit included a tailfin section that had four square fins and contained a battery and control and communication electronics, and a nose section that contained an EO seeker system. The two sections were linked by four long slender fins, or "strakes", and an umbilical conduit that ran along the length of the bomb from the tailfin section to the nose section. A HOBOS weapon based on the Mark 84 slick bomb was 3.78 meters (12 feet 5 inches) long, had a span of 1.12 meters (3 feet 8 inches), and weighed 1.016 tonnes (2,240 pounds).

The EO seeker head was based on either TV or imaging infrared technology, though it appears there were a number of experiments with other options. Like the Walleye, the seeker in principle had a "fire and forget" capability, in which the operator could lock it onto a target. After release, the bomb glided directly to the target on its own by maintaining focus on the target's contrast pattern. Also like the Walleye, occasionally "fire and forget" meant the weapon would forget where it was going -- but when it worked, it worked well.

* After the end of the war, the Air Force and Rockwell continued development of the weapon through the 1970s, eventually coming up with the improved "GBU-15/B EOGB", originally the "AGM-112", which had either a TV or infrared guidance system. The TV-guided "GBU-15(V)1/B" became operational in 1983, and the infrared-guided "GBU-15(V)2/B" became operational two years later. The weapon was fielded with the USAF and the Israeli Air Force, with the Israelis using it effectively in Lebanon in 1982.

GBU-20/B Planar Wing Weapon

The GBU-15/B was actually a family of weapons based on a kit of standard components, known as the "modular guided glide bomb" system. The Air Force also worked with Hughes to build a glide bomb with a pair of "switchblade" pop-open wings for extended glide range, but this "GBU-20/B Planar Wing Weapon (PWW)" never reached production.

The GBU-15/B was conceptually similar to HOBOS, with tail and nose sections that can be strapped to a 900 kilogram (2,000 pound) Mark 84 GP bomb. The kit could be attached to some other munitions, and was also evaluated with a cluster munition canister, though this option was never deployed. The standard GBU-15/B with a Mark 84 had a length of 3.5 meters (12 feet 10 inches), a wingspan of 1.49 meters (4 feet 11 inches), and a weight of 1.125 tonnes (2,500 pounds). The EO seeker permitted fire and forget operation, or command guidance through an "AN/AXQ-14" datalink, or the later "AN/ZSW-1" datalink. Datalink guidance allowed the bomb to be released from above an overcast layer, with the bomb falling under command guidance until it penetrated the cloud layer, where it then locked on to the target.

GBU-15 airdrop from F-4 Phantom

The GBU-15/B had a distinctively different appearance from HOBOS, however, with triangular nose fins on the seeker section and large truncated delta fins on the tail section, with both sets of fins arranged in a cruciform pattern. As a result, the GBU-15/B was sometimes referred to as the "cruciform wing weapon (CWW)". The larger fins gave the GBU-15/B a longer glide distance than HOBOS.

The GBU-15/B was carried by the F-15E Strike Eagle and by the F-111 Aardvark, now retired from US service. The F-111 made very effective use of the GBU-15/B during the Gulf War, dropping 71 of the weapons to seal off oil manifolds wrecked by the Iraqis to spew out oil, and other targets.

GBU-15/B CWW EOGB

GBU-15/B kits were developed that feature smaller "short chord" wing, and the kits could be used on the BLU-109/B 900 kilogram (2,000 pound) penetrator warhead. This gave a "Chinese menu" of possible weapon configurations:

   designation      warhead     wings  guidance
   ____________________________________________

   GBU-15(V)1/B     Mark 84      long     TV
   GBU-15(V)2/B     Mark 84      long     IR 
   GBU-15(V)21/B    Mark 84     short     TV
   GBU-15(V)22/B    Mark 84     short     IR 
   GBU-15(V)31/B    BLU-109/B   short     TV
   GBU-15(V)32/B    BLU-109/B   short     IR 
   ____________________________________________

It appears that little or no work is now being performed on enhancing the GBU-15 series. It is unclear how many GBU-15s remain in US service, but they do apparently persist in inventory for the time being.

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[5.3] THE PAVEWAY LASER GUIDED BOMBS

* The invention of the laser in the early 1960s led immediately to excited visions of "death rays" that could shoot down enemy missiles, but such "directed energy" weapons proved a major challenge, and are only now beginning to seem practical. The laser did, however, have significant short-term potential for use in combat.

