Model rocketry is a hobby that is enjoyed by people all round the world. There are plenty of instructables out there on how to build rocket engines, parachutes and the rockets themselves so I decided to make an instructable on how to build cheap and reliable ignitors - after all your rocket won't get off the ground without them.
The first thing I want to bring up is safety as some parts of this process can cause injury or damage to property. I take no responsibility for an injuries sustained in undertaking any aspect of this instructable. I recommend you take every precaution including wearing eye and other protective equipment as there is soldering involved.
Step 1: Materials You'll Need:
You'll need a few materials to get you started. Most of the items can be purchased from either a hardware or electronics store. The good thing with these ignitors is that they only require very limited amounts of the materials so you can make a lot for only a small outlay.
• Thin steel based electrical wire - I recycled mine from an old computer, but you can probably get it from electronic shops as well. I tried working with copper based wire but it didn't work (I think this was because the resistance of copper is too high).
• Nichrome wire - I purchased mine off ebay and got about 30 metres for $7 (AUS). You probably won't need a length this long though as you only use just over 1cm per ignitor. Nichrome wire is usually 80 nickel and 20 chromium and is a highly resistant metal. The thinner you can get the wire the better. It is measured in AWG and the higher the number (gauge), the thinner the wire. I used 36 gauge wire, but if you can get 40 it would work better.
• Heatshrink tube - I purchases mine from Jaycar, but this will be available at any electronics stores. You don't want the diameter to be too big as when it shrinks you want it to seal the two wires together. You only use about half a centermetre per ignitor so a metre length would be more than sufficient.
• Gunpowder or black powder - I won't tell you where to get this but you won't need much as it is only to dip the ends of your ignitors in. A pile as big as your thumbnail should be enough for twenty ignitors or more.
• Electrical tape - doesn't need to be great quality.
Step 2: Tools You'll Need:
To make the ignitors you only need a few basic tools, a soldering iron and a bit of patience!
The tools you need include:
• Clippers or scissors
• Wire strippers
• Super Glue
• Testing leads with Alligator clips
• A soldering iron
• A "third hand" soldering iron tool helps
• A lighter
• Two 9 volt batteries connected in parallel
• A spoon
Step 3: Prepare the Materials:
First up what you want to do is cut your electrical wire into lengths of about 4cm (just over an inch) and strip about half a centermetre of the plastic off each end leaving the wires exposed. For each ignitor you will need two 4cm lengths of electrical wire.
Cut your nichrome wire into 1cm lengths and set it aside. For each ignitor you'll need one of these lengths of nichrome.
Cut your Heatshrink into half a centermetre lengths and set aside. For each ignitor you'll need one of these lengths of Heatshrink.
Cut your electrical tape so that it's about the same width and length as your Heatshrink. One piece of electrical tape per ignitor.
If you prepare all of these materials in bulk it will speed up the process later on.
Step 4: Start Making:
Take one length of your electrical wire and one length of your nichrome. You will need to twist the exposed wire around the nichrome. I find the easiest way to do this is to splay the exposed wires so they form a fan, place the nichrome in the middle of the wires and to twist the wires around the nichrome. The reason you twist the wires together is the nichrome is hard if not impossible to solder so when you solder the electrical wire later on it holds the nichrome in place.
Grab another length of wire and using the same method twist the nichrome into the exposed wire. It's important you make sure the electrical wires aren't touching each other, but having said that you want as little gab being "bridged" by the nichrome as possible. The less nichrome carrying the current the quicker your ignitor will go off (and the more likely it will be to go off). I find if you have about 2mm between the wires it is a good gap and gives pretty consistent results.
Step 5: Soldering:
Using your third hand soldering tool (if you have one) hold the two wires taught and solder the two exposed wire ends. Use as little solder as possible to solder the nichrome wire so it is secure - you don't want a big blob of solder hanging off so you can't fit the ignitor past the nozzle of your rocket but you don't want so little solder that if you pull the wires apart they come loose from each other. Again make sure that the only thing connecting the circuit is your nichrome wire - you don't want it shorting out before the nichrome.
Take your piece of electrical tape and wind it round one of the exposed wires. This is to prevent the exposed wire from touching when fold the two wires so they run parallel with each other. Your nichrome should form a horseshoe shape.
Step 6: Wrapping It Up:
Take your Heatshrink and put it over the nichrome end of the wires leaving only the nichrome section exposed. Using a lighter shrink the Heatshrink tubing into place. This holds everything in place and makes it easy to slide it up past the nozzle in your rocket engine.
The last thing to do is to dip the nichrome into the superglue and then into the gun/black powder. You only want a light coating of black powder on the top of your ignitor as lugged doesn't necessarily make it more effective and can make it difficult to slide it past the nozzle if it's too big.
To set off the ignitor all you should need is two 9 volt batteries. Having said that if you have access to bigger capacity batteries that pull more current such as remote control car batteries you will find your ignitors will work more effectively.
Setting the ignitor off is as simple as connecting the batteries in parallel, connecting the alligator clips to the batteries and then connecting the positive and negative terminals to the ignitor. After a short delay you should see the ignitor spark up!
And you're done! I'd appreciate your feedback and I'll take any suggestions you make for improvement on board :)