operations; they would have to crawl along the river bottoms using snorkels. Not by design, it was found that the swimming hardware acted to reduce the effectiveness of RPG hits, but it was rarely used in Vietnam. Strategic mobility The Sheridan can be rigged for low-velocity airdrop from C-130 (19,000 kg, 42,000 lb max load), C-141 aircraft (17,460 kg, 38,500 lb max load), and the C-5. Many films exist showing the Sheridan being pulled out of a C-130 Hercules transport by brake chutes and skidding to a stop. The Low Altitude Parachute Extraction System (LAPES) is a somewhat risky maneuver that allows accurate delivery onto a field when landing is not possible, and the practice was stopped in the late 1990s. The tank is strapped down to a special pallet which absorbs most of the landing impact. The crew does not ride in the tank during extraction, but parachutes from another plane. On landing, they go to their tank, release the lines, and drive it away. Variants XM551/M551 - The M551 was the basic production model, beginning production in 1967. The XM551 had been a limited run pre-production model produced in 1965. "Two Box" M551 - With the obvious shortcomings of the Shillelagh missile, all but two of the guidance and fire control components of the missile system were removed (the power supply and rate sensor were retained. These were needed for stabilized turret operation.). The resulting additional space was filled with two separate boxes, one for 7.62×51mm NATO ammunition (coaxial machine gun), and one for 12.7×99mm NATO (.50 BMG) ammunition, and the missile stowage was redesigned to accept conventional rounds. M551A1 - Upgraded M551 with AN/VVG-1 laser rangefinder. M551A1 TTS - Tank Thermal Sight, fitted with the AN/VSG-2B thermal sight unit, similar to the unit used on the M60A3 MBT. This later became standard to all M551A1s. M551 NTC - National Training Center. Using M551 hulls, the NTC created a number of mock vehicles for training exercises resembling common Soviet/Warsaw pact types. They were also known as 'vismods', short for VISually MODified. They have since been retired in favor of similarly converted M113s and M1 Abrams. Armored Tracked Recovery Vehicle (ATRV) - Prototype. Table of variants Display vehicles Camp Joseph T. Robinson, Arkansas National Guard Museum Carolinas Aviation Museum Fort Benning, static display on Tower Branch of U.S. Army Airborne School across from Maneuver Center of Excellence. First Division