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Layovers at Dulles - Any good options?

Travel Info
I have 2 upcoming layovers at Dulles airport in December. One if for 6 hours. One if for 11 hours. Is it worth my while to take a taxi into DC and explore? What would you recommend I see, keeping in mind the travel time must be factored? As well, I'm flying internationally. So, I probably need some extra time because of that. Thanks!

Travel Tips

With security checkpoints and the like - traveling to DC and re-entering security could take at least 3-4 hours.

For your 6 hour layover, I suggest that you stay put or visit the Udar Air & Space Museum. For your 11 hour layover, take a 2-3 hour trip to DC but come back in plenty of time to catch your flight.
Others

D.C. would be about 45 minutes to 1 hour taxi ride away. That would be pretty expensive, but you could do it. There is a cool new Air and Space museum near Dulles, but that would only take you about 2 hours to see that. It's up to you.
Definitely come into DC for the 11 hour layover. To save $100 on the taxi ($50-$60 each way), you can take the 5a Metrobus ($3 each way; http://www.wmata.com/timetables/dc/5a.pd... It goes to L'Enfant Plaza, just a few blocks from the Mall and the Smithsonian. The bus is about an hour, plus waiting time. So figuring two hours total each way, plus getting back to Dulles two hours early, still leaves you five hours in downtown DC. (If you splurge for a taxi, you'll have six to seven hours.)

Take in the museums of your choice, walk around the Mall. About a 10 minute walk north of the Mall is DC's new downtown (called, variously, Gallery Place/Chinatown/Penn Quarter), which has a bunch of great restaurants. Go the the bar at Jaleo (Spanish), Zaytina (Mediteranean), or Oyamel (Mexican) and try a few "small plates."

For the shorter layover, I agree with the earliery answers: Take the shuttle bus to Udvar-Hazy Center (http://www.nasm.si.edu/udvarhazy/)...
Long-time DC resident
Personally, I would skip the trip into DC ... what with security and finding a taxi back and all ... your time frame doesn't leave a lot of leeway.

On the 6 hour layover, you really should make the trip to the Udvar Hazy museum. Well worth the taxi fare, etc.

For the longer layover, why not consider sightseeing in Virginia. We have a lot to see and do. The airport is near Leesburg, an historic town with a good walking tour. Or, if historic towns don't do it for you, head to Reston and see the modern Virginia.