![]() The upper bubble is an ordinary Comet circular single bubble, and the lower bubble is not pressurized. The British military Nimrod is also a very obvious double bubble, but different. That is one very complicated piece of structure. The "problems" on the 747 is where the double bubble part blends gradually into the circular (single bubble) part. Even if the floor on the upper deck also is a cross beam on the forward double bubble part. The mostly circular fuselage on the 747 (especially with front loading door) is in a totally different league concerning outsize cargo. If anyone of the floors were removed the fuselage would break apart when pressurized. It needs the two floors to keep the three bubbles in perfect bubble shape. The A380 triple bubble design makes it a lausy outsize cargo plane. Neither double bubble nor triple bubble designs require extra strength, except that the floors must be designed to take the tension at the bubble junctions. It uses both floors as cross beams - one floor exactly at each junction between the three bubbles. Resisting such deformations requires extra strength which adds extra weight. In other shapes (including the WhaleJet's ovoid), the pressure loads change the shape (not just the size) of the fuselage. In both circular and double-bubble (assuming the floor attaches exactly where the two bubbles meet) designs, the skin is simply in tension where metals are strongest. The crew rest areas in the 777 are a good example of using otherwise unproductive space in a circular cross section. Several other factors affect the cross section, such as the inclination of the "walls" in the passenger cabin (you want them to be as close as possible to vertical in the area adjoining the seats) and the required volumes for systems. The tradeoff is that a non-circular cross section will increase the weight of the fuselage slightly, since a circular section is the ideal shape to resist the pressurization loads. The same goes for the A380, if the fuselage were circular it would be huge with a lot more volume than necessary, with the extra volume in places that are not practical to use. To fit the desired dimensions of the E170 into a circular cross-section would mean a significantly larger frontal area and commesurate increase in drag. The explanation lies in balancing the desired internal dimensions (pax cabin and cargo compartment) against the drag generated by the fuselage. The E170 cross section is a double bubble, and there was no use of old tooling whatsoever. ![]()
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March 2023
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