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Superlight: Engineering the floating hydropneumatic high performance speargun

Thread Status: Hello , There was no answer in this thread for more than 60 days.
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popgun pete

Well-Known Member
Jul 30, 2008
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The advent of carbon fibre offers a solution to the problem in engineering a floating hydropneumatic, high pressure shooting speargun which until now has always seemed to be an impossible task. The firing valve operated guns all need an open rear end to the inner barrel tube which means that the outer tank is always the structural element holding the gun together rather than the inner barrel.

The clue as to how it can be done is in the last version of the Alcedo “Hydra”, namely the “Hydra Sprint 62”. If we look at its internal construction, which is of a modular form, we can see that like the later Aquatech most of the “extra” weight in the gun is in the thick bulkheads. Now in all the guns to date these parts are cast or machined from aluminium alloy, but they could be replaced with carbon fibre elements with the gun re-engineered to suit this new material. The question with carbon fibre is can a screw thread cut into it take high axial loads? If it cannot then the screw threads need to be engineered out of it and the construction modified accordingly as that is why the forward section is not also marked as a carbon fibre replacement on the Alcedo Superlight diagram. The problem screw thread is indicated, otherwise all the carbon fibre elements are trapped between the two outer body circlip locations. We know that this circlip holding system is adequate as this mid-handle gun operated at 100 Bar and could shoot for prodigious distances underwater.

A better candidate for Superlight conversion is the Aquatech and if founder Sergiy Kravchenko was still contactable then I would be discussing this with him instead, however the concept is getting its first airing here as he seems to have vanished from the Internet as an active presence. The problem with the Aquatech is that it may not lend itself to a circlip end bulkhead holding system, in which case the large diameter screw threads will need to be retained. This can be done by using threaded metal bushes which are annular rings that trap the carbon fibre bulkheads in their centres. A example is shown on the Aquatech Superlight diagram. Another aspect that will need engineering revision is the elbow connection from the auxiliary hydropump to the gun’s locking chamber, if the locking chamber body is itself a carbon fibre element then a strong connection to it must be retained as during gun charging this component is exposed to maximum gun pressure, that pressure being 1,500 psi (approx. 100 Bar) or even more. With extra strokes of the hydropump lever the gun could be charged well above 100 Bar which would mean that the spear would be driven to “cavitation sheath” generating velocities, decoupling it for some flight distance from the hydrodynamic drag effects of water. The problem with doing this is the gun then operates in the region of rapidly diminishing returns as more and more gun power buys little further advances in performance and the gun structure may fail alarmingly if over-zealously pushed beyond its tested limits. As always follow the manufacturer’s operating instructions and, importantly, read them in the first place!

Due to the high pressure operation the cylindrical outer tank needs to be retained as a metal component, ideally titanium which has been used in the 35 mm tank diameter Aquatech guns. This metal tubing is the main structural element in the gun and must not fail or the gun will eliminate any bystanders as it comes apart in a very big way.

With these changes the releasing valve, servo operation (due to the trigger “pilot” valve) hydropneumatic guns would be floaters and probably the most powerful underwater weapons in the submarine arsenal, yet easily and safely loaded by anybody regardless of physical stature or strength, with the all important charging to shoot operation conducted from the rear end of the gun.
Alcedo Hydra SuperlightRR.jpg
Aquatech Black Sea Superlight.jpg
 
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Most of this stuff went over my head and I don't know how relevant this would be, but I wanted to bring up my experience with paintball gear. We use carbon fiber pressure vessels that have 800 psi regulators screwed into them. The vessels need to get hydro'd periodically like scuba tanks, but the reason I mention it is because they have been in regular use for a long time and are charged to 4500psi regularly. I would think construction using carbon fiber would be feasable given this information, and if a carbon fiber paintball tank can hold 4,500 psi with a reg screwed in then it shouldn't be difficult to engineer a pneumatic speargun with that material and a pressure requirement of 1kpsi.
 
I will check it out, but one thing to remember is the cubic capacity or “water volume” of the tank. A larger volume tank has a lot of energy stored in it because there are many more gas molecules in the tank and thus more of a big bang if and when the vessel fails. Scuba tanks that have blown up in scuba filling stations due to a tank wall failure have totally demolished the store and at times killed people. The pressure is important, but so is the volume. Expellable gas guns like paintball guns are only shooting a light projectile, not a long metal spear, so the energy requirement is much less. I am not saying the tank could not be carbon fibre, but generally large volume tanks are fibre wound vessels with a metal sub-structure underneath, in fact fibre wound gas tanks (oxygen) go back a long way for use in WWII aircraft in order to minimize weight. So one must be careful in making any comparisons as what works OK in one application may be unsuitable for use in another at a different scale. The “cost” of any failure also has to be set against the individual application and the likely consequences. I am certainly no expert on pressure vessels, but there seems to be plenty of info about such composite tanks on the Web.
 
