Re: Space drives.
Actually, it isn't. It's the equation for a constant-acceleration "fly-by"--- t=sqrt(2d/a)
for a Brachistochrone trip, it's t=2*sqrt(d/a)
{In the above, "sqrt(x)" means the square-root of x, and "d" is distance.}
BTW, for a trip involving acceleration, coasting, and deceleration:
D is the total distance,
t' is the time spent accelerating (= the time spent decelerating)
a is acceleration.
Total time=(2D/at')+(t'/2)
One thing to remember about all these formulae: They are meant for situations where acceleration is constant and in a straight line, and the ship is not effected by any force other than it's own acceleration. When you throw in varying acceleration and gravity effects...well, it become rocket science.