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F-4D from the 435th TFS armed with two GBU-10s

F-4D from the 435th TFS armed with two GBU-10 LGBs

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Comparison of Guided vs. Conventional Weaponry

Research and Sources


F-105D Thunderchief of the 357th TFS over Vietnam,6 February 1970

Less than 1% of the bombs dropped during the Vietnam war were guided or 'smart' bombs and yet they were crucial to the conduct of the war. Of the 26,690 guided bombs, laser-guided ones dominated being 94% of the total (over 25,000). The rest were made up of TV-guided (electro-optical) weapons such as the Walleye and the Homing Bomb System (HOBOS.)

Analysis showed that, using unguided bombs, F-105 Thunderchiefs achieved a 447 ft (136 m) CEP against defended targets, 365 ft (111 m) against targets undefended by AAA. CEP stands for Circular Error Probable - a measure of weapon accuracy, often defined as the radius of a circle in which 50% of the weapons launched will impact. This has to be put into context though: for example, a bomb release altitude of 6,000 ft (1830 m) and a 40 degree dive angle means a slant range of 9,300 ft (2835 m). Slant range is the straight-line distance from an aircraft to its target, taking into account the aircraft's altitude. This comes with a pull out altitude of about 3,500 ft (1067 m) while going 500 knots (926 km/h) true airspeed to avoid enemy fire. In this case, the bombs have to travel around one and a half nautical miles (2.78 km), aimed but unguided to strike the target or more likely miss. Laser guided bombs, on the other hand, had a CEP on the order of 27 ft (8 m) or better. A 2,000 lb (907 kg) bomb has a blast radius that's equal or greater than the average miss distance with laser guidance so most targets are destroyed or heavily damaged even when there's not a direct hit.

Source for launch profile data of free fall bombs: "Dragon's Jaw" by Stephen Coonts and Barrett Tillman, Kindle edition, 2019

To illustrate the difference precision guided munitions can make, witness the saga of the Dragon's Jaw bridge. Literally more than a thousand free-fall or unguided bombs were aimed at the Thanh Hoa Bridge, also known as the infamous Dragon's Jaw. The US Air Force and Navy pummeled this bridge for years with 873 sorties while losing eleven aircraft in the process. Although damaging it, the massive structure still stood - this was the best that could be achieved with regular bombs. None of this is meant to be a criticism of the bravery or determination of the flight crews involved.

However, on 13 May 1972, in one iconic mission the Dragon's Jaw is rendered unusable by aircraft carrying 24 laser-guided bombs, including nine of the 3,000 lb (1360 kg) variety. But, it was the US Navy that took the weakened bridge out for good in a subsequent mission with 2,000 lb (907 kg) Mk-84s and Walleye II glide bombs. Although this marks the dawn of the use of PGMs for many - their use in Vietnam goes back four years before that, TV-guided (electro-optical) weaponry goes back a year further still. Often, a footnote, the radio-guided Bullpup air-to-ground missile (AGM) first saw combat in 1965 and was also used against the Thanh Hoa Bridge. Unique in this list, in essentially relying on the enemy to provide guidance in the form of radar emissions are the AGM-45 Shrike and AGM-78 Standard ARM.

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The BOLT-117 and GBU-10, GBU-11, and GBU-12 Laser-Guided Bombs

BOLT-117 Laser Guided Bomb

A project to create a relatively low cost laser guided weapon resulted in the BOLT-117 consisting of mostly off-the-shelf parts 'mated' together. A 750 lb (340 kg) M117 general purpose (GP) bomb functioned as the warhead, an AGM-45 Shrike control activator was also utilized, and finally, a laser seeker was mounted on the nose. The BOLT-117 had 'bang-bang' guidance where the control fins deflected fully for every guidance correction. This was inefficient aerodynamically and gave the weapon a wobbly flight path to the target. The GBU-10 with a Mk-84 2,000 lb (907 kg) bomb represented a more refined product with better performance. Although it still utilized the 'bang-bang' guidance method, the control fins were now in the front as canards, this made for an improvement in accuracy. Also the fact that it was now based on the Mk-84 GP bomb, yielded greater target destruction potential: the 2,000 lb (907 kg) bomb had more than twice the explosive power of the M117. In fact, the GBU-10 was destined to make up 84% of all laser guided bombs used in the conflict.

