A launch vehicle has a main rocket engine with two identical strap-on motors, all of which fire simultaneously during the operation. The main engine delivers a thrust of $6300 \mathrm{kN}$ with a specific impulse of $428 \mathrm{~s}$. Each strap-on motor delivers a thrust of $12000 \mathrm{kN}$ with specific impulse of $292 \mathrm{~s}$. The acceleration due to gravity is $9.81 \mathrm{~m} / \mathrm{s}^{2}$. The effective (combined) specific impulse of the vehicle is____________ $\quad \mathrm{s}$.

A launch vehicle has a main rocket engine with two identical strap-on motors, all of which fire simultaneously during the operation. The main engine delivers a thrust of $6300 \mathrm{kN}$ with a specific impulse of $428 \mathrm{~s}$. Each strap-on motor delivers a thrust of $12000 \mathrm{kN}$ with specific impulse of $292 \mathrm{~s}$. The acceleration due to gravity is $9.81 \mathrm{~m} / \mathrm{s}^{2}$. The effective (combined) specific impulse of the vehicle is____________ $\quad \mathrm{s}$.

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