Has anybody tried building high-efficiency cannon using a valve such as the FV-3? With an accumulator connected to port 2, source connected to port 3, and cannon to port 1, the system would charge the accumulator in the home position then discharge the accumulator into the cannon while cutting off the supply gas if I'm not mistaken. Size/weight aside, couple that with an interrupter/sliding breech (that need not have a perfect seal) and that could offer improved gas efficiency. Thoughts?
That is a very interesting idea. It would depend if the accumulator charge by itself is enough to shoot the round. The accumulator would be sized to give just enough gas to do the deed. It would also prevent multiple cannons from "stealing air" from other cannons in the firing circuit. The downside may be a longer recharge time which would reduce rate of fire. Also, I could see a problem of not enough gas to move multiple rounds, such as a misfiring cannon that loads two or more rounds in the breech. Still ... your idea sounds like something to try out. Hmm ....
After a couple more moments of thought, I believe it would depend critically upon having a properly-sized (probably somewhat large) accumulator to be able to fire at a conservative gas pressure under "worst-case" climate conditions, being "mis-fire resistant" or perhaps even needing a simple pressure transducer to detect a minimum required accumulator pressure for firing. Who knows, the slower recharge could be a good way of better regulating rate of fire for the larger caliber cannons. The gas efficiency sure is enticing though.
I'm somewhat confused... is this for a Big Gun cannon, or a Small Gun cannon? I can tell you right now that a #10-32 thread port will not flow enough gas to fire a Big Gun cannon. You need 1/8NPT to fire one barrel of any caliber. The valve for that is MJV-2 or JEV. And for multiple barrels (of any caliber), you really need to build your own valve. There are plenty of different valve designs, with varying degrees of gas efficiency, and other tricks you can do (such as closing the breech).
The intent of the inquiry is for a Big Gun gun system that isolates firing gas from the rest of the gas system at the time of firing without having to go to a custom-machined assembly, in the interest of improving gas efficiency. As for specifics, I believe that the FV-3P uses 1/8NPT instead of 10-32.
Surrender and join the other hobby... Machining I am looking at the FV-3P (but my catalog might be old), which has 1/8 outlets and supply. The confusing thing from the web is that the outlets are on opposite sides of the inlets. If it's for cruisers and destroyers, maybe it will have enough "umpf" to fire two of the smaller rounds. Similar to the Arizona gun... it might be a viable concept. Mike3
How does using this new valve improve efficiency over a standard MJV-2 and MAT-2 setup? Does it have a higher flow rate? Does its internal gas routing permit better efficiency? Right now, when I look at it, all I see is a different valve in place of the MJV-2 valve on an Arizona cannon. An MJV-2 valve can flow enough gas to fire up to one 1/4" round, or, theoretically, two .177" bbs. Couple an MJV-2 with a MAT-2 volume chamber, and you've got yourself the beginnings of a very efficient and powerful cannon. I used three of those, chained together, as a 7/32" stern cannon in a Scharnhorst. Even without a closed breech, it operated just a little above 100 PSI.
Correct me if I'm wrong in how I'm seeing the way this works (although my current projects are being built for Big Gun, my past experience is only with the piston-selector gun that was in my USS Gearing) but in traditional Big Gun cannon systems, during firing, there is no isolation between the cannon and the gas supply system nor a system that closes the supply of gas into the cannon after a particular volume of gas discharge (unless you count releasing the trigger), making it possible for the gas system to loose propellant as long as the trigger is held down even after the projectile has been fired, however briefly. By isolating the gas used for the firing (in a manner not unlike some paintball markers) from the rest of the system, this source of propellant gas loss no longer exists. My thought was that based upon an assumption that the flow characteristics are at least no worse than the MJV valves so by preventing the discharge of excess gas into the cannon, there could be an efficiency gain. Before I get carried away with trying to chase down what may be the least significant sources of cannon system inefficiency, how do each of the various elements of a typical Big Gun gas system (tubing, accumulators, flow rate/restrictiveness of valves, etc.) stack-up in terms of influence upon the efficiency of gas usage?
