DC Metro doesn't go out this far, so you'll have to drive. it's not that far from the Dulles Airport & easy to get to. you pay to park. there is no entry fee for the museum itself. wear comfortable walking shoes.
if you go to the observation tower, you can use an app to know what flights you are seeing landing at Dulles.
If you own or manage Smithsonian National Air and Space Museum Steven F. Udvar-Hazy Center, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.