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Airframes are usually designed for semi-Monocoque construction. The skin carries the primary stresses to keep the fuselage rigid.

enter image description here

https://www.abbottaerospace.com/aa-sb-001/22-aircraft-specific-design-features-and-design-methods/22-12-53-fuselage/22-12-2-monocoque-type

The skin must be continuous, so continuity defects (hatches, windows) are kept small. There are no picture windows or French doors on aircraft. Loads are carried around the smaller defects by reinforcing frames. For a large hatch, the hatch itself can carry structural loads but the hatch dogs and hinges must be designed to transmit loads between fuselage and hatch.

A good illustration of this principle is a cardboard shoe box. Without its lid, the box has little torsional rigidity. With the lid, it is rigid.

The Space Shuttle has a startlingly large defect in its fuselage: the cargo bay doors.

enter image description here

https://tatourian.blog/2020/02/20/space-shuttle-orbiter-structures-thermal-protection-system-tps

Although the FOREWARD FUSELAGE AFT BULKHEAD is a closed member, the “WING CARRY-THROUGH FRAME” clearly is not. Torsional loads on the airframe will need to be carried by the deck of the cargo bay unless the forces are carried through the cargo bay doors and their attachment hardware.

The designers are faced with either

  1. Build the carry-through frame section of the fuselage so rigid (=heavy) that the cargo bay opening does not flex
  2. Design the doors and their attachments adequately to carry torsional airframe loads.

Is the Space Shuttle an open shoe box? Or a closed shoebox?

This issue will need to be addressed by SpaceX as well.

enter image description here

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  • $\begingroup$ Not sure it was an either/or. But leaning towards #2. See nasa.gov/centers/johnson/pdf/… $\endgroup$ Commented Feb 12, 2023 at 18:14
  • $\begingroup$ @OrganicMarble ... thanks for the excellent reference. "The payload bay doors were an integral part of the fuselage structure… the doors provide strength when the fuselage encountered loads from bending, twisting, shear, internal pressure, and thermal gradients … If the latches did not fully engage, the doors could not be relied on to provide strength during re-entry for fuselage bending, torsion, and aerodynamic pressure." This would make the core of an excellent answer. $\endgroup$
    – Woody
    Commented Feb 12, 2023 at 19:05
  • $\begingroup$ Structural design is way out of my wheelhouse. $\endgroup$ Commented Feb 12, 2023 at 19:40
  • $\begingroup$ See also space.stackexchange.com/a/18931/6944 $\endgroup$ Commented Feb 12, 2023 at 20:26
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    $\begingroup$ @OrganicMarble ... that reference to mandatory operation of cargo bay latches now makes sense. When I first saw that, I wasn't sure what problem a partially latched hatch would cause. But the reference to limiting stress if one latch was unsecured makes sense if the hatch carries loads. $\endgroup$
    – Woody
    Commented Feb 12, 2023 at 22:48

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I found a concise answer in Space Shuttle Orbiter Structures & Mechanisms.

From Section D Mid-Fuselage and Payload Bay Doors

Primary body-bending loads are carried by the sill and lower longerons together with the side and lower skin panels. Ascent thrust loading is carried by the sill and lower longerons which transfer this load to the forward fuselage and mid-fuselage skins. The payload bay doors do not carry body-bending loads or thrust loads (fore/aft loads). However, when closed and latched, the payload bay doors act together with the mid-fuselage to form a closed torque box to carry body torsional loads.

(emphasis mine)

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