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From what I have read concerning the blimps on the southern border of the US, it is that they are tethered. From my own observation of several transitting at low level altitude, they porpoise through what appears to be several hundred feet of vertical airspace. Cheaper perhaps than a ridgid design, both are fighting the winds aloft. As I also understand it, the new Zeppelins have a ridgid spine with gas bags attached. Not a lot of added weight in exchange for attaching larger thrusters.
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