Big gun cannon construction questions.

I am building some big gun cannons for my DKM Bismark. So far I have ordered the parts for the magazines from pololu.com But I wasn't sure how to build the bases for the cannons and what material to use for them. Any ideas?
I was also wondering whether the servos for the cannons have to be water-proof. I know this may sound like a stupid question, but water-proof srevos tend to be very expensive and I was hoping that maybe the cheap servos could still handel the water. Do they?
Thank you in advance,
-Dexter

 

Ahoy Dexter,

Cannon bases are usually built from machined PVC or ABS plastic. Unless you have your own lathe and drill press, making your own is going to be difficult. If you are seriously interested, your best option is to draw up a design that a machine shop can produce, order a bunch (probably 25+ bases), and sell the extras to other people in the community. There's been a huge shortage of Big Gun cannons recently, and if you stockpile a generic cannon base, that would go a long way towards helping the hobby grow. Another option is to 3d print cannon bases from ABS plastic, if you have a 3d printer available...

For servos, they do not have to be "waterproof". It turns out that fresh water does not directly harm electronics, only the corrosion caused by extended exposure. I use standard, non-waterproof servos all the time. To prepare for sinking, I drill holes in the case to ensure water can drain out as quickly as it enters. I then coat the circuitboard with a protective layer of skotchkote. CA glue or epoxy resin also works, but those are more permanent modifications. When the ship sinks, the servos flood, and don't hold their position quite as accurately. When I recover the ship from the bottom, I pour out the water and the servos drain out, returning to full capability. Another option, if your pond is fairly shallow, is to seal the servo case yourself using an o-ring.

 

Thank you for the help.
I have some valves that I am not using at the moment, so I was wondering if this would work for a BG cannon base:
Here is another picture of the parts separated. Any advice on whether this will work?

 

What if I use a higher air pressure? Will it then still be to low or is it that the opening in it is too small?

-Dexter

 

It is an issue of the size of the ports/ internal passages, and hoses that would feed them. The typical big gun cannon is somewhat air hungry. We are also limited to 150 psi air pressure as well (by the rules for safety) so there is a limit to increasing the pressure. Running a high air pressure is usually not necessary if all other factors are good (accumulator size and sheer flow capacity of all components in your system - depending on how everything is configured, you are typically limited by the smallest cross section that air has to flow through beyond the accumulator.

 

I have heard that a MAV-2 with a sufficiently large accumulator is capable of firing a single .177" bb, if you have a VERY long barrel. But it is not suitable for any practical cannons. The average Big Gun cannon's buna-ball valve has a cross-sectional area for airflow of 1/2" to 5/8", which is larger than some valves in nuclear reactors. It needs that big of a cross-sectional area to have enough airflow. That is why we machine them from PVC or ABS, because no commercially-available valve has sufficient airflow in a sufficiently compact size.

 

I am not very good in drawing, so the idea of drawing plans and having copies made makes me worry if it will turn out well.

Would it work if I ordered a solid plastic rod from here http://www.iplasticsupply.com/materials/abs-cycolac-absylux-royalite-sheet-rod/#ABS%20Pricing . And then drill in the holes for the air flow?

 

I realize it's been a while since this thread was active....likewise a good many years since I was active and building gun systems. That said, when I was building systems I started to experiment with combinations of valves and sizes of reservoirs. When you think about it, it is the pulse of high pressure air that propels the projectile. Once the ball-bearing clears the end of the barrel any air left after that is essentially....wasted. So....exactly how much air and how big a pulse is actually needed to push the bearing to the velocity necessary to do the job? As a result of my now ancient experiments I was able to virtually halve the size of the reservoir tanks of my systems. This allowed me to have very effective systems that fit in much smaller platforms, i.e. Destroyers, Torpedo Boats and I was working on 1/144 scale submarines when I had to leave the hobby for a while (military career/life/family), but now I'm back and getting back into the game. Not only that but my onboard CO2 primary tank lasted considerably longer for its given volume. Less waste is more bullets down range....I love the smell of Balsa in the morning!!

Also I noted that pressures now allowed are 150 PSI. Back when I began in SCBG we limited pressures in systems to 100PSI. So for you Math types, with 150 PSI operating systems, how many cubic inches of compressed air is necessary in how small of a reservoir system is minimally necessary to effectively penetrate 1/8" of boat hull? Hmmmm.....back to the laboratory Igor!