Using a laser as a weapon itself places enormous demands on device physics and energy supply, but the fact that a laser beam can be precisely pointed and remains tightly organized ("coherent" in laser terminology) over long range meant that it could be used as a precise pointing device. A laser could be strapped to a telescope with crosshairs so that the beam could be focused to "illuminate" a particular target to "mark" or "designate" it. The fact that the laser also generates a narrow range of colors ("monochromatic") also meant that the light reflected off such a target could be easily detected by simple sensors through a filter lens. A guided weapon could be fitted with such a sensor, with the sensor linked to feedback-control mechanisms so that it would home in on an illuminated target.

The idea of using a laser to designate targets apparently was devised in 1960 by two civilian engineers, David J. Salonimer and Norman Bell, at the US Army Missile Command in Huntsville, Alabama. The two were interested in building laser-guided artillery shells, and conducted studies on laser designator and seeker systems. Salonimer managed to get a little funding, and worked with Weldon Word of Texas Instruments (TI) to modify a Shrike anti-radar missile, discussed in a later chapter, as a laser-guided surface-to-surface weapon. The experiment didn't work out, but the idea of laser-guided weapons didn't go away.

Inspired by the Shrike experiment, Martin Marietta performed experiments of their own with laser targeting systems, and in 1964 demonstrated such a device to the Air Force, leading to a modest contract to TI the next year for an experimental demonstration of a "laser-guided bomb (LGB)". TI engineers built a laser seeker, based on an airflow test probe fitted to the nose of a bomb on a universal joint. The unit looked like a badminton "birdie", and so was called a "birdie head". It controlled the movement of four fins, which were originally fitted to the tail of the guided bomb.

The seeker had an optical sensor, shielded by a filter lens that was transparent to laser light but blocked light of other wavelengths. The sensor was a simple array of photodiodes, arranged in quadrants. The quadrant that picked up the most light energy activated the fins, which were operated in a "bang-bang" control mode: they were either deflected completely to one or the other limit of their range of movement, or remained straight. The fins operated in pairs arranged symmetrically around the guided bomb to shift the weapon up and down, or left and right.

Early tests were conducted with M-117 bombs with moveable tailfins, resulting in the TI "BOLT-117", the first LGB, which was tested in April 1965. Results were poor, but much improved accuracy was obtained with the use of the more aerodynamic Mark 84 "slick" bomb, and control fins attached to the nose of the weapon, instead of the tail.

The TI group, working on a shoestring budget with the USAF Armament Development & Test Center at Eglin, conducted tests through the rest of 1965 and 1966. After some work, the bombs became very accurate, though due to the bang-bang control scheme the bombs did have a tendency to bob up and down along the laser beam until they locked on target. Despite the lack of gyrostabilization systems, the bombs generally locked on solid after a few seconds.

TI was finally awarded a contract for 50 "Paveway" guidance kits, where "Paveway" is sometimes said to be derived from the "Precision Avionics Vectoring Equipment (PAVE)", though it seems more likely that the name was arbitrary and the acronym was invented after the fact. Prototype LGBs were sent to Vietnam in 1968 for operational testing, with mixed results. F-4 Phantom fighter-bombers were used in the tests, with the "weapons system officer (WSO)" in the back seat marking a target with a hand-held laser system designated the "Airborne Laser Designator (ALD)". It proved very difficult to keep the laser aligned on the target, but half the LGBs hit the target anyway.

* The tests led to the "Paveway I" munitions. They consisted of a kit that was attached to ordinary Mark 82 225 kilogram (500 pound), Mark 83 450 kilogram (1,000 pound), and Mark 84 900 kilogram (2,000 pound) bombs. The kit included a laser seeker head attached to four control fins in a cruciform arrangement, which was attached to the front of the bomb, and a set of four larger fins, also in a cruciform arrangement, which was attached to the rear of the bomb to provide some limited glide capability. All the LGBs used the same seeker head, but had different fin assemblies to accommodate different types of bombs.

The LGBs had their own power supply, consisting of a thermal battery for the electronics, along with a hot gas-driven actuator to move the seeker head. They did not require any electrical connection to the aircraft. They could be launched by any aircraft that could carry ordinary bombs of the same size, and could be guided by a laser designator on the launch aircraft, on a spotter aircraft, or operated by ground forces. The Air Force developed a laser designator for the F-4 under the code name "Pave Knife". This consisted of a laser slaved to a TV camera and allowed the Phantom back-seater to hit a target while the pilot concerned himself with evasive action.

