Earlier today I was reading Battle for the Abyss (one of the Warhammer 40,000 novels) and a battle was described between two spaceships where the villains used torpedoes described as "Unguided and slow, but still dangerous because they were tipped with drill heads" so that they could bore through the target's hull. Now, neglecting the parts of the story that are obviously outside of the realm of reality (ie spaceship battles), would such a weapon actually be of any use? I assume that how it works is that the torpedo reaches the surface of the target and then drills in. But that causes a problem: wouldn't the torpedo's body just start spinning around with the drill bit stuck in the target? In order for it to drill in, it doesn't just need force behind it, but also needs force holding the body in place so that the torque of the drill's motor spins the drill bit rather than the torpedo's body. But this is in space, so it doesn't have that, and it would just harmlessly spin itself around while sticking to its target's armor because equal and opposite reaction would mean that the torque of the armor on the drill bit is equal to the drill's motor's torque. Or, am I just thinking about this the wrong way, and enough force behind the drill bit while it spins (from the torpedo's motor) will still make it able to bore into the target? Alternatively, the designers of this exotic weapon could make it so that the missile penetrates the target by way of its kinetic energy (neglecting the point that if you're able to do that you likely don't need the drill bit) and then starts drilling. But wouldn't you then run into the same problem? Instead of spinning around harmlessly outside the target, it now just starts spinning inside the hole it made. tl;dr: Would putting a drill bit at the tip of a missile confer any benefit to its ability to penetrate its target?