Time-of-Flight
Time-of-flight is the period of time between a gun's discharge and the impact of the projectile against land, sea, or other object.
Generally, time-of-flight is considered as that when a shell would strike the water, as in all but the shortest range firings the abbreviation of a projectile's flight by hitting a vessel before it reaches the water is quite small.
Due to aerodynamic drag, the time-of-flight increases more and more quickly as range increases. For instance, a 1,250 pound British 13.5-in shell with a 4 CRH head would take 14.28 seconds to reach 10,000 yards range, 23.8 seconds to reach 15,000 yards and 34.98 seconds to reach 20,000 yards.[1]
By 1908, the Royal Navy was trialling Time-of-Flight Watches to assist spotters in identifying their own shell fall. The watch would be set to the given gun range, started when the guns fired, and would ring a bell at the moment the shells should fall.[2]
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