Distance between two objects from velocity vectors

Two objects have velocities given by $\dot{\tvec{r}}(t) = \tvec{i} + \cos(t)\tvec{j} + \sin(t)\tvec{k}$ and $\dot{\tvec{s}}(t) = \tvec{i} + 2\cos(2t)\tvec{j} - \sin(t)\tvec{k}$, where components are measured in m s$^{-1}$.

Given $\tvec{r}(0) = \tvec{i}$ and $\tvec{s}(0) = -\tvec{k}$, the distance, in metres, between the two objects when $t = 2\pi$ seconds is