This is a lot of fun! Your games are almost pure gameplay, and this feels good. Not sure I like that holding down left or right without space bar still activates the rocket pack though. It's useful for things like mines, but it doesn't feel exactly intuitive for other hazards. I like all of the hazards except for the double mines. Those almost made me quit, because playing those levels was just a waiting game, moving and detonating then getting out of the way. C-12 was the worst offender. I would also say that I'm fine with rooms where timing and waiting is critical, but having a second room attached to those with a high speed section that probably requires many attempts isn't fun. Those should be separate in my opinion. C-3 killed me the most, easily 40 times, that second room is tough. Stopped at D4.
Your SFX are really good, and the music is good enough, doesn't get annoying after many repetitions, which is good. The menus are nice and the brief story threads are cute and brief, which is good. Your art is the probably the biggest drawback to this. Your game is going to have a tough time differentiating itself from helicopter flash games already, the flat shaded art isn't helping. I know it's pretty late into development, but I would experiment with the more simple and juicy aesthetic that's been popular for indies for a few years. INK is a good example. Bright, contrasting colors and simple geometry might lower the bar for what people would classify as good art for this. Just a suggestion, I really like the gameplay, keep up the constant progress!