Answer
It can be launched at $90^{\circ}-.66^{\circ}=89.33^{\circ}$.
Work Step by Step
We call $\theta$ the amount below 90 degrees that the rocket can fire. Thus, we find:
$x = \frac{v_0^2}{g}sin(2\theta)$
$50,000 = \frac{4600^2}{9.81}sin(2\theta)$
$.0231 = sin(2\theta)$
$2\theta = sin^{-1}(.0231)$
$\theta =.66^{\circ}$
