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UndermineBriarEverglade
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Det cap questions: e-match nichrome, selecting a primary
Hi everyone! Just getting into energetics. I want to figure out a safe det cap design so that I can play with secondary explosives.
First, I already built a remote ignition device that uses a digital timer to discharge a 9V battery through a 3Ω resistor. Tape to a match, set it to
5 minutes, get comfortable behind cover. Very precise. However, the resistor sometimes cooks to a higher resistance before whatever it's attached to
is set off. Approaching a dud is no big deal with pyrotechnics but scary with explosives.
I'd like to improve it with a nichrome coil as a heating element, dipped in NC lacquer (acetone and smokeless powder) and ground-up
match heads so that the heating element and match can't separate. Is this reasonable?
What thickness nichrome should I use? What are you guys using? I'd like to keep the current at 3A - this is a comfortable current for my
batteries to provide without dropping the voltage below what the timing chip needs - but that needs a very long length unless I pick a thin and
fragile wire.
Second, what can I do to make a SAFE det cap that's not going to explode in my bag on the way to the field? As a beginner I would really rather not
synthesize a primary explosive at all. Mineman's pyrotechnic detonator looks a little complicated for me.
Can I make a det cap by cooking off confined commercial firearm primers? I know cartridges can bang around in a backpack forever
with no danger.
If not, can I scavenge lead azide/etc from commercial primers to avoid having to synthesize a primary?
I'm probably gonna make ETN for the cap output segment. Are there designs that use ETN as the initiator as well? I read that you have to be
careful casting ETN because it becomes dramatically more sensitive while liquid. So perhaps someone has made a cap that melts and detonates a small
amount of ETN. DennyDevHE77 says some detonators are made without primaries:
| Quote: |
All detonators without a primary explosive usually work as follows: a low-density charge of a sensitive secondary substance (PETN or ETN) is affected
by a heat flux from a high-temperature pyrotechnic composition (thermite / powder / something else). As a result, layered combustion begins in the
charge of the secondary explosive, then it quickly turns into convective (gorenje gorenje gorenje between the particles of the explosive), and then
the combustion breaks down into a detonation wave. Therefore, the charge must be low-density in order to accelerate the transition of combustion to
detonation |
For interest, here are some pics of my pyrotechnics igniter
 
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Sir_Gawain
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First of all, I would never trust a timer circuit enough to plug it up while holding an explosive charge.
You can cook off ETN but it’s hard to make it reliable. I’ve had some success with initiating ETN with copper oxide thermite (iron oxide just
didn’t work).
As for primaries, I would recommend NHN. DDNP also works quite well.
If you are looking for top safety and reliability, build a simple EBW detonator. Using a stun-gun module to charge a microwave oven capacitor to 4kv
and discharging through a very thin wire has initiated ETN every single time I’ve tried it(>25 times).
“Alchemy is trying to turn things yellow; chemistry is trying to avoid things turning yellow.” -Tom deP.
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UndermineBriarEverglade
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Wow that EBW setup sounds easier than I thought. So you're using one of the cheapo ebay modules? Are you using a switch for that 4kV or just touching
wires together once it's charged up?
This particular board is only for pyrotechnics: it is designed to fail "on" if it starts to run out of battery. I will have to redesign it with all
the safety stuff in the other direction. I'd probably never trust somebody else's gadget - especially all the godawful analog timers - but I'm doing
the design/code/assembly/testing myself so I figure it isn't much more risk than the actual synthesis.
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Sir_Gawain
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Yeah, I’m using one of the cheap EBay modules. With a single 18650 the voltage levels out at around 4kv; the bleeder resistor in the capacitor
keeps it from going higher.
For switching it, I at first used a triggered spark gap, then switched to a large relay which worked better.
The detonators I use with it are composed of 1 g of ETN: 0.25g very low density, 0.25g pressed, and 0.5g melt cast. The body is made from 0.25 in ID
aluminum tube sealed on one end with pressed aluminum foil. The EBW head is made from a 0.25 in long piece of .25 in acrylic rod with two holes
drilled in it to feed the wires through, and epoxied in place.
Edit: Here’s a post I made describing the construction in greater detail.
[Edited on 6-14-2024 by Sir_Gawain]
“Alchemy is trying to turn things yellow; chemistry is trying to avoid things turning yellow.” -Tom deP.
