Making Inexpensive Ejection Charges
Please note: This information was provided by PerfectFlite. Please visit this site as they sell some very good products including electronics at a good price. http://www.perfectflite.com/index.html. Note: you can find Grains to Grams conversion chart HERE. I have tested these little device's with 100% success. I can just make up five or ten pre sized charges for my rockets and just snap them in during flight prep. They work really well with the MissleWorks RRC2 altimeter. They are easy and fast to make. I recommend that you test these before using. The first couple I made, I didn't shrink the heat-shrink all the way. This allows the powder to move away from the leads and might not blow. It also seemed to not have the pressure to blow out a nose cone because it was not tightly packed by burning at a slower rate. I have tested them with my level two rocket for both nose cone and fin can deployment and they worked great. I highly recommend them. You can make them your self with an old string of Christmas lights and a few other materials.
Making inexpensive ejection charges for small to medium sized rockets using miniature Christmas tree bulbs
Download .PDF version of these instructions
A number of factors influence the selection of ejection charge igniter's when using electronic deployment of recovery devices:
- Reliability. The igniter must ignite the black powder charge, or the recovery device will not eject.
- Low Current. The igniter must fire at a low current and voltage to minimize the requirements (and ultimately the weight) of the igniter battery.
- Size. The smaller the ejection charge, the easier it is to fit it into the rocket’s airframe. Smaller devices also tend to be lighter, which will improve the overall performance of the rocket.
- Ease of Use. The ejection charge and igniter should be easy to replace in the field to allow for multiple flights without an excessive amount of preparation time.
- Cost. The ejection charge and igniter should be as inexpensive as possible without compromising the above criteria.
This document describes the fabrication of ejection charges and igniters using readily available parts that fit all of the above requirements. They will fire on as little as 2 Volts and 0.2 Amp of current, so the ejection charge battery can be as simple as two “N” alkaline or two to three “1/3AA” NiCad cells in series. A single “N” size 12 Volt type 23 battery can also be used, but has a much lower Amp Hour capacity and will need to be replaced more frequently.
Charges
of various sizes can be constructed using this technique, ranging
from a 1.5” long, 0.25” diameter version suitable
for up to 1.6”
airframes and weighing in at under 1 gram to a larger version for up
to 3” airframes that measures 1.5” long by
0.38” diameter and
weighs just over a gram. A simple plug-in socket arrangement allows
for easy installation and removal in the field, and only adds a gram
or two to the overall weight.
The total cost of each charge
including igniter, all materials, and black powder is under $0.25.
Fabrication time is approximately one minute per charge and requires
no special tools.
Extensive testing has demonstrated superb reliability. Dozens of ground tests and over 80 flights have been logged without a single failure. The low current characteristics of the igniter also allows the use of redundant charges wired in parallel for an even greater degree of confidence.
You will need:
3/16” 1/8” ID Silicone tube
1” 3/8” diameter (before shrinking) heat shrink tube
A suitable quantity of FFFFg black powder
1” sq Non-flammable recovery wadding
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Take the bulb and inspect its leads. You will probably find that they are coated with oxidation. Using a knife or piece of sandpaper, remove the oxidation until the leads are bright and shiny. Do not damage the leads. |
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Cut a ~ 3/16” length of 1/8” ID Silicone tube and slide it along the bulb until it reaches the flange. Most of the filament and lead wires inside the bulb should be covered. |
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Carefully snap off the end of the bulb with pliers. Do not damage the filament wires inside. Check continuity at this stage. You should get ~2 Ohms. |
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Slip a piece of heat shrink tube over the silicone. Do not allow the heat shrink tube to extend past the bulb flange or the bulb will not fit in its socket. |
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Heat the end of the heat shrink tube that is over the silicone tube to secure it. If you are making a large charge, leave the remaining section of heat shrink unshrunk (full diameter). For smaller charges, shrink the entire length of tubing to the desired size. When the tubing is cool, fill the end with black powder and plug with flameproof wadding. |
Note: Ejection charge fits in socket with tight friction fit. Due to the low weight of the charges, this is sufficient for most applications. If you expect very high acceleration, you may want to fabricate a clip that holds the charge in place against acceleration/deceleration forces.
Warning! To protect your fingers, always insert/remove live charges with a pair of long nosed pliers. Do not hold charges between your fingers!
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Views of the avionics bay with bulb socket shown from inside and outside. Apply silicone or hot melt glue around the outside of the socket (inside the bay) for added strength and to seal out ejection gases. |
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