Anyone Curious, Skilled and Equipt?

[gunsonwheels]

In the May 20, 1976 issue of Machine Design there was an excellent article on the development of the velocity collar which appears on all modern artillery and tanks.  Everytime an ammunition lot cha
anges, velocities are sufficiently different that target accuracy (velocity) has to be re-determined for the new lot.  Used to be several rounds fired were required to do that.   Someone determined the barrels stretch a minute amount (called "hoop stress") as the round progresses up the barrel.  Attaching strain guages (SG) at a know distance from each other to detect the stress and presenting their data to a computer to measure time between hoop stretches instantly gave the velocity.  The collars contain the SG's, magnets, springs and coils (to produce electricity to power the collar from the recoil) and a transmitter to send the info to the fire control computer.
AND...
During this same period Matt Swittlick did his breech pressure studies for the NSSA.  They wanted to be sure they were NOT overstressing original 19th century tubes (of which they have many).
AND...
I was studying "upset" at a sand pile in a gravel pit.  "Upset" is the sealing and swelling of a sabot or skirt or whatever into rifling by a muzzle-loaded projectile.  It became apparent when that sealing took place, breech pressures went up dramatically.  The report, recoil and other factors all supported higher pressure.  And the dug-out slug witnessed whether upset/sealing had occurred or not.
SO...
Ever since that time, I have wanted test results using a cross-hatched strain guage at the bottom of a barrel to measure breech pressure combined with a series of strain guages mounted up the barrel every few inches all fed to a computer to gather data as a round is fired and progresses up the barrel.  Just how much pressure, where, and when sealing took place would/could all be accuratley disclosed.

Anyone of us/you ever do anything like this? ? ? ? ?

[keith44]

From what I have been able to gather, even ammunition ballistic labs do not go to such lengths due to time and expense.  Most of the programs I've seen only collect data from one or maybe two input sources.  You could build a data trend with analog inputs and track the data that way, but I think the price will cause a scaling down of your project.

That's the down side.  On the other hand - What a great idea!!  If you can pull it off, others will want to have you test other weapons.  It would be interesting to watch the data stream of a projectile traveling the length of the barrel.  A friend of mine has a theory of a pressure spike at the muzzle due to air compression as the projectile tries to exit the barrel and inertia of still air in front of the projectile causes compression.

Good luck with this one.

Keith

[Co. Batguano]

So the muzzle swell on these old guns might not just be a "for style" thing?  Since cannons and the like have been around for several hundred years, I figure that the design was made that way often for a real, and practical reason, rather than simply a "guys' fashion statement".  cannons do make the ultimate accessory to any wardrobe don't they?  they go with anything, and only come in black!  Never make you look fat either.

[RocklockI]

The muzzle swell was a necessary item in the early days , the muzzles needed the reinforcement to prevent cracking .

Even the old 'hoop guns'(not the hoop that was mentioned earlier) have them ,it's easy to see why they got started when you look at one .     Welcome Co. Bat.........                
                 
Gary

GOW  there have been lots of studies that sawed off 1" sections and the velocities are noted .
I realize that's not the sliding scale you are looking for .

I'd ask "The Germans" they made some "chamber guns with charges along the length of the barrel .

 

[Cat Whisperer]

I would think that one sensor would suffice, as the pressure in the entire burning area would be equal.  It would of course rise and fall, but that would be experienced throughout the length of the burning powder area. 

The muzzle swell is needed (even for modern cannon reinforcement is employed) because at the end of the tube there is metal only on one side - not as everywhere else where there is metal infront and behind the point in question.  (It's obvious when you do a finite element analysis of the stress-strain.)  Modern howitzers will often have a portion at the muzzle counterbored - where the pressure at the end is much reduced to the end, making the counterbored portion functional not to contain the high pressure but to reinforce the immediately preceding length.

Modern (US) howiters are logged for EVERY round shot and what charge was employed.  Then at something like 4,000 full-load equivalent rounds it is replaced.  Further, when it is approaching the end-of-life # of rounds the tube is inspected after EVERY shot.

I don't buy the pressure spike at the end - as the air in the tube would be compressed all the way to the exit point of the round - it would ramp up to that point.

[gunsonwheels]

The only reason for SG'g up the barrel is to detect the projectiles position... what I would like to see would be a table showing the pressure (from the breech SG) at each position as it moves up and out.  A breech reading at each position in the barrel.  A sudden increase in breech pressure at a position should indicate "upset"... if it happens near the muzzle end of the barrel... not too good... if it happens near the bottom... very good as both pressure and spin will be maximized. 
Strain guages are fairly cheap and computers are too.  The software is a little complex but redily doable.  Everytime I run a survey for doing it the cost of the high speed analog to digital converters reqired to interface the SG's to the computer sink the boat. 

[Cat Whisperer]

Ahhhh.  Read you lima charlie.

Position of course would be skewed by the response time of the metal flexing, but knowing the latter is a direct indication albeit slightly off.

Software is easy.  I have access to LabVIEW, which can measure analog voltages easily.  Response time would be a function of the DAQ.  That may present a problem if wishing to do this outside the lab and at the range.  That would limit us to a USB-6008 (on hand) or USB-6009 (could be ordered) with 4 differential voltage inputs or 8 RSE.  I think I could scan across all inputs to make readings at the same time.  Scan rate is the issue.  If we go to a series E card (PCI slot in desktop) scan rate is about a meg and series M cards to 10 Meg Hz.  (Obviously 120vac source needed.)

That leaves the sensors and signal conditioning.  What do you recommend for strain gauges?  Getting above 0.1v would be VERY good to get away from nominal clutter and noise.

[Cat Whisperer]

There was an article a couple of years ago about an FM transmitter and accelerometer that was built INTO a 20mm projectile designed to measure the acceleration (hence velocity and position) of the projo while INSIDE the barrel.   Hmmm.  High tech is OK.

[Winger Ed.]

As I recall, the Oehler 33 Chronograph folks make a chamber pressure strain guage for rifles that might work for what you're doing.
I don't know how expensive they are, but they might be affordable without a budget like NASA.

[GGaskill]

The Oehler model that does pressure measurements is no longer available to my knowledge.  I have one that I bought to do BP cannon pressure measurements with but have not yet done any.  It cost about $800 as I recall.

[Cat Whisperer]

That gives you one strain gauge (Oehler).

What pressure ranges and outputs of the strain gauge itself would be appropriate?

(LabVIEW easily inhales -10 to +10v.)  20mA loop?