Correct me if I am wrong, but I thought Tsar Bomba was the largest nuke ever tested, not the largest nuke to exist(raher, existed, tehre was only one, shame that they never built the original 100 mt one...)
You might be right, as I'm no expert in the field. I wiki-ed nuclear weapons yields to find a figure. In general nuclear bombs only get into the double digit MTs, don't they?
It doesn't really that much. Whether it's Little Boy, Tsar Bomba, or a nova cannon blast, you still don't want to be at ground zero.
Stop the planets rotation?
You'd have to attach the nova cannon to the planet at a 90 degree angle for that, and the force isn't enough anyway. Since Ordinatus Armageddon clearly isn't attached to a planet, as it in fact has tracks, firing a nova cannon would rather send it hurtling through the air like a toy car in a cyclone.
I was thinking of the recoil compensators you'd need to build into the Ordinatus. The nova cannon fires an actual shell - unlike laser weapons, there is a kinetic impact. Apparently it can be fired on a several mile long spaceship without sending the ship flying backwards, but those ships are very large and have considerable forward thrust to counterbalance. (One of those links mentioned attack speed for a Chaos cruiser being around 75% of lightspeed, I think.)
In an Ordinatus, I don't think you could fire a nova cannon without sending the structure flying. A lance, sure, since it's a recoilless laser, but not a nova cannon. Too big.
Perhaps the weapon in the Ordinatus is a smaller version? There are mentions of mass drivers and the like being mounted in ordinary weapons batteries, which would be an appropriate place for a sort of scaled down nova cannon. That way it could still be a starship-grade, nova cannon-esque weapon but a little more sane in scope.