The museum displays a Titan
II Mark 6 reentry vehicle. The Air
Force Museum displays such a reentry vehicle atop their Titan II.
The U.S. Centennial of Flight
Reentry Vehicles page has a bit of information on the development of
reentry vehicles for ICBMs, including a bit on the Mark 6. Apparently, the
"phenolic nylon (a nylon cloth impregnated with resin)" nose cap was about two
inches thick, while the "special plastic" on the rest of the vehicle was only
about 1/4 inch thick.