While doing some ASME code calculations for a pressure vessel subject to external pressure I was wondering what'd be the optimum L/D ratio for a vessel that is subject to buckling under external pressure. For a given volume & Pressure. Could a general L/D ratio be derived or would it change with scale & imposed pressure. It's hard to use conventional optimization since a lot of the buckling design seems a mix of tables, formulae & charts. In general to prevent buckling larger wall thickness helps but on the other hand both small L & D are good. Wondering if L or D dominates. i.e. a long vessel or a shorter stouter vessel. Any idea? Of course, this is assuming stiffing rings aren't an option.