After a successful combat evaluation in 1968 LGBs were used with increasing frequency. Indeed, 1969 saw 1,612 of the new weapons dropped, with 57% scoring direct hits. Larger still, was the GBU-11 based on the massive 3,000 lb (1360 kg) M118 demolition bomb - the largest bomb seeing 'normal use' in the USAF inventory. This had an unmatched blast effect that was devastating to a lot of targets. 28 of the 186 M118 bombs dropped on NVN targets were laser guided during April 1972 to January 1973. In the other direction, seeking a smaller and more maneuverable weapon for use against targets such as tanks and trucks came the GBU-12 with a Mk-82 500 lb (227 kg) bomb as warhead. Many of these weapons saw use against vehicles on the Ho Chi Minh Trail (HCMT). Even the B-52 Stratofortress got involved in the war on trucks for a short while, from 21 November to 28 December 1970, B-52G bombers made 59 drops with 500-lb (227 kg) laser guided bombs from 40,000 ft (12192 m). This resulted in the destruction of 35 trucks with a good probability of damaging more. For whatever reason this 'experiment' using heavy bombers against trucks was not mainstreamed. During Operation Nguyen Hue, the Easter Offensive in 1972, GBU-12s were used to blunt the tank thrusts of the North Vietnamese with great effect. Although flying just 10.4% of the sorties, the F-4D's armed with LGBs were responsible for 22% of the tanks destroyed. The US Air Force dropped 6,000 LGBs with 84% guiding and 64% scoring direct hits during the Easter Offensive. Between 1 February 1972 and 28 February 1973, 10,651 laser guided bombs were dropped in Southeast Asia (SEA.)

F-4D 435th TFS with Pave Knife laser pod 1973

The key to the laser guided bomb is the laser designator, without it the weapon would just fall to earth like a regular bomb. At first there was the Airborne Laser Designator (ALD) or "Zot box" which was gyro-stabilized and mounted in the back seat of an F-4 Phantom II. The Weapons Systems Operator (WSO) would aim the Zot and 'paint' the target with the laser (designate the target.) This would create a large cone or 'basket' in the sky, bombing aircraft could then drop their LGBs in the basket and the weapons would guide themselves to the target. Of course, during this time, the illuminator would have to circle the target at about 12,000 ft (3657 m) above ground level (AGL) and continue to 'lase' until impact, a process that could take 15-30 seconds once in place. This put that aircraft in a position where anti-aircraft artillery (AAA) could engage it.

Later in the war, with better laser designation systems, the altitude of 'lasing' would increase to more like 18,000 feet (5486 m). This is representative and the minimum cloud base for successful employment of Paveway was 2,500 ft (760 m). Arriving in Southeast Asia (SEA) in mid-1971 the Pave Knife, a 1,200 lb (550 kg) wing mounted targeting pod marked a huge improvement in laser designation, fixing many of the issues with the Zot box. Aircraft with Pave Knife could now carry LGBs and self-designate as well as designate for other aircraft. The designating plane was also able to conduct normal defensive maneuvers and still maintain the laser on the target, thanks to the Pave Knife pod, which had a steerable laser with a nearly 180 degree freedom of movement. All of this made for a more effective and survivable system. This would prove vital, when attacking heavily defended targets in and around Hanoi and Haiphong. where AAA sites and surface-to-air missiles (SAMs) existed in great numbers.

Click here to see full specifications of Laser Guided Bombs.

