Took me a bit to figure out the gameplay loop but once I figured it out I actually thought it was pretty creative. I did think the bullets should be able to take out the birds, too, and you should be able to pick up the gems by just moving over them, but I understand if that would make the game too easy.
The only real criticism that I have is I wish the game had more of a story- It's the storytelling round after all.
The gameplay loop of clicking to fuel the jet, and then trying to collect more energy to upgrade the jet to fuel it faster and and fly further was really satisfying and addicting.
Climbing through the different stages of the atmosphere gave the game a nice solid feeling of progression.
I could see this game becoming even more addictive if the climbing stage had more and more elements of a vertical shmup as you progress through the different stages of the atmosphere.