| Quick Links |
| Home |
| Propellants |
| Rocket Motors |
| Rockets |
| Rocket Electronics |
| Ground Support |
| Launch & Static Tests |
| Software |
| Links |
| Motor Class Table |
| E-Mail Scott |
|
| Don't Click Here! |
E-match type initiators:
I had been using the soda straw style deployment charges with great success, they were fairly easy to make, very reliable, inexpensive and fired on very low current. But some of my new rocket designs required another approach. The PIRM2 for example, required a very small initiator to set off a small charge to release a pin, the x-mas tree bulbs were just too big to work. I needed a new initiator, hopefully one that would fire on low current. Nichrome wire seemed to be the best answer. I ordered some 36 and 40 gauge nichrome wire from an EX rocket supplier, and a roll of 40 gauge nichrome from McMaster- Carr.
The nichrome wire doesn't solder very well, but I found I could use a little plumbers flux on the wire from McMaster-Carr and it soldered nicely to copper wire. For some reason the other nichrome wire I ordered was very difficult to solder, even with flux. So I stuck with the 40 gauge from McMaster-Carr. The wire came on a 2 ounce spool, and has 3,000' on it, yes 3,000 feet! That should last a long time.
Here's a picture of the spool of nichrome wire and a completed initiator.
I'm still using the Cat. 5 cable for the initiators. The cable is cut to desired length and one end is left 1/4" longer than the other. Then both ends of the wire are stripped back just enough to allow the nichrome to be wrapped around it, about 1/8". Then the nichrome wire is wound around the lower exposed copper wire 5 times. Then 3 turns are made around the insulated area of the longer wire. Then the nichrome is wrapped 5 times again around the end of the long wire on the bare copper.
A very slight amount of flux is applied to each joint area, then the soldering iron is touched to some rosin core solder to tin the soldering iron tip. The tip of soldering iron is touched briefly to each joint. Too much heat can melt the nichrome wire through the insulation, so care must be taken to just touch the iron to the joint. I test each joint with a pull on the free ends of the nichrome wire to check the solder joint. The nichrome is then cut from the spool and the ends trimmed. The nichrome wire isn't as fragile as it looks, it's very strong considering its small diameter.
I finish by dipping them in a thinned out pyrogen like I use on my igniters. While I wouldn't have to use the pyrogen for igniting a black powder charge, I use the pyrogen for several reasons. They do throw a small ball of sparks and fire so that insures any black powder in the area will burn. It also makes the head of the initiator a little larger so they don't fall through the wire holes. And lastly, it insulates the bare wire ends to prevent the initiator shorting out in an aluminum container.
When finished I test them all for continuity and to see how many ohms they read. I look more for consistency than an absolute value when reading ohms. I generally test one initiator out of a batch. With a good 9 volt battery they will fire at around .44 amps. I've used theses in a lot of flights now, and haven't had a single failure yet. I test them again before installing in the rocket, then again after hooking them up to the electronics module or altimeter.
I should note, these initiators are used to set off a black powder charge in a canister, or a charge in the PIRM2 release mechanism. They are not used alone for deployment (way too small).
I had been simply using my igniters, wrapped in a piece of recovery wadding with a measure of black powder. The problem was, in cold weather the 9 volt battery would not supply enough current to the igniter. Deployment either did not happen, or was delayed. The result was a couple of nice core samples, and a couple of late deployments that were rather dramatic.
I was now building my own electronic timers, and using the timer with a 9 volt relay to get maximum power to the igniter. That also meant using two separate 9 volt batteries. There had to be a better way! After revisiting R. Nakka's site, I decided to try his Christmas bulb igniter, only using it in a larger version for parachute deployment. A quick test with the bulb open, indicated the filament would burn easily from just the 200 ma output of the 555 timer. Just what I needed! A few more tests indicated a 3" length of soda straw held enough powder to eject the nose cone on my 3" rocket.
Building the deployment initiator/charge is very simple. The fact that both the initiation and charge are together, in one nice compact unit, keeps things simple.
 |
|
Completed Deployment Charge |
2.5 volt Christmas Light Bulbs
Twisted Pair Wire (cat. 5 cable)
Soda Straws .25" outside diameter
Fiberglass Wool or Recovery Wadding
Solder
Hot Glue
Fine Sand Paper
Black Powder
Start by cutting your cat. 5 cable to whatever length you need, then strip about 1/2" from both ends of the pair. Cut your straws into 3" lengths.
Now take your bulbs, and use the fine sand paper to clean the wires coming out of the bulbs, this is necessary to allow the solder to stick to the fine wires.
Next, remove the top of the glass bulb. I use a roto tool with a fine grinding wheel. But you could just break off the top too. Check the filament of the bulb now, if it broke while cutting the bulb open, discard the bulb and start over with a new bulb. Be careful, and wear safety glasses at all times.
I usually make a loop in the wires coming out of the bulbs to make soldering easier, but you could just solder the cat. 5 wire to the bulb wires. Just make sure you have a good solder joint.
Insert the bulb into the straw until the base of the bulb is just into the straw. Use hot glue to seal the bulb into the straw and to keep the bulb wires separated. If there is excess bare wire on the leads to the bulb, use a small amount of tape to insulate the wires. Twist the loose ends (not the bulb ends) of the cat. 5 wires together for safety.
Fill the straw with black powder from the open end. Tap the straw gently to settle the black powder, making sure the black powder goes into the bulb and is in contact with the bulb filament. Fill the straw to within 3/8" of the top. Now make a small ball from your glass wool or recovery wadding, and insert it into the top of the straw. Pack it in firmly. Now seal the top with a drop of hot glue.
There you have it, a deployment charge that will fire on a half dead 9 volt battery!