Before everything else, it is not because the thread is notified in category "game physics" that I'm repporting a physic-due bug; this is not a physic bug, but a glitch that the barrel recoil is hurting the ground... and that happen if you fire vertically.
So, in conclusion, artillery equiped tanks are flying when your barrel is adjusted vertically.
Knowing the power of these barrel recoil, I think the tank should either be reversed to the groung, or its barrel should implode due to the firepower. Therefore I'm quite surprised (in the positive sense of the therm) to see that the recoil of the barrel is affected/supported by the physic when hurting the ground... and making your tank flying high, but the problem is why it hits the ground. The physic is logically reacting to this, which I agree, but in a too sensitive way.