Quite interesting, well done! Just to echo what has already been mentioned, the belch timer seemed to detract from the experience a bit - I found myself just rapidly clicking the mouse in order to trigger it. One extension that may be interesting would be to remove the threshold, so instead of being unable to burp until the timer fills up, you can burn at any time but the strength will vary (depending on how long you've waited). The wait-time -> propulsion relation could even be non-linear (e.g., if you wait 2 seconds instead of 1 second, maybe you get 2.5x or 3x the propulsion, instead of just twice the propulsion). Overall, nicely done! :)