# Homework Help: Need help finding required Propellant Fraction for equation

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1. Mar 19, 2015

1. The problem statement, all variables and given/known data
Consider a cryogenic chemical propulsion system with Isp (specific impulse) = 450 s
Enter the required propellant fraction as a proportion with at least two decimal places.

The ΔVelocity is provided. ΔV = 9.6 km/s

2. Relevant equations
Rocket equation:
ΔV = Vex x Ln (initial mass/final mass)

3. The attempt at a solution
I'm atrocious at math, yet this question has come may way nonetheless. Do I just reverse the standard rocket equation one uses to find the Delta Velocity?
Propellant/final mass = ΔV x Isp?
I need smart folk.

2. Mar 20, 2015

### Staff: Mentor

In the rocket equation you listed, where does Isp fit in? Is there another relevant equation, or perhaps one that is more relevant to your problem?

3. Mar 20, 2015

Apologies I forgot the actual equation. The rocket equation I figured was necessary anyway, as no mass information is provided.
Here:
Propellant Fraction:
mprop/mi = 1 - exp(-ΔV/Vex)

4. Mar 20, 2015

Isp is part of the Exhaust velocity equation
Vex = g0 Isp
In fact, I think a better question would be - How does one input exp(-ΔV/Vex) on a calculator?

5. Mar 20, 2015

### Staff: Mentor

It depends on the kind of calculator you have. If it's a standard scientific calculator, enter ΔV, divide by V, change the sign (usually marked +/-), and then hit the ex button. Some of the fancier calculators these days allow you to enter the formula in the display.