Infinitengines "Dreamair" pneumatic speargun

The new diagram shows the connecting cable as Dyneema, the piston (10 square centimetre in area) being an oval shape in what must now be an oval bore. Previously the piston was a cylinder sliding in a cylindrical bore with flanking side tubes that contained pressurized air and were cross-connected to the central tube at the front, the cross-section of the alloy barrel being similar in shape to what is used in the Sporasub "One Air". Otherwise the concept seems to be unchanged, this version having just the one roller axle and cable wishbone. The vacuum at the rear is necessary as if water filled the barrel tubing after the piston advanced against the air pressure in the front end of the gun then on the piston's return journey during the shot the water would have to be pushed out of the tube thus greatly slowing the shot. Thus for the gun to work the barrel needs to be a completely sealed system at both ends, the piston not being exposed to ambient pressure and no air transferring across the piston from front to rear. The dark spaces at the gun ends on the new diagram are not openings to the tubing as the roller/drum axle must be located in the pressurized area and for that reason I think the same applies at the rear end, i.e. it is closed off. Otherwise the gun would have to push a large column of water rearwards when it fired.

The compression ratio of the gun needs to be considered; with air from the central cylindrical bore being forced into the crescent shape cross-section side tanks by the piston forward movement a realistic compression ratio is possible, but if the oval piston version has no side tanks then the compression ratio will be higher. The edges of the tank shown on the diagram may still indicate voids or chambers on the sides, however their volumetric capacity will be reduced by an oval-shaped central bore.

For the investment required in producing specialized barrel tubing and the fabrication of spiral track winding drums or "CVT" rollers the "Dreamair" gun seems an unlikely business proposition given the prospect of low efficiency in its operation and the potential for unreliability where the various cables connect to these drums or rollers.

Judging by this Greek forum post the necessary components have been made for the single axle and twin axle versions of the "Dreamair" speargun.
http://www.spearfishingforum.gr/viewtopic.php?f=19&t=41714&start=300
It will be interesting to learn if the assembled guns function as intended. The photos appear to have been taken recently (13/02/2016).

The sealing takes place on the axles, the internal cables do not cross any pressure boundary as they are connected to the inner pulleys at the front end and to the piston at the rear. Both these items are inside the pressure chamber.

It appears that the trigger is going to be pulled in earnest on one (or two) of these guns in the not too distant future. Good to see them in the marine environment at last and away from the workbench (photos are shown as a detail taken from the larger original images on the infinitengines Facebook page).

Assuming everything works OK, and I am sure that it will in the short term, I think that from an operational perspective there needs to be the following features in any production version of the “Dreamair” speargun.

There needs to be a capped pressure bleed valve connected to the rear of the air tank. In the event of air leaks rearwards past the oval piston the necessary vacuum developed in the rear tank during gun cocking will be eliminated to a certain degree. A way to remove this unwanted air would be to periodically bleed the air out of the rear end when the gun is in a discharged condition with the wishbone back at the muzzle and the piston at the back end of the tank. The reason for a pressure tight cap is to prevent the bleed valve sucking air from the atmosphere when the piston advances forwards and more importantly water when the gun is submerged. Any water drawn into the rear tank will stop the piston fully moving back (due to hydraulic lock); any air and that air will compress until its pressure balances with the pressure in the front of the tank. Pressure equalization will be achieved because the rear tank section can operate as a high compression ratio pump given the large size of the tank bore, the piston stroke and the small “dead space” in that “pump”. The “health” of the gun could be periodically checked by monitoring any build-up of pressure rearwards of the piston by letting it out as the piston will be trying to drive that air into a small volume which is the “dead space”. A slow pressure build-up over time would indicate that the air loss to the rear end would need to be reversed by adding more air to the front of the gun using its hand pump, which I think is attached via an inlet port under the muzzle.

The wishbone cables during the shot are pushed laterally outwards at the front end by the spiral tracks on the outer winding drums and over time will wear, even in the presence of lubricating water. The user will need to replace these cables at certain intervals and to do this the outer drums need to be removable so that the cable anchors can be changed over to accept the new cord. The length of the spiral tracks on the outer drums determines the draw of the wishbone and will be matched to the gun length. I expect that side-to-side synchronization of the spiral tracks will be ensured by keyways on the axle so that they will only fit in one orientation on the axle. Synchronization is required as the drums on both sides need to haul in cable at the same rate from side to side, otherwise the spear will be pulled off the shaft guide track alignment. The outer drums should be removable with the air tank still under pressure as the roller axle sees the same pressure on its ends, i.e. ambient pressure. You can think of the axle being like a balanced valve in a scuba regulator.

As for the inner cable (or cables) this may have an easier life and only long term testing will determine what the durability will be as externally there will be no way to assess their state of wear, if wear occurs in the oil (if there is any) lubricated environment of the pressure tank.

tromic said:

Pete, have you seen this: https://forums.deeperblue.com/threads/hydro-roller.106509/
Also an interesting design idea.

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I just had a quick look at the Facebook page and it seems to convert band powered motion of a water pump of some sort to spit the spear out of the gun, but that comment is just straight off the top of my head as I have not seen anything but the main photo.

