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Automatic Piston Hybrid Cannon Design

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Automatic Piston Hybrid Cannon Design

Unread postAuthor: lordBenio » Thu May 28, 2015 10:51 am

Hello everyone!

I have started on a automatic piston hybrid cannon design that I have been mauling over for sometime now.

Some of my design criteria:
-60 rounds per minute
-3" bore
-Hybrid (of course)
-High mixes (200+)
-Turret mounted

So further more, here is the cannon that will, I hope, come alive some day.

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This thing will be a brute with a barrel at a 3" bore and a length of 20'.
Barrel will be made out of aluminum, construction unknown at this point but I have ideas on how to make it.

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Working on back from the business end of cannon is the action.

Image

The action consists of the action housing, bolt and bolt actuator. The action only loads a round into the chamber.

Image

As we get farther away from the barrel is the manifold system. It serves as a means to direct to the flow of gases from the combustion chamber to the action. Since I would like the cannon to turret mounted the manifold system has to be flexible yet rigid to withstand the pressures of high mixes and to allow the movement of the action and barrel due to recoil.

To achieve my 60 round per minute firing rate, one combustion chamber would slow me down due to the amount of propellant and air needed to get to high mixes consistently. Therefore I using four combustion chambers which should allow me about 3 seconds to refill one chamber in a cycle.

Image

Now the combustion chamber design that I have chosen is 6" diameter and 6' long. Attached to the combustion chamber is the valve, which is a self-venting auto-sealing piston valve design. I choose to go with auto-sealing design to allow the chamber to be closed before the next chamber is to be opened.

Image

I bet all of you are wondering what the heck I am planing to shoot out of the thing. The answer is simple, an APFSDS.

The round is 5lbs solid steel 2' long kinetic energy projectile. Btw this is the coolest part I have model yet besides part of the manifold system.

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I plan on using hydrogen as the propellant made on board by a massive hho dry cell generator.

Now for some theoretical figures given by HGDT with MAPP gas which I expect to be higher with hydrogen.

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Things I need to work on/finish: (NOTE-At the time of writing this all models are rough ideas)
-loading system
-metering system
-turret mounts and the turret it self
-refine all the models (add o-rings, make it more eye appeasing, etc...)
-different projectiles (optional)

This cannon is still WIP and will be for many years.

I will appreciate all feedback to refine my design before I build it and I will be glad to explain anything about the design that is unclear. I would also enjoy if everyone helped me with a name for this cannon.



P.S. I already have an idea for it's little brother.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Fnord » Thu Aug 06, 2015 11:38 am

Just checking in and came across this semi-old post. Bump.

Ignoring material considerations and inevitable destruction of moving parts, I'm wondering where one would acquire the 5 megawatt DC power supply needed to run this gun at full power? And also how to deal with multiple megawatts of waste heat being radiated by the gun and dry cell.

We need some kind of Pipedream award for ideas of this improbable caliber.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Ragnarok » Thu Aug 06, 2015 12:03 pm

Fnord wrote:I'm wondering where one would acquire the 5 megawatt DC power supply needed to run this gun at full power?

A bolt of lightning!

As entertaining as the idea of an automatic 200x H2/O2 hybrid is, I think Fnord's right about the idea being more than a little infeasible.
But heck, if you turned it into a single shot system, getting a 2 kilogram penetrator up to a kilometre a second would still be one hell of a show.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: mobile chernobyl » Thu Aug 06, 2015 1:45 pm

I reckon someone just learned how to use a CAD platform dangerously well and went to town on this design!

I hope not at least, but the moving parts are going to need some more explanation as to how they will survive the wear and heat generation.

Good luck! I've definitely let my mouse lead the design ahead of my current capabilities more then a few times :D
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: lordBenio » Thu Aug 13, 2015 11:27 am

Fnord wrote:I'm wondering where one would acquire the 5 megawatt DC power supply needed to run this gun at full power?


Fnord, could you please explain how you got this number.

True 60 rpm is pretty fast and infeasible, so I might take the firing rate down 90% to 6 rpm, which still would be impressive.

mobile chernobyl wrote:I reckon someone just learned how to use a CAD platform dangerously well and went to town on this design!


Thank you.

