Topic: Propulsion / Aerospace
Why specific impulse of a rocket engine increases in vacuum compared to sea level?
- A. Combustion chamber pressure increases in vacuum
- B. Mass flow rate of propellant decreases in vacuum
- C. Exhaust velocity increases in vacuum
- D. Mass flow rate of propellant increases in vacuum
Answer: C. Exhaust velocity increases in vacuum
Explanation: Specific impulse is directly proportional to the effective exhaust velocity ($I_{sp} = V_e / g_0$). The effective exhaust velocity is given by $V_e = v_e + \frac{(P_e - P_a)A_e}{\dot{m}}$. In a vacuum, the ambient pressure $P_a$ becomes zero, which maximizes the thrust term and thereby increases the effective exhaust velocity. Thus, the specific impulse increases.