A cruise missile, small--say 10 feet long, making 4000 mph at 1 mile altitude- Would it make a boom loud enough to break windows? The smaller the craft, the less the boom, correct? Probably would be a tough one to answer; no aircraft I'm aware of is that small, making that kind of speed. The X-15 could do Mach 6, but at what, 20 miles up. Any ideas about what the skin of a craft built for low altitudes could be made of? Something made of the same material as the shuttles tiles? At that speed and low, one mile altitude, would the air be ionized and the skin glow? How would it appear from the ground, in other words?