Now as to the heat generation, true it would get hot after continuous firing, but the temp spikes to roughly 2100C but only for 10ms. To combat this problem, I think at least the combustion chamber wall could be water cooled but the if the barrel had a water jacket on, it would bend due to the weight of the water and it already bends about .25in off center. The bend from the weight of the barrel as it is now I think I can compensate for it with software for targeting.

chernobyl, which moving parts are you talking about. The bolt is going to be operated with low pressures compared to what is being generated. The piston is the only part I assume you are talking about and granted the design is only in its infancy stages, I haven't thought about certain aspects just yet.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Ragnarok » Fri Aug 14, 2015 10:24 pm

lordBenio wrote:Fnord, could you please explain how you got this number.

Checking his numbers, I think he's short by an order of magnitude. (Although it is past four in the morning here, so maybe I'm the one that's wrong).

Each chamber requires ~280 moles of gas to pressurise to 200x. (33.4 litre chamber, ~24 litres per mol at RTP, multiply by 200 atmospheres)
Each mole of water takes a theoretical minimum of 286 kJ to dissociate, but this produces 1.5 moles of gas.

So, (280 moles / 1.5) * 286 kJ per mole of water = 53 megajoules.

At your target rate of fire of one shot per second (and if you're not generating it "live", then I'm wondering how you plan on storing that much high pressure flammable gas mix), that translates directly to wattage: 53 megawatts.

A gas generator using the full potential of a typical 3 kW home mains socket might generate enough gas for four or five shots a day.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: jrrdw » Sat Aug 15, 2015 5:08 am

A gas generator using the full potential of a typical 3 kW home mains socket might generate enough gas for four or five shots a day.


Needs a flux capacitor! :bom:
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Re: Automatic Piston Hybrid Cannon Design

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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Ragnarok » Sat Aug 15, 2015 9:14 am

I'm not sure how that would help - we've already ascertained those need a minimum of 1.21 Gigawatts to function properly.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Fnord » Sat Aug 15, 2015 9:54 am

Ragnarok wrote:Checking his numbers, I think he's short by an order of magnitude. (Although it is past four in the morning here, so maybe I'm the one that's wrong).

Your numbers are closer than mine, since I didn't do any 'real math' to come up with them:

1.6 megajoules muzzle energy / gun efficiency (assumed an insane 40%) / H2+ O2 cell efficiency (assumed 80%)
In the real world an order of magnitude short is completely realistic.
-----

Other things to take into account:
Hydrogen/oxygen mixes detonate readily under most circumstances. And at the pressures you're dealing with, that detonation wave essentially makes the tensile strength of your chamber arbitrary and irrelevant. The inertial/mass of the chamber walls is the only thing holding it together. Add a buffer gas like nitrogen/helium.

Your cell needs to be able to withstand the same pressures if you want to avoid destroying it. When you're pumping that much energy around, the chances of an accidental ignition at any point in the system become an increasing concern.

At full power, a single chamber contains the same energy as over 25 pounds of TNT. Have you ever seen anything close to that exploding in person? It's... um... significant. Enough to level an average two story house.

You need to do a separate calculation to find the velocity/KE of your valve pistons, then make sure you can slow them down in a way that doesn't fatigue or destroy the material they're hitting. You'll also need to do this for your action parts.

Remember that aluminum becomes about as strong as dried playdoh past a few hundred degrees C. And unlike steel, it just fragments when stressed. If you want to water-jacket the barrel, use a guy-wire attached to the muzzle end to take the tensile load (as is done in a heavy crane).

Flat end-cap flanges are a no-go in this type of design, they need to be roughly half-spherical or you'll have massive stresses where they're bolted on. Alternatively, make them several times your normal wall thickness.

edit: spelling
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Last edited by Fnord on Sat Aug 15, 2015 5:04 pm, edited 1 time in total.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Ragnarok » Sat Aug 15, 2015 2:30 pm

Fnord wrote:Add a buffer gas like nitrogen/helium.

Helium is a potentially interesting buffer gas, as it happens. While it doesn't add any combustion energy to the gas mixture, its very low molar mass (as it's monoatomic, 4 grams per mole, as compared to water vapour at 18 grams per mole) means that it has relatively little impact on the heat capacity of the mix.

As such, you generate almost the same temperatures and pressures as before you added the buffer, but you do it in a gas that's much less dense, giving the gas a notably higher speed of sound; As many flow restrictions are related to Mach number, this could potentially boost velocity in some set-ups.

