Combustion Curve Investigation Cannon

[Lentamentalisk]

decided that rather than hijack the "Any theories for Latke's data collapse?" thread, I would make my own here.
Here is the sketch up of the cannon and measuring devices in order to collect more data on which we can speculate:

The general principle is this:
1) press the ignition, recording is started
2) sensor 1 sees the buildup of pressure behind the piston/ammo
3) piston starts to move, causing a pressure wave in front of it, which sensor 2 sees
4) the piston moves over sensor 2, and it sees the 0psi air trapped between the O-rings. The time is recorded and can be used with the time from sensor 3 to figure out how fast the piston was moving
5) rinse and repeat for the rest of the sensors.

From this data hopefully we can see whether the front pressure wave has any effect, if the back pressure suddenly drops off at some point (due to state change or what ever,) and hopefully many other things.

I live very close to CAL, and CAL day is coming up so I will take my plans over there, schmooze up a physics professor, and see if I can borrow some of their equipment.

If it is impossible to get that many sensors, do to their high cost, then I will just put them in one of the positions at a time and shoot off a hell of a lot more shots.

As for consistency, I was thinking that I would put the entire metering system in an ice water bath to keep it at a constant temperature.

What do you think?

[D_Hall]

I don't think you'll see your zero psi, but that's not particularly significant because you WILL see a step function increase that will allow you to discern the same information.

Beyond that... Sounds great!

[Lentamentalisk]

I don't think you'll see your zero psi.

It all depends on how many cycles the pressure gauge does, and how responsive it is. If it turns out that the gauges that I get access to wont be responsive enough or don't cycle fast enough, then I was thinking I'll just fill in the piston with Bondo or something to keep the gauge from "bouncing".

Things I could use help on are designing the ammo, so that it won't break on impact with the backstop, and designing a backstop that won't break my ammo. The ammo would also preferable be able to be altered in weight so that I can test the effect of mass on all of this.
If I do end up shooting this think off, it will probably be in a lab because CAL won't just let me run away with their priceless equipment, so I need it to be very controlled, and not have ammo flying all over the room. Also, what scale should I make this on? I was thinking just making the chamber out of 3" pvc, and the barrel out of 1.5" pvc, to make it as standard as possible. I could theoretically also make a metal one, (though it would cost a lot) to see what effect heat transfer has on the graph.

Are there any other things you guys think I should be setting this gun up to test for?

[D_Hall]

I'd think that something similar to Latke's rubber diaphramed projectiles would be about ideal... But skip the diaphram and just use some duct tape to increase the projectile diameter a bit as required. The point being that wood is cheap, plentiful, and durable.

As for variable weight, just drill a hole through the projectile and put a machine screw through said hole. Want more weight? Throw a stack of washers on the screw.

Backstop.... I'm thinking cardboard. Lots and lots of cardboard.

[Jared Haehnel]

Backstop.... I'm thinking cardboard. Lots and lots of cardboard.

Or phone books...sand bags....

Its not very technical but wouldn't a spud give pretty decent results? There is not to much variation between one spud and another except for a slight difference in weight...

Edit...look below me...D-hall's got a good point on the sandbags...

[D_Hall]

As one who deals with sand bags on a daily basis... I'd avoid them if at all possible. They're cheap and versitile, but they make a friggin' mess!

[Lentamentalisk]

Its not very technical but wouldn't a spud give pretty decent results? There is not to much variation between one spud and another except for a slight difference in weight...

variable weight, and surface area, and heat transfer, and friction due to the dryness of the spud... Horribly variable.

given the maximum speeds that Burnt Latke being around 500-600fps, and pressure sensors every foot, If I want even remotely useful data, I will need at least 10 cycles per 1 foot section, so at the fastest point (which studies have shown to last for much of the time) I will need at least 5000 cycles per second. If it is not possible to get this accurate with the sensors that I manage to get out of Cal's LBL, then I will basically scratch the entire project.

As to the size, I was thinking keep it at 3" chamber to 1.5" barrel, since that seems to be the most common, and so most relevant.

So far the only varialbes I have come up with are barrel length, ammo weight, spark gap position, and chamber/barrel materials. Do you guys have any other suggestions as to what I can test?

[Jared Haehnel]

Fuel mixes...it would be interesting to see how much you can bee off before it would have a a dramatic effect on performance.

Another good addition might be chamber fans...does size matter? Does placement matter....How much better is it, if you leave it running when the gun is fired. 8)

[Lentamentalisk]

Does placement matter?

This is a bad idea I hate to say. If you put your fan anywhere but in the back you seriously risk having it exploded out the barrel with your projectile. Your other ideas sound intriguing though.

@D_Hall: If I am unable to do all of these tests, due to time constraints or lack of large numbers of sensors, which of the tests do you want most to aid your combustion software?


There is a high probability that the sensors will be rather temperature sensitive, so I was wondering if any of you knew what temperature the gasses are in the gun, and whether they will cause any issues. In other words, how do I convince the people up at LBL that I won't destroy their precious sensors? Also, how do I convince them to let me use their equipment at all?

[D_Hall]

@D_Hall: If I am unable to do all of these tests, due to time constraints or lack of large numbers of sensors, which of the tests do you want most to aid your combustion software?

Measurements to be taken (in order of importance):

0) Projectile mass and diameter.
1) Chamber pressure.
2) Muzzle velocity.
3) Barrel pressure at breech.
4) Barrel pressure at (say) 24" downstream.

Beyond that, the Latke tests are a pretty good model. It would just be nice to have more complete data!

