First, weight. NASA engineer Mark Moore says that, as a rule, conventional aircraft grow roughly three pounds heavier for every extra pound of payload they're supposed to carry, whereas, "VTOL aircraft grow about five to six pounds heavier for every extra pound of weight." Now picture a VTOL aircraft that happens to be armored and packing four soldiers and their equipment. The required compact, rotorless propulsion system means, Moore says, a "hurricane-speed flow field" of rocks and debris upon takeoff. So much for stealth. And all of this likely would do little to increase security. Transformers may be able to leap over IEDs, but insurgents can simply aim RPGs at the wings, which on one proposed Transformer design are laden with fuel tanks.