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 Here is my present launch controller. My first controller came with the Aeronautics lab, and had a styrofoam (no kidding) housing that didn't last all that long. It was followed by a dual rocket launcher that I built from scrap parts in a metal housing. The dual launcher had a safety switch (not a key) a selector switch, and two launch toggle switches. The third launch controller I built, was purpose built for pyrotechnics. I was the tech director for a high school play which required an explosion from the basement. I built a box with a safety key (very important to prevent accidental firing by an actor leaning on it) and an arming switch and firing button for each of three channels. For the play, rather than rockets, the box fired Estes rocket igniters in 35mm film canisters partially filled with black powder. "Pyro" Pete Schilling manned the controls, and the smoke clouds it created looked great when enhanced by the theater lighting. It wasn't until I apprenticed under pyrotechnitian Chuck Craft that I learned what I had done was completely illegal. My current launch controller is built to conform to the NAR safety specifications. It was very simple to build, and if you are getting into rocketry you may well consider building your own versus buying one pre-built. The cost difference isn't all that huge, but you can end up with a higher tech looking (IMHO) launcher, and an "I did it myself" feeling, which is one of the reasons I like building my own rockets. Building a launcher like mine requires only basic electronic skills - soldering wires to components, drilling/cutting mount holes in the case, and stripping wire. Here is an excerpt from the NAR Safety Code, section 8: The system I use to launch my model rocket will be remotely controlled and electrically operated. It will contain a launching switch that will return to "off" when released. The system will contain a removable safety interlock in series with the launch switch.Of my three original launch controllers, only my third complied. Its safety drawback was that it was possible to remove the safety key and leave it armed. My current launch controller also has this additional safety measure. My launching switch is a push-button that automatically returns to the off position, and the "safety interlock" I used is a momentary, round security keyswitch. Additionally I have a continuity light which glows to verify that my battery has power and that there is a good electrical circuit through the rocket's electrical igniter. The schematic for my launcher looks like this:
In order to visualize how it all goes
together, I drew this diagram:
The following is a parts list I used for my launch controller: 270-378A - Insulated Alligator Clips (10)The parts listed are Radio Shack part numbers. I started by drilling mount holes for the keyswitch, pushbutton, and lamp, as well as the cable. I chose the particular box because it had a both a plastic and metal lid. If I later decide to change this to a two channel launcher (using the cable shield as a common line and the two cable conductors for each channel) I can drill different switch locations in the metal lid. With the components mounted in the lid, I soldered an alligator clip to each conductor on one end of the cable. The other end of the cable I slid into the box, and tied a knot in. The knot forms a strain relief so tugs on the cable don't strain the soldered connections to the components. I then soldered the wiring together as per the diagram above. The 9 volt battery, on its clip fits inside the case. The keyswitch I used is, I believe a discontinued item number. In the year 2000 Radio Shack catalog, the same key switch is listed as RSU 11898509. RSU items must be ordered from your local store, by phone, or the internet, and are shipped by US mail. The stock number 49-523 didn't register in the store's computer, and the switch had been hand marked at $2.99 which was a bargain compared to the $9.99 plus shipping that I was expecting to pay. I had considered making a safety key with a phono plug, as many home-builders do. This would have saved me a couple of dollars if I'd paid the normal price on the keyswitch. The round security key looks a lot cooler though, and it's also safer. Since it is a momentary switch, it is only on while you are holding it. When you release pressure on the key, it twists back to the off position. In this sense it acts like a "dead man" switch. It's impossible to set the launch controller down or drop it and have it still be armed to where a bump on the launch button can cause ignition. That is a shortcoming of the Estes, and many other conductive key controllers. In total, the parts set me back just under $20 from Radio Shack, and compared to some of the commercial launch controllers that sell for $30 or so, I think I have a much higher quality product. |
On
the left is the 9 volt battery (I prefer them to multiple AA or C cells
- just less to have to deal with. SW1 is the arming switch, L1 the
continuity lamp, and SW2 is the launch button. CL1 and 2 are the
alligator clips that connect the launch controller to the igniter.
It's pretty simple.
The
operating principle is straightforward. When you turn the keyswitch
electricity flows from the battery, through the switch, through the bulb,
through the clip, through the igniter (which is installed between the two
clips) and back into the battery. The electrical resistance provided
by the light bulb is enough to limit the amount of current flowing into
the igniter to a level too weak to set it off. When you press the
firing button, it creates a bypass for the current around the bulb.
Since electricity always follows the flow of least resistance, the full
power passes through the switch and into the igniter setting it off.
It is important to note that I designed this system to work with Estes
and similar igniters. Flash bulb style igniters and pyrotechnic squibs
require less current to fire, and turning the key switch may provide enough
current to set them off (in that case, simply don't rely on the light -
hold down the button, and turn the key to fire). Either way, if building
your own launch controller, test fire a number if the igniters you plan
to use to make sure both the firing and safety systems operate reliably.