If the launch point is the origin, and the trajectory starts off at angle $\theta$ and velocity $v$, then under unit gravity it follows the parabola $$y = x \tan \theta - [x^2 /(2 v^2)] (1 + \tan^2 \theta)$$ and the envelope of all such trajectories is another parabola: $$y = v^2 /2 - x^2 / (2v^2)$$