"This patent application discloses a defense system for protecting a static structure, such as a building in an urban environment, from an aircraft attack. It is a further object of the present invention to provide an air defense system which can be fired at close range against an incoming aircraft to deter its attack, and which uses ammunition that becomes harmless after a short distance. It is a further object of the present invention to provide an air defense system which does not require explosive devices of any kind. The air defense system broadly comprises at least one launching device for discharging a nonexplosive, fluid payload at an incoming aircraft. Each launching device is mounted to an exterior wall of the static structure. In a preferred embodiment of the air defense system, the launching device is a water cannon mounted to an exterior wall of a building. In a typical system, an array of launching devices is mounted to an exterior wall of a static structure that may be subject to attack from an aircraft."
Defense Technical Information Center (DTIC): http://www.dtic.mil/dtic/