Bowling Ball Mortar: Seeking technical feedback

Discussion in 'Cannon' started by Pyroflume, Aug 12, 2019.

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  1. Aug 12, 2019 #1

    Pyroflume

    Pyroflume

    Pyroflume

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    About six years ago, I purchased a homemade bowling ball mortar from a man who had experience constructing bowling ball mortars. The mortar is made from a gas tank and has a welded-on breach piece with a chamber that holds up to about 4 oz of black powder.

    I love the mortar but have toyed with the idea of building an improved mortar someday, and I’m looking for feedback about whether my ideas may be feasible.

    The one thing I really don’t like about my current mortar is A) how dirty it gets when fired, B) how it is susceptible to corrosion if not cleaned immediately (and even if it is cleaned well, since I live in a humid environment), and C) how heavy it is (it weighs 300 lbs, making it very stable when fired but also very challenging to transport).

    My primary question is whether any alloy of titanium would be suitable for a cannon? I am aware that titanium and steel have very different properties in terms of strength, brittleness, weight, deformation, fatigue, elongation, ability to handle stress, etc. I am also aware that titanium, for a number of reasons, usually makes a poor choice in applications with pressure spikes (e.g., as a rifle barrel). However, there are many different alloys of titanium and I am wondering if any of them might be suitable for use as a bowling ball mortar? The appeal of titanium is it’s lighter weight and tremendous corrosion resistance. (And yes, I know that titanium is very expensive and difficult to tool and manufacture...so my question at this point is hypothetical). If titanium is a poor choice for a mortar, what about stainless steel or some other metal allow that is more corrosion resistant than normal steel?

    Another question is about powder. Currently I use FG black powder in my cannon (up to 4 oz max). Would any less corrosive blackpowder substitutes be suitable to use with a bowling ball mortar (for example, blackhorn 209)? I’m aware that blackpowder and blackpowder substitutes have very different properties (in terms of burn rate, pressure spikes, etc). I think I already know the answer to this question but would appreciate hearing from anyone who is more informed and knowledgeable.

    Another question is about ceramic coatings for a steel bowling ball mortar. If I painted a bowling ball mortar in something like high-temp cerakote (designed to withstand temps up to 1800F), would that potentially protect the steel from corrosion? Would the coating stay on, considering the heat and pressure of firing the mortar?

    My final question is whether there are any reputable manufacturers or builders of bowling ball mortars other than Coaches Club Cannons (see http://www.coachesclubcannons.com/Cannons/Bowling-Ball-Mortar)?

    Also, I’d be very grateful for not getting flamed for asking “stupid questions” or being seemingly lazy for not wanting to clean my mortar. I ask these questions in earnest in order to become better informed and to promote safety.

    Thanks in advance for any feedback and advice!

    - Dave
     
  2. Aug 12, 2019 #2

    Loyalist Dave

    Loyalist Dave

    Loyalist Dave

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    Well first, A, B, and C, above describe the very nature of a very large, black powder mortar. Black powder is dirty (only when compared to modern powder). Black powder residue is corrosive. Black Powder mortars need to be stable when fired, thus their mass. It's tough to get away from those and still have a mortar, I think....

    Next, most if not all of the black powder substitutes are actually a nitrate similar to potassium nitrate, plus some charcoal or coal. They either reduce or omit the sulfur to make them less prone to deflagration and thus are no longer considered an "explosive" but merely flammable. If you take Pyrodex, and remove the sulfur and charcoal, but keep the graphite..., you get Black MZ, which the company says is virtually non-corrosive. It's probably the lack of sulfur. Maybe that will help you?

    Cerakote would help the exterior of the mortar resist corrosion, but I don't think it would last in the chamber or where the ball sits. I think the sudden spike in pressure would cause fissures, perhaps too small for the naked eye. These would lead to corrosion which would undermine the adhesion of the Cerakote. BUT you could contact them and ask. They'd know what it can and can't do.

    Have you thought of Stainless Steel? Not lighter, but it would help with corrosion. I think the mass you actually don't want to lose too much.

    Perhaps if you reduced the outside diameter THEN had it wrapped in carbon fiber filament? Perhaps that would hold the pressure, but drop the weight down enough for you ??

    LD
     
  3. Aug 12, 2019 #3

    Zonie

    Zonie

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    I'm going by memory and prices have increased greatly since I retired but as I recall, the unmachined titanium forging we used to make our centrifugal compressor wheels cost well over $1500 each. With inflation, that would be over $2400 today.

    These were about 11 inches in diameter and 10 inches tall.

    Titanium, in order to even began to stand a chance of being used as a mortar would have to be a forging. The likelyhood of finding one that would be the size you would need rates up there with finding a chicken with a full set of teeth. Tooling for making a large titanium forging could easily cost over $150,000.

    I think one of the new synthetic black powders based on ascorbic acid could greatly reduce the amount of fouling in your mortar. These powders are known for their clean burning and weak pressures. I'm sure they would be safe to use in your mortar and sometimes they can be found rather cheaply.
     
  4. Aug 13, 2019 #4

    Pyroflume

    Pyroflume

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    LD, thanks a lot of your response! Helpful information. LOL...yes, as you pointed out, A), B), and C) pretty much describe the very nature of a large black powder mortar. But perhaps with some technical innovation, I can reduce a little bit of A), B), and C)? I like the idea of stainless steel. I wonder who might be able to collaborate with me to brainstorm a design and build it? Unfortunately, I don't have the tooling, welding, and machining capability or know-how.

    - Dave
     
  5. Aug 13, 2019 #5

    Pyroflume

    Pyroflume

    Pyroflume

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    Zonie, thanks a lot for the info. You're absolutely right that titanium would be crazy expensive! But perhaps a titanium mortar would make an interesting family hand-me-down for generations to come? LOL. It's bound to last virtually forever. BTW, I still have to do some research to find out if its a viable idea, but high-strength 3D printing is now possible with titanium. If I remember correctly, it involves vaporizing powdered titanium using a laser. Anyway, apparently high strength aerospace parts such as rocket nozzles are being manufactured this way, so maybe it would be more feasible than forging titanium? Also, I'm really glad to hear I can probably start using a black-powder substitute that is less dirty. That will definitely help with mitigating corrosion, and I'm guessing that without sulfur, there is less likely to be the rotten egg smell that I have to endure as I drive home for hours.

    - Dave
     
  6. Aug 13, 2019 #6

    Zonie

    Zonie

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    IMO, the smell of sulfur goes along with shooting historic old guns. Without it, something seems to be lacking.

    Protecting your mortar from corrosion from black powder isn't that difficult. The various salts that are made when black powder burns are all water soluble so just thoroughly washing out the bore with water will wash them away.
    Then, all it takes is to apply a coating of oil to keep the oxygen away from the metal. An aerosol can of light weight oil will do that.
     

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