Originally Posted by
mickjoebill
May I imagine a seamless Carbon Fibre tube being extruded or spun...
How much weight would that concept save?
You can't "extrude" CFRP. You can "pultrude" it (as for carbon arrowshafts) but that puts the fibres parallel to the long axis, and the principle stress in a pressurised fuselage is the hoop-stress from the pressure. The fibres in a pultruded fuselage run the wrong way to react this, so the tubve would be weak for its weight.
I'm not sure what you mean by "spin", but you could lay fibre onto a former in the way they do for glass/carbon pressure vessels (used in chemical engineering). It's an expensive process as essentially each fibre is individually placed, and you'd need an autoclave large enough to take an entire fuselage to cure it off. But the main problem with this kind of structure is that you'd need to add subframe assemblies to mount things like wings, tails, engines, floors etc, and these don't integrate well with that kind of structure. Working out how to do doors, windows and access panels is a bit of a challenge as well.
The other option would be to use dry-woven tube (aka "sock") which could be laid over a former and tensioned to shape. I suppose you could also put it in a female mould and inflate a balloon inside it to create the shape, but that sounds like a very fiddley process to get right while the resin is curing. This is how carbon tent poles are made. The snag is that the actual fibre orientation is a function of the local curvature of the shape rather than being a specific angle determined to suit a stress requirement. You'd also have no fibres running axially down the tube, so it would be poor at taking bending stresses. Again, doing doors, windows, hatches amd mounting points would be a real challenge, so it would probably need longerons and some kind of hoop-frames to attach things to.
Carbon is a high-strength material, but only in tension. That means that you would end up making thin sections (because you don't need them any thicker to take the loads - that's where the weight-saving comes from), and these thin sections would need to have stringers or other supporting elements to give them the stiffness to take compression loads without buckling. So what you end up with is a complex structure - just as we do with metal, wood or any other material. I'm afraid there aren't any short cuts!
PDR