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Etanol
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This is a perfectly selected scheme and its design.
Use a fishing line or a magnetic switch and a magnet to safely activate this.
The best nichrome thickness is 0.15-0.2 mm.
NHN is too unreliable and has a large delay in detonation. I made it in different ways and refused it.
Use HMTD, lead or silver azide, silver nitrotetrazolate, copper diazoaminotetrazolate, Ni-aminoguanidine perchlorate, Ni or Cu 4-aminotriazole
perchlorate, Ni or Cu carbohydrazide-perchlorate. These substances do not fail.
Perhaps DDNP, BNCP and Cu glycinate-perchlorate are suitable. I did not work with them.
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dettoo456
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@UndermineBriarEverglade, are you in the US or EU? If you’re in the US or can order from Alibaba or other pyrotechnic suppliers in your country,
I’d recommend just buying the pre-made E-matches (10ft or longer). They aren’t perfect quality and can have duds but are much easier to deal with
and are consistent (consistently bad or good) from different suppliers. The e-matches conveniently have a 1/4” OD red cap on the end which helps to
fit snugly in whatever 1/4” ID tubing you’d use - just requires a small dab of glue and a crimp to make a strong seal to the cap body.
As for the cap body itself, thin aluminum works well and is used widely in industry but I’d prefer stainless for corrosivity protection and
strength. Thermocouple/thermowell/temperature sensor tubes (sealed at one end) are basically perfect for this. Aliexpress has a couple 7mm OD x 50mm
length stainless tubes available for a reasonable price.
For an output charge, pressed PETN or ETN (ETN needs to be highly washed and acid-free) is good. Transfer can be loose fill/low density PETN or ETN,
and for the initiating charge, I’d use AgNTz.
If you aren’t comfortable making AgNTz, SADS could work (though I don’t know if it corrodes Al or stainless) or try making a perovskite complex
from something like AgClO4 or NaClO4 and piperazine.
Also, if this is your first time working with anything energetics wise - make sure you take every precaution you can. Don’t hurt yourself or others.
[Edited on 14-6-2024 by dettoo456]
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fx-991ex
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If you want to do nichrome wire dipped in NC, what i found is NC alone wont do much.
What i do is i dip it in NC lacquer, let it dry, then i dip it again in NC lacquer and then in some black powder(meal powder if possible) i let it dry
and then one last drip in NC lacquer to seal it.
I guess black powder could be replaced by other pyrotechnic composition, like ammonium perchlorate or flash powder.
Make sure to solder wire to the nichrome wires, otherwise the resistance will be wrong.
[Edited on 14-6-2024 by fx-991ex]
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UndermineBriarEverglade
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Thanks everyone. Sir_Gawain I really appreciate the schematic and detailed cap design. I'll build something similar. I will spend a little while
researching a solid-state solution but high-voltage SSRs are probably too expensive and I've got some beefy relays lying around. What is C2 for,
inrush current on the relay? (edit: oh yup I see)
Noted, thank you Etanol. If I can't get Gawain's EBW to work I'll plug this in to one of the length/target temp calculators and probably use it with
NC + smokeless powder per fx's recommendation.
Dettoo, I'm in the US, but hate to order anything but the most innocuous components online. I've had stuff seized before so I'm on the customs
watchlist already  . I've heard that an inner layer of a plastic straw inside
metal caps can be a good idea. I'll have to check compatibility of all the metals I'm using (bridge wire too) with my ultimate choice of chemical.
This will be my first time working with EM. I have read the warnings about peroxides, Life after Detonation, etc. I will doing syntheses as small as I
can manage, keeping all my appendages away from the work, and using double boilers if I need to heat but would appreciate any suggestions for safety
equipment - face shields, etc.
[Edited on 2024-6-14 by UndermineBriarEverglade]
[Edited on 2024-6-14 by UndermineBriarEverglade]
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Yorty2040
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The copper hexamine perchlorate complex (CHP) is a very safe primary, and easy as shit to make with common pyrotechnic supplies available in the US.
You might need to use a bit more of it than you would of a nitrotetrazolate salt or nickel aminoguanidine perchlorate:isopropanol complex if you're
trying to set off ETN, but it's cheap and easy to make, so using more of it isn't too big of an issue for most applications.
