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A few air blaster questions

 
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A few air blaster questions
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chris_ngale
Junior Member


Joined: 13 Dec 2016
Posts: 32
Location: Cambridge/Durham



Post A few air blaster questions  Reply with quote
With the Rayven afterburner concept reduced to a motor swap, cage realignment, LSK barrel and paintjob - only a couple of days' work really - I'm contemplating the realities of making a decent, solid air blaster in an empty Longstrike shell I have - the insides are knackered, the previous owner tried to put too much spring into it and it started spitting orange plastic instead of foam!

I have a couple of 4Bs knocking around but I haven't had the time to crack them open yet (and they're currently 200 miles away until I get back to uni) so I'm not sure if they're gen 1 or gen 2 - they're circa 2010 so it could be either. Obviously if I have a gen 2 I could do the typical 4B/LSK integration, and with a gen1 it would just be a question of ingenuity and a decent paintjob, but there's a definite appeal in making something unique and preferably full of shiny things. As such I've put together a rough plan:




For the compressor I have my eye on one of these: https://www.amazon.co.uk/d/Portab...Compact-Air-Compressor/B00BYORVMG - I reckon that with the shell and pressure gauge off, the compressor itself should be nicely sized to fit into the magwell. For the piping I could use nylon pipe, but 6mm aluminium or brass would be so shiny. The breach would probably be 9/16ths brass for a loose fit, but I've never barreled an air blaster before, so if anyone has any better ideas I'm open to them. I'll ideally be firing Waffles but I'm worried the wide rubber tips will drag on the sides of the tube.

Basically, the pump would fill the tank until the pressure switch triggered. Then, when the trigger was pulled, the microcontroller would open the solenoid for a set time, making the QEV release the air tank into the barrel, then close it again and re-prime the tank. Rinse and repeat. The ATMEGA328 microcontroller is the same chip out of an Arduino, meaning I could very easily use my Arduino board to program it, then just stick it in a socket soldered to a piece of stripboard. Using a microcontroller to time the air release seems excessive - but what in Nerf isn't? - but it should also mean a very consistent air release - same pressure and same release speed every time. It also leaves the opportunity to fit flashy LEDs, shot counters, things that go beep when you turn it on...

The thing I'm having the most trouble with finding is the air tank. Above all I want the finished product to be safe, so I don't want to touch PVC for the tank construction, even though it'll be contained in the shell. As I mentioned, I have a bunch of 4Bs lying around, but I'm not convinced they'd handle the pressure any better. Obviously the pressure I can work with would be restricted by the pressure the compressor can handle - since the gauge reads up to 5bar I'd be inclined to use 4bar - possibly with a bike pump mounted on the underside as an 'overdrive'.

I was thinking of using large diameter copper pipe with a soldered end cap. Using some rudimentary calculations, if I take my 60m/s target with the 1g dart weight, I end up needing a muzzle energy of ~2J, call it 3 because not all of that would be transferred to the dart. Using the tank's stored energy as being 0.5*Pmax*V (gauge pressure) I need about a 1e-5m3 tank, which in real terms translates to a 3cm length of 22mm copper pipe. This seems awfully small, and obviously I haven't accounted for mechanical losses or heat loss here (I'd guess I'd probably be after about quadruple the size in the end), but you get the gist of where I'm coming from. Obviously the questions this raises are 1. Is this safe? and 2. Would this work, or have I got something terribly, horribly wrong? Also, is there anything much simpler that I've completely skipped over?

Finally, legality. I have absolutely no intention of making this thing in any way war-friendly, it's a "cos-I-can" design, so that's no concern to me. I've spent some time delving through the most up-to-date copy of the Guide on Firearms Licensing Law I could find (April 2016, https://www.gov.uk/government/upl..._Licensing_Law_April_2016_v20.pdf) The sections which appear most relevant to my application are points 2.45 through 2.47, which declare:

    - Air guns, air rifles and air pistols are exempt from the certification requirement if they are not of
    a type declared specially dangerous.
    - The Rules provide that any air weapon is ‘specially dangerous’ if it is capable of
    discharging a missile so that the missile has, on being discharged from the muzzle of the
    weapon, kinetic energy in excess, in the case of an air pistol, of 6 foot lbs or, in the case
    of an air weapon other than an air pistol, 12 foot lbs.


12 ft-lbs translates to 16J in proper numbers, which is way, waaay over my intentions - even the lower 6 ft-lb limit for pistols is above the energy those famous 450fps 4B LSKs achieve at the muzzle. I've heard of the 1J Airsoft limit (commonly translated to 130FPS in Nerf terms) but within this document this only appears to apply to the sale of firearms without a license, rather than their use. Could somebody clarify this, and whether I've missed anything important? The last thing I want to do is get myself into a legal mess for the sake of a toy.

Ta in advance Smile
Mon Apr 03, 2017 3:40 pm View user's profile Send private message
Andrew_Aitchison
Junior Member


Joined: 16 Apr 2016
Posts: 80
Fav. Blaster: Stampede
Location: Surrey



Post Reply with quote
I wonder if the air compressor would fill up the tank quicker than 3 manual pumps.
Also I love modding and we are both at durham uni so we need to meet up.
Mon Apr 03, 2017 3:52 pm View user's profile Send private message
chris_ngale
Junior Member


Joined: 13 Dec 2016
Posts: 32
Location: Cambridge/Durham



Post Reply with quote
Andrew_Aitchison wrote:
I wonder if the air compressor would fill up the tank quicker than 3 manual pumps.
Also I love modding and we are both at durham uni so we need to meet up.