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Tropic Moon III

B-57G of 13th Bomb Squadron Ubon RTAFB, 1970

B-57G of 13th Bombardment Squadron, Ubon RTAFB, 1970

Tropic Moon III was a program conceived in 1967 to convert the B-57 Canberra jet bomber into a self-contained night attack aircraft with the addition of low light level television (LLLTV) and forward looking infrared (FLIR.) The converted planes were also given a laser range finder/illuminator and a forward looking radar (FLR) during the three years it took for conversion and training. Finally on 17 October 1970 a Tropic Moon III aircraft flew the first B-57G combat sortie over Laos and on 24 October the 13th Bombardment Squadron, Tactical records the first truck kill by a B-57G. A pilot would fly at high altitude to reach the search area. After coordination with the area controller and nearby aircraft the pilot descended to 6,000 ft (1829 m) above ground level and flew the B-57G at 250 knots (463 km/h) true airspeed. Often the operating altitude was increased to 8,000 ft (2438 m) whenever there was more than a quarter moon visible. Then the search was on for trucks and vehicles below.

An evaluation of the first three months of operations (17 October 1970 - 15 January 1971) showed that in 543 sorties, the B-57G crews sighted 759 trucks, attacked 565, and destroyed 363. It should be noted these results include many sorties on which no trucks were sighted. The Canberra armed with up to four 500-lb (227 kg) laser guided bombs could be deadly. For example, during the period April through June 1971, 306 GBU-12 LGBs scored 215 direct hits (an impressive 70% rate). Of the 73 bombs that failed to guide, eight were caused by crew error, nine by weather, 18 by equipment malfunction, and 38 to unknown factors. Equipment deficiencies were common to the Tropic Moon aircraft, their suite of advanced electronic gadgets were cutting edge at the time and often malfunctioned. In July 1971 the Seventh Air Force ordered the B-57Gs to fly normal daytime bombing missions over Cambodia, this lasted until 10 November.

Tropic Moon III aircraft carried and used other weapons besides LGBs including M35 and M36 incendiary cluster bombs, 750-lb (340 kg) general purpose (GP) bombs and, for a short time, canisters of BLU-26 cluster bomblets released from Hayes PW4/4A modular bomb dispensers. Not all was good though since the Canberra had limited fuel and no inflight refuelling capability it's loiter time ended up being limited to less than an hour once over the search area. Consequently, the key measure of trucks destroyed per sortie fell short when compared to AC-130 gunships - also working the Ho Chi Minh Trail (HCMT.) Finishing strong, in the final three months (January through March 1972) the B-57s were credited with the destruction of 369 trucks, two 37mm guns, two 57mm guns, a tracked vehicle, and three tanks. They also made two roads cuts and caused 1,474 secondary explosions and 1,255 fires. These almost certainly destroyed or damaged additional supplies and vehicles. Tropic Moon III reached it's conclusion on 12 April 1972 when the aircraft left Ubon RTAFB, Thailand for Clark AFB, Philippines, the first leg of their return to the US.

Source: "The B-57G -- Tropic Moon III 1967-1972" Office of Air Force History, 1978

Source: "Ho Chi Minh Trail 1964-73" by Peter E. Davies, Osprey publishing (ePub edition), September 2020

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OV-10 Bronco firing a white phosphorus smoke rocket

OV-10 Bronco firing a white phosphorus smoke rocket

The OV-10 Bronco is a twin engine turbo-prop aircraft used in light attack and observation roles. The USAF utilized the Bronco primarily as a FAC (Forward Air Controller) platform. Beginning in 1966 the 23d Tactical Air Support Squadron (TASS) was first deployed to Nakhon Phanom (NKP) Air Base, in northeastern Thailand. They started out flying O-1s, then O-2s and finally, by 1968 OV-10 Broncos to patrol the HCMT as FACs. This mostly involved sighting and marking targets for strike aircraft to attack. The 23d TASS used the aerial call sign Nail.

