The Double Bubble Comment Rescue/Response

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Oscar Gordon

A Navy Turbine Tech who learned to spin wrenches on old cars, Oscar has since been trained as an Engineer & Software Developer & now writes tools for other engineers. When not in his shop or at work, he can be found spending time with his family, gardening, hiking, kayaking, gaming, or whatever strikes his fancy & fits in the budget.

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27 Responses

  1. Awesome. Now we know what the hull it’s all about.Report

    • Avatar Oscar Gordon says:

      Man, I try to write a nice, high quality post, and within 2 hours it’s pun’d, and probably ain’t worth a shilling now.Report

      • Avatar Kolohe says:

        A lot to chew on with this double bubble post.Report

      • Seriously, it was great. I don’t quite see the overlapping vectors canceling out, though. Are you starting by assuming a vertical wall at the overlap and then showing that would be unnecessary?Report

        • Avatar Oscar Gordon says:

          Well, it’s pressure, not force, that is what is key here. The vectors are more to help visualize what is going on than it is an true representation. Pressure works uniformly in all directions, so inside the tank wall the vectors don’t cancel so much as they don’t exist.

          Truly the reason they don’t use an ellipse is mostly the windows and seating. If it was a straight up question of structure, an ellipse would be fine.

          PS Thanks. I’m glossing over a lot here, but I hope the trade-offs at play are more obvious.Report

    • Avatar LeeEsq says:

      You put your right foot in, you put your left foot out. You do the hokey pokey and that’s what its hull about.Report

  2. Avatar Anne says:

    Yes thanks Oscar great explanation for the areonautically challengedReport

  3. Avatar Brent F says:

    Articles like this often don’t get huge commentary on them because there is typically isn’t much for a commentariat to say in response. But don’t let that lack lead you to believe this sort of thing isn’t read and appreciated on its merits.

    The best thing the blogosphere can offer is things written by experts for the educated layman. We could all use much more of that around in every topic. Its like what Vox proported to try to do but couldn’t, because Vox is written by the same class of non-expert journalist type that already wrote for newspapers, not by people actually knowledgeable enough to really explain something on a particular topic.Report

  4. Avatar Kolohe says:

    My question is: just making a single bigger cylinder with a bigger diameter (whether circular or elliptical) is a volume vs surface area issue, correct?

    Or is the biggest advantage maximizing the diameter of the centerline with less ‘wasted’ space?Report

    • Avatar Oscar Gordon says:

      It’s all about maximizing centerline, since that is were the people sit. If I can nearly double the passenger capacity without having to double wing and engine sizing, I potentially have a huge market advantage.

      The importance of how much passenger windows drives design can not be overstated. If It was all about packing bodies in the hull, we could extend environmental controls to the lower cargo deck and pack a bunch of passengers in there as well.Report

      • Avatar Kolohe says:

        Frankly, I think the dogs that travel in the cargo hold have it better than the people that travel in economy these days.Report

        • Avatar Oscar Gordon says:

          Cargo holds are noisy, and not well heated, but if you got a good set of headphones and a warm coat…Report

          • Avatar Kolohe says:

            once took a Mac flight on the old C-141’s. (Travis to Mcguire, or maybe Travis to Dover) There was a baby next to me that cried the whole time, but I didn’t hear a thing thanks to the ambient noise and earplugs.Report

    • Avatar DensityDuck says:

      “is the biggest advantage maximizing the diameter of the centerline with less ‘wasted’ space?”

      Yeah, that’s it. Cargo is more often wide and flat than it is square, so you care less about overall cross-section area than you do about the maximum width (or height) that can fit inside the aircraft.Report

  5. Avatar notme says:

    Interesting, I knew this had been used in subs but didn’t know about its use in aircraft.Report

    • Avatar Oscar Gordon says:

      Yes, same concept, but reverse direction. Airliners are higher internal pressure, so the hull is in tension. Subs have higher external pressure, putting the hull under compression. The structural design is different between the two, but both are taking advantage of the environmental stress to help stiffen the structure.

      Also, notice how subs don’t have any fecking windows. Seriously, windows drive structures guys nuts.Report

  6. Avatar Jaybird says:

    This post was really awesome, I presume. I wish I was thinking something other than “me dum” while reading it so I might be able to know for sure.Report

  7. Avatar Oscar Gordon says:

    Oh! One final thing that affects all of this – regulations!

    The FAA does not tell aerospace companies how to build airplanes. Boeing can slap together whatever design it thinks will fly and sell well, but it has to meet the FAA regs, which are all about keeping the airplane flying in one piece and getting passengers & crew safely to the ground. Designs that are uncommon are not only challenging for the engineers designing them, but also challenging for FAA regulators to evaluate. Ergo, uncommon designs have an additional expense of being harder to get certified.

    Everyone and their dog in the aerospace business knows how to evaluate a cylindrical hull such that it will meet FAA regs. Same with elliptical hulls, and even Double Bubbles (since they’ve been done in the past in vertical arrangements). Something like the stretched cylinder, however, is very uncommon and will present a challenge getting it certified. So even if the structure was easy to evaluate, the cost of the regulatory process might bust the design before it ever got off the drawing board.

    I saw lots of really cool airliner designs discarded for this exact reason. Boeing and Airbus are nothing if not conservative when it comes to design. That conservativeness is actually one of the reasons the 787 did not do as well as it should have.Report

  8. Avatar DensityDuck says:

    I think the issue is more subtle than corners causing a stress concentration. As you point out, you can smooth out the curve and avoid that issue. It’s a question of the skin being in bending rather than pure tension.

    A sphere, when pressurized inside, is put in tension all over. The same thing happens for an oval or for an ellipse; while those shapes have variation in the state of tension due to the changes in curvature, it’s still all-out, only stretching the skin.

    The problem with the double-bubble arrangement is that the reentrant-curve “pucker” between the circles is going to want to open up–the pressure will try to flatten it out. This puts the interior surface at that location into compression, rather than tension; so now you need to worry about your ribs buckling (something that doesn’t happen with non-reentrant curves) as well as the fact that you have a larger stress gradient between outside and inside (which has implications for fatigue life).

    Boeing’s Blended Wing Body concept is using an elliptical pressure vessel.Report

    • Avatar Oscar Gordon says:

      Exactly, and one reason the horizontal arrangement hasn’t really been done (trust me, I know how much those wings can flex, and I don’t care how stiff the wing box is, flexing wings will cause a body to flex the same way). I think being able to build more with materials like carbon fiber alleviates some of those concerns, especially fatigue (assuming the engineers design structural components like they are made out of carbon fiber, rather than treating it like black aluminum, which is one reason a certain high profile design is struggling with weight issues).Report

    • Avatar Oscar Gordon says:

      Boeing’s Blended Wing Body concept is using an elliptical pressure vessel.

      And I can’t tell you how anxious people are about the windows, or rather, that if it doesn’t have enough windows, no one will want to fly in it, and no, you can’t actually let people sit out near the wing tips so they can have a window seat.Report