As mentioned in the last post, I had drawn up a schematic for the prototype launcher (simple, I know…). The next part was building the circuit, which was first put onto a breadboard, so any changes I might decide to make could be easily done: just pull out the offending component, and stick a new one in.
The first launch was done using the prototype on the breadboard, and this was what we achieved:
As you can see in the video, the breadboard was hooked up to my bench power supply. The rocket went up without a hitch. Retrieving it was another story…
Knowing my simple circuit worked, I then proceeded to make a permanent version. Below is a poorly-built first attempt:
Inside there is no circuit board, just the required wiring and two resistors to limit the current going to the LEDs. Very simple. The component list is as follows:
1 x ABS Black Enclosure
1 x Red LED
1 x Green LED
2 x SPST toggle switches
1 x SMD momentary switch
2 x 390Ω resistor
1 x PP3 battery clip
This was hooked up to a 9V battery (a PSU acted as one in the photo). There are three switches: one to turn the circuit on, indicated with a green LED; one to arm the circuit (so to prevent misfires), indicated with a red LED; and a momentary-launch switch. So, in order to launch a rocket, three switches must be on. Thus, no accidents can occur.
All was well until I tested the circuit with the actual rocket. When attempting to use it, I noticed two issues. When the launch buttons were pressed, the LEDs went out, and, more importantly, the rocket didn’t launch. A purpose-built rocket launcher that cannot fire a rocket is quite useless, so, back to the drawing board…
After doing some research, and reading a guide to home-made launchers by (t)Estes (which can be found here), I realised that the igniter must supply “adequate electrical current to cause ignition”. It seems I was wrong in my last post, saying it didn’t require much current or voltage! But with the low resistances involved, it will not take a high voltage to push the required current. The igniter must reach a temperature of at least 288 degrees Celsius to ignite the rocket booster and to achieve this it needs—2A or more. 2A is a lot of current, and a PP3 cell cannot provide it. A typical alkaline PP3 battery has a capacity of 550 mAh, which clearly isn’t enough. This meant I had to go with the traditional AA cell, which has a capacity of 2000 mAh (this is why they use these them in series in remote control toys: because they last a lot longer, as well as supplying more current). This wasn’t a big problem to solve.
The next one was the LEDs going out. This happened because they were shorted out, sincee electricity always takes the path of least resistance. This meant that I would need to implement two separate circuits… But more on that next time