The Fence of Incredulity

Sometimes, out of boredom, I trawl the depths of Reddit for entertainment. There are a few gaming-related subs that I visit on these trips, such as /r/WorldofTanks, /r/Eve, and /r/citiesskylines. Beyond these Reddit subs, I don’t venture too far as I have no idea what dark corners of humanity bring me in the void of Reddit.

In this game, World of Tanks, you play as an arcade version of a tank crew in various tanks – so it is basically pixel tanks shooting other pixel tanks.

However, now and then, a nugget of brilliant comedy gold shines through and blinds readers (aka Redditors) into submission. This post at /r/WorldofTanks, was called [OT] 4chan determines the velocity necessary for a walrus to penetrate a T-72M’s upper glacis.

2015-08-01a
Click to enlarge!

4Chan is another user board/forum on the Internet, but I would like to provide the warning that you should not go there. I find Reddit safer (at least these specific gaming-related ones) and interesting to read.

In all earnestness, a reply made on Reddit itself paved the way for this 4Chan post into the Halls of Legends.

 [–]Cidious[NARWL](Scrubwhisperer)Sabot_Noir 220 points 4 days ago*

So obviously OP didn’t account for Walrus normalization and basically set up the Walrus to impact at 0 degrees from normal which is fine I guess…

However, I am more disappointed with the failure of OP to calculate the yield deflection of the steel (you use the elastic modulus which is about 200GPA or the elongation at break which is 22%) and further OP failed to appreciate the consequence of this failure on their calculations. I’m using SAE 4340 which has very similar properties to MIL-DTL-46177 RHA the current US army standard. At 22% elongation the metal would be deflected into the tank by a distance of 1.05m if we apply that elongation along the armor plate’s short axis.

Additionally OP Failed to account that the metal under elastic deformation (read as prior to reaching the yield stress) acts like a spring with the internal stresses and thus also the force opposing the Walrus increasing in an approximately linear fashion. This means that OP has overestimated the required energy by a factor of 2 no matter whether we accept the other assumptions. Not to mention that since we only expect to see the yield stress at the plastic deformation threshold we have no bearing on weather the yield stress would even be reached by a deformation of .5m. From above we may conclude that 0.5 meters would only bring the plate half way to deformation and thus OP has further overestimated the energy required by another factor of 2.

I’m giving OP a C on this assignment because I want them to understand that they can do better. I will however allow OP to come to my office DURING OFFICE HOURS, to discuss some extra work which would demonstrate that OP has a more complete understanding of the principles at work here (do another problem right and I’ll make the grade a solid B).

That said I want to commend OP on attempting a Graduate level problem as an undergrad; the attempt to apply statics principles to a highly dynamic situation is as heroic as it is whimsical. For a more realistic evaluation we would have to consider the heat generated by dynamic strain and how that could lead to thermal softening which would concentrate the deformation into shear bands leading to cracking and failure.

We could also get a more accurate picture by representing the Walrus as a hyperbolic distributed load instead of a point force since the Walrus is large and mostly liquid upon impact.

Finally we don’t actually have to assume the Walrus is combat reinforced if we instead are willing to consider the effect of extreme sheer on the liquefied Walrus and how it would restrict the dissipation of kinetic energy and create a thermal energy hazard to the crew were any of the walrus were to enter the crew compartment through even a small hole in the armor.

Edit: TL-DR A sonic Walrus impacting normal to the UFP of a T-72M would likely result in a crew kill by penetration and infiltration of the tank with superheated walrus gasses.

Never, I tell you, never have I laughed so much I snorted coffee out of my nostrils.