Operational considerations further decrease the useful payload of a helium-inflated airship. As an airship rises, its lifting gas expands; an airship that begins a flight with its gas cells fully inflated must therefore release gas as it climbs to keep the cells from bursting. Because hydrogen is easy to manufacture and inexpensive to buy, hydrogen airships often began flights fully inflated to maximize payload and released hydrogen as they climbed. But since helium has always been a rare and expensive gas, helium airships began their flights at only 90-95% inflation, thus reducing payload, to allow their gas cells to expand without releasing helium. In addition, hydrogen airships compensated for fuel burned during flight simply by releasing hydrogen; helium-inflated ships, on the other hand, required heavy water-recovery apparatus (to recover water ballast from engine exhaust), which further reduced the useful payload available for fuel, passengers, and freight.

(Helium blimps do not need to vent helium to maintain equilibrium; they employ internal ballonets, or air sacs, which can be inflated or deflated to maintain the blimp’s shape and buoyancy.)

Ryan also ignores any other alternatives between current jet transports and ships — he doesn’t consider slower, less-expensive airplanes or faster, more-expensive ships.

I skimmed his thesis and did not see any description of how lift would be controlled as the weight of the airship changes by a few hundred tons.

For example, on page 30, an airship drops 200 tons of cargo, yet has to rev up his engines to take off.

In real life, it would be like releasing a cork you held 3 feet underwater (assuming the airship was close to neutral buoyancy before dropping the cargo).

If the engines were providing 200 tons of lift before the drop-off, what you really have is a helicopter and why bother with all the gas?