|Quoting Tdscanuck (Reply 7):|
Different loading. A dome is in pure tension (away from the rim), a flat bulkhead is in bending. That's the main reason the flat bulkhead is heavier.
I think the bending issue is the main reason flat bulkheads are far more robust structures compared with a pressure dome. Applying bending loads means you have to deal with compressive loads as well as tensile loads.
Tensile loads are a stable configuration, and are relatively easy to design for. The problem with bending is the compressive part of the loading. You have the additional scenario of buckling and instability to deal with, which is trickier to design for, and often requires additional material mass to ensure stability.