This calculation is based on an algorithm and formulas in the
March 1995 Physics Teacher, Volume 33, Number 3, p 150-157:
Soda-Bottle Water Rockets by David Kagan, Louis Buchholtz and
Lynda Klein.
Note that for small values of "Launch Gauge Pressure", the calculation
may use the square root of a negative number - a "NaN" (not-a-number)
to the Javascript interpreter. Just raise the pressure value and this problem
should go away.
This model was modified to include friction effects using:

F=0.5*c*d*A*v^{K}

where:
- c = Friction constant of surrounding fluid (depends of object shape)
- d = Density of surrounding fluid
- A = Area of rocket cross section
- v = Rocket velocity
- K = Velocity exponent