Glass Diode Ignition

I am new to rocketry. I have been experimenting with KNO3+Sugar with medium success (no explosions, but also nothing over 100meters high).

I have been using a fuse of yarn soaked in Rcandy with ok success. However I also happen to be into electronics so would like to build a electronic system. I plan on pumping 12volts into a diode, causing it to heat/explode (and if I have to I will include a match). I have three questions:

1) Do I put the diode at the start of the rocket fuel or near the end (see image).
2) How do I connect my 12v source to the e-fuse, as to not cause the launch to burn a foot of my power cable that is giving power to the diode?
3) How do I connect my 12v source to the e-fuse, as to allow it to disconnect on take off without slowing down or tethering the rocket?



thanks
Matt

Use a bit of nichrome wire instead of a glass diode as your heating element. Use more than a foot of cable, like 30 more feet.

No worries about being new. We all start somewhere. Your questions are things that everyone in the hobby deals with. I'd strongly recommend getting involved with a rocket club, and/or purchasing and flying a commercial kit (F or G motor size), and looking at how they work things like ignition systems, ignitor placement, and electrical leads. There are many options out there that work as great solutions. You can then take the things you learned from that experience and apply it to your own development. But to give you a start here's some common practices:

I'm not familiar with exploding diodes, That doesn't mean they won't work. By all means, give it a try! But, like what was said above, most of the time we use Nichrome as a heating wire. It has well known resistance and thermal properties, and if you select the proper gauge and voltage source it will get hot enough to ignite any sort of pyrogenic mixture. Typically you dip the nichrome in some sort of pyrogen, (EG a mixture of black powder and nitrocellulose lacquer) so that you have a reactive mixture in direct contact with the wire. Once the wire gets hot, the mixture ignites, and in turn ignites your propellant. There's good igniter instructions on lots of the main sugar rocket websites. I'm sure you're familiar with James Yawn and Richard Nakka. If not google them, and check out their instructions.

1) I don't see the image, but for igniter placement, you always want it at the forward most point in the engine, this will let the hot gas generated by your motor's ignition to pass along all the exposed propellant on its way to the nozzle exit, ensuring rapid ignition of the whole motor.

2) Burning power cables tends to be inevitable. So use something cheap and replaceable for the last few feet close to the motor. Most hobby systems have some sort of electric squib (called an Ematch) with several feet of thin solid core leads attached. Those leads then get connected to the better wire that leads to your ignition power source. Typically you connect between the two with alligator clips. The thin leads of the Ematch are not reusable, as they're consumed as soon as the motor lights.

3) The whole ignitor package is not attached to the motor, it is expelled out the nozzle as the motor pressurizes, and then your rocket is free to fly! Of course it's very important to make sure your ignitor is small enough to easily pass through the motor and out the nozzle. Things go wrong very quickly once the nozzle gets plugged with an oversized ignitor.

All of this depends on your set up. I'm just telling you what you would do if you were using a typical mid or high power system. Like I say, you can get a good start with the hobby stuff, especially if you can join a club, and learn some good safety practices along the way.

Good luck!

I haven't seen a diode used, but I know at least one motor manufacturer supplied small carbon resistors. When you pump a couple amps of 12V through them they burn quite hot.

One trick commonly used is to use a relay box at the pad and have the remote unit trigger the coil. That way, the high current circuit is kept shorter, which reduces losses due to wire resistance. There are designs in the electronics section here if you are interested.

I'm not sure what the availability or legality of some of the things we use in the US are like in Australia, so you will want to research that a bit. Hopefully someone else from there posts and fills you in on that.

Generally, with sugar/AP you want to start the motor burning at the forward end of the motor, furthest from the nozzle. For the clips to connect the leads, use smooth jaw alligator clips. That way, even if the rocket takes the igniter with it, the leads can let go. Often a bit of masking tape is used to hold the igniter in the motor at the right height. It doesn't take much, perhaps 1cm^2.