It's improved tube (once you have enough energy, you can improve tubes to increase their max width and height). This is labeled as Ball 1^2 etc, so that's why N^4. We should probably hide this meta-upgrade before you first improve the tube as to not cause confusion.
Was spending all my money on the ship upgrades. Quickly stumbled into a situation when it's impossible to gain any money from missions due to the minimum crew requirements, and purchase of a crew requires money. I believe this has to be tweaked because a user shouldn't be punished for actions that are allowed, and negative consequences of which were not communicated prior.
Good concept, like it! I would make XP progress slower several times, because you quickly end up in the 30-40 levels, when a level up doesn't bring much value any more.
"You haven't any chest left" may be better phrased as "You don't have any chests left". Correct me if I the first version is still considered appropriate
What, did you expect me to add something and not have initial bugs? :V