Theories on the cause of the explosion have ranged from sabotoge to gross incompetence.... None of these have been conclusively proven (or disproven) and many a heated debate has been had discussing the probable cause.... Here is a short write up from Battleships by William H. Garzke, Jr. and Robert O. Dulin, Jr. Copyright 1995, Naval Institute Press...
Orders were given to Turret II to commence firing. The crew of this turret prepared to load all three guns with dummy shells and D846 propellant, unlike the type D845 propellant used in Turret I. The D846 propellant, normally restricted to service 1,900-lb High-Capacity (HC) shells, was being used as part of a continuing developmental program to control and perfect the velocity and long-range accuracy of the 16-in guns. Official prohibitions against using the D846 propellant were based on the desire to limit high pressures in the guns, although reduced charges had been fired in these and other battleship guns during World War II. This special five-bag charge (the guns had a maximum capacity of six bags) resulted in a maximum pressure of about 38,700 psi, well below the normal service pressure of about 49,700 psi, and much less than half the theoretical strength limit of about 90,000 psi. The D846 charges would not create an inherently unsafe condition for firing this gun.
The breeches for the two outer guns were closed and locked within 17 and 44 seconds of the order to load and then elevated to their firing positions. Their powder hoists were lowered to the powder flat. GMG3 Robert Backherms was the rammerman for the center gun and was new to his position. GMG3 Richard Lawrence was the cradleman, and GMG2 Clayton M. Hartwig was acting as the gun captain for the center gun in place of Lawrence. The projectile for the center gun was rammed into the rifling, and three powder bags dropped into the loading tray. In accordance with normal practice, Hartwig should have inserted a small lead pouch behind powder bag number 1 and then shoved powder bags 1 and 2 into the breech, allowing room for two more powder bags to fall into the loading tray. Hartwig might have placed another small pouch behind powder bag number 2 before the last three bags were pushed into the gun by the ramming mechanism. These two lead pouches were to "de-copper" the guns. (Note 1) It is unlikely that Hartwig would have delayed the gun-firing process had he missed placing one of these pouches. Rammerman Backherms reached for the lever to activate the rammer that would push the five powder bags into the center gun. His actions sent the powder bags at a low ramming speed some 24 inches farther into the gun chamber than prescribed, so the last bag was not at the correct distance to the firing charge in the breech door. This spacing, well beyond the maximum of 4 inches, was undesirable because it can contribute to inconsistent ballistic performance or to a misfire. The last bag should have been positioned so that the mushroom of the plug was just touching the ignition bag of the last charge when the breech was closed.
The following conversation ensued in Turret II that morning:
GMCS(SW) Reginald O. Ziegler (turret captain)-"Left gun is loaded. Good job! Center gun is having a little trouble. We'll straighten that out."
GMG3 Richard Lawrence Center gun cradleman -"I have a problem here; I am not ready yet."
Ziegler, now shouting to LTJG Robert M. Buch-"Tell plot we are not ready yet. There is a problem in the center gun!"
GMG3 Richard Lawrence (with annoyance) - "I'm not ready yet! I am not ready yet!"
Unidentified Seconds later- "Oh my . . ."
It should be noted that Lawrence had signed off as the captain of the center gun, and Hartwig had acted as the gun captain in his place. The gun captain had complete control of the loading and ramming of a shell and powder into a gun. Hartwig was believed to have been hunched over the breech door, presumably to investigate what happened. A few seconds later, a voice stated over the turret's intercom circuit, "Oh my . . ." The breech of the center gun had not been closed, and the rammer was still in the barrel when smoke, unspent powder grains, flame, and hot gases burst out of the open breech. Hot gases swept through the lower turret substructure and erupted through the three gun ports, the vent ducts, and the rangefinder hoods (see Figures 7-1 and 7-2). The gun bloomers (Note 2) were ripped from the turret's faceplate and blown away from all three gun ports. Thick, hot, gray smoke billowed forth, scorching the teak deck beneath Turret II, which was pointing to starboard. The antiflash seals in the scuttles to the powder magazines prevented a much greater calamity, a catastrophic explosion that likely would have destroyed the Iowa. Eleven crew members in the lower magazines, outside the rotating turret substructure, survived the holocaust, which killed forty-seven of their shipmates in the inferno above and within the turret substructure. One of the crew in the magazines of Turret II turned on the sprinkling systems in the magazines, and seven to eight minutes after the powder fire occurred in the turret, Captain Moosally ordered the magazines flooded. The powder and BL&P shells in the right and left guns were not affected and were later removed by fire-fighting teams. Powder for the next salvo had already been placed in the turret's lower revolving structure, according to procedure. Working its way into the lower substructure of the turret, the fire scorched some of these powder bags, and some caught fire. The flooding of the magazines extinguished these fires.