There are high volume (scuba tank volume or a tad less) carbon fiber tanks rated for +200bar. I don't know if the are all a thinner alu tank wrapped with unidirectional fiber or if some are fully composite now.
 
Well I don't doubt that carbon fibre can replace many things, but for the purpose here the outer tank has other jobs, like supporting the grip handle and transferring recoil effects into the diver's hand. Spearguns need to be able to be dismantled for periodic maintenance and a metal tube based gun can support the range of fasteners required. Remember current carbon fibre tank guns such as your own modified examples and the Salvimar "Darkside" only bear the pressure trying to blow the tank wall outward, a not inconsiderable task, but they don't hold the gun together, that is the job of the inner barrel tube.

As I have examined both the "Hydra" and the "Black Sea" guns (I have the former on indefinite loan for technical examination as it was hopelessly jammed and I both own and helped design the latter) then I have a good appreciation of the variables involved. My BST800-A "Black Sea" gun is well balanced in the hand, has good recoil resistance and has amazing range and accuracy. The titanium tank means that with next to nothing in the front end of the gun there is a buoyancy effect that lifts the nose of the gun (when discharged of course) and the balance in the hand is achieved by the counterweight of all the stainless steel in the rear end of the gun. The downside is if I drop the untethered gun to seize the twitching victim then the "Black Sea" gun plunges straight to the bottom. Fernando Abella, who was "chief test pilot" of the "Black Sea" gun (or "chief test guinea pig" depending on your point of view), in one of his first shooting experiences with the gun was somewhat careless and was nearly KO'd by the recoil with the result that the prototype "Black Sea" headed to the distant bottom. A big game Tuna hunter, Fernando had to engage a scuba team to find and recover the "lost" gun from the not exactly benthic depths, but it was beyond freediving range on an impromptu basis.

While not a "show stopper" as a "sinker", the "Black Sea" gun would be a much more useful gun if it was a "floater" and it is not that big a step to replace the bulkheads, where appropriate, and the rear support arm for the hydropump lever with carbon fibre substitutes. For that matter the "Alcedo Sprint" could be similarly upgraded, although as a "Classic" layout gun I don't see it as a floater. Bear in mind that the "Alcedo" gun was so powerful that the instructions state that the spear stop rings for the line slide are consumables as they eventually succumb to the impacts. The designers state that they tried shock absorber springs on the spear tail and other methods, but to no avail. What they did not try, but we know about today, was hydro-damper line slides which are both line slide/slider and stop ring combined. As readers of my ramblings here will know the "Black Sea" gun has an appetite for stop rings as they get hammered when going from zero to maximum spear velocity in an instant. I have not tried them, but my gut feel is that with even the tiniest bit of grit a hydro-damped line slide would be almost a permanent fixture on the "Black Sea" spear tail until a gas torch and sliding hammer were brought to bear after the termination of the dive. Bashing the stop ring free with my dive knife butt and a special metal tube made for the job is only possible because there is near zero metal overlap, unlike the situation with a hydro-damped line slide.
 
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I have recently purchased an incomplete "HYDRA SPRINT 62" body, so now I can find out how practical a "Superlight" conversion really is.
 
I have recently purchased an incomplete "HYDRA SPRINT 62" body, so now I can find out how practical a "Superlight" conversion really is.
And the answer is not very practical at all as it turns out some of the modules that fit inside the outer metal tank are already made from plastic. The alloy outer tank or body tube is relatively thick in wall section so that even if all the parts inside it were gossamer light then the gun would still not float. The Alcedo "Hydra Sprint 62" is not as bulbous as the Alcedo "Hydra II" was, but it is still a heavy speargun and you would not want to lug it any further than you had to when going for a shore dive. The inner rubber pipe that serves as a boundary between water and air is torn in places as I suspect compressed air pressure trying to push the rubber pipe through the numerous ports of the vented metal tube has perforated it. In the “Hydra Sprint 62” gun’s operating instructions it is suggested that the gun be stored with freshwater inside it under slight hydraulic pressure to keep the rubber pipe held off the vented metal tube or alternatively let all of the compressed air out of the gun. Evidently whoever owned this gun, or their successors, never bothered to do that preventative maintenance and over time and with a lot of use the rubber pipe has just started to disintegrate and crack up.
Alcedo Hydra Sprint disassembly R.jpg
 
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This is what the Alcedo "Sprint 62" speargun looked like before full disassembly, note the front barrel is missing and the gun is not operational.as the rubber pipe is busted.
HYDRA INSPECTION RR.jpg
 
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