No. All of the Smithsonians in DC are free.
No, during the week off peak time(September through February) no fee no pass needed. On weekend you may need a pass but no fee.
No, but check online before attending. I believe on busy days they require you to get a (free) ticket online the day before, so as to control the crowds a little.
Nope. They do accept donations, but it's only a suggestion.
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