Played it on steam, very cool concept so far. Can definitely use more polishing, but its just a demo. Shame that you cant freeplay after completing the last mission.
A few things to add:
A symmetrical wing/attachments placer (maybe that you need to unlock first)
Non-reusable fuel rockets, so that you need to make another one or a bunch at once.
The need to buy as many attachments as you need, instead of just one and getting an infinite amount of them, of course these can be reusable.
Clearer instructions from the water rocket to the fuel ones.
Obviously more upgrades and or attachments but I believe thats WIP.
Cant think of anything else at this moment