I'd separate out the desire quite easily: I don't have any desire to build said rocket, so any consideration of doing so is strictly hypothetical. I don't need to think about it and I can spend my time pursuing my natural and necessary and my natural and unnecessary desires.
If for some reason I found that I had the desire to build said rocket, I wouldn't need to give it much thought. I'm retired, and at this point in my life there's very little possibility that I could complete such a project. I'd just intuitively consider it, for me, to be unnatural. In point of fact any decision would pretty much be subconscious. If I were to analyze why, I would find that the potential pain that I'd experience in doing such a project would be far beyond any pleasure that I might experience.
I think that the key here is that I've already considered, very generally, what I feel is unnatural for me. Because of this it's almost become a personal prolepsis: I have a preconception (which may or may not actually be a prolepsis
) as to what is unnatural for me. Running for president, becoming a billionaire, solving the great problems of physics are other things that I find in my unnatural category if I rummage around.
Now imagine, if you will, that I'm a 40 year old rocket scientist. First I can ask myself, "would I like to do this?" (do I have a desire to do this?) If I don't, end of story. If I do, then I can ask myself why I'd like to. What are my motivations? Am I motivated by the money I might make? By the fame it might bring? By the excitement of the challenge &c.... Then I can analyze each of those motivations (desires) in terms of the pleasure and pain involved.
At least for the moment, I think this comes down to two basic points:
1) The goal is always pleasure. If we get away from that, we'll lose our way. (PD22) In evaluating desires, the end goal is pleasure. But desire and pleasure are different. Examining and understanding our desires helps us to attain pleasure.
2) Understanding our categories of desires in a sense becomes our personal value system. This system runs in the background and simplifies our life by directing our thoughts and actions to what is most important to us.