# Homework Help: Rocket Fuel Problem

1. Mar 7, 2012

### Bugeye

1. The problem statement, all variables and given/known data
A warlock testing a rocket broom takes off from a standing start and flies at fixed elevation. The warlock and fueled rocket system start out with mass Mi. The rocket fires with exhaust speed u, losing mass at a constant rate R, and we’ll assume air drag is negligible. If the rocket broom is to reach a velocity equal to 0.6u, what minimum fraction of the initial mass must be devoted to fuel?

2. Relevant equations
Conservation of momentum

3. The attempt at a solution
I really don't even know how to start. I looked at this topic but I do not understand how the fuel has initial momentum, or how to apply any of that in another situation

2. Mar 8, 2012

### tiny-tim

Welcome to PF!

Hi Bugeye ! Welcome to PF!
The question means that the difference in speed between the rocket and the fuel that's just gone is u.

So call the mass "m" (as a function of t), and write a formula for what happens when a mass dm of fuel is expelled in time dt …

what do you get?

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