A key part of Pave Nail was the PAVE SPOT pod which incorporated low light level ability with a level of magnification for day or night target identification and laser designation. It was stabilized and gimbaled for moveable tracking. The pod was mounted on the OV-10 Bronco's centerline instead of the usual external fuel tank. To compensate and bring the aircraft's range and endurance back up to an acceptable level the fuel system was modified to allow two wing mounted drop tanks to be carried. To test Pave Spot two OV-10s were flown to Eglin AFB, Florida in November 1970. After system installation the planes flew 16 missions and using laser guidance delivered six inert Paveway bombs. The average miss distance was 25 ft (7.6 m) proving Pave Spot's viability. Eventually 13 OV-10 FAC aircraft of the 23d Tactical Air Support Squadron (TASS) were modified for the program. Before that there was a combat evaluation with four aircraft. From July through September 1971 they flew over northern Cambodia from the Ubon RTAFB and Pave Spot was put through it's paces. 79.5 percent of the bombs hit within 40 ft (12 m) of the designated targets. After this successful showing, the four aircraft were moved up to Nakhon Phanom (NKP) Air Base to begin sustained combat operations over the HCMT.

Although intended to allow searching for troops, supplies, and trucks along the Ho Chi Minh Trail (HCMT), the Pave Spot's narrow field of view frustrated many attempts to do so. The Pave Nail crews often relied on binoculars for sighting instead. With difficulty pilots could talk the Weapons Systems Operator with the Pave Spot, onto the target. Under a related program named PAVE PHANTOM several dozen F-4 Phantom IIs were upgraded with the LORAN system. This allowed them to work with the Pave Nail aircraft and bomb in weather conditions requiring instrument only flying.

Operationally, the Weapons Systems Operator (WSO), using Pave Spot, acquires the target and designates it with the laser. This gives a readout of LORAN target coordinates, elevation of the target, and slant range. The pilot then passes the LORAN coordinates and the elevation of the target to the F-4, who in turn feeds this information into his LORAN bombing system. Once run-in headings and timing are agreed upon the WSO begins to 'lase' the target, ten seconds prior to bomb release, for bomb acquisition and guidance. January through March of 1972 squadron records show that 23d TASS Forward Air Controllers attacked 237 AAA sites and claimed eighty-two destroyed - representing the full range of 23 mm, 37 mm, 57 mm, 85 mm, and 100 mm guns. They also directed 767 major road cuts, and damaged or destroyed more than 300 trucks and bulldozers. The fact that the Pave Nails were also on call for search and rescue (SAR) missions reflected the Armed Forces' commitment to providing every resource to the safe return of air crews. Overall, the Pave Nail aircraft suffered a high 20% loss rate, primarily due to the OV-10s unsuitability to flying in high threat areas.

Source: "PAVE NAIL: there at the beginning of the precision weapons revolution" by Darrel Whitcomb. (Essay)

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Specifications for Laser-Guided Bombs (LGBs)

NameGBU-10/B PavewayGBU-11/B PavewayGBU-12A/B Paveway
TypeLaser-Guided Bomb
ManufacturerTexas Instruments
In Combat196815 Oct, 1969?
# Used in Combatsome 21,000 (84% of all LGBs)28 1?
GuidanceSemi-Active-Laser Homing
Guidance kitKMU-351BKMU-370BKMU-388/B
Warhead2000 lb (907 kg) MK 84 bomb 23000 lb (1360 kg) M118E1 bomb 3, 4500 lb (227 kg) Mk 82 bomb 5
Total Weight2080 lb (943 kg)3066 lb (1391 kg)650 lb (295 kg)
Length14 ft 2 in (4.32 m)13 ft 9 in (4.19 m)10 ft 6 in (3.20 m)
Diameter18 in (46 cm)25 in (63.5 cm)10.75 in (27.3 cm)
Fin Span3 ft 9 in (1.14 m)4 ft (1.22 m)3 ft 3 in (0.99 m)
Range 1,650 - 20,000 yards (1510 - 18290 m) 6??

1 28 of the 186 M118 bombs dropped on NVN targets were laser guided during April 1972 to January 1973.
2 Mk-84 makes a 49-foot (15 m) wide, 13-foot (4 m) deep crater in medium soil with 946 lb (429 kg) explosive.
3 M118 with 1888 lb (856 kg) Tritonal explosive - a demolition bomb with a thin-casing and a greater blast effect but less cratering and penetration ability.
4 M118E1 is only different from the M118 in having threaded lug wells - similar to the M117 series of bombs.
5 Mk-82 warhead is 192-196 lb (87-89 kg) explosive.
6 Depending on launch altitude. Info from "A Compendium of Armaments and Military Hardware" by Christopher Chant

Estimates vary by source, especially range and cost.

