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> we've dug in and haven't found much evidence that supply is actually drying up

Interesting. Is this a case of people over-hyping the problem or just that prices are going to rise to a point that people will stop wasting it?

> the best way to build a hydrogen blimp is to get lots of flight hours on a helium version first

Can you just put hydrogen into an existing helium blimp or would you need to design a new one based on flight data?

I was under the impression they required radically different gasbag designs due to the explosion hazards (not to mention the increased lift)

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> Is this a case of people over-hyping the problem...

As best I can tell, this is the case. The price of helium has gone up quite a bit, but non of the commentary links that to the actual production volumes of helium or size of remaining reserves.

> Can you just put hydrogen into an existing helium blimp

We'd need to modify the envelope design, but the rest of the design wouldn't need to be modified (aerodynamics, controls, propulsion, etc.)

For h2, depending on where some of your electronics/electrical systems are installed, you may need hazardous area rated gear ...equipment rated for use in flammable/explosive atmospheres. ATEX for Europe, AEx for North America, IECEx for rest of world.

> Can you just put hydrogen into an existing helium blimp or would you need to design a new one based on flight data?

The issue is more about the permeability of hydrogen gas. It can seep through all kinds of materials, including metals, and then embrittle them.

Does it embrittle rubber or plastic balloons? Hydrogen is so cheap and easy to make, you could have a single bottle that feeds into a blimp, refilling it when enough has seeped out.

It chemically reacts with a lot of things. Graphene coatings are being explored - powdered graphene oxide seems promising, but permeability without a contiguous layer of something that can block h2 molecules is a "laws of physics are laughing at you" level issue. If large super high quality graphene sheets ever become practical, they can make almost perfectly impermeable envelopes.

But yes hydrogen is cheap enough that refillable bottles and more leakage is cheaper than helium in terms of just the features of lifting gas. Hydrogen reactivity makes for other materials, engineering, and maintenance challenges that can be ignored with helium.

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