Secondary explosions briefly hampered the fire-fighting efforts, which succeeded in extinguishing the last of the fires within ninety minutes of the initial explosion. The fires and explosions consumed some twenty-five of the forty-five bags of propelling charges stored on the deck in the powder flat, in addition to two or three of the five bags that had been loaded into the center gun. (Note 3)
The force of the deflagration was so intense that it propelled the 2,700-lb BLIP shell some 44 inches into the rifling of the gun, where it stalled because the energy of the burning propellant had exited into the turret {see Figure 7-3). The center gun's rammer chain first began to move backward in its housing some 23 inches and then collapsed from the forces generated by the burning powder bags. The fifteen sections of the rammer were propelled backwards out of the gun and toward the turret officer's booth. The Mk 3 computer was destroyed, the optical rangefinder was wrecked, and much of the turret's interior structure and equipment was damaged or destroyed.
Despite exhaus-tive analysis of the physical evidence and post-accident experimentation through September 1989, the Navy was unable to exactly duplicate the accidental explosion that was believed to have occurred in the Iowa's turret. It is important to emphasize in these trials that the D846 propellant and black powder were also tested to determine their ignition properties. For example, a cigarette lighter required more than nine minutes to ignite the black powder through the quilted patch on the powder bags. The powder grains that make up the powder bags took 2.5-3.5 minutes to ignite, depending on whether they were inside a polyethylene wear-reducing jacket or the plain silk material. It proved impossible to ignite the powder bags by ramming or dropping them from heights of 40 and 100 ft. The powder grains were also insensitive to electromagnetic radiation. Post-incident analyses of the charges in the lowa's powder magazines confirmed that they were in a safe, stable condition.
A primer {similar to a 0.30-caliber blank rifle cartridge), fired from the breech block, normally ignited a bagged propelling charge. The gun was designed to prevent the primer from being fired until the breech block was closed and locked. Each bagged charge had an ignition pad containing black powder sewn onto its base and was quilted to spread the black powder evenly so that there would be virtually instantaneous Within milliseconds ignition. Each bag ignited the next in sequence, that is, it burned from the end much like a cigarette, creating a pressure that would propel the shell out of the gun. A test was done to determine if a preignition of the primer could have initiated the explosion, but preignition was proved to have been unlikely with an open breech block.
Tests were done at Dahlgren in 1989 on a 16-in/50 Mk 7 gun, using five bags of D846 propellant and the techniques employed in the lowas to load and fire these guns. Overrams of the type that had occurred in the lowa's turret were replicated without incident.(Note 4) Systematic tests were conducted: charges were initiated at each bag location, and measurements were taken of the pressures and of the movement of the projectile up the barrel. These tests determined that the point of ignition that caused the projectile travel in the center gun was most likely located between bags 1 and 2 or bags 2 and 3. Finally, the investigators decided to remove the rammer and its chain from the center gun of Turret II and take them to Dahlgren, where they were reassembled in a fixture similar to a 16-in gun. As the rammer and its mechanism were taken apart, each part was closely examined for the slightest evidence of failure; nothing indicating failure was found. In May 1989 a test was done with a timing device similar in size to the lead pouch and placed between bags 2 and 3, and the investigators found that this arrangement closely replicated what had occurred on board the Iowa.