Making CHP complex:
https://youtu.be/ZV9Pv3Jdtak?si=QtUejgvA7YI9bQYR
If you can't find aluminum or stainless-steel thermocouple covers, the casings of ballpoint pens make a good substitute if you can compress the
contents.
[Edited on 14-6-2024 by Yorty2040]
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dettoo456
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Also, I’d refrain from casting ETN in general. The performance gains are low compared to the waxed, crystalline, pressed material.
And as Yorty mentioned, CHP can be a capable primary, though I’d view it as a better booster than primary, and a fairly weak booster at that.
The primary you use as an initiator is only required in very very small quantities. For example, a 300mg (or larger) pressed PETN output, followed by
a 50mg loose PETN transfer charge, only needs 15-20mg of AgNTZ. NAP, SADS, AgN3, or DBX-1 would perform just as well in around that same 15-20mg
range.
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greenlight
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I'm still a fan of aluminium cased caps filled with 700mg recrystallised, pressed PETN base charge, 200mg PVA coated lead azide pressed primary and a
final layer of granulated milled black powder with the ignition bridgewire nestled into it to ensure ignition.
Have had them stored for 7 years before and they never fail.
Am I getting too old and behind the times these days, is the beloved ead azide not favoured anymore? I know she's toxic and extremely sensitive and
more so if not synthesised properly, but she's a damn good initiator with remarkable DDT properties.
Be good, otherwise be good at it 
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ManyInterests
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| Quote: |
I'd like to improve it with a nichrome coil as a heating element, dipped in NC lacquer (acetone and smokeless powder) and ground-up match heads so
that the heating element and match can't separate. Is this reasonable?
What thickness nichrome should I use? What are you guys using? I'd like to keep the current at 3A - this is a comfortable current for my batteries
to provide without dropping the voltage below what the timing chip needs - but that needs a very long length unless I pick a thin and fragile wire.
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The way how I make my e-matches is I use a regular match (any size will do) and I put the nichrome wire over it and each wire tapped to the sides
(separately, to prevent a short). This has proven to be an extremely reliable way to make an e-match for me and I almost never have any failures.
For the thickness of e-matches. I've experimented with 0.08mm, 0.1mm, 0.15mm, 0.20mm, and 0.25mm. I would not recommend going above 0.25mm since you
need a LOT more power to get that to light up vs the smaller ones. the 0.15mm and 0.20mm are the best in my opinion since they are thick enough to be
easily managed, but also thin enough to not have to worry about the battery power being insufficient. the thinner ones are simply too bouncy and while
they can be used, I would rather use something thicker that is easier to work with.
| Quote: | | If not, can I scavenge lead azide/etc from commercial primers to avoid having to synthesize a primary? |
Theoretically it can be done. I read in ragnar's homemade detonators that it takes around 5 large pistol primers to give enough material to make a detonator. Now I need to make it
clear about this, I don't know about that first hand since I never made a detonator that way. I did remove priming compounds from toy caps (I intend
to use them to make strike-anywhere matches), and when one goes off (rarely for me) it is a nasty thing. Not injurious I just hate it. I can't imagine
how much more powerful a large pistol primer going off will do.
| Quote: | | Also, I’d refrain from casting ETN in general. The performance gains are low compared to the waxed, crystalline, pressed material.
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I was informed that it is much more superior to regular recrystallized ETN (in methanol to give it an extremely fine powdery texture). also would it
be generally denser?
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dettoo456
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Cast ETN is denser and will outperform crystalline ETN, but the gain in performance doesn’t seem high enough to matter. Especially if the ETN is the
output charge of a detonator, 100-500m/s difference will not be a big deal. Even 5500m/s or lower EMs can be strong enough to initiate a secondary
main charge, so the 7.5km/s from pressed ETN is fine.
And the increase in loading amounts (from simple density calculations) would likely verge on <50mg more in a cast ETN filled detonator vs a pressed
ETN filled detonator - not too big of a deal if you’re working with 700mg total output EM.
On the other hand, large charges of over maybe 50g may benefit from noticeable improvements in performance with cast ETN over pressed, but I’d never
want to melt that much ETN anyways.
