Your problem is not that tricky : you do have a formula that will give you the next position (at
t+dt), given the current one.
Now say the projectile is at
P : use the formula to get the next point
D = (NP-P) is the direction vector at t.
Now depending on what you seek :
• Angle ? Get the angle (atan2) of this D vector, to have the angle of your projectile.
angle = atan2 ( ( D.y), (D.x) ) ;
• Vector ? Normalize the Direction vector to have the normalized direction of your missile.
dLength = sqrt( sq(D.x) + sq(D.y) ) ;
D.x /= dLength;
D.y /= dLength;
(Rq : normalized means it has a length of one.
P is the rear of the missile, and
D is the normalized direction, missile's head is at
P + missileLength * D );
( Another remark : from your post it seems that you do not compute the velocity vector. But if you do, the missile direction is this very velocity vector, just take its angle or normalize it to get what you want. )