The concept of a problem is that if rockets continue to jettison fuel, at some point they will get a very small momentum because their mas will be so small. (Bit of a silly problem, there's a lot of mass in the rocket itself) Anyways, the idea is that you would take the derivative of the momentum and find the time at which you should stop ejecting fuel. There are some solutions floating around, but they all say that momentum is m times the common velocity equation (exit velocity times the natural log of the ratio of initial mass to remaining mass). I said that mass was the (initial mass minus the rate of mass loss times time), which gives me a terrible derivative. The solutions I've seen just left mass as m. Is that a reasonable thing to do?