first of all this code is not mine i am trying to understand how the equation for m_u works. this comes from a master thesis called Trajectory design for a Titan mission using the Direct Fusion Drive as shown it is a different variant of the tsiolkovsky rocket equation i believe for the propellant weight on the rocket in this case it is being used to plot the change in delta V relevant to mission length so Delta-t is total number of seconds in 0 to 15 years. can anyone link to elsewhere this equation can be found

to make it clearer the equation is

where Ve is (Isp * Gravity constant)