Check the boardgamegeek page to find a lot of useful information and discussions about Leaving Earth
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This page is a small tool that let you check your itinerary. It just makes sure your rockets were powerfull enough to let you pass all the stages of your itinerary
This example can be found in the rulebook, at the Mission Planning chapter, pages 34 to 36. It describes how you should fill the form to launch a rocket to the moon, with a final payload of 1.
For each stage, you have to fill the total payload so far, without the rocket(s)' payload used to reach this stage. Then, just add the rocket(s) needed to reach this stage from the previous one