If you are still designing solid rocket motors using Kn spreadsheets, it is time to step into the 21st century with FPRED. This software package calculates the thrust-time and chamber pressure-time curves based on the propellant properties and propellant grain geometry you select. Instead of blowing up a motor on the test stand, you do it safely on a computer. Design the motor and hit the calculate button to see how your motor does. All you do is adjust the grain pattern until you get the motor performance you want and then make it, knowing it will work the first time.
You can also input an ignition delay time with FPRED. The software simulates the gradual ramp up that is typical of most amateur motors plus it includes the blow down or tail-off thrust after all the propellant has burned out of the chamber. FPRED models all the common solid propellant geometries used in the amateur rocketry community. If you want a neutral thrust pattern, simply click the "Neutral Length" button after entering your propellant bore and outer diameters. FPRED will calculate the precise neutral cartridge length required and enter it into the input for you. FPRED's neutral cartridge length is not an estimate, but an exact calculation of the neutral length based on the geometry you input.
Easy To Use FPRED Screen
FPRED incorporates advanced features such as the calculation of divergence losses and flow separation inside the exit cone. In the picture above, FPRED indicates in red letters that the flow has actually separated from the exit cone walls during motor operation. The exit cone diameter is too large and is overexpanding the exhaust gases to such low pressures that the gases detach from the wall. But FPRED doesn't leave it there, it also recommends an exit cone diameter for optimum expansion. In this case, it recommends a diameter of about 0.48 inches. When the same motor is run again with a 0.48 inch diameter exit cone, the flow now stays attached to the exit cone walls and the thrust coefficient (Cf) is increased from 1.21 to 1.26 (Figure below).
FPRED outputs key design parameters and makes the input simple. You can input the theoretical C* value and a combustion efficiency to get the delivered C* rather than having to calculate it. With divergence losses and flow separation losses now calculated by FPRED, we have added a simple exit cone efficiency factor to account for two phase flow and thermal losses. We added two additional graphical outputs to help you design your motor. They are propellant weight and thrust coefficient as a function of time during the motor burn.
FPRED exports thrust, chamber pressure, propellant weight and thrust coefficient as functions of time to EXCEL. You can export all of these parameters to a CSV file, which can be read by EXCEL or other software programs. The exported data is clearly labeled within the file. Now, you can make plots of these parameters using EXCEL for importing into reports.
If you want to see how your new motor design will fly in your rocket, click on "Save Thrust Curve" at the top of the screen and FPRED will save your thrust - time curve and propellant weight to a file. Simply read this file into FLIGHT and see how your rocket flies. You can work back and forth between FPRED and FLIGHT with the software exchanging the data for you until you get the motor design that is just what you want for your rocket.
wRasp & Motor Weight FPRED Option Page
If you want to use another trajectory program like RocSim, you can do that, too. Just save the thrust time curve along with the motor data in the wRasp format option in FPRED. You can select the motor diameter, length, ejection charge delay and input the motor weight based on a fixed weight or by mass fraction. If you work with inches for your motor diameter and length, FPRED will convert that for you to millimeters for use in the wRasp format.
With FPRED you can see the chamber pressure and thrust of your motor as a function of time. Simple hit the graph button after running your motor case and an on screen graph of either curve. You print out the curve or save it for use in virtually any document or presentation software package.
FPRED Chamber Pressure-Time Curve
For The Rocket Camp Course 2 Motor
FPRED has been used by Wickman Spacecraft & Propulsion Company to design motors in the "T" class and beyond. It is an accurate and reliable software package for predicting the motor chamber pressure as a function of time and the corresponding thrust levels. Casper College and NCSD students used the software to design the "Q" class motor used in the Pathfinder rocket launch (left). Both the static motor and flight motors were successful. FPRED is used in the CP Technologies AE-101 Motor Design class. FPRED has been used to design literally thousands of solid rocket motors without failure.
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