Golf ball challenge! Help appreciated.

[mysticjbyrd]

So is the idea to keep the angle constant and to only vary the distance you pulled the sling back?

I would have to question the accuracy of a home made sling shot even if we had a perfect person shooting it. I would think if you are off on the length of either of elastic bands by even a small margin, than it could drastically change the trajectory of the ball.


Of course I have not owned a sling shot since I was 5, so what do i know.

[jimmy101]

Use air, not propane. The regulator is a high quality regulator designed to hold a stable pressure and not wander like the cheap air tool regulators. My little regulator on my compressor sets somewhere within about 15 PSI of where it is set. My welding regulator on the other hand holds set pressure to within maybe 1/4 psi or less.

The problem with any regulator is the precision. Like I said before, accuracy is basically irrelevant.

If a good reg can hold the pressure within 1/4 PSI the question is how reproducibly can you set it to a particular pressure? Can you even read the gauge to 1/4 PSI (probably not).

If you use a gauge, I would lock it at a particular pressure and never change the pressure. If the pressure is changed, even if it is set back to where it was, then the gun should be recalibrated.

The advantage of a liquid/gas pressurized mix is that you don't need the regulator and you don't have to worry about setting the pressure to a particular value. The system has a built in regulator.

No pressure regulator (that a spudder can afford) is anywhere near as accurate, or precise, as a $5 digital thermometer.

There is the problem of releasing a cloud of propane … so put that doobie out.

BTW, here's a very cheap digital pressure gauge (it's a flow regulator but can be used as a gauge.). Precise to ~0.5 PSI. (But not that accurate.) $12 http://www.harborfreight.com/cpi/ctaf/d ... mber=98426

A more expensive ($25) and more accurate one (though the precision is still 0.5 PSI). An 0.5% pressure error (1 part in 200), assuming a chamber pressure of 100 PSI, might be OK. Be interesting to see how much difference GGDT predicts in the range for 100 vs. 100.5 PSI. http://www.harborfreight.com/cpi/ctaf/d ... mber=96392

For comparison, here's a $5 digital thermometer that's accurate to 1 part in 300 and precise to better than 1 part in 600. http://www.harborfreight.com/cpi/ctaf/d ... mber=95382

[Technician1002]

The advantage of a liquid/gas pressurized mix is that you don't need the regulator and you don't have to worry about setting the pressure to a particular value. The system has a built in regulator.

You do realize you are dealing with a refrigerant? Drawing gas changes the temperature and pressure. Telling the ref to wait 1/2 hour between shots while I stabilize my temperature is not going to cut it.

We deal with some electronic temperature controls at work designed to keep a chemical bath within 0.1 degree C. It takes about 20 minutes after disturbing the chemical for the control to stabilize after a process rocks the temperature. In the field you won't have the luxury of that type of temperature control with an insulated tank and constantly circulated bath.

[MRR]

My 2 cents....

Use a piston or sprinkler valve setup and pilot it with a blow off valve.

I thinks that would be an easy, cheap and reliable solution to gain consistent shots.

Keep it simple! 8)

[jimmy101]

You do realize you are dealing with a refrigerant? Drawing gas changes the temperature and pressure. Telling the ref to wait 1/2 hour between shots while I stabilize my temperature is not going to cut it.

We deal with some electronic temperature controls at work designed to keep a chemical bath within 0.1 degree C. It takes about 20 minutes after disturbing the chemical for the control to stabilize after a process rocks the temperature. In the field you won't have the luxury of that type of temperature control with an insulated tank and constantly circulated bath.

Don't need all that. All you need is a large enough heat sink. A typical Bernzomatic cylinder dropped into a 10 gallon cooler filled with water will be rock stable for this usage. Don't need to heat or cool the bath, just need to have a bath big enough to not change temperature significantly.

The amount of liquid propane that is converted to gas for a gun this size is going to be pretty small. it won't take all that much heat to do it and the temperature of 10 gallons or so of water isn't going to change significantly, or probably even measurably. If my math is correct you would have to suck 160 KJ out of 10 gallons water to get the temperature to drop by just 1C. The heat of vaporization of propane is no doubt a lot bigger than the heat capacity of water but the mass of the water will be so much greater than the propane that the temperature change is not going to be insignificant. (The heat of vaporization of propane is about 1/7th that of water, so it is pretty easy to get it to vaporize.)