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Comparison of Guided vs. Conventional Weaponry

A 1968 DOA report showed that for specific point targets, more than 20 times as many targets could be hit using Paveway I laser guided bombs than could be hit using 750 lb (340 kg) freefall bombs in the same number of sorties. Data adapted from HQ PACAF Directorate, Tactical Evaluation Project CHECO SEA Report "Second Generation Weaponry in SEA" 10 September 1970

Target10 ft (3 m) by 100 ft (30m) bridge
WeaponSorties 1Cost 2
MK-84 LGB 31$71,000
MK-84 Freefall 4191$3,180,000
M117 LGB 57.5$420,000
M117 Freefall 664$1,082,000
Walleye 72.5$112,620
AGM-12C Bullpup 89.5$268,000

1 Sorties Required to Get 50% Probability of Hitting Target.
2 Dollar Cost to Get 50% Probability of Hitting Target.
3 MK-84 LGB two bomber passes, two bombs per pass, 1 illuminator.
4 Mk-84 Freefall bombs delivered in pairs
5 M117 LGB 7 1/2 bomber sorties, 7 1/2 illuminators.
6 M117 Freefall bombs delivered in groups of six.
7 5 Walleyes delivered in five passes.
8 Bullpups delivered in 19 passes.

The AAA figures below represent actual experience gained from the use of 322 Paveway LGBs, and from 1,366 sorties delivering general purpose bombs and cluster bomb units. The highlighted items below mark the most cost effective weapons: guided or conventional for a particular target type. They show that for lines of communication (LOC) the use of unguided weapons is actually more cost-effective. Interestingly, the vast majority of targets hit by GBU-10 laser guided bombs in 1969 were roads and fords (lines of communication). Fords are shallow places where a river or stream may be crossed by wading or in a vehicle.

Data adapted from HQ PACAF Directorate, Tactical Evaluation Project CHECO SEA Report "Second Generation Weaponry in SEA" 10 September 1970

TargetAAA 1
Type of weapons usedConventionalLGBs
Sorties per gun destroyed5.081.48
Flying cost per sortie$1,662$1,662
Weapons cost per sortie 3$5,950$11,678
Total sortie cost$7,612$13,340
Cost per AAA destroyed$38,650$19,700
TargetLOC 2
Type of weapons usedConventionalLGBs
Sorties per cut3.081.15
Flying cost per sortie$1,662$1,662
Weapons cost per sortie 4$2,460$11,678
Total sortie cost$4,122$13,340
Cost per LOC cut$12,700$15,360

1 AAA Anti-Aircraft Artillery
2 LOC Lines of Communication
3 MK-82 500-lb (227 kg) bombs and Cluster Bomb Unit against AAA Target
4 12 MK-82 500-lb (227 kg) bombs against LOC Target

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Research and Sources

"Operation Linebacker I 1972 The first high-tech air war" by Marshall L. Michel III, Osprey publishing (ePub edition), 2019
"Rolling Thunder 1965-1968 Vietnam's most controversial air campaign" by Richard P. Hallion, Osprey publishing (ePub edition), 2018
"Linebacker The Untold Story of the Air Raids over North Vietnam" by Karl J. Eschmann, Lume Books (Kindle edition), 2018
"Dragon's Jaw: An Epic Story of Courage and Tenacity in Vietnam" by Stephen Coonts and Barrett Tillman, Da Capo Press (Kindle edition), 2019
"The Long Road to Desert Storm and Beyond" by Major Donald I. Blackwelder, School Of Advanced Airpower Studies, May 1992
"Second Generation Weaponry in SEA", HQ PACAF Directorate, Tactical Evaluation Project CHECO SEA Report, 10 September 1970
"Linebacker Operations September - December 1972", Project CHECO Office of History HQ PACAF 31 December 1978


Southeast Asia War Gallery of National Air Force Museum

Getting Closer: Precision Guided Weapons in the Southeast Asia War (National Air Force Museum)

National Archives Catalog


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