The modified commercial Backpack Subsystem (UM21) contains a backpack, patrol pack, and butt pack. The backpacks state-of-the-art internal frame affords a stable platform sufficient to carry 120 lbs. effectively. The backpack transfers load efficiently from the shoulders to the waist and provides adjustments to fit the 5th - 95th percentile male SOF operator. The butt pack attaches to ELCS or the backpack; the patrol pack attaches to the backpack, and is compatible with the other SPEAR BALCS components.
In 1963 Natick Laboratories developed a tropical rucksack for the Army of the Republic of Vietnam (ARVN).
Military specification MIL-R-43323 covers one type of rifle butt pocket with strap used with field packs.
The lightweight rucksack with aluminum frame was developed by Natick Laboratories for cold weather and mountain use as an initial effort to develop lightweight load-carrylng equipment in 1961. It weighed 3 pounds (1.4 kg) as compared to the 7.5 pound (3.4 kg) cotton rucksack with steel frame which it replaced.
Based principally on the conclusions and recommendations of 'A Study to Reduce the Load of the Infantry Combat Soldier', 1962, and 'A Study to Conserve the Energy of the Combat Infantryman', 1964, a Quantitative Materiel Requirement (QMR), was established in 1965 calling for the development of Lightweight Individual Clothing and Equipment (LINCLOE).
Military specification MIL-F-43356 covers an aluminum frame and cargo support shelf used with the lightweight rucksack.
Military specification MIL-C-41823 covers a camouflage cover for the lightweight rucksack.
Military specification MIL-R-1619.
The field pack cover is intended to be used over the medium and large field packs to provide camouflage.
Military specification MIL-C-43830 covers the material, design and fabrication of a field pack cover.
Military specificatin MIL-S-43828 covers the requirements for a strap consisting of nylon webbing, a metal fastener and a metal hook.
Military specification MIL-F-43834 covers a lightweight aluminum pack frame and a detachable aluminum cargo support shelf.
Military specification MIL-S-43835 covers waist, shoulder, and chest straps for use by ground troops in attaching and adjusting LC-2 pack frames and field packs.
The combat patrol pack was designed for short missions and offers 1200 cubic inches of cargo space in two compartments.
The tropical rucksack is a nylon version of a cotton rucksack for ARVN forces. It has a longer back and more capacity.
DA Pam 21-54 is a soldier's guide for the use and care of fragmentation protective body armor.
Military specification MIL-R-43373 covers a lightweight rucksack made of OG-106 nylon fabric.
Military specification MIL-R-43574 covers the tropical rucksack.
The medium field pack is designed to carry up to 50 pounds of existence loads which include clothing, personal items and rations.