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Comment by whatsupdog | Hacker News Reader

I think bigger question is, how do you bring the blimp down without dumping out helium?

Airships have air bags inside. Same deals as submarines. They take in and out ambient stuff into the bags to control buoyancy.

What ambient stuff is available at the altitude where a zeppelin typically flies?

there is generally plenty of ambient air in the earth's atmosphere

And how can that ambient air be stuffed into bags to control buoyancy?

https://en.wikipedia.org/wiki/Air_pump

An air pump will not compress the air such that it becomes dense enough to overcome the buoyancy of the helium. The idea is ridiculous.

Tell that to the airships I guess

so balloons appear to have negative mass, it's actually just the result of having lower density than the air. the upward force balances out with the gravity where the lbs/in^3 figure of its entirety matches that of ambient air. it's exactly the same as how an empty tank underwater floats, and a water filled tank underwater sinks.

or I guess one could say it's the bottom side getting more compressive load from air than the topside, given the observable effect, whatever floats our zep...

The stuff the Zeppelin/Blimp floats in

And how do you propose that stuff be stuffed into the bags to control buoyancy?

Attach hooks to the bottom of the blimp and send a guy in a heavy sled with hooks on it, with helium balloons attached to it, to the bottom of the blimp. Attach the sled to the hooks on the blimp, then get the guy to pop all the sled's balloons. The blimp will land on the ground gently, if the math is right.

you could compress the helium into tanks?

This. Buoyancy is related to volume. Reduce the gas volume, increase its (relative) density, bouyancy goes down, ship goes down.

Same way submarines adjust dive depth without dumping air overboard.

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