Paul Hill, a NASA aeronautical engineer, spent years quietly researching the AAV phenomenon, and his book, Unconventional Flying Objects: A Scientific Analysis, was published after his death. This page summarizes his findings with regard to the notable absence of sonic booms in supersonic AAV reports:
“In an effort to examine the force-field propulsion hypothesis yet further, Hill analyzed a number of cases involving near-field interactions with an apparent craft in which some form of force was in evidence. These include examples in which a person or vehicle was affected, tree branches were parted or broken, roof tiles were dislodged, objects were deflected and ground or water were disturbed. Under close analysis the subtleties of these interactions combine to point unequivocally to a repulsive force field surrounding the craft, while discriminating against propulsion mechanisms involving jet action, pure electric or magnetic effects, or the emission of energetic particles or radiation (although the latter may accompany the propulsive mechanism as a secondary effect). Further detailed investigation indicates that the particular form of force field propulsion that satisfied observational constraints is what Hill labels a directed acceleration field; that is, a field that is, in general, gravitational-like in nature, and, in particular, gravity-canceling.3 Such a field acts on all masses in its sphere of influence as does a gravitational field. Corollary to this conclusion is that observed accelerations ~100g relative to the environment could be sustained without on-board high-g forces.
One of the consequences of the above identification of field propulsion type by Hill is his conclusion, supported by detailed calculation, computer simulation and wind-tunnel studies, that supersonic flight through the atmosphere without sonic booms is easily engineered. Manipulation of the acceleration-type force field would, even at supersonic speeds, result in a constant-pressure, compression-free zone without shockwave in which the vehicle is surrounded by a subsonic flow-pattern of streamlines, and subsonic velocity ratios. An additional benefit of such field control is that drops of moisture, rain, dust, insects, or other low-velocity objects would follow streamline paths around the craft rather than impact it.”
UFO physics, UFO propulsion, technology
I assume that AAVs don't start fires when they crash because they're not actually hot - there are number of possible explanations for why they often glow. For example, our atmosphere is mostly nitrogen, and nitrogen glows when stimulated with a high-frequency electrical field.