There is a high probability that the sensors will be rather temperature sensitive, so I was wondering if any of you knew what temperature the gasses are in the gun, and whether they will cause any issues.

We routinely fire rocket motors with gas temperatures on the order of 5,000 F and burn times measured in seconds (not miliseconds). Our sensors are not heat tolerant.... In other words, heat tolerance is a total non-issue if you protect your gages.

How do you protect them? Put a blob of silicon grease over the sensor element before installation. The grease provides an insulative layer to protect the gage from the hot gases while at the same time transmitting the pressure to the sensor element.

Works like a charm.

Also, how do I convince them to let me use their equipment at all?

On that front, I have no idea. Maybe finding a grad student in need of thesis work?

[Lentamentalisk]

Measurements to be taken (in order of importance):

0) Projectile mass and diameter.
1) Chamber pressure.
2) Muzzle velocity.
3) Barrel pressure at breech.
4) Barrel pressure at (say) 24" downstream.

can you clarify this? What should I be comparing these against. I was thinking that I should be comparing everything to muzzle velocity, but you put that as one of the things I should be testing. What I think you mean from this is that I should test the chamber pressure, barrel pressure at breach, barrel pressure 24" down, and muzzle velocity for a projectile with variable mass and diameter (or do you want fixed projectile with the variable barrel length?)

[D_Hall]

Measurements to be taken (in order of importance):

0) Projectile mass and diameter.
1) Chamber pressure.
2) Muzzle velocity.
3) Barrel pressure at breech.
4) Barrel pressure at (say) 24" downstream.

can you clarify this? What should I be comparing these against. I was thinking that I should be comparing everything to muzzle velocity, but you put that as one of the things I should be testing. What I think you mean from this is that I should test the chamber pressure, barrel pressure at breach, barrel pressure 24" down, and muzzle velocity for a projectile with variable mass and diameter (or do you want fixed projectile with the variable barrel length?)

Sorry, I was saying that those are the things I'd like to see measured. I marked projectile stuff as "0" because, OF COURSE that has to be measured (But Latke didn't so I explicetly mention it. I find that to be an unfathomable oversight on his part.) Beyond that, if I could have ONE piece of data, I'd want the chamber pressure profile. Beyond that, if I could have TWO pieces of data I'd want the chamber pressure profile and the muzzle velocity. Beyond that.... Well, you get the point.

Now, if you're asking about things to vary, I'd vary barrel length and projectile mass. Maybe 2 or 3 different masses and 2 or 3 different barrel lengths. Latke tried a buttload of barrel lengths, but I really don't think that's necessary if you have good data. The thing is... He didn't have good data. At all, really.

[Lentamentalisk]

so are we basically ignoring that weird knee, or should we get more accurate data around those lengths to analyze it better? Or would ammo mass and a few varying lengths be enough?

[D_Hall]

so are we basically ignoring that weird knee, or should we get more accurate data around those lengths to analyze it better? Or would ammo mass and a few varying lengths be enough?

You asked (or at least I *thought* you asked) what I'd like to see if you had to cut corners. So I did. Obviously, capturing the knee would be great, but if you look at the knee it shows up in seriously long barrels. How often do we see such barrels? Not very often. So while I think the knee would be great to investigate, I would set priorities on shorter stuff.

Sooooo... If you find that you DO manage to procure enough instrumentation to investigate long barrels, by all means do! But if it's an "either/or" proposition, I'd rather see the shorter barrels done *right*.

Note: If you do a test *right* you shouldn't have to do a gazillion tests the way Latke did. Record ambient temperatures and pressures. Try to keep your gun/tanks/etc. at ambient temps. IE, control everything to the best of your abilities. Seriously, one very tightly controlled test is more useful than 10 sloppy tests.

[jimmy101]

Lentamentalisk

Sounds cool, but finding pressure gauges that respond fast enough is going to be pretty difficult.

A 1KHz pressure transducer would need, for example, a 90% response time of less than 1mSec. That is pretty fast for a mechanical/electrical device. The transducers are going to be at least a couple hundred bucks each.

I wouldn't thing temperature affects would be all that great. The mass and heat capacity of the sensor is probably pretty large, and the duration of the high temperature is pretty short, so the transponder really doesn't change temperature all that much. Besides, most gauges that respond this fast, and to this type of pressure range, were probably designed for high temperature excursions anyway. In additon, there is a good chance that a suitable transducer is already temperature compensated. (Usually by using two sensor elements configured in a way that the temperature changes tend to cancel out.)

Instead of pressure sensors you might consider strain gauges glued to the outside of the barrel. Since they aren't in the combustion envelope then the temperature changes are not a problem. There are small piezo resistive elements made for real guns that are just glued to the barrel. You could use several glued to the barrel at various places. You might take a look at http://www.shootingsoftware.com/pressure.htm for an example of a shooters pressure recording system using this type of sensor. The same source sells just the strain gauges (3 for $85) if you want to home-brew the rest of the electronics. It is probably not obvious that an external strain gauge would be accurate enough but it looks like they are considered to be sufficiently accurate to help serious shooters gain data on the detailed pressure versus time response in a steel rifle barrel. I would think it would work on PVC (as long as the PVC doesn't expand too much). Since they work on real guns the time response must be amazingly fast.

Of course, in a real gun with a steel barrel, the expansion of the barrel is fairly small. Things might work out though, a spud gun operates at much lower pressure but the PVC will expand much more than a steel barrel.

To calibrate the setup you'll probably need some way to plug the barrel and a way to pressurize the gun. Perhaps a threaded fitting on the muzzle and schrader valve on the chamber.