So as an example; a second J1 takes 20 tons of hydrogen fuel. If you are carrying ammonia in the collapsible tank you only need to carry 11.6 tons.
That's displacement tons, of course. Liquid ammonia is much denser than liquid hydrogen, and the quantity that can be processed to extract 20 tons of hydrogen actually masses over 113 tons. When you process it you get 20 tons (mass) of hydrogen and 93+ tons of nitrogen.We looked into this
on the Traveller forum at SJGames back in February '08. Of the forms in which to store hydrogen which anyone came up with over there, the one that got the most hydrogen into a given volume was metallic hydrogen, which (assuming that it can be made and stored) would theoretically squeeze somewhere between 11 and 60 times as much hydrogen into a given space and with no mass overhead. But there's no evidence that it is available in teh Traveller
Confining ourselves to materials actually existing, the most compact form for storing hydrogen anyone found was ammonium bifluoride
, which is 1.87 times as compact as liquid hydrogen. (But it is 11.3 times as massive per unit hydrogen, and is a bit of a chemical nightmare.) The solution I liked best was a 1:2 molar mixture of lithium amide and lithium nitride, which is safe to handle, 1.85 times as compact as liquid hydrogen, and yields up its hydrogen on simple heating. It is 9.7 times as massive as the hydrogen it includes.