Some of the 'controversy' around the identification of the aircraft that hit the Pentagon on 9—11 has been caused by the engine parts that appear in the wreckage photos.  The assertion has been made that an aircraft other than a Boeing 757 was involved, because of the relatively small diameter of the turbine disk shown (on the order of 2—3 feet), when compared to the larger diameter of the fan disk on a 757 (on the order of 6 feet).  Thus, some claim that this 'proves' that the aircraft that hit the Pentagon was an older aircraft, one that used smaller—diameter jet engines. The "documentary" film Loose Change makes the case that " a certain flywheel observed at the site seemed too small to have been part of the aircraft's engine turbine."

This is an invalid conclusion, drawn largely through lack of understanding of the relationship between the basic types of jet engines, and how their components would behave in a 600—mph impact against a massive barrier.

Modern jet airliners almost exclusively use turbo—fan jet engines.  To understand what a turbo—fan is, it is best to first understand the configurations of earlier jet engines, which were called turbo—jets.  (I have hyphenated the two terms for emphasis, but from here on in I'll just write them as 'turbofan' and 'turbojet', as they are commonly written.)

A turbojet is basically a tube with a compressor turbine at the front, a combustion chamber in the middle, and an exhaust turbine at the rear.  The front and rear turbines are connected via a long shaft.  The turbine components can occur in various numbers depending on the design of the specific engine. 

This is the classic design that was defined in the first jet engines built nearly 70 years ago.  It simply burns fuel to produce hot gases that turn the exhaust turbine, which then turns the front compressor that forces fresh air into the combustion section.  Of course there is no sequence like I have used to explain it: the whole process is continuous, and will settle into equilibrium at a certain amount of thrust being generated by the hot gases coming out the back, depending on the amount of fuel made available by the throttle setting.

The turbofan is a refinement of the turbojet that is designed to make it more efficient.  The basic assembly, usually call the 'core', is the same tube with the same components.  At the very front end of the core, however, the turbofan engine has yet another turbine mounted on the central shaft, called a 'fan'.  The fan is larger in diameter than the front inlet of the core, and therefore also larger than the compressor turbine found there.

The large fan turbine is essentially a very high speed propeller.  In addition to pushing air into the core, it also moves a large volume of air around the outside of the engine, generating a great amount of additional thrust.  For reasons that are beyond this discussion, this is a more efficient strategy for developing thrust than putting all of the air through the core.  It also has the secondary benefit of surrounding the hot jet of gases from the core with a tube of cooler air pushed back from the fan, which reduces the exhaust noise.

Now this is all very interesting (to me anyway) but the relationship of the turbojet to the turbofan has a consequence that is very relevant to the 9—11 conspiracy theories.  Bottom line: inside every turbofan engine, there is a turbojet engine!

That turbojet engine (aka the core) not only has all the traditional components of a classic turbojet, but it is much smaller in diameter (in modern designs) than the outer diameter of the fan.  What visually appears to be the 'outside' of the engine is actually the large hollow duct (or shroud) that channels the thrust of the fan for even more efficiency.  That is the reason older jets have slim—looking engines (like an A—3 Skywarrior does!), but newer jets have fatter—looking engines.  Note that early turbofans were still fairly thin, because they didn't have a fan that was very much bigger than the core.  Over time, fans have gotten bigger and bigger, as 'high—bypass' designs have come to dominate, and engines have gotten fatter and fatter as a result.

That's why finding some things that look remarkably like parts of a turbojet at a modern crash site really isn't too surprising.  They are parts of a turbojet — the ones that comprise the core of the aircraft's turbofan engines!

But, what happened to those big fans and shrouds?  Well, the large fans are less resilient than the turbines in the core in many significant ways.  They operate in cool air, so they don't need to be so incredibly heat—resistant. 
And their blades are much longer and proportionally thinner than those in the core, so they have less inherent strength.  The same goes for the shroud that channels the fan's thrust.  It is essentially streamlining sheet metal that contributes little to the structural integrity of the engine.  In order to service the 'engine' (that is, the core) large parts of the shroud can usually be removed by opening some latches or fasteners, and swung out of the way or taken off altogether.

So, in a violent crash, the fan and shroud could be expected to be consumed to a much greater degree by impact and fire than the core of the engine. 
That engine core is designed to operate in conditions that are not unlike a continuous 'crash' — extreme heat, velocity and pressure, while the fan and shroud are designed to operate in the same atmosphere we live in.

I am sure some non—core parts were recovered on 9—11 at the Pentagon, but I haven't checked all the photos, because that's not my point.  I do not claim that this proves a 757 crashed there.  But what appear to be the remains of a turbojet engine in the wreckage also don't prove that one did not.