Well, in the grand scheme of things, the efficiency lost by having the accumulator open to the gas supply is negligible. In fact, there are quite a few larger sources of lost efficiency that can be considered "negligible". Before I continue, I must inform readers of a few things. First, I am not a cannon-design expert. I have not spent years studying hydrodynamics of gas flow, or taken "Cannon Design 501 for experts". Much of my claims are based on real-life observations in the field and reported statistics from other people, and the rest is purely hypothesis to explain my field observations. I have not run any scientific experiments to prove or disprove my claims, and for all I know this could all be a load of @#$%. So don't accept my statements as fact, think about them a while and decide their validity for yourself. Secondly, all of my claims are based on my definition of "efficient" as the smallest number of CO2 molecules required to fire a cannon at its maximum allowed foam-penetrating power. Because Big Gun cannons are not permitted to penetrate through 2" of foam, the ideal state of a cannon is to penetrate far enough that the ball is ALMOST breaking the back surface of the foam, but not quite showing itself. This does not necessarily mean lower pressure is good. At first glance, it may seem that using a lower pressure is good, but that is not necessarily the case. For example, one person I know had a four-barrel cannon that would reach its maximum legal penetration at 80 PSI, using 8 cubic inches of accumulator volume. Another four-barrel cannon could reach its maximum legal penetration also at 80 PSI, but this cannon only used 4 cubic inches of accumulator. The second cannon used fewer molecules of CO2 to fire the same payload, and thus is more efficient. What this post seeks to explain is why one cannon is more efficient than another. And now, without further ado, here is a list of the top causes of reduced efficiency. 1) blow-by Blow-by, or the gas that leaks around a projectile in the barrel of a cannon, is the leading cause of efficiency loss. Most of the propelling force that accelerates the projectile in your cannon is caused by a pressure differential, with higher-pressure gas behind the shot and lower-pressure gas ahead of the shot. When gas leaks around the projectile, or "blows by", it reduces the pressure behind the shot and increases the pressure ahead of the shot, causing a double reduction in the cannon's efficiency. Blow-by can be reduced by using tighter tolerance barrels and a tighter tolerance breech. The downside to this is that you increase the probability of jamming, either from dents and dings, or from grit when you sink. 2) air path The path that the air takes from the accumulator to the breech, and then out of the cannon, has a huge effect on your cannon's efficiency. The cause for this is twofold. First, as gas expands to take up volume, its pressure drops. A twisting, volumous air path from the accumulator to the breech means that the pressure will drop before it even starts pushing the shot out of the barrel. Second, pressure differential isn't the only motive force that accelerates the shot. The second motive force is the impact of a pressure wave of gas from the accumulator. Our cannons don't operate on an ideal gas. An ideal gas doesn't have mass. On the other hand, air and CO2 do. That mass can be used, if accelerated and rammed into the back of the shot you're trying to fire. However, every direction change in the path of the gas flow will reduce the speed of the gas when it hits, reducing the initial effectiveness of the pressure wave. For example, the standard Indiana cannon includes four 90-degree direction changes. A more efficient Canister cannon has 3 direction changes. A super-efficient "Stomper" Canister cannon has only 1 direction change, and one particular Submarine Torpedo cannon design I've developed has exactly 0 direction changes. There is one more element of the air path that must be addressed, and that is flow rate. The entire gas system must be capable of flowing gas fast enough to accelerate the shots at their maximum possible rate. To do this, there is a general rule of thumb I follow. Take a look at the cross-sectional area of any given section of the air path. In an ideal cannon, the cross-sectional area is largest at the buna-ball valve, and smallest at the ends of the barrels. Remember that even when the air path splits in different directions, it's the total area, not each individual part. In my cannons, I am satisfied if the cross-sectional area of the air path is constant. The trick to getting efficient flow (and enough of it) is choosing the right valve shape and size. However, the downside is that all of the more efficient valve shapes require more precise and difficult machining. That's why BDE mass-produces Indiana cannons, rather than canister cannons or Stompers. 