Why did shells fired from Danish frigate explode prematurely? Explosives expert gives his take on the malfunction
In the past week, the Iver Huitfeldt frigate has given new life to the debate on the state of the Danish defence and resulted in the dismissal of former chief of defence Flemming Lentfer.
The frigate engaged hostile drones from the Houthi movement over the Red Sea on 9 March. Since then, OLFI has reported on a faulty radar system on the frigate, which meant that the crew had to defend themselves with autocannons rather than the longer-range ESSMs (Evolved SeaSparrow Missiles).
The problem is that up to half of the shells that the crew fired at the hostile drones exploded immediately after they left the barrel of the frigate’s two OTO Melare 76 mm autocannons.
“During the engagements on Saturday the 9th, up to 50 percent of the 76 mm shells exploded in the vicinity of IVER without any effect on the hostile targets. All shells in the standard war equipment are more than 30 years old,” the commanding officer of Iver Huitfeldt, Sune Lund, writes in a leaked document, according to OLFI.
Expiry date
Chemist, explosives expert, and former army officer Peter Hald works as a safety manager at the Department of Chemistry at Aarhus University. He believes that the cause of the premature detonation probably lies in the radio fuse, which arms and detonates the shell.
“The fact that half of the shells that are fired detonate shortly after they leave the barrel strongly indicates that they’ve exceeded their expiry date,” Peter Hald says.
This stems from the fact that the second half, which did not detonate too early, reached the targets that Iver Huitfeldt engaged. They probably would not have done that if the source of the malfunction was either the radar control of the shell or the tempering.
Tempering is a term used to describe the time setting on a grenade—how long it takes from firing to explosion.
“The primary purpose of a fuse is to prevent the shell from exploding at the wrong time. Its secondary purpose is to trigger the explosion when all the conditions are right,” Peter Hald says.
Incorrect tempering
Premature detonation can be caused by several different sources of malfunction, as fuses can be made in hundreds of different ways. However, it is not possible to obtain information on which type of fuse was used on Iver Huitfeldt. The Danish Ministry of Defence Acquisition and Logistics Organisation’s press officer says that the organisation cannot comment on the subject at this time.
“I don’t know what type of fuse they used, but if it’s for aerial target shooting, then it’s presumably an electronic fuse: either one that is timed when the shell is fired or a fuse with a ‘proximity tube’ that measures the distance to the target and activates when the shell is close enough,” Peter Hald says.
One of the sources of malfunction could be an issue with the electronics in the fuse. This can, for example, cause the tempering to be set incorrectly.
Incorrect tempering can be caused either by errors in the software or by a defective chip in the shell. If the tempering, for example, is set to have a flight time of 0, it will detonate prematurely.
“It (the shell, ed.) has been instructed to detonate, and the moment that the fuses—which are supposed to prevent it from exploding in the tube—are triggered, it detonates,” Peter Hald says.
Old sensors
Since half of Iver Huitfeldt’s shells reached the intended range, it is more likely that the accidental detonations were due to outdated hardware.
“The other possibility is that the radio fuse has become too old, which can result in the radio transmitter in the fuse thinking that it has reached the target before time,” says Peter Hald.
Normally, the shell works by emitting radio signals towards the object in its crosshairs. The shell explodes when its radio signal is reflected back to a receiver with sufficient strength.
“Banana peel”
Peter Hald does not think that the problem was caused by the TNT in the explosive device.
“If it’s stored properly, it’s rarely the explosive that is the problem. My guess is that it’s the mechanics or the electronics, depending on how it’s made,” he says.
Normally, when making ammunition, manufacturers do everything in their power to ensure that they do not explode prematurely, especially during transport, loading, or in the barrel. However, it has happened before. This can result in a broken barrel, i.e. the metal cracks and takes the shape of a banana peel.
“It’s not unheard of for shells to explode in the barrel. If, for example, the ammunition is of poor quality or if the barrel is worn out and cannot withstand the pressure of the propellant charge, there’s a risk of the cannon opening up like a banana peel” Peter Hald says.
Wooden tube full of gunpowder
In the old days, people used a wooden tube that was packed full with black gunpowder, which was inserted into a hole in the shell. When the cannon was fired, the flame from the firing ignited the gunpowder cord, and it burned slowly and steadily.
After it had burned through, the shell exploded. It then had to fit with the shell reaching a specific place. The wooden tube with gunpowder, which was attached to the shell, was then cut into sections to the best of the gunner’s ability.