At full power, a single chamber contains the same energy as over 25 pounds of TNT.

... erm. Right. If you'll excuse me...

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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: lordBenio » Tue Aug 18, 2015 11:04 am

Ragnarok, I believe you are on the right track, but I think I would only need roughly 43 kW of power to achieve a descent rate of fire.
Yes, the chamber is 33.36L but the amount of fuel that is needed, which is "pure hydrogen" just to put some clarification out, is 133.44 L given by HHGT's load/mix calculator. Which @STP:

133.44 L / 22.4 L/mol = 5.957 moles of hydrogen

Given that the firing rate is 6 rpm and there are 4 combustion chambers, one chamber has 40 seconds to fill with hydrogen. Thus:

5.957 mol /40 s =.148 mol/sec of hydrogen

needs to be produced. Since 1 mole of water is need to produce 1 mole of hydrogen, we can assume that .148 mol/sec of water is need to be split and 287 kJ/mole of water is needed to split water, so:

.148 mol/sec * 287kJ/mol = 42.5kJ/s.

A watt is defined as a joule per second, so one can assume you would need 42.5 kW.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Ragnarok » Wed Aug 19, 2015 12:17 pm

lordBenio wrote:Yes, the chamber is 33.36L but the amount of fuel that is needed, which is "pure hydrogen" just to put some clarification out, is 133.44 L given by HHGT's load/mix calculator.

That's the amount of MAPP gas you'd need for a 200x mix; the amount of fuel needed is specific to the given gas mixture, and as HGDT doesn't have a Hydrogen/Oxygen option, it can't answer your question accurately.

The hydrogen required for a pure H2:O2 mix will be 2/3rds of the chamber (as opposed to about the few percent that MAPP gas will be - I forget the exact number). This results in 22.24 ambient litres of hydrogen per atmosphere (or X), so a 200x mix will need 4448 litres of fuel, not 133.44.

Given that the firing rate is 6 rpm and there are 4 combustion chambers, one chamber has 40 seconds to fill with hydrogen.

Calculating the power per chamber is fairly redundant given that if you want to fire sustainably at 6 rpm, you'll need to be able to refill four chambers in forty seconds, not one chamber in forty seconds.

Treating it as needing one shot worth of gas every ten seconds (or one second, as per your original design brief) is entirely reasonable from the perspective of calculating power requirements.

A watt is defined as a joule per second, so one can assume you would need 42.5 kW.

That's fairly easily disproven. Your planned muzzle energy is 1.677 megajoules. Fired six times a minute, that's 167.7 kW. Even with an impossible 100% launcher efficiency, you'd be breaking thermodynamics.

In any case, 42.5 kW is a lot. "Half a street" of a lot.

Sorry, but it's just not possible quite as you're envisioning.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: mobile chernobyl » Wed Aug 19, 2015 2:42 pm

Why bother with power off the street? Take a page out of University of Texas's book and investigate the use of a Compulsator... Rumor has it they've been working pretty closely with their gov't funded buddies in Dahlgren, MD to work on the power supply for the Navy rail guns! :wink:

https://www.utexas.edu/research/cem/IEE ... ations.pdf
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: Fnord » Thu Aug 20, 2015 11:40 am

In all honesty, if this is your first hybrid or first large-scale build with a college machine shop at your disposal, just go for a standard 10-20x mapp/air gun with a piston valve.
After that goal is achieved, the breech and fueling can be automated with a bit of pneumatic/EE work. 5-10 shots per minute is entirely reasonable if your compressor can keep up, even with a single chamber. You'll still be getting 100+kJ of energy, which is enough to destroy just about anything down range (and wake everyone up within a half mile).

It should be possible to get this design finished in a year or two if you have the right resources.

I say this because your current design is really just higher-energy version of the M2/M3/M6 tank gun from WWII. It was perfectly capable of going through 3"+ of armor or shot-shelling an entire squad of infantry.

Anything that consumes in the 1-10 MWh scale is going to cause your nearest substation to crap its pants wondering where sudden electricity demand of a town or small city came from.
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Re: Automatic Piston Hybrid Cannon Design

Unread postAuthor: lordBenio » Sun Sep 06, 2015 4:04 pm

Thing is, I wasn't planning on building this cannon for quite some time and dry cell electrolysis is dependent the amount of amps you put in and the number of plates used. So you can get high outputs of hydrogen.
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