3) open breech When the air from the accumulator runs into the shot in the breech, it has two different paths it can take. It can either push the shot currently loaded up and out the barrel, or it can push the next shot up and out the breech. Usually, the gas does both. However, gas that is spent pushing the next shot out of the breech is gas that does not push the loaded shot out the barrel, and hence is wasted. You can prevent this by closing the breech. I have seen various methods for closing the breech, each with varying levels of success. The downside is that a closing breech mechanism requires more maintenance to keep working than a cannon that doesn't have a closing breech. 4) improper accumulator size This is probably a shocker to some people, but maximum cannon efficiency is not achieved at low pressures. In fact, maximum efficiency is theoretically achieved at 150 PSI, once all other major efficiency reducers have been dealt with. Remember what I said earlier about the gas our cannons fire being a non-ideal gas? Using a higher operating pressure increases the impact of the pressure wave, and also provides a higher pressure differential in the breech. The way to increase operating pressure (without introducing other inefficiencies that would require it) is to decrease the size of the accumulator. A smaller accumulator requires a higher pressure of gas to fire. This can also be used to go the other direction, with larger accumulators yielding lower pressures, but once you go beyond 1.5-2.0 cubic inches per barrel you encounter the problem of diminishing returns. Going with a higher pressure reduces the total mass of gas that you use, but it also has downsides. First, higher operating pressures increase the probability of leaks, and increase the wear on components. Second, operate at a high enough pressure, and you won't have any adjustment room for when the weather changes and you need to increase pressure to maintain your maximum allowed penetration. Generally, people are happiest when their cannons operate around 100PSI or so. 5) speed of valve opening Remember what I said about how part of the motive force behind the shot you're firing is from a pressure wave? Well that pressure wave isn't very strong if the buna-ball valve just dribbles open. If it just decides, "I think I'll start to open" and finishes a while later, your pressure wave will be pretty weak. On the other hand, if it snaps open almost instantaneously, you'll have a much more effective pressure wave. Basically, if the cannon sounds like a very sharp, distinct "PUH!" then you're OK. If it sounds more like a "BLEAH" or a "HUH" then you're in trouble. There are two ways to overcome this. The first is to run your firing valves and your accumulators on two different pressures. Ideally, you would run 150 PSI to your firing valves, and then regulate down your accumulators to whatever pressure they need to fire at maximum legal penetration. The other way is to use a "negative pressure" cannon with a quick-exhaust valve on the actuator. Unlike regular cannons where you apply gas pressure to the actuator to fire, a "negative pressure" cannon fires when you REMOVE pressure from the actuator, hence the term "negative pressure". Simply using a negative-pressure cannon increases the rate that your buna-ball valve opens, and adding a quick-exhaust valve to the actuator brings the firing rate up to nearly instantaneous. The biggest downside to a fast-opening firing valve is the increased wear and tear of quickly opening and closing. Also, operating at two different gas pressures increases complexity and reduces reliability, and negative-pressure cannons are among the most difficult to manufacture.
An example for #2: try placing your fist against a someone's face and pushing as hard as you can. You'll find it isn't as painful as accelerating your fist before impact and transferring momentum to the idio... test subject's face. #4 needs some checking. I don't recall anyone has tested this, but it makes sense to me. If this weren't the case, then we would all use huge accumulators and massive, high flow valves to operate as close to 0 psi as possible, but I doubt such a cannon would have enough power. Also, to clarify: the "people" who are happiest when cannons are at 100psi are my brother and I, and we can't speak for other builders. I personally think this provides the best mix of power and low stress on components. #5 You can tell your actuator is fast when you hear a crisp "crack". I think I fell in love when I heard the sound of Warspite's new cannons. I think I may even bring my transport alongside to dance with her just so I can hear the symphony of her crisp, fiery voice before Davey Jones separates me from my beloved. I don't think that you'll notice any change in wear on the valve when you open it faster, the only downside is that operating on more than one pressure is more complex and reduces reliability.
