Richard Nakka’s Experimental Rocketry Web Site
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Introduction
to Rocket Design
Appendix J
Ejection Charge Design and Construction
Introduction
In this appendix, I will provide details on the design and
construction of the ejection charge, (alternately referred to as deployment pyro)
that I have used for my latest generation of EX rockets.
Design
and Construction
Figure 1 illustrates an ejection charge assembly. The same design
is used for both the apogee separation event pyro
and for parachute deployment pyro. The only
variant is the length of the casing, which accommodates the charge of deployment
pyrolant. The design of this ejection charge is intended to provide a degree of
containment. Containment allows pressure
buildup and aids rapid combustion of the charge and helps prevent dispersal of
unburnt pyrolant.
Figure 1: ejection charge assembly
The casing is a length of paper drinking
straw. Modern (biodegradable)
paper straws are actually quite rigid and strong. They have a polymer coating
on the interior that prevents them from becoming soggy in use. This mislabeled rocketry item
is ideally suited as a ejection
charge casing. The straw that I currently use for my Xi rocket has an OD of
8.6mm and a wall thickness of 0.4mm. Burst pressure is calculated to be nearly 800 psi (however is likely to be
somewhat lower as the tube is spiral-wound). Length of the casing is in the range of 51 to
55mm and holds a charge mass of 1.5 to 1.8 grams. The charge is granular Crimson
Powder (CP). Black Powder is
an alternative pyrolant. The measured bulk
density of granular CP is 0.79
grams per cubic centimetre. As such, it is straightforward to calculate the
volume required for a given mass of CP, or the required length for a given
diameter straw.
Figure 2: Paper drinking straws
Both ends of the casing are sealed with hot-melt
adhesive. At one end, the
adhesive also serves to secure the electrical leads. This end is prepared first,
after which the desired quantity of CP is loaded. The paper wad serves as a
thermal insulator, which is installed prior to sealing the other end with hot-melt
adhesive.
The bridgewire is the nichrome wire element that gets hot when
electrical current is supplied. The bridgewire is soldered to the copper lead
wires. I use no. 46 gauge nichrome wire
that has a diameter of 0.0015˛ (0.04mm). This is extremely fine wire, not much
thicker than a human hair. Figure 3 illustrates a sample of this wire, together
with heavier no. 38 gauge nichrome that I use for my motor igniters (purchased
from McMaster-Carr). For comparison, a standard sewing needle is
shown.
Figure 3: Nichrome wire samples compared to sewing
needle
I originally used no. 38 gauge nichrome wire for my ejection
charges. I became concerned with the reliability, especially in cold weather,
as I utilize a 9V battery for the powering the flight computers, and not having a separate dedicated battery for the pyro
charges. As it happened, some years earlier I had purchased a roll of this very
fine no. 46 gauge nichrome wire at Active Surplus on Queen St. in Toronto (an amazing place which,
sadly, shuttered its doors about a decade ago). At the time I thought it was
probably too fine gauge to be of practical use, but I couldn’t pass up on this
mislabeled rocketry item. So I decided to do some testing of this wire and
found that the current drawn by a typical 10mm bridgewire was only about 500mA.
The wire got very hot instantly and burned out in a fraction of a second. This
is exactly what is desired for igniting a pyrotechnic charge such as CP. After conducting
ground testing of ejection charge assemblies using this fine nichrome wire,
which verified its effectiveness, I started using this nichrome wire for my
rocket flights. Since that time, I have made hundreds of such ejection charges
and the reliability has proven to be nearly 100% (one failed due to an
imperfect solder joint that escaped detection).
It is, of course, not
necessary to use such fine gauge nichrome wire for ejection charges. I have used
what I had on-hand. But I believe it is fair to say that the finer the wire,
the higher the reliability can potentially be, especially with lighter capacity
batteries such as 9V cells.
The electrical leads should be solid, not stranded wire. I use 24 AWG or 26 AWG wire. For reliability, the nichrome bridgewire
must be soldered to the
electrical leads. Use acid-based
soldering flux (not rosin based) and solder
for stainless steel for best results.
Figure 4: Roll of no.46 nichrome wire (left) and
no.38 nichrome wire (right)
Quality control checks
of the ejection charge assemblies are important, as these are flight critical
items.
1)
Verify adhesive caps provide a complete seal with no gaps, using a
magnifying glass, in order to prevent pyrolant from spilling out and to ensure
proper pressure containment when the pyro fires.
2)
Verify integrity of solder joint of bridgewire to copper wire
leads using 30× magnifier.
3)
Once ejection charge has been assembled, do a resistance check (not a continuity check). Nichrome wire has
a defined resistance
per unit length, depending on gauge. For
example, my 10mm bridgewires typically have a resistance of about 10 ohms.
Figure 5: Apogee Primary (AP) ejection charge for
Xi rocket
Last updated January
6, 2025
Originally posted January
6, 2025