Scavenging primer EMs is a bad idea too. It’s way more laborious and dangerous than just making the EMs yourself.
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ManyInterests
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| Quote: | | Cast ETN is denser and will outperform crystalline ETN, but the gain in performance doesn’t seem high enough to matter. Especially if the ETN is
the output charge of a detonator, 100-500m/s difference will not be a big deal. Even 5500m/s or lower EMs can be strong enough to initiate a secondary
main charge, so the 7.5km/s from pressed ETN is fine. |
The more powerful the better. I made 6 detonators to test, but due to really cramming my cap bodies (1g of ETN + 0.3 to 0.4g etn/nhn mix) I hope that
putting in my ematch didn't press the mixture too much... and that the epoxy I put in doesn't hinder ignition, the only UXO I was a cap that I am
certain the epoxy got between the match and the mixture. I took some pliers to crimp the top of the cap, it slipped away for some reason (the steel is
quite hard) and even yanked the match out. I hope it'll be OK in the end. I just want them to detonate!
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UndermineBriarEverglade
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ManyInterests, I read that part of Ragnar's too but I think that actually scavenging primary from commercial detonators is kind of sketchy. It would
be much nicer to have a way to use the complete primers, since they won't go off from dropping, shaking, etc - basically anything short of a firing
pin. I might melt-cast some ETN and then break it up into powder again, but I probably won't bother since it doesn't seem important for detonators.
But I'm going to make Sir_Gawain's EBW design before I make any primaries. Waiting for parts to arrive.
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Microtek
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Firearms primer mixture is much more sensitive than a quality primary such as many of the metal amine perchlorate complexes. Several of these
complexes are also less sensitive than ETN, which is quite sensitive itself.
Regarding ETN and melt casting or pressing, you cannot ignore scaling effects: In a large diameter charge, cast ETN would probably outperform a
pressed one since a well cast charge would probably be the denser of the two (though it is not a given that a cast charge will be at max theoretical
density). However, in smaller diameter charges such as a detonator, the lack of crystal boundaries in the cast charge will increase the critical
diameter and decrease the detonability of the ETN (it does this in the large diameter charge as well, obviously but because of the large diameter,
critical diameter effects are not noticeable). Due to the high sensitivity of ETN, you would probably be able to initiate it anyway but eg. Roscoe
reported difficulties in initiating cast charges of ETN/PETN cast composite.
Taken together, the higher than normal critical diameter of cast ETN, the small diameter of typical caps and the relatively weak containment of cap
bodies, you might well end up with caps that perform worse and are less reliable if you cast the ETN.
If you plan on setting off larger charges, consider using redundancies in your initiating train. This way you can square (or cube with three
independent systems) the risk of a dud, so for instance a 90% reliable train would give a 99% reliability with two independent systems.
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UndermineBriarEverglade
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Quote: Originally posted by Sir_Gawain  | Yeah, I’m using one of the cheap EBay modules. With a single 18650 the voltage levels out at around 4kv; the bleeder resistor in the capacitor
keeps it from going higher.
For switching it, I at first used a triggered spark gap, then switched to a large relay which worked better.
Edit: Here’s a post I made describing the construction in greater detail.
[Edited on 6-14-2024 by Sir_Gawain] |
Tested it out (no electronic control, no EM yet) and this thing packs quite a punch! I'm using a step-up converter to power the relay from the same
battery used for the HV transformer and, eventually, control circuits. But I'm having a problem: as I charge up the cap, the voltage eventually rises
high enough to simply arc past the relay. Not sure if it's across the external relay lugs or internally across the contact, but either way it fires
after a couple seconds without ever powering on the relay. Did you just use a really wide relay, and/or one with a very large throw? I wonder if I
could use several relays in series to increase the total required arc length. I see that there are actually commercial high-voltage relays for pretty cheap as well.
How are you and freepatentsonline actually measuring these high voltages? I have no way to know how charged the cap is or what rated relay I need.
[Edited on 2024-6-23 by UndermineBriarEverglade]
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Sir_Gawain
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I'm using a very large relay that has an open contact gap of about 5mm.
To measure the voltage, buy a cheap amp meter and wire it in series with a 1 M ohm HV resistor. 1 mA = 1 kv.
