In this situation, m 1 represents the mass of the gas in the propulsion system, v 1 represents the initial speed of this gas, m 2 represents the mass of the rocket and v 2 represents the initial velocity of the rocket. On the other end of the equation, v 1f represents the final velocity of the gas and v 2f represents the final velocity of the rocket. Initially, both the gas in the propulsion system and the rocket are stationary, leading to v 1 and v 2 equaling 0. As such, the equation can be simplified to the following: 0 = m 1 v 1 f + m 2 v 2 f