23 January 2011

Operational Improvisation: Over-the-Shoulder Nuclear Bombing

F-84Gs of the 20th FBW had lightning markings, each squadron had its own color
In 1952 the Republic F-84Gs of the 20th Fighter-Bomber Wing crossed the Atlantic supported by aerial refueling to set up shop at their new base, RAF Wethersfield, in order to provide tactical nuclear strike capability for the first time to NATO forces in Europe. Just a year earlier, scientists and engineers at Sandia, one of the development centers in the United States for nuclear weapons, had developed the Mark 7 nuclear bomb, the first tactical nuclear weapon with an explosive yield of 20 kilotons. While the Mark 7 weapon would be carried operationally by many USAF and US Navy tactical attack aircraft, the first aircraft to carry the Mark 7 operationally also happened to be the first production tactical fighter to have not just nuclear capability, but also air-refueling capability. That was specifically the G variant of the Republic F-84 Thunderjet, which had an air refueling receptacle for a flying boom in the left wing root, a more powerful jet engine, and provisions for the Mark 7's special pylon that had the necessary circuitry for nuclear weapons delivery. 

The Mark 7 was the first American tactical nuclear weapon
The 20th FBW had been given six months to prepare for the move to Great Britain as well as to become the first tactical nuclear fighter-bomber unit in military history. At the time of the deployment, the F-84Gs and pilots of the 20th FBW were only versed in clear-weather weapons delivery more suited to the bombing ranges in the predominantly sunny southwestern United States, drops being made starting at 20,000 feet in altitude. The weather in Europe, however, was far from ideal for this sort of weapons delivery mode, with a predominantly cloudy maritime climate in the areas that the 20th FBW was expected to operate. I had posted this past September about the nuclear delivery role assigned to the McDonnell F-101 Voodoos of the 81st Tactical Fighter Wing based in RAF Bentwaters/Woodbridge in the late-1950s and early 1960s. Not unlike the conditions facing the pilots several years later that flew the Voodoo, the pilots of the 20th FBW were expected to navigate visually and by dead reckoning to their targets with only the most basic of navigational aids. By the time the 20th FBW had set up shop at RAF Wethersfield, a different form of nuclear delivery was needed and the wing commander, Colonel John Dunning, had sent some of his best pilots to Kirtland AFB in Albuquerque, New Mexico near Sandia, to find out more about a new weapons delivery tactic called LABS- Low Altitude Bombing System. Developed at Kirtland by Major Jack Ryan, it hadn't yet received much attention as most nuclear delivery tactics of the day concerned use by strategic bombers. Col. Dunning wanted his pilots in Europe to have every advantage possible and LABS offered that prospect. 

In a LABS run, an IP (initial point) is chosen that is a known distance and direction from the target and was most ideally located three miles away. The LABS equipment was quite basic- it was a timer with a gyro that was free to move about the pitch axis of the delivery aircraft. Having previously set the time from the IP to the pull up point near the target and the calculated angle of release beforehand, an aircraft on a LABS run headed towards the target at 500 mph at low level with the IP in between the aircraft and the target. Once the IP was reached. the pilot pushed the "pickle" button on the control stick which started the timer and a flashing red light on the gunsight was connected to both the timer and the LABS gyro. The pilot pulled into a steady 4G climb and at a precise point and angle (usually 25 to 30 degrees), LABS released the nuclear weapon which continued onward on a precalculated ballistic trajectory towards the target while the pilot pulled his aircraft into an Immelman loop and exited the area as fast as possible. In effect, LABS "tossed" the bomb towards the target. 

Diagram showing the over-the-shoulder bomb toss
On return to Europe, the pilots that trained in the LABS technique for the 20th FBW found that finding an IP near the target was challenging. It was noted that as the distance between the IP and target decreased, the ideal release angle of the Mark 7 bomb increased. If the IP was very close to the target itself, then the optimum release angle was 90 degrees. Pushing the idea further, the pilots of the 20th FBW worked out that if the IP was the target itself, then the optimum LABS release angle was 110 degrees and the bomb would impact right at the point where the pull-up maneuver was initiated. No IP was needed- the target itself was the IP. The bomb was released "over the shoulder" and would arc upward to 10,000 feet and more than a minute elapsed before it detonated, allowing time for the F-84G to rollout and accelerate out of the area in a dive. The USAF and the specialists at Kirtland AFB doubted if the average USAF pilot could carry out such a complex maneuver as the wings had to be absolutely level in the pull up or the bomb's impact point would stray away from the target. The operations officer of one of the 20th FBW's constituent squadrons had noted that the F-84G didn't even need a LABS gyro- the aircraft's own gyro started to "tumble" right past vertical and by complete coincidence, right at 110 degrees! Major John J, Kropenick, the ops officer who made this observation, came up with his own solution, the "Kropenick Autopilot" that was taught to all the pilots of the 20th FBW- two large rubber bands were hooked to the control stick on the run in, each one then looped over a cockpit light on the sidewall on each side. The tension of the rubber bands kept the stick precisely centered during the pull up and once the Thunderjet's own gyro tumbled, the bomb would be released. Pilots taught the method with the "Kropenick Autopilot" had bomb scores acceptable to the USAF given the 20-kiloton yield of the Mark 7 bomb.

By the time the LABS equipment had been fitted to the 20th FBW's Thunderjets, the pilots of the wing had gotten quite proficient at using the "Kropenick Autopilot" and made the transition to using the LABS equipment for "over the shoulder" toss bombing with a minimum of delay and fuss. 

Source: Aviation History, March 2011, Volume 21, Number 4. "Over-the-Shoulder A-Bombing; Cold War F-84G pilots improvised a surprising twist on bomb delivery" by David Rust, p54-57.

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