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Can launchers reasonably be equipped with wider payload fairings in order to carry larger (not heavier) payload? What would be the main problems with doing so, air resistance during the early slow part of the launch maybe? Or the balancing of the center of mass of the payload? Or is it just rarely demanded and therefore uneconomical to provide?

For example, everyone (except for the professionals involved it seems officially) get worried when they see animations of the complicated unfolding planned for the James Webb Space Telescope in space. The mirror is 6.5 meters in diameter. What would it take to launch that unfolded? Its 12.2 meter diameter sunshield would be the next step.

This is a follow up on this similar question/answer

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I believe the main problems are aerodynamic.

A wider payload fairing produces more drag, so you'd be able to take less payload mass to orbit; worse, the added drag is above the center of mass of the rocket, which decreases stability; at some point you'd need fins on the base of the rocket to compensate, adding still more drag.

The back end of the payload fairing has to be tapered back to the rocket body diameter to avoid turbulence, so the fairing is going to be longer, thus bigger and heavier, than a narrower equivalent-payload-volume fairing.

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There is a limit to the width of a fairing that can over hang the edge of a booster core.

Stability issues due to aerodynamics will quickly become an issue.

This is an advantage a wide single core large booster like SLS has over a Falcon Heavy style booster.

SLS will be about the same width as the shuttle main tank so 30+ feet wide. An overhang foe that width means a fairly large fairing is possible.

When you look at a CST 100 on top of an Atlas V booster it has fairly large over hang as well.

But even with all that there has been very few payloads demanding bigger fairings.

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