“Alchemy is trying to turn things yellow; chemistry is trying to avoid things turning yellow.” -Tom deP.
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dangerous amateur
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| Quote: |
If you are looking for top safety and reliability, build a simple EBW detonator. Using a stun-gun module to charge a microwave oven capacitor to 4kv
and discharging through a very thin wire has initiated ETN every single time I’ve tried it(>25 times).
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That capacitor is very far from ideal imho.
How can you say you initiated ETN 25 ttimes? Did you use a witnes plate or did you just judge the bang it made?
| Quote: |
With a single 18650 the voltage levels out at around 4kv; the bleeder resistor in the capacitor keeps it from going higher.
[/rquote]
The bleeder resistor or rather the failing capacitor?
Usually those are rated some 2XXXV...
[Edited on 24-6-2024 by dangerous amateur] |
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Sir_Gawain
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| Quote: Originally posted by dangerous amateur |
| Quote: |
If you are looking for top safety and reliability, build a simple EBW detonator. Using a stun-gun module to charge a microwave oven capacitor to 4kv
and discharging through a very thin wire has initiated ETN every single time I’ve tried it(>25 times).
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That capacitor is very far from ideal imho.
How can you say you initiated ETN 25 ttimes? Did you use a witnes plate or did you just judge the bang it made?
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They were tested against witness plates, but most were using them to initiate ammonal charges. I know the capacitor is very far from optimized. It
should be pulse-rated polypropylene film, but somehow the simple oil and paper caps work.
“Alchemy is trying to turn things yellow; chemistry is trying to avoid things turning yellow.” -Tom deP.
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Sir_Gawain
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| Quote: Originally posted by dangerous amateur |
| Quote: |
With a single 18650 the voltage levels out at around 4kv; the bleeder resistor in the capacitor keeps it from going higher.
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The bleeder resistor or rather the failing capacitor?
Usually those are rated some 2XXXV... |
The bleeder resistor. When it reaches a certain voltage, the resistor drains it away as fast as it charges. You just have to find an input voltage (on
the hv module) that makes it level out around 4kv. For me it was 3.7 v from one 18650.
These caps can actually handle way more than their rated voltage (>8kv).
[Edited on 6-24-2024 by Sir_Gawain]
“Alchemy is trying to turn things yellow; chemistry is trying to avoid things turning yellow.” -Tom deP.
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UndermineBriarEverglade
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I measured my output as 5KV from a 9V battery. Spent $10 on a reed relay rated for 4KV isolation and so far it hasn't arced unexpectedly and seems to
be firing properly. Of course you can't buy a 5KV/300+A rated relay anywhere so its lifespan will be limited. The relay can be powered from the same
battery running HV converter and the electronics.
Though it makes a nice bang I still have not cooked up any ETN to test if the bridgewire is really capable of detonating it. I've boiled down some
sulfuric acid but am having trouble sourcing ammonium nitrate. Found some ammonium nitrate cold packs on Amazon, but how to get them without getting
on a list? Going to use nitrate salt instead of mixed-acid method because nitric acid is also hard to obtain.
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fx-991ex
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You can add a capacitor across the relay/switch terminal to reduce arcing and make the contact last longer. Be carefull i dont know if this will be
safe with a fuse, i dont see why it would not but better be safe than sorry.
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UndermineBriarEverglade
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To be clear, the arcing I'm talking about was 5KV jumping the terminal before the relay was turned on, not arcing while making/breaking the
connection. I'm not worried about relay damage (think it's unavoidable putting that much current through it), just didn't want the thing to go off
unexpectedly!
This is what I have designed. Circuit board is size of the microwave capacitor, waiting for it to arrive. There's an green indicator to show that the
outputs are electrically disabled and it's safe to plug them in. Before throwing the switches to enable the HV converter and det cap, you check the
red/yellow indicators to make sure software isn't trying to turn on the output. But software is still safety-critical in the end. The charge pump
overdrives the relay slightly (datasheet says 1.1ms to turn on at 16V).
[Edited on 2024-7-19 by UndermineBriarEverglade]
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fx-991ex
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Nice circuit, good idea the charge pump for the 15Volt supply.
You could probably do without the "driver on" circuit by just disabling the pwm signal, i guess you know all that already.
[Edited on 19-7-2024 by fx-991ex]
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