For the ODE
$$ \dot{x} = f(x) $$
we have the 2-stage, second order SSP RK scheme (Shu, Osher, Gottlieb)
$$ x^{(0)} = x^n $$
$$ x^{(1)} = x^{(0)} + \Delta t f(x^{(0)}) $$
$$ x^{(2)} = \frac{1}{2} x^n + \frac{1}{2}[ x^{(1)} + \Delta t f(x^{(1)})] $$
$$ x^{n+1} = x^{(2)} $$
What is the scheme in case of the following ODE
$$ \dot{x} = f(x,t) $$
In particular I am looking for papers which deal with this case.