At first I was confused about if the star is 39 light years from earth, how come it would take just 41.8 years from the observer perspective (assuming me lookin

At first I was confused about if the star is 39 light years from earth, how come it would take just 41.8 years from the observer perspective (assuming me looking out of the window). But with pen and paper I see that you would be accelerating the first year up to speed of light, then 40 years at almost speed of light and then one year decelerating. What is the efficiency of some current hydrogen -> helium fusion engine?