http://www.youtube.com/watch?v=LtZhVElR2Dw
Kids and their air cannons will disprove the planes. Fire a hallow aluminum ballistic projectile into a steel grid. Build your own. See what happens when aluminum collides with steel. Learn how densities affect ballistics and a realistic collision expectation.
5miles per second is what it takes to cut steel.

A little heavier on the physics than I expected, but altogether good information to know!