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During the testing phase as well as during transport of the Space Shuttle on top of a 747, a boat tail was used:

enter image description here

Source

Here I found that this was mainly to reduce the turbulence that would strike the tail of the 747, thus increasing yaw authority.

However, I can also imagine that the boat tail would reduce the base drag of the Space Shuttle, thereby reducing the fuel penalty of carrying the Space shuttle.

Is there any research that shows that the boat tail also reduced the drag, rather than just providing a cleaner flow to the vertical tail?

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    $\begingroup$ Qualitatively, a reduction in turbulence will probably lead to a reduction in drag, and thus a reduction in fuel consumption. However, this was NASA's only option to transport the Shuttle so I doubt much attention was paid to efficiency. $\endgroup$ – user11516 Oct 15 '15 at 9:03
  • $\begingroup$ I don't think the fact that it is the only option removes the necessity to reduce drag? $\endgroup$ – ROIMaison Oct 15 '15 at 9:06
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    $\begingroup$ @Airsick too much drag, and you won't even take off. $\endgroup$ – Antzi Oct 15 '15 at 9:18
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    $\begingroup$ It's interesting to note that photos of the Buran on the An-225, which has an H tail, don't show any boat tail on the Buran. $\endgroup$ – fooot Oct 15 '15 at 14:43
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    $\begingroup$ Yes, and there are certainly other factors to consider. Just pointing out that if there were a drag reduction reason, the Buran may have used one. $\endgroup$ – fooot Oct 15 '15 at 15:11
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I have always been under the impression that it was mainly installed to protect the engines. However this blurb (and cool photo) claim it did reduce drag.

...reduces both drag and turbulence created by the piggy-backing spacecraft while also protecting the engines

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