Standby for the earlier flight from BOS-IAD which arrives at 10:12am. Head straight out to Ground Transportation curb 2E for the first shuttle to Udvar-Hazy at 10:30am.

Spend a couple of hours at the museum. When you return to Dulles early afternoon, it is a very quiet period for TSA security - probably won't take more than 5 minutes to get through security. Note that when you re-enter the Main Terminal from the Udvar-Hazy shuttle, head straight for the "Dulles Diamond Lane" between baggage carousels 7-8 - there is rarely any wait at all there at this time of day.

Be sure to get the shuttle schedule - links to it in a previous post in this thread.