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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?

ROIMaison
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    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. –  Oct 15 '15 at 09:03
  • I don't think the fact that it is the only option removes the necessity to reduce drag? – ROIMaison Oct 15 '15 at 09:06
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    @Airsick too much drag, and you won't even take off. – Antzi Oct 15 '15 at 09:18
  • My point is that unlike a revenue-earning 747 where fuel consumption and thus operating cost is a high priority, NASA had other priorities here. The carrier aircraft with the Shuttle aboard is going to be pretty draggy anyway, so there's little point in spending research dollars refining drag by 2 or 3 percent on a combination that will only fly half a dozen times a year. –  Oct 15 '15 at 09:35
  • @Antzi An empty space shuttle weighs 75t, Boeing 747's MTOW is 333t and empty weight is 162t. There's still a lot of space for fuel so that you are way below MTOW. My guess is that some added drag is not that much a problem. – yo' Oct 15 '15 at 09:38
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    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. – fooot Oct 15 '15 at 14:43
  • @fooot, that's probably because the An-225 was purposely built to transport the Buran. As such, the wake coming from the Buran was taken into account from the beginning. Therefore it has the large H-tail. As there is no rudder to 'protect' using a boat-tail, there is apparently no need for a boat tail. The 747's were already designed before, and were adapted to carry the Space Shuttle. – ROIMaison Oct 15 '15 at 14:51
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    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. – 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

Dave
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