Hey Kotori... I don't mean to dispute you, but I think you are an expert I think wookies idea is very similar to the arizona design, but slightly modified. I am having trouble figuring out the squiggles and hieroglyphics in the clippard catalog... If you could get a readily assembled double .177 I think it would be a good thing. Maybe too heavy for small boats... but more options mean more interesting boats Certainly if there were a Clippard part that could be adapted with work done by just a drill press and tap... the barriers to getting cannons without buying a 7x10 lathe and an x2 mill would decrease. 5) I've only seen one working negative pressure cannon system... but I like it. I have thought that once the initial pressure difference is overcome, and the traditional MP-7 opens, then the valve will have no back pressure resistance and open fully. I also had thought that the clippard actuator had a mechanical advantage... the listing for factor. I could be wrong. My first boat has 7/32 JCWhite tripples, and has a 3/8 or so ball. (I didn't measure too closely when I had to replace the plastic 15/32 32 thread with an insert) Maybe the whoosh instead of crack is that the instantaneous flow is reduced by poor flowpath. I had thought that the advantage of the negative pressure cannon was that the closing air supply also vented to the firing suppy in theory. Luck Mike3 horne
It's also much more difficult/expensive to build, especially bending barrels while maintaining tolerance throughout the bends. I wonder if anybody has ever tried using a butterfly valve? Seems to me like it could provide a very quick opening rate plus an efficient path & high flow volume. Also, force required to operate should be reduced since you don't have to overcome the pressure of gas on the other side, as with a poppet (which the Buna-N ball type really is). Also, as the magazine is depleted its ability to absorb (waste) more gas increases, making the problem worse the more rounds you shoot. Also, more complexity/cost to build in the 1st place. The tolerance necessary to prevent blow-by & leaks may make the mechanism so tight that it's difficult to operate. Phil Sensibaugh of MBG did a lot of testing on accumulator size (among other things) years ago. You might want to pick his brain over this (& probably other issues of cannon design as well). Perhaps more than just 2. Consider the butterfly valve, & there may be other options as well. Also, I'd add that system safety is a concern where QEVs (Quick Exhaust Valves) are used. I don't know of a simple way to "safe" the system at a single stroke where QEVs are used, since the immediate drop in system pressure generally causes the guns to fire, which is exactly what you want to prevent with a single-point safety in the 1st place. JM
Nope, I'm not an expert. Look at all the things I missed, that Gascan and John pointed out! Once I edit my previous post and add in their suggestions, THEN I'll be an expert Mike, you may be right about insufficient flow and poor flowpath in your cannons. A 3/8" ball is too small for a triple 7/32" cannon. Even assuming that your barrels have perfect 7/32" diameters, you're looking at a cross-sectional area of at least 0.1127 sq.in. Further assuming that, by "ball" you meant "aperture", that provides a cross-sectional area of 0.1104 sq.in. which is smaller than what the 7/32" barrels need. Expand the diameter of the barrels to real-world measurements, and you would have better results with a 1/2" diameter aperture in your buna-ball valve. John, you are right on about the safety concern with negative-pressure cannons. If you use the standard gas setup, then the moment you flip the safety switch you will drop the pressure to the firing lines, and trigger the cannons. There is a way to overcome this, but it is complex and leak-prone. Basically, what you need is a complex collection of carefully placed check valves and other fittings, that allow you to vent the accumulators without venting the firing lines.
A five port toggle switch will do the trick for negative pressure cannons, as I recall. You need a second air line coming from the accumulator to the safety valve, and it drains pressure through this second air line.
Thanks Gascan, that's good to know. I always have liked the negative pressure cannons. They Pop! instead of Fssssppppt!
Kotori, the old cannons always had some gremlins... I don't recall the exact diameter, but it was smaller than I had expected. The aperture vs ball concept is a good one. I wonder if a larger chamfer will help too, both for sealing, and gas flow upon opening. Don't race cars use some fancy three chamfer valve seat? Wookie, for small ships where gas efficiency is a concern, I think you are on target... Ham fisting the firing control will vent both the firing pressure and the supply pressure past firing the cannon, and if you are not careful, it will be enough for the balls to fall back into the riser and fire again. If you ever see a boat dribbling bearings onto the deck or over the side after firing... you now know what is happening. I definitely will be looking into the firing setup you described. Mike3
Somewhat tying into the Rookie Ship Design Project, if we move on to a smaller vessel next such as a Destroyer, balancing gas efficiency and the ease of cannon assembly will be paramount in conjunction with the need to keep the weight of components to an absolute minimum.
Not disputing, but I'd really like to see how. Seems to me that there needs to be some small delay between venting 1 side & the other. That might be achieved somehow using an extra accumulator in the system (sort of like a capacitor in electronics, to store the charge), or not, but I'd like to see it tested & documented to be sure. JM
