In December 1967, the U.S. Air Force began a testing program to use large bombs for explosively clearing jungle areas for landing of helicopters.
In October 1968, a C-130 crew from the 29th Tactical Airlift Squadron of the 463rd Tactical Airlift Wing flew a series of test drops while under the guidance of MSQ-77 radar controllers; additional test drops were made in December.
In March 1969, the 463rd commenced Project Commando Vault, and bomb drops became a regular occurrence.
Besides clearing the jungle and preventing the ambush of helicopters that were approaching the landing zone (the M121's blast diameter was 60 meters), the explosion also stunned the NVA or Viet Cong personnel within 500 meters and revealed or destroyed booby traps in the landing area.
[3] Despite this, the United States continued to use the M121 to clear helicopter landing zones in the jungle until stockpiles were depleted while a more powerful bomb was developed for jungle-clearing purposes.