Probably slower, but reduces the actions needed to prepare between shots to simply opening the breach, stuffing in a dart and closing it again, rather than having to twist your elbows round at an ungodly angle to prime a BBBB pump out the back of the LSK shell, or mucking up the lines of the blaster but sticking a shiny plastic handle out of the front.

Good grief, Durham must be a veritable hive of modders...
Mon Apr 03, 2017 4:04 pm View user's profile Send private message
OldNoob
Foam Data Collector


Joined: 07 May 2013
Posts: 4834
Fav. Blaster: Xtreme 180 Rapidstrike
Location: In the Boonies



Post Reply with quote
8.8 bar is MASSIVE, well over what the tank will take without going bang. This would also be much better off an airsoft or paintball HPA bottle.
I have also not seen any small enough 12v compressors that will fit in a LS mag well.

_________________
Big_Poppa_Nerf wrote:

Boff whats the damage? I have spent over 3 times my Nerf budget this month already. Part of me is trying to be a responsible parent/husband/house owner. The other half is just says 'Ahhhh, Screw it!'.
Mon Apr 03, 2017 4:15 pm View user's profile Send private message Send e-mail
chris_ngale
Junior Member


Joined: 13 Dec 2016
Posts: 32
Location: Cambridge/Durham



Post Reply with quote
OldNoob wrote:
8.8 bar is MASSIVE, well over what the tank will take without going bang. This would also be much better off an airsoft or paintball HPA bottle.
I have also not seen any small enough 12v compressors that will fit in a LS mag well.


This is why I'm trying to get a 'taste' for these things before committing, while what I can work out or read up on might look good on paper I don't have the experience yet. I based the 8.8bar (~130psi) off release valves which were readily available, and off the engineering toolbox guide to solder joint pressures in copper pipe, which suggests a joint rating of somewhere in the region of 12bar http://www.engineeringtoolbox.com...ratings-solder-joints-d_1162.html - my proposed working pressure is significantly below this, nor could the pump supply this, and it's very much just a last safeguard - and possibly also a handy pull-release air dump valve. At my year's internship at a tech design/consultancy firm before uni we were regularly handling high-pressure equipment - a plain old beer keg sits at 12(!) bar, which is more than enough to blow all the tiles on the ceiling off, as one of my colleagues discovered when they joined the company... so this is where my current feel for pressure ratings is coming from.

Figures for the pressures actually used in Nerf air blasters seem to be hard to come across - I've seen numbers like 40psi, 50psi, 90psi being thrown around but I have a suspicion most people just build themselves a PVC tank using rule-of-thumb measurements and without actually seeing what the pressure really is - I'd be interested to know the pressure rating of a stock 4B OPRV, for instance.

I'd thought about airsoft tanks but I don't know where I'd go to find an empty tank the right size - I guess just ring up some places locally and see if they have any empty tanks knocking around.
Mon Apr 03, 2017 4:51 pm View user's profile Send private message
OldNoob
Foam Data Collector


Joined: 07 May 2013
Posts: 4834
Fav. Blaster: Xtreme 180 Rapidstrike
Location: In the Boonies



Post Reply with quote
I would suggest joining the HPA facebook group. The OPRV and half the control circuitry in your design would be unnecessary with keeping the (less efficient) 4B trigger valve and just going to 35-40psi on the tank pressure, either utilising the stock OPRV or a fixed pressure regulator feeding the tank, with a gauge threaded in the tank to check pressure.
Also you will be creating a "barrelled air weapon" with this so bear that in mind. It will be comically over even the closed field limit for Britnerf sanctioned events as well, unless the pressure in the tank is set at a fixed, non user adjustable level that results in a sub 170 fps muzzle velocity.
UK Foam shot over 260fps with his air blaster, which hasn't got such a big tank as your proposed one.

_________________
Big_Poppa_Nerf wrote:

Boff whats the damage? I have spent over 3 times my Nerf budget this month already. Part of me is trying to be a responsible parent/husband/house owner. The other half is just says 'Ahhhh, Screw it!'.
Mon Apr 03, 2017 5:32 pm View user's profile Send private message Send e-mail
chris_ngale
Junior Member


Joined: 13 Dec 2016
Posts: 32
Location: Cambridge/Durham



Post Reply with quote
A fortnight till I can size it up against the LSK, but I reckon it looks promising...



The fan can go, it's just there to draw air into the casing. The pressure gauge is more tricky because it's fitted into a moulding which forms part of the pump's one-way valve, so that would likely need replacing to get rid of it - which means fabricating an at least semi-airtight component, it'd probably have to be a hand-machined aluminium component with an O-ring to seal. Runs nicely on the 7.2V NiMH I have lying around, I may run it on 2S instead of 3S in the end, I'll see how it goes filling the tank.


Tue Apr 11, 2017 10:14 am View user's profile Send private message
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