Britain's World War II offensive balloons
Most students of world war II are aware of the thousands of barrage balloons that Britain employed throughout the war to help protect cities, ports, and other important targets from low-flying Luftwaffe dive bombers and fighters. These large gas balloons measured about 64 ft (19.5 m) long by 34 ft (10.4 m) in diameter and were tethered to the ground by steel cables attached to winches and rose to heights of up to 5,000 ft (1,524 m).
In addition to making the German aircraft fly higher than planned, those that did fly low had to maneuver to avoid the barrage balloons and their cables, thereby cutting their accuracy. Those aircraft that did impact the cables or balloons were often destroyed or heavily damaged. During what was termed the “Blitz,” which was the period from September 1940 to May 1941 when Luftwaffe attacks centered on London and other large or important cities, 102 aircraft struck barrage balloon cables and either crashed or had to make a forced landing.
That history, including the iconic photographs of barrage balloons deployed near Tower Bridge, the Houses of Parliament, and other strategic structures and installations is well known (see Figure 1). What is less well known is what happened next.
On the night of 17 September 1940, during the Battle of Britain when the Luftwaffe was incessantly attacking England by air, a raging storm with gale-force winds ripped many of the barrage balloons away from their moorings. The balloons were carried by the winds over the North Sea toward mainland Europe, dragging their severed cables behind them.
Within hours, reports of electrical outages in Denmark, Sweden, and Finland began to come in. The balloons' heavy tethering cables had struck high-voltage overhead electric transmission and distribution lines, and the resulting short circuits caused power outages affecting electrified railroads and even whole cities. One balloon strike resulted in the toppling of the broadcast tower of the Swedish international radio service.
One such report said “On Wednesday evening the Swedish west coast witnessed a great ‘invasion' by foreign barrage balloons. Over the coast of the province of Halland, the barrage balloons came over in such numbers that at times the sky was lit up with sparks when the balloon cables touched electric wires.”
It occurred to Britain's leaders that if wayward balloons could accidentally cause damage to overhead electric power transmission lines, perhaps a purpose-designed and deployed balloon system could do even better.
In early 1940, the Air Vice Marshal of the Balloon Command, the organization responsible for the barrage balloons, wrote “Since the outbreak of war, I have had constant complaints from the electricity distributors regarding the damage done in this country by [barrage] balloons that have broken away from their moorings.” He went on to suggest that “…advantage might be taken of this to impede and inconvenience the enemy.”
On 19 September 1940, Winston Churchill, ever the out-of-the-box thinker, captured the sentiment when he wrote a comment on a memo to the War Cabinet “We may make a virtue of our misfortune.”
And so was born Operation Outward, an offensive weapon scheme that the British hoped would carry the battle to Germany and the occupied countries less expensively and more safely than British night bomber attacks.
The British military rightly assumed that balloon attacks would be very hard and expensive for the Germans to defend against. The balloon cruising altitude was high enough that even if spotted in the daytime, it would exact a significant cost in fuel and wear and tear on their fighter aircraft that were diverted to the task of intercepting and destroying the balloons.
Captain C.G. Banister, director of Boom Defence (an Admiralty group responsible for laying antiship and antitorpedo booms to protect harbors), a proponent of using balloons as an offensive weapon, continued to press the point. He suggested holding trials to determine what size balloon could be used, the particulars of the wire (length, diameter, material), the meteorological considerations, and most important, whether safety devices (e.g., circuit breakers) on the high-voltage lines would work well enough to make the scheme ineffective.
To help prove its case, the Admiralty ran trials using surplus spherical latex meteorological balloons about 8 ft (2.4 m) in diameter when inflated. Calculations based on the trials predicted that there would be between 10% and 75% chance of a balloon's wire coming into contact with a high-voltage overhead line during a 30 mi (48 km) transit along the ground.
Surprisingly, the trials showed that even a thin steel wire (much thinner than that used to tether the static barrage balloons), when drawn in sliding contact across two or more phases, could cause an arc as long as 15 ft (4.6 m) that would be maintained until the circuit breaker opened. In some cases, the arc's heat melted the aluminum outer layers and then the reinforcing steel center strands of the conductors. Further, even if not severed, the conductors would be so weakened by the arc that they would be susceptible to breaking due to increased load demands or even normal weather events such as wind, snow and ice. Then, even if the trailing wire was severed by the arc, the balloon with the remaining portion of the wire could be carried along with the wind to engage yet another electric line (see Figure 2).
A balloon's trailing wire could cause the same kind of phase-to-ground short that would occur from normal peacetime causes, such as conductors breaking. These resulted in tripping of circuit breakers and an inconvenient, but usually short-term, resultant loss of power. However, if the trailing wire caused a phase-to-phase event, it could be much more serious.
The British knew that the German high-voltage electric transmission system was protected by Petersen coils, which could not cope with phase-to-phase shorts of the type that would likely be caused by the balloons' trailing wire. Further, they knew that the German systems of that time used slower-acting circuit breakers, also not designed to handle phase-to-phase shorts. The British concluded that this design could lead to the destruction of the circuit breakers and transformers and cause even more catastrophic faults, such as wrecking an entire power generating station, which actually happened in 1942.
Even though the British electrical system had a more developed grid than that of Germany, which could make it more vulnerable, it also had faster-acting circuit breakers, and had proven itself more capable of tolerating (but not be completely protected from) hits from errant barrage balloon cables. This made the British less worried about the effects of possible retaliation and more encouraged about the potential of trailing wires as a weapon.
Further supporting the argument that trailing wires could be an effective weapon was the idea that since the balloons would be released in large numbers, they would be likely to cause numerous faults in the same area, thereby complicating the task of repair and further diverting valuable resources. Also, even a single balloon could cause multiple disruption events as its long wire dragged along the ground.
Considering the shortages of materials for repair of electric power systems in England and especially in Germany, it was clear that the consequential damage of a balloon strike could be much greater than that caused by a single bomb dropped from an aircraft.