It's free all you have to do is que up but groups I think need to purchase tickets if you want a guided tour, singularly I never payed as it is a Smithsonian
Tickets for time slots are acquired on the Internet. However if you're a veteran no ticket is needed. Each veteran is allowed one guest.
I just left there! Go online...way in advance to going. Reserve your free tickets...they have timed entries. Guided tours start every hour or so, but you are free to roam about.
Thanks! Your answer is awaiting moderation.