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Electrothermal/chemical gun: Updated with first test results

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Electrothermal/chemical gun: Updated with first test results

Unread postAuthor: DYI » Wed Sep 17, 2008 7:50 pm

I'm working on one of these for my grade 12 manufacturing tech project (the 11 and 12 classes are merged, but the 12s can get exempt from all practical work and be marked on their own project, if they come up with a suitable one).

Now, I'm starting this thread off as a question to the good people that are running the place: Is a thread about ETCGs appropriate in the off-topic section? I know that ETGs, railguns, and coilguns have been discussed here before, but ETCGs incorporate an aspect that may or may not make their discussion against the rules (although the rules here only prohibit the discussion of solid propellants used for spudguns, which this obviously isn't).

So, is this thread okay? If so, I'll post the design and turn it into a discussion thread tomorrow. If not, just delete it. I thought I might as well try, and see what happens, as it'll be the only project I'm working on until next spring.
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Last edited by DYI on Sat Oct 18, 2008 7:40 pm, edited 1 time in total.
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Unread postAuthor: brogdenlaxmiddie » Wed Sep 17, 2008 7:55 pm

I think it would be fine seeing as it hasn't been chastised before.
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Unread postAuthor: Fnord » Thu Sep 18, 2008 6:42 pm

Not trying to be a mod, but it would depend on what the function of the gun is. If you're using it to create plasma from a non-reactive substance (such as larda's gun does) it would be fine. If you're using it to enhance the burning characteristics of a propellant which contains its own oxidizers, then you may have some problems.

I would just stick it in Theopia at first.
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Unread postAuthor: rp181 » Thu Sep 18, 2008 8:30 pm

i remeber emailing pcguy about this when i was having troubles with my acc. no reply ( i just checked) even though i thought he said yes...... (dont trust me on that)

weird this came up, because today i just happened to get a certain white powder from school...
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Unread postAuthor: DYI » Thu Sep 18, 2008 9:55 pm

Well, more than 24 hours later, this topic is still here, which means either:
a) it's all good
b)not one member of the moderation staff, or PCGUY, has looked over the new topics in 24 hours. "a)" seems more likely than "b)", so here we go:

If you're using it to create plasma from a non-reactive substance (such as larda's gun does) it would be fine. If you're using it to enhance the burning characteristics of a propellant which contains its own oxidizers, then you may have some problems.


It's an Electrothermal-chemical gun, not a pure electrothermal gun (although the current design could probably be used as such).

The idea here is simple: get a projectile moving as fast as I can out of a launcher of given dimensions. In this case, those dimensions are: 0.375" x 8" reaction chamber, and 0.375" x 36" barrel. The current chamber design (shown at the bottom of this post) could survive a maximum of just under a million psi, and the aim is to hit at least half that.

The capacitor bank planned (4kJ at 800V to start) won't be purchased until next spring or summer (barring some unexpected financial windfall :roll:), and this element will obviously be necessary for the launcher to reach its full potential, but I believe that a goal of 1 mi/s for the chemical component alone should be the starting point.

I'm not going to start talking about the actual fuels yet, as I haven't got a specific go-ahead from anyone in charge to do so, but here's the general gist of it:

Image

Image

These pictures are neither accurate, or final, but they should show you what's going on with the design.

Could a mod please respond to this topic just to clarify things? Until then, comments on the chamber design are welcome, but I won't be here to answer them for at least another 24 hours.
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Unread postAuthor: rp181 » Fri Sep 19, 2008 6:59 am

your images seem conflicting, in one green is fuel, in the other red is. Im guessing the orange thing copper and the poly carb is to insulate? Why is there a big orange thing in the beging?

making an reliable spark plug will be hard.
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Unread postAuthor: Ragnarok » Fri Sep 19, 2008 10:19 am

DYI wrote:The capacitor bank planned (4kJ at 800V to start) won't be purchased until next spring or summer (barring some unexpected financial windfall :roll:), and this element will obviously be necessary for the launcher to reach its full potential, but I believe that a goal of 1 mi/s for the chemical component alone should be the starting point.

A mile a second is very optimistic from almost any chemical fuel, save one that's producing shockwaves or deliberately low density gas mixes.

If I were hoping to beat 1609.344 m/s, (Yes, I'm the kind of nerd that knows the metric conversion of a mile down to millimetres!) my own choices would probably be:
1) rely solely on electromagnetics for at least the final stage (and thus, probably a railgun)
2) a hydrogen/oxygen/helium mix hybrid to achieve low gas densities.
3) a pure ETG using carbon plasma (carbon having a lower atomic mass than the commonly used aluminium - should allow greater velocities, if it can be coaxed into working).

1 km/s a second would probably be a better and more manageable goal, unless you want to share some secret that the propellant is up to those tasks.
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Unread postAuthor: rp181 » Fri Sep 19, 2008 7:16 pm

not to long ago i was contemplating using an ETG as an injector for a railgun, why not an ETC? The plasma might even help establish contact of the rails. it would be very high maintance though.
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Unread postAuthor: DYI » Fri Sep 19, 2008 7:25 pm

A mile a second is very optimistic from almost any chemical fuel, save one that's producing shockwaves or deliberately low density gas mixes.


