ARSA Safety Code Promotes Safe Amateur
Rocketry With Maximum Freedom To Flyers
A safety code has been developed to permit the widest possible range of experimentation by amateurs. Some of the major differences with other rocketry organizations (1) there is no certification of members, (2) commercial and experimental motors are permitted at the same launches, (3) all experimental motors can be permitted to fly at a launch and (4) there is no certification of motors. Click here for a copy of the safety code in Adobe Acrobat PDF format.
Filing For FAA Waivers
All waivers must be signed by the appropriate FAA regional center. If you plan on going over 25,000 ft above ground level, the waiver must be approved by FAA HQ in Washington, D.C. . People filing waivers should state on the waiver the answers to three questions about their rocket . If the answer to any one of those three questions is yes, then you cannot file a waiver. Instead, you will need a launch license or waiver from a launch license from FAA HQ in Washington, D.C. The three questions are (1) is the total impulse less than total impulse of 200,000 lb-sec; (2) is the burn duration 15 second burn duration and (3) is the ballistic coefficient of 12 lb/in**2. The ballistic coefficient is the rocket's gross weight divided by the product of the rocket's cross-sectional area and drag coefficient.
To help members file FAA waivers, a blank copy of the required form is now available on-line.
Click here to download a blank FAA waiver form in Adobe Acrobat PDF format.
How To Estimate Nozzle Throat
Erosion With CE Phenolic Inserts
A good insulating, nozzle throat material is CE grade phenolic. It is commercially available from a variety of sources in rod and plate form. While not as good as bulk graphite nozzles, it is suitable for a variety of motor designs. The following graphs should be useful for the amateur rocket motor designer planning on using CE phenolic nozzles. The graphs are based on actual motor data and thermo-ablation models.
You will notice that the throat erosion is relatively flat at high throat diameters for a given pressure. As the throat diameter becomes less than 0.5 inches, the erosion rate increases at a given pressure.
Finally, the throat erosion is shown for a constant throat diameter and a function of pressure. This shows that the erosion rate is directly proportional to the chamber pressure for a given throat diameter. If you examine the curves closely, you will notice that the slope of the lines increases with smaller throat diameters. This clearly shows that as the throat diameter becomes smaller, the sensitivity of the erosion rate to pressure increases as well.
A Simple Tip for Easily Measuring
Black Powder Or Pyrodex Ejection Charges
Using a scale or balance to measure out the amount of black powder or pyrodex for your ejection is a major pain in the backside. Forget it under field conditions! A simple method to measure out 1 gram of powder is to use a 1/8 teaspoon (tsp). A heaping 1/8 tsp is just a shade under 1 gram of powder.
You can usually find a set of kitchen measuring spoons at discount stores like Wal Mart or in grocery stores. Buy a set and take off the 1/8 tsp measuring spoon and add it to your range box. Now, you are all set to measure your ejection charges.
Another tip is don't take the whole can of black powder or pyrodex with you to the launch. Take an old 35mm film cannister and fill that with your powder. Leave the powder can at home and take the film cannister with the black powder or pyrodex to the range. You now have a lot safer container to work out of.
How Much Black Powder Or Pyrodex
Do I Use for the Ejection Charge?
Estimating the amount of 4F Black Powder or Grade P Pyrodex for the ejection has in the past been "that looks about right". Using combustion data for black powder, a simple formula has been developed to calculate the amount of black powder required for a given ejection force on the nosecone. This formula has been checked with experimental data for a variety of tube sizes and nosecone weights and found to be fairly accurate. A key part in using this formula successfully is making sure you do not have significant ejection gas leaks in your body tube.
Black Powder Weight (grams) = (0.002 * F * L) + 1.0
Pyrodex Grade P Weight (grams) = (0.002 * F * L) + 2.0
F = Ejection force on Nosecone in pounds
L = Length of body tube to be pressurized by ejection charge in inches
The ejection force should include the friction force to pull out the nosecone plus a little more than the aerodynamic forces on the nosecone holding it in. If you set the ejection force equal to the weight of the rocket plus friction of the nosecone plus a pound or two, you should be ok.
When using this formula, it is important that your parachute fit loosely in the body tube and not be crammed into it. You should also pack the chute so that the nosecone pulls the chute out of the body tube as it is being ejected.
A Simple Formula For Fin Size on a Rocket
The size of the fins controls a rocket's stability and the amount of weather cocking (turning into the wind). The best way to determine final fin size is to do a center of gravity measurement and calculate the center of pressure. The center of pressure should be aft of the center of gravity by a distance of one body tube diameter or more. However, the center of pressure calculation assumes you know the fin size, but what do you use for a starting point? Simple formula are as follows:
For a three fin rocket:
Fin Area in square inches for each fin = 0.17 * [(d+0.5)*L]
d = Body Tube Diameter in Inches
L = Body Tube Length in Inches
For a four fin rocket:
Fin Area in square inches for each fin = 0.13 * [(d+0.5)*L]
A simple rule of thumb for fin width is that it should be at least 1.25 times the body tube diameter. These formulas are empirical based on a series of flights and should only be used as a starting point.
Low Cost Nozzle Material
Durham's Water Putty
We have conducted several tests with 1 inch up to 3 inch PVC pipe motors using nozzles made out of Durham's Water Putty with excellent results. The throat erosion ranges from 0.020 to 0.030 inches/second. At pressures above 250 psi, the water putty erodes at very high rates and it probably not usable unless you want a regressive thrust time curve.
Use threaded fittings were the Durham's water putty is going to be located. This gives the water putty something to hold on to as the expansion of the putty in the fitting is not sufficient to hold it in place during a motor firing. The mechanical properties of the putty are weak in tension and stronger in compression. So for larger diameter motors, support of the putty will be required. The Water Putty acts as a thermal insulator and is very erosion resistant.
The putty comes as a dry powder and can be bought in most hardware or building supply stores. Simply add water until the mixture is the consistency of plaster. Put it into a PVC fitting or your chamber and let it dry for a week or two before drilling the throat, entrance region and exit cone. It machines very easily if the moisture has evaporated out of the putty. It is very important to allow enough time for the putty to dry. Otherwise, your filing tools will become clogged with moist putty. There is no need for degassing while the putty sets up.