VR Pneumatic Recoil Rifle Stock: DIY PART 1: Intro

Ok so a few months ago I got a VR headset, then found Onward among other games like contractors and Arizona sunshine.  Great fun,  Pretty good weapon handling.  But.... still missing things.    One of the biggest was the feel of the rifles wasn't right.   Rants aside about vertical fore grips the next aspects of the virtual rifles that annoyed me was the  lack of recoil or incredibly weak trigger pull.  Anyway, why should you read this absurdly long post.  It tells how to make the following pneumatic recoil stock for VR games for crazy DIY'ers who want a forcetube, but want more than 60N force and they want it right now!'ish.  To be upgraded with a forcetube fire control board once their kickstarter is over.  Go support it!

DISCLAIMER:   Use at your own risk.  You assume all responsibility for any injury or property damage incurred.  I include this for informational purposes only and what you do with this information is on you.  If you lack sufficient expertise to vet the information before trying to replicate it you are highly likely to cause injury or death to yourself or others near you.   Don't burn your house down, don't smash your fingers in it,  don't get blind by shrapnel,  or deafened by the concussion or air venting, always wear safety goggles, hearing protection or any other PPE that may be advisable.  Have a fire extinguisher on hand, and someone who checks in on you. Don't electrocute yourself be careful of lipos or other high density energy sources as they can explode or cause fire if you don't know what your doing, don't puncture them etc etc. Don't try this at home even though I did.  And if you do always start at low power with the test apparatus far from you and or a safety barrier  between you and anything that may fly off of it directly or indirectly.  You've been warned.  The forces being used here are dangerous.   Any person consultations are theoretical and you are to use your own judgement as to whether or not you follow any advice.  If you are not an adult stop reading this and go ask a responsible adult for guidance.

EDIT: Note do not use LiIon batteries. The stock kicks too hard and after 10k rounds or so compacts the foil in the batteries.  Use a tethered 12V supply parallel to the airline or NiCd  NiMH or other batteries proven to take a beating with radio controlled vehicles.  (dont use LiPos without great dampening)

Part 1: Intro and disclaimer
Part 2: Trigger and grip mod:
Part 3: Fore grip Mod:
Part 4: Modifying the Haptic for a signal
Part 5: Pneumatic Recoil parts and Fire Control Circuit 
Part 6: Scent Module

But, First lets discuss what felt recoil is.

Actually before that,  Why not just go buy a stock from someone who makes VR rifles.  Well they usually dont incorperate hand controllers so the game has no idea what your hand is doing if you take it away from the rifle.   And for immersion that one is pretty big to me.  Plus ego says I want to build it.  Ok back to felt recoil.

Lets leave the bulk of it to someone else to discuss who should know what they are talking about.  Searching the web this guy seemed to take a decent examination of it so Ill make him the gold standard out of laziness and not  knowing better myself.

https://precisionrifleblog.com/2015/07/01/muzzle-brakes-recoil-primer-test-equipment-rifles/

But just in case they ever take it down, Im reposting a few of their images below.

Self explanatory, lots of energy produced  and if I recall the rough rule of thumb is not much more than 1/3 of the energy goes into the bullet and we can crudely just say the momentum of the rifle at the end equals the momentum of the bullet and at worse we are off by a factor of 2x.  And we are playing video games so I can stomach 2x easily.

Now for most games I imagine I would want something between a 22lr (very weak recoil) in an unforgiving wood stock  and a 223 round (roughly equivalent to 556) with a direct impingement AR style bolt.  Maybe later we want to "upgrade" to perceived recoil of a larger cartridge in a solid wood stock (ouch).


As his article states there is argument over what is the closest measurand of perceived recoil but again I want ballpark cuz its a game and the controllers can't take a hard recoil.

But I can work with total energy, Or impulse or whatever.  Its basic math and we are spitballing.
The thing is many of these curves measure in the hundred of newtons or footpounds  in fractions of a second... so yea, spitballing.

More searching found a table of rough energies.

So if life were ideal we would have between 0.2 and 5 ft-LbF  or  about 0.3 to 7 joules. 

Now measuring the area under the curve or roughly force times time gives impulse, or change in momentum per unit time.


ForceTube Recoil:============================
So to get really sloppy with math lets look at forcetube.   (correct me if I have any of this wrong)
What we know of their prototypes (the production should be more powerful)
Theirs works by applying a near continuous force from the transducers to the buttpad insert over a distance of 5mm.  It is direct force to the shoulder counter balanced by your pull of the stock into your shoulder.
F~   60N     Force of the solenoid/spring
M~ 2.5kg  weight of entire stock and controllers.
dx ~ 5mm   Max displacement of the shoulder insert  or *2 for total return per cycle
dt ~ 50ms   Lets assume they can get up to 20 rounds per second to simulate nearly all full auto.

Lets calculate a few different ways to get a rough envelope to account for estimate errors.

Energy / work
F * dx = 60*.005=0.3 Nm     ---this came to the highest so Ill just assume this value  as the max raw energy.
1/2 M * V^2  = 0.5 * 2.5 * (.01/.05)^2 = .05 Nm    --it assumes accelerating the entire stock using the velocity of the insert and assuming the same velocity of the stock... a bad assumption yes.

Impulse  --I dont have a chart above but may be useful to compare later.
F*dt = 60 * .05  = 3 Ns   --- again we will take this one as the highest
M*dV =  10mm/50ms*2.5kg  =  0.01/.05*2.5=0.5 kg m/s

To compare to the chart above  .3Nm converts to 0.22 ft Lbf  so about that of a 22lr!
or at worst  .05Nm =>  1/6th that of a 22lr


Pneumatic Recoil:===========================
The pneumatic recoil, as its presently designed, accelerates an 8oz mass at 60psi to slam into a glass filled nylon and firm rubber butpad (1cm thick in total) directly into the shoulder.  So rather than a solid force you feel a shock or more of an momentum transfer.  I chose 8oz as that is the weight of a light AR15 bolt according to google.
Ok the specs on my parts are.
Pressure 60Psi peak
Piston diameter 1 1/16"    website says for 60PSI it delivers about 50lbf
F = 50lbf or 222N
Mass of bounce weight  8 oz  (1/2 lb)  or 0.227kg
Mass of stock with everything  5.5lbs  or 2.5kg
dx  2" throw per direction  or 50mm
dt ~ 70ms for forward and return round trip

Energy / work
F*dx = 222N * .05m = 11Nm 
1/2 M * V^2 = 0.5 * 0.227 * (.1/.07)^2 = 0.23Nm

So at most, assuming the pneumatic actually delivered max pressure over the entire stroke the total energy would be on par with 8.1ftlbf  or about an AK  or as little as 0.17ftlbf  or 90% of a 22lr at worse.


Now looking at raw energy especially since these work by different mechanisms its difficult to actually compare them.  without having both on the same load cell with the same reference mass.

But one upgrade I may implement soon is a sliding butt plate that the mass impacts directly and it slides freely of the stock so then all of the momentum does transfer directly to the shoulder, and for the past few mm you get a full 200+ newtons of force.  This will take some careful design to ensure it doesn't shatter or get bound.