Due to fuel considerations, the largest portion of the time of flight will be spent cruising at top speed, since constant acceleration for such a long trip makes very large demands on fuel supply. The primary limiting factor on this top speed will be the proportion of mass necessary to devote to fuel. Solving the above relativistic rocket equation for mass ratio (Isp in m/s):
The rocket also needs to decelerate back to its original speed by the time it reaches its destination. Therefore we have two stages and:
Therefore,
Finally,
In the above Pfuel is the fuel proportion necessary to achieve the cruising time of T. A cruising time of approximately 300 years will require 87.7% of the ship’s mass to be fuel when it leaves the solar system. Assuming that the current thrust-to-Isp ratio stays approximately the same in future VASIMR prototypes, we can assume that the mass flow rate will be 1.05 x 10^-5 kg/s. From this, a reasonable correction to the flight time to account for acceleration is about 25 years.