Barrage balloons, (or
'blimps', as they were also known), were a familiar sight in the skies over
Britain during
World War II, when they were used around strategically important targets as a form of
defence against enemy
aircraft.
Tethered by a
steel cable, the
hydrogen-filled
balloon could be raised and lowered between heights of 500 and 5000
feet by means of a
winch.
Flown either in groups or in isolation, the idea behind the barrage balloon comes from observation balloons, which were commonly used during World War I to help direct artillery fire and, at sea, to spot dreaded U-boats. The concept behind the barrage balloon was simple: Both the barrage balloon itself and the cable it was attached to impeded dive bombers and strafing runs from fighter aircraft. To avoid collision, the enemy would be forced to fly high, which reduced the accuracy of their attack. Also, strategically-placed barrage balloons could be used 'channel' aircraft towards anti-aircraft guns.
Deployment of the barrage balloons was organised under the aptly-named RAF Balloon Command, who also trained the operating crews. Balloons were launched from both fixed and mobile sites, (i.e. the backs of modified trucks), in addition to being launched at sea to protect naval vessels.
To give an idea of the size: The most common British barrage balloon, known as the 'LZ', ('Low Zone'), was approximately 62 feet long and had a maximum diameter of 25 feet.
Although use of the barrage balloon declined after World War II, the spectre of 9/11 has resurrected interest since they represent a cheap, reliable and easily deployable method of protecting static targets.
References
http://www.bbrclub.org/Links.htm
http://www.fishponds.freeuk.com/puckle.htm
http://news.bbc.co.uk/1/low/world/middle_east/1562423.stm