what effect did jimi hendrix use in woodstock to make that noise that sounds like bombs dropping?

Answer 1

Simply his talent. He was hitting the lower E and A strings and stretching the notes to go from a "higher" pitch to a lower "pitch" combined with a bit of feedback from his amp to create the "reverb" sound of a bomb. You can also sample more of this man's talent when he has his guitar "speaking/singing" the national anthem.