The Paveway I kits were designated as follows:

Kits were also developed for other unitary bombs and cluster munition canisters, but it is unclear if these other weapons were fielded. The LGB kits only cost a few thousand dollars US, and were produced in great quantity. Tens of thousands of them were used in Vietnam, proving themselves in the North Vietnamese ground offensive into South Vietnam in the spring of 1972, and the LINEBACKER bombing campaign late in that year. 225 kilogram (500 pound) Paveways scored direct hits on North Vietnamese tanks, and 900 kilogram Paveways destroyed bridges that had survived repeated conventional bombing raids.

laser-guided bomb in test

* The initial Paveway design gave way to an improved series, known as "Paveway II", in the early 1970s. These weapons featured a enhanced but simpler and cheaper seeker head, and pop-out fins for the rear assembly to improve the weapon's glide characteristics, and make it easier to fit to aircraft. The new LGBs based on the Paveway II were given the following designations:

The British RAF also adopted a 450 kilogram (1,000 pound) Paveway II variant known as the "Mark 13/18", which was used by Harrier strike aircraft during the Falklands War in 1983. GBU-10C/B bombs were used by F-111 strike aircraft in the 1986 EL DORADO CANYON punitive raid on Libya.

The US Navy wanted to increase the range of their Paveway II weapons, and so devised a variant fitted with the solid rocket motor from a Shrike missile. This weapon is based on the Mark 83 bomb and is known as the "AGM-123 Skipper II". A Skipper II was used in 1988 during the Persian Gulf convoy operations to sink an Iranian frigate.

* The Paveway II required the launch aircraft to operate from medium altitude so that the bomb would have direct path to the target. This left the aircraft vulnerable to ground defenses, and so in 1976 the USAF issued a requirement for yet another generation of Paveway weapons, the "Paveway III".

The Paveway III used larger rear fins and a "smart" seeker head that incorporated a microprocessor-based digital autopilot, coupled to a more sensitive seeker with a wider field of view. Unlike earlier Paveway seekers, the seeker head on the Paveway III did not pivot, and the weapon did not use "bang-bang" control, instead following as straight a path to the target as possible. Bang-bang control was simple, but wasted much of the bomb's kinetic energy and reduced its range. The bomb could be released at low altitude and then sail almost level into the target area, where it could then dive directly into a target. However, tests of the improved LGB kit with 225 kilogram (500 pound) bombs showed such weapons to be unstable, so the USAF only fielded the 900 kilogram (2,000 pound) version, with test and evaluation completed in 1986.

The 900 kilogram (2,000 pound) Paveway III was known as the "GBU-24/B". The kit could be fitted to the standard Mark 84 bomb or the hardened penetrator BLU-109/B bomb. An improved Paveway III kit was used with the F-117 Stealth fighter and was known as the "GBU-27/B". The USAF worked on an "Advanced Unitary Penetrator (AUP)" munition externally similar to the BLU-109, but achieved twice the depth of penetration by the interesting method of shedding its outer layers. It was not adopted for service, apparently because even better ideas were in the works.

Confronted with deeply-buried Iraqi command bunkers, the USAF found that even Paveway IIIs built around the hardened BLU-109 bomb weren't good enough, and a crash program was put in motion to build something better. The new "bunker-busting" bomb, the "GBU-28/B", was prototyped in 17 days, using scrapped 203 millimeter (8 inch) artillery barrels with Paveway III kits attached. A single test drop was performed at the Tonopah Test Range in Utah, with the bomb burying itself so deep that it could not be found. More GBU-28/Bs were quickly built, airlifted to the Gulf, and immediately used to attack hardened Iraqi installations. Each GBU-28/B weighed 2,250 kilograms (5,000 pounds). It consisted of a guidance kit attached to a BLU-113/B core munition, mentioned in an earlier chapter.

The GBU-28/B was also used in the American war in Afghanistan in the winter of 2001:2002, carried by B-52s and F-15Es, which are the only aircraft currently qualified to carry the weapon. Only a limited batch was built in the first place, and stocks ran low during the campaign. The Air Force has since obtained a batch of "GBU-28C/B" bombs based on the new BLU-122/B penetrating bomb.

US laser-guided bombs

Some 9,000 Paveway II and III munitions were dropped during the Gulf War. Video recordings taken by targeting systems showed Paveways and other smart munitions dropping into the front doors of aircraft shelters and falling into the ventilation shafts of buildings, blowing them apart with satisfying effect. These proved a striking and impressive propaganda tool for US forces. Videos that showed the gruesome effects of high explosive blasts on humans were, understandably but a little dishonestly, not released to the public. One Paveway was even used to blast an Iraqi helicopter out of the sky, an action-movie stunt that probably amused one side of the transaction immensely.

