The VB series munitions were in general more conceptually similar to the German Fritz-X weapon, consisting of finned bombs, and were more successful than the GB munitions. All the VB weapons were developed by the USAAF Air Materiel Command at Wright Field, Ohio.
The first eight weapons in the series, “VB-1” through “VB-8”, were modified standard aerial bombs. The VB-1 was based on the standard M44 450 kilogram (1,000 pound) high explosive bomb. It was fitted with a new tail featuring a gyrostabilization system, a pair of rudders, and a tracking flare. This weapon could only be steered right and left, and so it was named “AZON”, for “azimuth only”. The later “VB-2” was a 900 kilogram (2,000 pound) bomb with the same guidance scheme, though apparently it was not build in large numbers. A radio control system conceptually similar to the German Kehl-Strassburg? system allowed a bombardier to direct the bomb with a joystick. This radio control system had five channels, allowing five bombers to drop and control a AZON simultaneously.
Design work on the VB-1 began in the summer of 1942, but the weapon’s gyrostabilization system proved ineffective, and in early tests the AZON showed a tendency to roll, making its control surfaces useless. Eventually the bugs were worked out, and test drops showed the guided bomb had between one and two orders of magnitude more accuracy in hitting targets than an unguided bomb.
These successful tests convinced USAAF officers of the merits of guided bombs. There had been considerable resistance to the idea because the AZON wasn’t a “fire and forget” weapon: once dropped, the launch aircraft had to remain on course while the operator guided the bomb into the target. Since this has remained a problem through the entire history of air-launched guided weapons, it was not an unreasonable concern.
Those who accepted the usefulness of guided glide bombs, on the other hand, found the restriction to “azimuth only” guidance too severe, and wanted to wait until a fully-guided next generation weapon was available. However, the test results, and the effective use of the Hs-293A and Fritx-X on the Allies in the late summer of 1943, decided the issue.
A total of 15,000 AZONs was built through 1944. It was fielded in early 1944 in Europe, and was used operationally in attacks on targets in northern Italy. Since AZON, as its name implied, could be steered side to side but not up and down, it was suited to attacking long, narrow targets like bridges and viaducts. Results were unimpressive. First, AZON was difficult to use. Its effectiveness depended greatly on the training and skill of the operator in guiding a bright point of light to a target kilometers away.
Second, one bomber would drop multiple AZONs on a target, with the bombardier directing the first and hoping the others would follow. However, aerodynamic imbalances in individual bombs ended up dispersing the bomb pattern widely over the target. In fact, the dispersal was in general greater than it would be for unguided bombs. Unguided bombs rolled on the way down, reducing the effect of aerodynamic imbalances, while AZON was specifically designed not to roll, allowing imbalances to build up.
Weapons developers at first experimented with linking all the AZONs in a bombload together with a cable. This caused a whipsaw interaction among the bombs while they fell that made the bombs completely unguidable and generally snapped the cable. The engineers then tried a nylon rope, whose “rubber band” action made the interaction between the falling bombs even more interesting. They finally gave up and decided that only one AZON would be dropped by one bomber in each pass on a target, meaning only five AZONs could be used at a time, which was less than optimal for attacks on tough targets.
• AZON went into action against the Japanese in Burma in late 1944. Allied sea control was so tight that the Japanese could no longer send supplies to Burma by ship, and so supplies were sent by train from southern China instead. The northern Burmese terrain is very rugged, and the rail routes required many bridges. That made these bridges very high priority targets for the USAAF. The Japanese understood this perfectly, and set up heavy anti-aircraft defenses to protect the bridges. That meant that USAAF heavy bombers had to bomb a target that was difficult to hit while dodging Japanese antiaircraft fire.
AZON seemed like a good solution, and a total of 459 of them were dropped, destroying 27 bridges. The fact that a bomber had to stay on course in the face of flak while its bombardier guided the AZON into a bridge was a dangerous nuisance, but the USAAF came up with a clever solution. Bombers were often escorted by P-38 Lightning fighters flying “top cover” over the bomber formation to provide protection against enemy fighters. The higher altitude of the fighters gave them some protection against flak, and the Japanese concentrated their fire on the bombers anyway, as they were the greater threat. A number of P-38s had been modified with clear “droop snoot” bomber noses to accommodate a bombardier, with these aircraft operating as “leads” for a flight of conventional P-38s carrying bombs and targeting for them; when the lead dropped its bombs, all the other Lightnings did as well. Some of the “droop snoot” P-38s were fitted with an AZON controller. A bomber would drop an AZON and then take evasive action, while the bombardier on board the droop-snoot P-38 would guide the AZON into the bridge.
• The attacks on Burmese bridges showed that radio-guided guided munitions had some potential, and advocates hoped that the next series of fully guided bombs would be even more effective. This series consisted of the fully guided “VB-3” (450 kilogram / 1,000 pound) and “VB-4” (900 kilogram / 2,000 pound) “RAZON (Range And Azimuth Only)” guided bombs, characterized by a double box tail. About 3,000 were built, but the weapon did not reach operational status during World War II.
The USAAF experimented with other VB weapons that never got out of the test phase:
The “VB-5” was a VB-3 RAZON with a simple EO light contrast seeker.
The “VB-6” AKA “Felix” was a 450 kilogram (1,000 pound) bomb with an infrared seeker for homing in on blast furnaces and the like. It had a single tail and it is unclear if it had single or double axis guidance.
The “VB-7” and “VB-8” were both radio-controlled, with a bombardier targeting the weapons through a TV seeker. Details are unclear and they may not have even been tested.
The “VB-9” through “VB-12” departed from the earlier weapons, since they did not resemble modified bombs. They were known as the “ROC” weapons. This name was derived from the great bird of THE ARABIAN NIGHTS, but also seems to have reflected a control algorithm in which the bomb “rocked” its wings to stay on course.
The VB-9 was another 450 kilogram (1,000 pound) glide bomb with relatively long cruciform wings and tail, and had an active radar seeker that turned out to work poorly against ground clutter. The later VB-10, VB-11, and VB-12 were of similar size, but used a circular airfoil and were fitted with TV, infrared, and radio command guidance systems respectively.
The “VB-13”, known as “TARZON” and later redesignated “ASM-A-1”, was the only one of these later VB weapons to actually see action. It was a monster, a modified license-built British Tallboy deep-penetration bomb weighing 5.44 tonnes (12,000 pounds) and 6.4 meters (21 feet) long. It had a circular forward wing and a box tail, with four rudders on fins within the tail. It was, like the German glide bombs, radio-controlled and had a flare on its tail for optical tracking.
