Baseline Rocket Design Parameters

The information on this page is to help you design your rocket that we will fly in May. Sample input files for Aerolab, FPRED and CG Calc are listed below. They can be used as starting points for your design. The weights and locations in the CG Calc file are as accurate as is available at this time with exception of the weight of the motor and length plus the root length of the your fins. You will need to change the weight and length to reflect your motor design from FPRED. The Excel or html page called Motor Weight will enable you to calculate the weight and length of your motor. When you get to the point of running Aerolab, you will want to change the fin root length in CG Calc as well. The Aerolab input file has all the correct dimensions with the exception of your fin design. You will need to change the fins to reflect your design. The Fpred input file is correct except for adjusting the throat diameter, bore diameter, number of cartridges and cartridge length to reflect your design.

The maximum allowable altitude will be 9,000 ft above ground level. I would recommend going for a lower altitude of around 5,000 ft as you will have a long enough walk to retrieve your rocket at that altitude. Also, the rocket will not be visible to the naked eye above 5,000 ft and you will need binoculars to track it above that altitude and down to the ground again. If you lose sight of the rocket, you lose the rocket because you have no idea where it has landed.

Key Parameters

1. Maximum Chamber pressure = 450 psi (2" motors) and 500 psi (2.5 inch motors)
2. Minimum throat diameter = 0.375 inches
3. Maximum throat diameter = 0.5 inches (2" motor) and 0.75 (2.5" motors)
4. Initial thrust to weight ratio = 5 (minimum)
5. Rocket inert weight (less motor) = 6.2 lbs
6. Launch rod length = 12 ft
7. Minimum velocity off the rod based on 30 mph (44 ft/second) wind
8. Maximum altitude = 9,000 ft above ground level
9. Launch elevation = 5,200 ft
10. Launch angle = 2 degrees
11. Drag coefficient = 0.35
12. Exit cone efficiency = 100%
13. Exit cone angle = 0 degrees
14. Combustion efficiency = 77%
15. Fixed thrust coefficient (Cf) = 1.2
16. Propellant outer diameter = 1.55 inches (2" motor) and 2.05 inches (2.5" motor)
17. Maximum bore diameter = 0.875 inches (2" motor) and 1.125 inches (2.5" motor)
18. Motor length = propellant length (cartridge length multiplied by number of cartridges) plus two inches
19. Rocket body diameter = 4.05 inches
20. Distance fin extends from body tube = 6 inches (Minimum)
21. Minimum fin root attached to body tube = 5 inches
22. Center of pressure should be aft of center of gravity by one to two body diameters

If you are seeking altitude, then you want a slightly regressive thrust-time curve. High enough thrust to come off the rod in a stable condition and then power back to increase burn time, which translates into altitude. If you just want a normal flight, then you can go with the neutral length for the propellant cartridge which will give you a fairly constant thrust output.

The design procedure is to design a motor on FPRED with adequate initial thrust, then save the thrust-time curve for the FLIGHT trajectory program. Input the thrust-time curve into FLIGHT and adjust the inert weight for the correct light off weight. Note that the lift off weight is the sum of the inert weight and motor weight. Set the Flight program for zero angle of attack. If the altitude is too high, you will need to shorten the burn duration by reducing the amount of propellant in FPRED. Make the appropriate changes in FPRED and then save the new thrust-time curve for use in FLIGHT. Input the new thrust-time curve into FLIGHT and rerun the trajectory. Keep doing this until you come up with a design that works for you.