>I'd love to find the original reference and re-read it. Considering that > 99.99999% of all mass transport we humans do involves air and/or water and/or surface friction rockets in a vacuum can be neglected.

I figure it was some concern to Arthur Clarke :).

>>Though, it would be nice to have a comparison for dirigibles. These things may sound promising.
>
>A jet powered dirigible now there's an idea worth investigating. *bg*

I was thinking of the same. Now with helium being slightly less flammable than hydrogen :), jets could actually do some good. But then, I'm not sure a dirigible would ever be a fast thing, because of its huge cross-section.

>Seriously though the He only overcomes gravity, the propulsive force is relatively inefficient piston driven props.

I'd rather see it go hybrid. The whole upper and side surface of the balloon can be covered with solar cells (not the rigid glass regular ones, but the flexible sheets developed by that guy from Island, or better), so it would provide additional energy. And with good meteorogical data, it could always pick a height where it doesn't have to fly against the wind - well, provided it doesn't collide with airplane routes, of course.