The 20 foot diameter would only be relevant were an aircraft solid. If it were then the laws of physics dictate that it would punch a hole roughly consistent with its own dimensions, provided the force and mass were greater than the static object. A plane however is little more than a hollow tube, and one in which a compartively weak and combustible metal features prominently in its design.
As i understand it a plane flying laterally into a heavily reinforced building, and at a considerably lower speed than the WTC aircraft will involve a different set of dynamics, which means American 77 can't really be compared with American 11 and United 175
A normal aircraft lands at about 115 - 130 mph (I believe?), even if they accelerated once they'd levelled up its difficult for me to think they were doing much more than 200mph, and probably accounts for why they failed to penertrate, much beyond the third wall of the Pentagon. In order to lose the altitude they did, they had to perform a pretty complicated manouevre which would have meant losing a lot of air speed in line with controlling the crude figure of 8 descent they performed. Their mass was obviously the same as the other planes, but with their force likely to be between 33% and 50% of those involved in the WTC attacks, the plane was never likely to be as destructive. Essentially they were attacking a stronger target, with at least half the destructive effectiveness. Given time lapses involved with speed differentials (fractions to the naked eye, but significant in physics) another thing regarding momentum and energy would seemingly apply. It has a scientific name I believe (but I don't know it).
Just draw yourself, or imagine a cross section of an aircraft cabin. Put in the floors, seasts, backage hold, anything you like. What you have is a low density, and low mass, concentration surrounded by a thin outer ligh tmetal shell of a fuselage. On impact, all the destructive energy is transferred to the nose. The nose is obviously a smaller area than the fuselage and thus able to concentrate that energy more destrutively on a smaller local area. From the moment of impact, the aircraft is slowing up (losing energy). Not only is this happening, but as the nose cone disintegrates the destructive force is being spread over a wider, and less dense area. Without any substance inside the cabin (its largely hollow) the increasingly degraded energy is transferred to the outer fuselage. Instead of having a focal point, the energy would be transferred to the shell of a tube (a toilet roll). Once the velocity dips below a critical figure in line with the plane slowing up with impact, a point will be reached where the toilet roll no longer has the required force and mass to generate a puncture consistent with its own dimensions. In other words the thin shell will effectively peel back, and the momentum generated by the initial incursion will act as the hole through which the remaining energy would be transferred. In other words the planes shell just breaks up when it can no longer destroy the mass of the building, but continues to move forward with momentum. 'The immovable object proved stronger than the irrestible force'. however the forward momentum is continued but now seeks out the point of least resistance. In this case the initial penertraion provided by the nose cone impact
The only possible area of a plane that might have had sufficient mass to punch its own hole after the initial point of impact, would have been the engines. The wings are essentially a brittle item and would snap off, and conceivable fold back at a lower speed and thus also follow the line of momentum. In doing so of course they would also release gallons of aviation fuel, a lot of which would have been spent within fractions of seconds. The 'splash' effect of the vapours would have presented a visibly impressive air burst fireball, but it terms of destructive capacity this would have been pretty short lived, and almost certainly looked more destructive than it was. Although the fires didn't burn with anything like the same intensity as the WTC's, (suggesting to me that a lot of the fuel was indeed lost in a flash over and in seconds). they did burn for about 24 hours, and then with a red glow rather than a conventional fuel burst orange/ yellow. This is pecuiliar, but could also have been molten metal? associated with aluminium
The reinforced Pentagon was obviously built to a much higher specification than the WTC. Unlike the WTC the use of class was minimal and even then it was used, it was the best available to modren science. With the shock being absorbed across a series of walls it was always likely to present a much more robust target, and better able to withstand any such impact beyond the immediate point of contact. The fire management systems weren't wiped out either, unlike the WTC, although I believe they were rendered pretty useless at the point of impact?
Lets not forget also that Reagan International Airport is about a mile away from the Pentagon. Many of the eyewitnesses routinely see aircraft taking off and landing on a daily basis, and in DC this will be every few minutes. Indeed some can even recount the livery of the airline and identified it as AA, and were able to name the make and model of aircraft. Crucially they're also used to seeing aircraft at take off and landing speeds. An anti aircraft missile such as the Patriot for instance travels at between Mach 5 and 6. I'm not so sure what an anti ship cruise missile travels at, they vary? Alright I'm lying a bit, even the 'Iranian Sunburn missile' travels at Mach 2 (yes I said Iranian Suny - its not widely known about!!!) I think Excoet is similar if not faster? In any event, they would not only have had to mistake something that would have generated a sonic boom, (which would have been heard and recorded quite widely), the DC area isn't short of science based industries and academic facilities. They'd also have to confuse it with something travelling at the very least, 7 times faster than any aircraft they'd have seen over the skies of the capital.
Also there's the addtional issue of the cellphone calls made from flight 77, most famously of course by Barbara Olson (a neo con media personality who was allegedly due to have a book published 2 days later about corruption in the Clinton White House, by way of an aside
). Although in the confusion of 'Clear the Skies', it is possible that planes might have turned up at secret bases, as they appeared to be landing anywhere in the NORAD command area, the theory still has to account for the missing people. Ground radar would still have been able to track them, but 7 were reported as "unaccounted for". We now know that one of these was an incoming Korean 747 who were operating on the wrong radio frequency. They were forced down over Anchorage or similar. Conspiracy theorists might run with the idea that the 4 missing planes, were amongst these 6 left, and they were in the process of being flown off to secret locations, if the radio controlled "phantoms" are supposed to have replaced them for attack purposes. Personally, I think this is hysterical nonesense, and a completely unnecessary over elaboration.
I'm surprised how much attention this missile strike theory gets to be honest. To me it exhibits a lot of the classic symptons of permissable red herrings, designed to misdirect critical investigation, and discredit advocates.
Having said that, when in doubt I just fall back on something else and that usually reminds me <_<