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Exit flaps are handy for things like radial engines, but the NACA duct loses streamlining if it is not in use. It still creates a high pressure pocket, but can only spill it back over a sharp edge.
In general, internal flow is pernicious and should be avoided as much as possible. On the Voyager flight of 9 days, they kept adjusting the cooling air intake to the minimum, and still reckon it cost them 20% of the overall drag.
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There is no excuse for a land vehicle to weigh more than its average payload.
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