Jet Propulsion/Streamline

In fluid dynamics, a streamline is a line which is everywhere tangent to the velocity of the flow. This implies that if u is the velocity vector at a point and dr is the vector in the direction of the streamline, then their cross product is zero.

u x dr = 0.

This translates to,

$$ \frac{dx}{u} = \frac{dy}{v} = \frac{dz}{w} $$

where u = ui + vj+ wk and dr = dxi + dyj + dzk.