The Time Hack: A Series on the Military, and Watches - by Jon Custis
Nearly 50 years ago, during the North Vietnamese Armyʼs 1972 Easter offensive, 33 year old Marine Captain John Ripley accomplished a staggering feat of valor and blunted an offensive surging into South Vietnam, while serving as an advisor to the 3rd Battalion of the South Vietnamese Marine Corps.
Across a period of nearly three hours, he dangled, shimmied, and crawled under the Dong Ha Bridge and emplaced a reported 500 pounds of explosive charges on the bridgeʼs I-beams while the enemy swept the wooden decking above with tank and small arms fire. On one of his last trips, Ripley crimped blasting caps to lengths of time fuse, primed the explosives, and set the fuses alight with matches before scurrying to safety on the friendly side of the Cua Viet River.
The resulting explosion shattered the middle span of the bridge and prevented some 20,000 NVA troops and tanks from easily pushing further south. For his “heroic actions and extraordinary courage”, Ripley was awarded a Navy Cross, the second-highest decoration which is awarded to a member of the United States Navy, Marine Corps, or Coast Guard for valor in combat.
Ripley survived the war, achieved the rank of Colonel, and commanded the 2nd Marine infantry regiment before closing his career as the senior Marine instructor at the U.S. Naval Academy. His story remains a legendary tale learned by every new Marine Officer during basic training.
The elements of the system Ripley used to set off the explosives is still in use by Marine infantry assaultmen, engineers, and explosive ordnance disposal technicians: a blasting cap crimped to a length of time fuse, connected to a fuse igniter.
The fuse igniter is pulled to create an initiating flame that burns down the time fuse to the blasting cap. That aluminum alloy cap holds a small amount of sensitive RDX explosive, and when the flame reaches this small charge, it detonates the cap and any military-grade explosive the cap is inserted in or taped to.
The ability to measure time is essential to the safe use of time fuse in non-electrical explosive initiating systems. M700 time fuse in use with the U.S military burns at a nominal rate of 40 seconds per foot. The fuse is actually marked with yellow bands at every 18 inches, for an easier calculation of 60 seconds of burn time between every mark.
When a Marine wants to calculate the length of fuse required to get to safety before the boom, a little math is required, backed up by a watch. A three-foot length of fuse is cut from the coil and tested to ensure it falls within five seconds of the 40-seconds-per-foot burn rate. A fuse igniter is pulled and the fuse is timed as it burns. Any watch that can track elapsed time will suffice, but most Marines favor a PX-bought digital Timex or Casio GShock. In a garrison environment, even smartphones get whipped out to achieve the task.
Once the burn rate of the time fuse is confirmed, the required distance to reach safety is walked and timed (or estimated), with the resulting time plugged into a simple formula. This produces the required fuse length to allow for a safe withdrawal before fillings in teeth rattle, craters are blown, and bridges collapse.
Time required (min) x 60 (sec/min)
———————————————— = Fuse length (ft)
Burning rate (sec/ft)
The roll of M700 fuse is cut to required lengths, and those pieces subsequently crimped onto blasting caps. Once the blasting caps are inserted into the explosives of choice, the safety pins on fuse igniters are removed and the igniters pulled. The resulting spit of flame and puff of smoke signals the countdown to detonation and the satisfying crack and billowing smoke that brings a smile to any Marineʼs face.