You hit the nail on the head with that one :D

I can't tell you for sure if the propellant's up to the task, but I can guarantee that, if it isn't, it's certainly close to it. I've sent you a PM about the matter.

your images seem conflicting, in one green is fuel, in the other red is. Im guessing the orange thing copper and the poly carb is to insulate? Why is there a big orange thing in the beging?

making an reliable spark plug will be hard.


Perhaps, next time, you should take time to read and comprehend the description and the titles on the diagram. :lol:

Green is the ignition component, a metal powder which conducts electricity from the electrode to the chamber wall material, and vaporises to a great extent in the process. Red is the fuel in both diagrams.

The "big orange thing in the beginning" is what stops the copper bar from leaving the back of the gun at about Mach one after each shot, as there are no threads on it or the insulating material.

A reliable spark plug will be hard, but you're looking at what is essentially the best idea I've got right now.

1 km/s a second would probably be a better and more manageable goal, unless you want to share some secret that the propellant is up to those tasks.


1km/s is nothing, I had an airsoft gun I threw together from spare parts this summer shooting up to 3000 fps (2700-2900 on most shots). No, I'm looking to see just how fast the average hobbyist can get something of a decent mass travelling without spending huge sums of money or needing highly specialised equipment, and I believe that, with a certain amount of careful planning and construction, I can far exceed what I've achieved right now.
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Unread postAuthor: brogdenlaxmiddie » Fri Sep 19, 2008 8:41 pm

I like what I see... Even if I don't fully understand the whole thing but still. Looks pretty impressive its going to have a half-million PSI runnin' through it.
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Unread postAuthor: rp181 » Sat Sep 20, 2008 11:58 am

hehe, i get it now =)
what fuel do you have in mind?
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Unread postAuthor: Hotwired » Sat Sep 20, 2008 4:50 pm

DYI wrote:1km/s is nothing, I had an airsoft gun I threw together from spare parts this summer shooting up to 3000 fps (2700-2900 on most shots).


Thats some evil airsoft gun capable of firing a pellet at 2.67 x the speed of sound.

Unless you meant air gun.


On the design:

1000 m/s is pretty achievable, just under 3x SoS.

1609 m/s is not as likely, over 60% faster. The fastest commercial rifle cartridges using solid propellants can chuck their .22 rounds at 1200m/s. Rounds can go faster but people seem to prefer to have rifling left in their barrels after a couple of hundred rounds instead of a more zippy bullet :P

I'm wondering how close to being an ETCG this will turn out to be as opposed to an elaborate alternative to a primer and firing pin.

Something new at any rate :)
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Unread postAuthor: DYI » Sat Sep 20, 2008 5:33 pm

Unless you meant air gun.


Nope, it's an airsoft gun, as it fires airsoft BBs.

1609 m/s is not as likely, over 60% faster. The fastest commercial rifle cartridges using solid propellants can chuck their .22 rounds at 1200m/s. Rounds can go faster but people seem to prefer to have rifling left in their barrels after a couple of hundred rounds instead of a more zippy bullet


The 120mm gun on the M1A2 Abrams MBT fires an APFSDS round at over 1mi/s, using conventional systems, so I can't see what the problem here is.

And in the interest of gauging barrel errosion (and preventing premature death) of the gun, I've decided to use an easily replaceable inner sleave in the barrel, probably just some cheap SS tube to start with. If there is errosion, it should be caused almost solely by the propellant (which I'm still not discussing, as this topic STILL hasn't been read by a mod), as the projectiles will mostly either be ball bearings, or plastic slugs. Either way, it's smoothbore, so I don't need to worry about the rifling going away.

As far as being an "elaborate alternative to a primer and a firing pin", I suggest you try to make both this electrode design, and a firing pin system that could survive a million psi (as well as the maximum number of rounds the gun will fire in its lifetime, possibly a few thousand, as the use of a primer and such neccessitates this), and tell me which one caused you more grief. I can almost guarantee that, for an experimental gun at least, this is as good (and simple) as ignition gets.
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Unread postAuthor: Ragnarok » Sat Sep 20, 2008 5:46 pm

Hotwired wrote:1609 m/s is not as likely, over 60% faster.

Not to mention, nearly 160% more energetic, and that's the important thing... two and a half times the energy needs to be sunk into the propellant gas to keep it up to speed with the projectile.

Rounds can go faster but people seem to prefer to have rifling left in their barrels after a couple of hundred rounds instead of a more zippy bullet

Ah, the old fight between the advantages and disadvantages of higher velocities... a fight which means even subsonics still have their advantages. A subsonic 11.43x23mm Auto rimless cartridge is, although still hard to suppress, perfectly capable of being so quiet you're more likely to hear the bolt being worked on a well suppressed carbine.
(There's also a certain appeal of knowing you didn't have to break the sound barrier to do the job, which is why I'm experimenting with subsonic darts)

Anyway, I'm nonetheless intrigued to know what can be done, even if I'm not exactly raring to tread the same paths.

DYI wrote:(which I'm still not discussing, as this topic STILL hasn't been read by a mod)

Then PM MrC or jrrdw then!!!
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Unread postAuthor: Hotwired » Sat Sep 20, 2008 5:59 pm

Haha, firing an airsoft pellet does not an airsoft gun make :lol:

I was hoping it wasn't that kind of propellant really.

You missed the last point, it was whether the design would be closer to an electrothermal-chemical gun (as in actually boosting power by better ignition of the propellant through plasma) or just conventional electrical ignition causing a normal propellant burn.
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