Knowing no more and purely from a technical perspective it is possible that this gun uses a roller system to drive a water pump for say a half gun length stroke on the top deck. If that pump stroke has a volume equivalent to a full gun length barrel at a smaller diameter which contains the spear then the water in the pump will increase in velocity as it leaves the pump bore and enters the barrel. This is because the mass flow rate has to stay the same as the water flows through the gun due to the incompressible nature of water. The same velocity increase in seen in sliding annular piston guns where the large diameter annular piston (which has an OD the same as the air tank ID, say 30 mm) moves in the tank and along the outside wall of the inner barrel (say 9 mm ID) at a slower rate and a shorter travel distance than the spear does when it is ejected from the inner barrel as the same mass flow rate condition applies. At first glance you would think that this is a very useful “accelerator” for increasing spear velocity until you realize that the water restriction posed by the smaller barrel resists the water flow coming out of the gun’s larger diameter “water pump” (driven by the annular piston acting as a moving bulkhead as the compressed air expands in the air tank). That is it is a form of hydro-braking; same as with the muzzle restriction imposed in wet barrel pneumatic guns in order to retain the piston in the gun. Any transfer between modes of propulsion in a force transmission system takes energy out of the system and reduces the overall efficiency. If you can afford to throw energy away then you can obtain good results, but the energy would be put to a more useful purpose if the system relied on a direct band drive. The spear tail would need a pressure seal on the extreme tail, similar to hydropneumatic guns and would be retained in the barrel without a catch as all the valve operated versions of those guns do.

Another example of a hydro-elastic gun was the SOS “Ringo”, there the rubber stretch (actually a “Vulkollan” polyurethane bladder) was used to jet water out of a short, large diameter flexible bladder with resistive walls into a slimmer diameter and longer barrel that raised the velocity as water sped up going into the smaller tube. I expect this Hydro-Roller gun is based on the same principle, but will suffer from inefficiency as all such guns do. Hydropneumatic guns throw away energy due to inefficiency, but return good performance by piling on the air pressure used in the gun. In the Hydro-Roller it looks like the doubled-up fat band battery located under the muzzle is scaled up to provide plenty of stored energy, but it will be throwing energy away as the laws of physics still apply.

The absence of further news on the “Dreamair” is perhaps an indication that there are problems. I admire the heroic and highly imaginative vision that has pushed the “Dreamair” gun so far that it is now a seeming physical reality, but it may be running into friction problems with the oval piston and oval bore barrel tube that the piston runs inside. Besides the piston having a large contact length on the seal’s periphery (I think the piston would need two sealing rings to resist piston tilting) which will add to sliding friction, there is the smoothness of the internal bore to be considered. If you look at an inclined angle along some cylindrical tubing then you will see a slight wave in the wall as the metal tube is extruded or pressed out in short sections from the forming die, causing a tiny change in diameter at regular intervals. You don’t want a sliding piston catching on such steps even though they are small. Polishing a bore to a constant diameter is not easy with a cylindrical bore, it will be even more difficult for an oval or elliptical bore. The tubing used in the Sporasub (Omer) “One Air” was the same three chamber layout as that which was originally proposed for the “Dreamair”, i.e. a central cylindrical bore with front windows connecting into the crescent shaped side tanks which allowed a long piston travel as air was compressed during gun cocking into the side tanks. The”One Air” is reported to be ceasing production, so maybe that same tubing could be used for the “Dreamair”, then the sliding piston can return to being cylindrical again. Increased inefficiencies will gobble up energy in the “Dreamair”, so to confine such losses to the roller system everything else needs to be close to perfect. The gun will work, the question is can it work well enough? I certainly hope that it does as it will be a triumph against the odds seemingly against it. Incidentally the “One Air” should have had a 13 mm or 14 mm inner barrel, the gun’s increased tank mass was wasted on a 11 mm diameter inner barrel. “Bean counters” may have had an undesirable influence on these important design considerations.

Well I cannot believe that I did not hear the champagne corks popping as the "Dreamair" has been in action against targets as seen here from the Facebook page. Available later in September, however as September is nearly over the guns may be on sale already, but where to look?

https://www.facebook.com/infinitengines/?fref=ts

The operating cycle for two-stage loading is shown on this schematic for the "Dreamair" speargun with twin axles and dual wishbones.

tromic said:

I suppose there will be water hammer effect like at hydraulic ram, on the left side of the piston!?

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No water inside, unless it leaks in through the rear end when the piston is pulled forwards by drawing the wishbones back. The chamber system is completely sealed, but I think a one-way valve is needed at the rear end to let anything out if some compressed air leaks past the piston seals into the rear chamber as then it can just vent off to the environment. I mentioned this valve earlier in a previous post.

I have now seen the final form of the alloy winding drums, both the inner and outer sets and they look like fine metal sculptures with their spiral track helical form, in fact they look very expensive items to make and must account for most of the gun's productions costs. Plastic parts can be churned out once the moulding dies are ready for production and the moulding conditions have been worked out (time, temperature, feed pressure, etc.) and the alloy barrel tubing is a dedicated extrusion, so it should just need chopping off to length.

Whether the gun needs more than 22 atm "cocked to shoot" pressure to deliver its desired performance I don't know yet. The compression ratio is around 2.0 from memory and the start pressure is 11 atm (absolute) as atmospheric pressure does not influence the gun's performance, the tank being a sealed system. Piston friction in the oval bore will determine how successful the gun is, as while other parts can be replaced, such as the long cables, the barrel bore is essentially fixed, but polishing may be needed to reduce drag on the piston seals.