Ammonia aircrafts, now an evidence - failure of the hydrogen aircraft is mostly forgotten now. on paper hydrogen had 250% more energy than kerosene, alas even in liquid shape storage was a PITA.
They hoped a 747 would be large enough to house all those fat hydrogen tanks, but they were wrong, and not much room was left for passengers and cargo. The aviation industry was lucky somebody at NASA thought about Ammonia as a possible successor.
Ammonia is an honest-to-god compromise between kerosene and hydrogene. NH3 - nitrogen and hydrogen, but no carbon.
Can someone remember all those studies on nuclear airliners ? Who needs to burn kerosene when you can just heat air through a nuclear reactor ? an aircraft running on
hot air, now that something.
Even though public opinion burned the concept at the stake, it led to Molten Salt Reactors now replacing BWR and PWR with far higher safety coefficient
(the Japanese Monju MSR endured a 9 magnitude quake and a mind-boggling tsunami yet it safely stopped without a glitch)
More generally kudos to the U.S Army and their Energy Depot concept of 1963. Nuclear energy to make hydrogen, methanol and ammonia from air and water.It was a major breakthrough.
http://www.alternatewars.com/Fiction/SF_Tech/SP-263-650050.htm