Ha, it's an inverted submarine!
http://en.wikipedia.org/wiki/Submarine#Submersion_and_trimmi...
How would they descend otherwise, release helium and reload before taking off?
> “The biggest advancement is our buoyancy management system that allows the vehicle to operate autonomously - without the hundreds of people that you saw grabbing the Zeppelin type airships”
or this:
> The system gets around a major drawback of traditional airships [...where] ropes and docking stations are needed on the ground to stop a vessel floating off.
Too wasteful, and uneconomic. The way they control their buoyancy is by compressing the helium to reduce buoyancy, then later releasing it from a special storage tank to increase buoyancy. BTW this is how submarines control their buoyancy -- they displace water with air in set-aside volumes made available for this purpose, which increases the craft's overall buoyancy. Then, when they want to descend, they recompress the air and allow water back into the set-aside areas.
They mention that on the descent, they mix outside air with the remaining helium in the ballasts.
Aren't you going to lose a lot of the helium in this process?
Wouldn't it be easier to have a sealed bladder that inflates/deflates with pressure changes?
Anyway you're right: a scientifically literate journalist would probably have explained the rationales behind this non-obvious choice.
> They mention that on the descent, they mix outside air with the remaining helium in the ballasts.
If that's true, I gave them too much credit for intelligence. :)
> Helium is expensive, and is only going to get more so in the coming decade or so.
Yes, true, and it's yet another example of something we could be paying attention to before it's too late. But we probably won't.
> Wouldn't it be easier to have a sealed bladder that inflates/deflates with pressure changes?
I have to confess that to me this seemed obvious, because it's more or less how a submarine works (the submarine doesn't have a flexible bladder, it just pushes water around with air, but it's the same basic idea).
The plan for the Hindenberg (a rigid frame airship, of course, not a blimp) was to have concentric gas cells -- spherical gas-bags inside the truss framework. Each outer sphere full of helium would contain a large inner sphere of hydrogen. The He would effectively insulate the H2 gas from atmosphere, reducing the fire hazard, while taking advantage of the superior lift provided by H2; to descend, they could simply vent hydrogen (via a valve out the top of the lift cell). Unfortunately the US helium export ban put paid to that plan, forcing them to use only hydrogen. The rest is history.
(Another key breakthrough in the early noughties (of the 20th century) was the invention of Blaugas -- a gaseous combustible fuel that was of neutral buoyancy in air. You could store fuel in gas cells and pump it straight to the engines without having to make any trim adjustments.)
Blimps -- non-rigid airships -- use concentric gas bags as well: there's an inner cell hidden within the outer lift cell that can be inflated or deflated with air to maintain internal pressure (keeping the outer bag taut but not over-pressurized) and regulate lift. But the ability to compress or liquify helium is indeed a valuable new technique that will reduce operating costs enormously for future airships.