The muzzle energy of this gun is going to be what, 0.2 KJ or less? Even figuring 10% efficiency that is not terribly significant compared to the 160KJ to get the water to change temp by 1C. And that is assuming all the guns energy comes from vaporizing the fuel, it doesn't even take into account the energy released during depressurization.

There's going to be at least a couple minutes between shots since the gun needs to reloaded etc. When the target distance is changed there is probably an even longer pause.

I don't see anything in the rules about how fast the gun has to be fired.

Even filling with a highly precise regulator will still require that a fixed amount of time passes between pressurization and firing. When filling the compressed air will heat or cool, depending on how it is done. That means the chamber pressure will drift once it is disconnected from the gas source. Easiest way to deal with that is to wait several minutes to equilibrate. If you leave the gas source connected then there is still a filling affect and the density of the gas in the gun will drift as the temperatures responds and equilibrates.

[rcman50166]

I hate to defeat the purpose of the forum, but a slingshot would simply be the best. If accuracy is truly a problem you can build a guide rail for it. There are many ways to limit shift in the x and y planes. The deviation of accuracy is probably close to that of a cannon in the end anyways.

[mysticjbyrd]

Accuracy and precision are the only problems.

How are sling shots the best? Quite a bold statement w/o any evidence or even an argument.

Is it the best because I can make one in an hour? Or because it is the most accurate? How about precision?

[rcman50166]

Accuracy and precision are the only problems.

How are sling shots the best? Quite a bold statement w/o any evidence or even an argument.

Is it the best because I can make one in an hour? Or because it is the most accurate? How about precision?

The theory is simple. If you prevent shift in the X and Y plane, the only direction the projectile can travel is in Z. given Z's angle and velocity. You can calculate the distance to the target. Of course you will need to compensate for real world variables like wind resistance, humidity, air density, etc. You precision will lack due to those variables, not the slingshot itself.

To prevent shifts in undesirable directions I would personally have 4 L beams around the projectile with the sling positioned inside. The ball will be stable and it will account for perpendicular shifts as well as ball spin (the sling prevents rolling inside of the guide rails) After that the only thing that needs to be made constant is the distance the sling is drawn. That can be remedied with a simple gate latch.

[Reactor4]

rcman50166, I just now noticed this, but, your quote by Nikola Tesla is the same one I used as my senior graduation quote! It is such a powerful statement and I love it!

[D_Hall]

rcman,

You are aware that even slingshots are subject to temperature differentials, right?

Put it this way... Calibrate even your perfect slingshot on an 80 degree day. Day of competition is 60 degrees? Congrats, even your "perfect" slingshot is going to miss.

(Elasticicity of polymers is subject to temperature...)

In other words, you're right back in the same boat as the pneumatic cannons.

[rcman50166]

rcman,

You are aware that even slingshots are subject to temperature differentials, right?

Put it this way... Calibrate even your perfect slingshot on an 80 degree day. Day of competition is 60 degrees? Congrats, even your "perfect" slingshot is going to miss.

(Elasticicity of polymers is subject to temperature...)

In other words, you're right back in the same boat as the pneumatic cannons.

Who says calibration can't be on the same day? Besides, in the end slingshots are cheaper and easier to built. They are also more reliable.

[jimmy101]

I suspect that no matter how it is done it would be best to recalibrate on the day of the shoot. Heck, I would have a PC handy and recalibrate after every competition shot as well.

[rcman50166]

I suspect that no matter how it is done it would be best to recalibrate on the day of the shoot. Heck, I would have a PC handy and recalibrate after every competition shot as well.

Haha that's the spirit

[Reactor4]

the one and only problem i see with building a slingshot, is that when calibrated, you don't really have something to refrence off of, like a pressure gauge on a pneumatic. I like the idea of a slingshot more than a cannon, however the slingshot (in this case) may need a little bit more engineering than a run-of-the-mill one.

[Moonbogg]

Use a mechanical catapult or even a trebuchet to do it. Metal springs or wound ropes should be at least a little less susceptible to temperature change (at least I think. Can't wait to be proven wrong)