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Wow, I want to try that, what a cool idea. You are using a little irreplacable helium, but it's in use at party stores around the world so it's not that irreplacable.

Is there a reason why helium would be preferred over hydrogen? I know very little about the topic, but would have guessed hydrogen to be cheaper and easier to obtain (never mind replace), and should offer higher performance. Doesn't seem like safety would be hugely high on the priority list.

Price delta -> orbital speed delta.

Interesting, but what's the risk of your balloon + satellite being destroyed by the United States Air Force?

A HAB won't even approach space; you'd be lucky to get roughly 1/3 of the way there before the balloon burst.