a) Define the control volume to surround the rocket. Because the hot exhaust gases cross the boundary, this is an open system. As the propellant burns, a mass flow exits through the rocket’s nozzle and the mass of the rocket decreases (changes with time), so the system is unsteady. The nozzle is very hot compared to its surroundings, so there is heat transfer across the boundary, but it is negligible compared to the energy flow associated with the hot exhaust gases. The stored energy in the rocket propellant is converted to kinetic energy in the rocket’s nozzle.