What do you mean by storing the pointer to the single mainjet type in a global?
Please answer this question:
Do you only have ONE main_jet type instance in your game, or do you have several?
If you only have one, then:
Global jet.main_jet
Creates a global pointer to it which can be accessed anywhere in the program. You then call load_mainjet() which creates the main_jet instance and sets jet.main_jet to point at it.
You can now access this main_jet type instance anywhere in your program. If you are getting errors with this, please post the code that causes the error.