All right here goes:
Facts we know about Dhanush trials:
1) During development a shell burst in the barrel. It was attributed to old ammunition being about 12 years old and possibly air bubbles in the shell that could have reacted to the shock. However shelf life of M107 shell is 15 years(OFB page on M 107
http://ofbindia.gov.in/products/data/am ... /lc/29.htm ) so should not be an issue. Air bubble could be but how did it pass the many QA steps? Not plausible.
2) During User Exploitation Trials (UET) in May 2017, a shell burst soon after exit from the muzzle.
3) During UET in July 2017, the shell struck the muzzle brake
4) Muzzle strike was encountered early on the SOLTAM upgrade of the 130mm barrel with 155 mm 45 caliber barrel. Dhanush barrel muzzle is larger in diameter than the SOLTAM upgrade barrel. Yet muzzle strike occurred.
Additional facts that could have bearing on the issue:
5) The shell is M107 type which is quite common. Made by OFB
6) The fuze is assumed to be M572 which is also quite common. Made by OFB
7) We do not know the charge that was used nor the range. Was it maximum or minimum range?
8 ) We do not know the debris field. Were the fuzes recovered? What was their condition? Expended or intact? And the barrel condition or borescope measurements?
Any common cause/phenomenon has to explain all these three instances or most of them.
Over the last two weeks I read many reports online and have three theories to postulate that can explain what happened. I will start with the most complex to the simplest.
I) Balloting of shell in gun tube
Initially the shell contacts the tube along the driving band and is held in place. The shell c.g. is forward of the driving band. About 40% as its a conical structure with a weighted nose. There is clearance between the shell and the tube or else it won’t be launched and get stuck. When the charge gets fired there is sudden force behind the driving band which makes the shell hit the sides of the tube. However there is another phenomenon. The shell is spinning along its axis which may be close to the tube axis but is not the same. The hitting the tube sides is due to yaw of the spinning projectile. However it becomes unstable if the shell center of gravity (c.g.) is off center due to manufacturing issues. Even a small offset could grow the instability in the roll axis. Becomes a major concern in heavy projectile with largest charge as that induces most forces. To put thing is simple terms, think of the spinning shell as a spinning top (lattoo). The top spins true at the right velocity. When speed is slow it tilts. And if the top nail is off center also it tilts. Now image the spinning top in a tube. The spinning top will hit the tube sides. That’s what is happening here. And the top is moving along the tube. The forces induced are such that the shell breaks up and could exceed the detonation shock limits of the explosive. This has happened many times in the Great Britain, US and we don’t know who else. When the energy is low it could hit the muzzle brake as it exits. So this phenomenon explains all three instances. So it has to be high charge with an off-set in c.g. of the shell to explode or low charge with offset in c.g. of the shell. Either case off-set shell will cause this problem of muzzle strike or shell burst. For those interested they can google for “yaw motion in projectiles in gun tubes”. Plenty of literature is available.
Side-slap is more prevalent in worn out guns when the clearances are more due to wear. All the above events 1), 2), and 3) have occurred in new barrels and hence side-slap is ruled out.
II Shell Fall Back
Shell fall back is the phenomenon where the shell driving band is not engaged in the tube inner diameter for whatever reason and after the charge is placed behind it falls into the chamber when the gun is elevated. The worst case is minimum charge with gun elevated for high angle fire to reach nearby targets. In case of fall back three things could happen: a) projectile leaves the bore but falls short b) projectile breaks up as it leaves the muzzle, and c) projectile detonates in the bore as it comes in violent contact with the area round the commencement of rifling. So here the phenomenon is for smallest charge and high angle fire. Again explains most of the events 1, 2, & 3. Shell fall –back is cautioned in many artillery manuals. So Indian Artillery and GCF would be aware and so this is less likely.
III Fuze design features
M572 is a long in production fuze and has been in service for a long time. It has two features that are pertinent. The M572 safing mechanism unsafes when the shell spin rate is minimum 2000 rpm and up to 25,000 rpm. The Dhanush muzzle velocity of 885m/sec ensures the M572 safing mechanism un-safes at muzzle exit. It has a wound spring that also has to unwind to arm it. So it is very remote that the M572 can arm the shell in the muzzle. However Janes Ammunition handbook reports that South Africa found M572 some case arms in long tubes like the GC-45 which is the precursor to the 45 caliber genre. If the charge is sufficient to give the shell enough rotation to spin and meet the safing requirement of >2000 rpm and the travel is the tube is long enough to unwind the spring, the shell would become live. This would explain only the 2) where the shell burst soon after muzzle exit. Again it is likely for 2 but will not cause muzzle strikes not shell burst in the barrel.
IV Conclusion
We looked at unusual phenomenon of balloting and shell fall back and the M572 fuze design. In all cases it is the shell and not the gun that could be at fault. And between the shell and the fuze, its more likely it is the shell and the charge being used in the gun. We don’t know what the QA at the shell factory is regarding center of gravity offset measurements but that is a place for improvement for safety of the operators. A further look at the operating procedures with respect to ramming could be useful. ARDE Pune has the capability to perform balloting analysis of the shell and tube systems as evidenced by many papers from them in Defence Science Journal. It would be useful to have them conduct an analysis to confirm the phenomenon. Also would recommend not use the M572 in long guns (>45 caliber) and confine usage to the 39 caliber guns till inventory is over.