Launch Control :

The launch circuit is typically a rather simple circiut that provides the voltage required to ignite the ingnition charge in the combustion chamber. In this simple system all you would need is a battery connection and maybe a toggle switch if you want to get elaborate. As with everything else, our system is a bit more complicated than the ideal lanuch circuit, thus the launch control will be a little more complicated as you can see in the rough schematic below.

Schematic of the launch circuit.

For safety reasons the controls must first be armed with a keyed switch, so even with the battery or power supply connected to the control box there will be no output through the launch switches until the circuit is armed. When the circuit is armed, two LEDs will indicate that the switches are armed and again for safety reasons, there will be a warning strobe on the gantry that will start flashing to alert people in the area. An internal voltmeter is used for diagnostic purposes during the prelaunch phase, by using the toggle switches we can measure the charge on the battery and the output voltage on each channel without sending that voltage to the ingition charges. If a battery is being used, this would allow us to verify the voltage on the battery during the prelaunch phase. If the voltages are not as expected, then we can fix the problem... to find out that one channel was dead or had some odd voltage during the launch sequence would just be bad. To launch the rocket we will have to remotely open the pressure valve, possibly wait for the system to pressurize, and then ignite the ignition charge in the combustion chamber. Although the system pressurization time might be nearly instatenous, we decided to put the pressure valve and ignition charge on separate channels in case there was some lag time between the two events. From the water and pressurization tests, we found there was about a 3-4 second delay form the time the switch was activated to the start of the flow of oxidizer through the nozzle heads. So, having separate channels for the valve and ignition charge was necessary, although a delay could have been used if we wanted to use one ignition switch. So, that is the launch circuit and below is the circuit laid out on a breadboard, everything seen there will then be mounted into a small metal box.

Breadboard layout of the launch circuit.

Design Considerations:

The launch box is really not all that complicated, but there are a few thing you should consider when designing the controls. The first consideration should be how much voltage and current is required to ignit the ignitors. For this you should do some bench tests to get the resistance of the ignitor and figure out the voltage required to ignite the ignitor. Once you know that you can use Ohm's laws to calculate the current required.

The second consideration will be how far away from the launch site does the control box need be. Below is a table of typical safe distances. The length of wire going from the launch control to the launch site will add a specific amount of resistance depending on the gauge of wire used. Typically 22 gauge wire is used, but any gauge wire can be used as long as it can support the current running through it. If you are just running a wire to a battery, then you add the resistances together and calculate the voltage required to ignite the ignitor.

Estimated Total Impulse. (N - Sec)	Minimum Distance From the Rocket. (ft)
160 - 320	100
320 - 640	100
640 - 1280	100
1280 - 2560	200
2560 -5120	300
5120 - 10240	500
10240 - 20480	1000
20480 -40960	1500

The third consideration is all the components you put in your control box. The LEDs, IC chips, and toggle switches will have voltage and current limits, so you might need to install fuses to avoid overloading them. You also need to design your circuit in such a way that you don't end up reducing your output current and voltage. Everything you put in the circuit will have some resistance, so remember that resistors in series add together and resistors in parallel will add as the inverse of each resistor.

Resistance in Series:

Resistance in Parallel:

If the is a power source or a battery then the voltage will be nearly constant. The resistance in the components will reduce the output current, which will drop even more as it travels through the length of wire going to the launch site and thus reducing the current going through the ignitor. So, it is always best to keep the circuit as simple as possible so that the voltage in is the same as the voltage out, especially when using a battery. Remember a battery will start at some voltage and will decrease at a rate relitive to how much it has to work.


Final Configuration:

The launch control box can be seen below. All the components are mounted onto a thin sheet of particle board and the board is mounted into the control box. The box used was a small cash box that you can find at any office supply store. The box is set up to allow us ta attach any type of DC power source using banana clips. Originally we planned on using a 12 volt battery, but due to complications with the electric matches we decided to use resistor ignitors which required a higher voltage. Instead of using a bank of batteries, we used the DC power source shown in the image below. This power source effectively ignited the resistors every time and the ignition times were extremely consistant.

Input and output ports of the launch control.	The inside of the launch control box.
The power supply and control box.
Internal wiring of the control box.	Testing the control box.