During the Anglo-American invasion of Iraq in the spring of 2003, US forces used 225 kilogram (500 pound) LGBs filled with concrete as "minimum collateral damage" munitions to perform precision attacks on targets in built-up urban areas. These weapons were designated "GBU-45/B". The British also used practice LGBs filled with concrete during the campaign. The idea of turning an LGB into a "smart rock" was dreamt up by the Israelis during their squabbles with the Palestinians. The Americans later fielded a "Low Collateral Damage Bomb", which was a core munition with a composite case and modified explosive filler to reduce damage outside the immediate impact area, allowing the weapon to be used in built-up areas.

Most Paveway IIs have been updated with Paveway III technology. Hundreds of thousands have been produced, and tens of thousands used in combat. New LGBs continue to be developed: in 2008, Lockheed Martin demonstrated a 45 kilogram (100 pound) LGB named "Scalpel" for the US Navy and Marines. As the name implied, Scalpel was intended as a low-collateral-damage precision strike weapon for close combat, and its light weight makes it ideal for small UAVs. It is unclear if there is any commitment to production, however.

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[5.4] LASER-GUIDED ROCKETS

* Given the development of laser guidance kits for existing unguided bombs, not surprisingly there was a push to develop such kits for existing unguided rockets, though oddly it didn't happen until the 21st century. The Army and General Dynamics worked on a guided Hydra-70 system, designated the "Advanced Precision Kill Weapon System (APKWS)", nicknamed "Hellfire JR" after the Hellfire laser-guided antitank missile. The idea was to update the Hydra-70 with a laser seeker. The program suffered from "requirements creep" and was canceled after suffering from bad test results.

The Army scaled back their requirements and ran a second competition for the APKWS, with BAE Systems winning the contract. Initial all-up tests were in 2007. The APKWS amounts to a module, the "WGBU-59/B", that is screwed between the warhead and rocket motor of a standards Hydra-70 rocket, with the module providing guidance using four pop-out fins directed by a "distributed aperture semi-active laser seeker (DASALS)". The baseline rocket configuration uses the standard M151 HE warhead.

A sensor, or "eyeball", for the DASALS is mounted on each of the four pop-out wings, instead of the nose -- a nose-mounted sensor wouldn't be practical since APKWS uses ordinary Hydra-70 warheads. There are little control surfaces on the ends of each of the four pop-out wings. Before launch, the wings are sealed inside the seeker module, only popping out after a half-second following launch; this helps prevent damage to the seeker from the launch of another rocket in the launch pod. The seeker, which has a 40-degree field of view, then acquires the target, remaining on lock by balancing the inputs from the four eyeballs. BAE Systems says the APKWS costs about 15% as much as a Hellfire, though the low cost is partly due to the large existing stockpile of Hydra-70 rockets, all of which are easily converted to a guided configuration with the APKWS.

APKWS & WGBU-59/B

The US military is very enthusiastic about APKWS, the effort becoming a multiservice program, with the US Navy taking charge to get the weapon into service -- likely because the Army was so heavily committed in Iraq at the time. The weapon entered low-rate production in 2011 and was fielded in 2012, with initial deployment on US Marine Cobra helicopters in Afghanistan. It has proved very reliable and highly effective.

Although the APKWS can't inflict much damage on heavy armor and hardened targets, its low cost and small blast radius would be a particularly handy weapon for the sort of dirty little wars currently in fashion. It can be carried by any platform that can carry the Hydra-70 with little crew training; laser designation can be provided by ground forces or other platforms if the carrier platform doesn't have the capability. Its light weight also makes it an excellent weapon for small drones.

* APKWS isn't the only effort to build a guided 70 millimeter rocket. Lockheed Martin lost out on the competition for APKWS II, but went ahead on development of their "Direct Attack Guided Rocket (DAGR)" weapon anyway. DAGR has been successfully test-fired. Unlike APKWS, DAGR features a nose-mounted seeker, derived from the Hellfire anti-armor missile; it is unclear if the nose-mounted configuration limits compatibility with standard 70 millimeter rocket warheads. Lockheed Martin has developed a four-pack launcher that can replace a single Hellfire on a standard Hellfire launch rack, and has also conducted trials for ground launch.

DAGR ground launch

Other similar efforts have included:

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