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Updates & Corrections

"How To Make Amateur Rockets"

The following are additions and corrections as of December 26, 2010.

The book states that only phase stabilized ammonium nitrate (PSAN) should be used in ammonium nitrate composite rocket motors. It is no longer available in the United States to the consumer. However, you can still make ammonium nitrate composite propellant motors by using ammonium nitrate in the prill or granular form. If you buy prilled ammonium nitrate, you can grind it into a powder using a grinder with chopping blade action such as a coffee grinder. You can also use a ball mill or hammer mill. To maintain the integrity of the propellant grain after casting, you will need to minimize exposure of the propellant to temperatures below 32F and above 110F before use. A good rule of thumb is about 15 exposures to either end of the temperature extremes. More than 15 exposures could result in the propellant grain cracking resulting in over pressurizing the motor chamber.

In older copies of the book, Figure 15-4 should be changed to show pins 3 and 5 connected. This will provide a ground for the amplifier through the battery.

Additional Information

Load cells come with a low millivoltage output. You can make a simple amplifier to boost the voltage output for use in the DATAQ DI-194 unit. A schematic diagram is shown below using an INA-125 chip. The chip pin numbers are shown in the schematic. If you right click on the schematic diagram with your mouse, you can save the schematic on your computer in a larger image format for better viewing. After right clicking on the image, select the "save picture as" option.

The resistor shown in the schematic is shown as a variable resistor. We actually recommend that you make it a fixed resistor based on the amplification requirements for your load cell and data acquisition system. Variable resistors can shift their position due to handling and vibration causing your recorded values to be off. You should use a high precision resistor with a tolerance of 1%. The value for the resistor can be best illustrated with the following example.

Example: You want a 0 to 6 volts output for the range of your load cell. Your load cell specifications are a rated output of 3 millivolts/volt excitation with a full scale load of 200 lbs. The recommended excitation voltage is 10 volts DC. We will supply the recommended excitation voltage of 10 volts DC.

The output voltage of the load cell with 10 volts excitation will be (10 volts) * (3 millivolts/volt) = 30 millivolts = 0.030 volts. When a load of 200 lbs is on the load cell, it will output 0.030 volts DC. We want to amplify that to 6 volts so we determine the required amplifier gain as follows:

Amplifier Gain = (Desired voltage)/(Load cell output voltage) = (6 volts)/(0.030 volts) = 200 = Gain

The value for the resistor can be found with the following equation:

Resistor (ohms) = (60,000)/(Gain - 4) = (60,000)/(200 - 4) = (60,000)/196 = 306 ohms = Desired resistor value in schematic

It probably is not possible to get a single resistor at 306 ohms. You can just pick the resistor value closest to it and use a 300 ohm resistor. You can also hook multiple resistors in series to obtain the desired resistance. In this case, you could use a 300 ohm resistor connected to a 6 ohm resistor or any combination of resistors to give you 306 ohms.

On page 1-4, the equations at the top of the page should be changed to the following:

Distance (X) = Old Distance (X) + ({Old Vel.(X) + 0.5 * [Accel. (X)] * [Delta T]} [Delta T])

Distance (Y) = Old Distance (Y) + ({Old Vel.(Y) + 0.5 * [Accel. (Y)] * [Delta T]} [Delta T])