# Minimal width of a carbon fiber shell to hold a flying airship

In Aves Æternæ: Building Perpetual Flying Machines you can find the specs for an airship with a rigid carbon fiber shell. It would be around 4m long and 2m wide and high, weigh around 10 kg empty (0 kg filled with hydrogen); and the shell would be built of carbon fiber panels of around 250 g/m2, which is not a lot.

For commercial grade carbon fiber the tensile strength seems to be around 5 GPa and elastic modulus of 200 GPa, according to wikipedia, and seems to be corroborated by commercial sources.

Would such a structure be able to sustain itself, empty? Would it crush, assuming it is filled with hydrogen and therefore holding no weight? How can I compute the max wind speed that it could withstand? I have a background in Physics. I have read a bit about buckling but cannot figure out how the Euler equation (or any other really) apply here. Thanks in advance!

• Perhaps the hint is in the title: "perpetual". Mar 16 at 16:02
• "Perpetual" since it does not need any external sources of energy: runs with solar and wind. NASA calls such a concept "long endurance" which is more accurate: months or even years in the air. Mar 16 at 18:53
• will the single bag be glued hydrogen-tight to the hull at some locations? I'm asking this because you'd need a spar to connect both wings, therefore two holes in the bag, or have two bags.
– user47135
Mar 16 at 20:36
• If need be, yes. I was assuming that for this size the wings can be just attached to the hull, unless I have reason (or even better calculations) to suggest that the wings need further support. Mar 16 at 23:45

... assuming it is filled with hydrogen and therefore holding no weight?

1. Hydrogen is lighter than air but it isn't weightless.
2. The force on the shell won't be related to density but will be related to the pressure differential, ΔP, between the outside and inside. By filling to atmospheric pressure the ΔP will be zero.

The problem I see is flushing out the air. With an inflatable you can squash it flat and then fill with hydrogen. With a rigid that's not an option - unless you but a bag inside and a fill port at each end. Open the hydrogen end valve, fill the air-end with air to squash all the air out of the hydrogen end. Then vent the air end and fill the hydrogen end (with hydrogen). Minimum waste. The bag should probably be attached on a ring half-way along the shell.

• A bag is the proposed solution, PVA has minimal leakage and very low weight (40 micron thickness). So carbon fiber should be viable then? Mar 16 at 15:02
• I haven't a clue about carbon fiber and the article link doesn't work. I imagine the challenge is how to evenly support any load. In an inflated balloon you would have some rigidity due to inflation pressure forcing the shape into a circular cross section (maximum area for a given circumference). Running at atmospheric pressure you won't have that so any load is going to distort the frame. Mar 16 at 15:51
• In case there was no bag, couldn't a fill port on top and empty port at the bottom make hydrogen replace air by pushing air down?
– user47135
Mar 16 at 16:51
• @user721108, there's some discussion on the stratification on chemistry - and there's the whole other problem caused by mixing turbulance - but think for a moment - how happy are you about injecting pure hydrogen into a container filled with a 21% oxygen gas mixture? Mar 16 at 17:04
• Link corrected now, sorry: pinchito.es/2024/aves-aeternae Mar 16 at 18:47