Good, but could be better
This is a very cool game. Nice music, nice graphics, decent gameplay.
The biggest problem, as a lot of people have mentioned already, is the Endurance meter. It adds nothing to the gameplay, for one thing. The need to return to the nest to create ammunition (er, "fire"), coupled with the limit on how much hydrogen and sulfur the dragon can carry back with it, effectively serves the same purpose as an Endurance meter: the player will be returning to the nest frequently whether that meter is there or not.
Assuming that there is some kind of compelling reason for having it, though, you should definitely make it deplete much more slowly. It's truly annoying to suddenly start flying around like a granny dragon in the middle of gathering elements, or returning to the nest.
What would make this more interesting? Make it a little more like SimDragon! Since you're the last surviving dragon, I don't see how any amount of military success on your part is going to save the race. No reproduction = no survival. You need to raise the stakes a little: make it so your spouse dragon is at home guarding the whelps. This could also play into some interesting gameplay elements: the more battles you win, the more time you buy, the more your whelps grow up and start having whelps of their own. You'd have stats on the growing population of your nest, and you'd see the effects on your colony if you start letting too many attack helicopters make it through. There could be rewards for having more whelps: maybe they help defend the nest. Maybe they'll harvest nearby elements for you. Something like this would spice things up a great deal.
Good luck!