Complex sentence: The main one was the Mk-54, a cylinder 40 by 60 centimetes and weighed 68 kilograms, fired by a mechanical timer it had a variable yield from 10 tons up to one kiloton.
Split into simpler sentences: The main one was the Mk-54, a cylinder 40 by 60 centimetes that weighed 68 kilograms. It was fired by a mechanical timer and had a variable yield from 10 tons up to one kiloton.