A rocket in space where gravity is negligible has a mass (including fuel) of $5000 \mathrm{kg}$ . It is desired to give the rocket an average acceleration of $15.0 \mathrm{m} {\mathrm{s}}^2$ during the first second of firing the engine. The gases leave the rocket at the speed of $1500 \mathrm{m} {\mathrm{s}}^{-1}$ (relative to the rocket). Estimate how much fuel (in kg) must be burnt in that second?