Author Topic: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic  (Read 516 times)

Offline johnf

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Dave
I saw a vacuum cannon with ping pong balls at Richard Hulls place a few years ago from memory it went at around 2000 ft per sec and had no trouble going through water filled soda cans barrel was about 16 feet long. I think it used sacrificial thin mylar windows. Always wondered about doing it with golf balls??
« Last Edit: October 23, 2020, 05:26:18 PM by Hydron »

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #1 on: October 20, 2020, 05:08:29 PM »
You should check, whether it is inside the rules of table tennis to punch a hole into the opponents racket.

Wrt simulation: A one dimensional equation is:

d^2y/dt^2 = c^2 * (1+dy/dx)^(-gamma-1) * d^2y/dx^2

The derivatives are partial derivatives. y is the displacement of air molecules relative to their initial position x. So y(x,t) is 0 for t=0. Gamma is the adiabatic exponent and c sound velocity. This equation differs from the Navier Stokes version in the way, that it describes displacements instead of the state at a fixed point of space. The pressure, e.g.

p = p0 * (1+dy/dx)^-gamma

which is a function of x, is at the place y(x,t) + x. p0 is the atmospheric pressure.

For small dy/dx the first equation becomes the usual linear wave equation. That won't work to describe your experiment.

I'd consider using helium to break records. Air mass in the barrel, which also has to be accelerated is probably larger than the ball mass.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #2 on: October 21, 2020, 05:00:34 AM »
"d^2y/dt^2 = c^2 * (1+dy/dx)^(-gamma-1) * d^2y/dx^2"

For discrete simulation, would the array of elements be specific quantities (fixed number of moles) of gas along the length, where the discrete gas quantities change position and length as the simulation progresses?  Could that handle a moving boundary condition at one end (position of the ball)?  I'll need to think about the physics in more detail, but that's a ways out.  Want to finish Tesla coil projects first.

Yes, helium is one of the changes we made to improve speed beyond the record "Myth Busters" TV show had achieved a couple years earlier.  The down-side of helium is its high gamma, but low density more than compensates.  Our unofficial max speed of 873m/s is faster than the existing official Guinness record.  The official record is about to increase, however, as someone recently hit 924m/s in preparation for an official record attempt next month.



David Knierim

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #3 on: October 21, 2020, 07:17:07 PM »
Exactly, the idea is to divide the gas column into initially equally long sections. Every section contains the same amount of gas throughout the calculation. Pressure gradients provide forces on the sections and pressure itself is calculated by the length of the section and the adiabatic compression law. The boundary condition to the ball is given by its mass adjacent to the gas volume.
The boundary to the gas supply is not clear to me though, as there is additional gas mass entering the system. I need to think about this. Perhaps you can't avoid using Navier Stokes. It's more complicated, since you have to solve a system of coupled partial diff eqs.

Offline davekni

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Re: QCW with replaceable ferrite-core primary
« Reply #4 on: October 21, 2020, 08:21:55 PM »
My intention is to simulate the entire system, so no gas entering.  Time=0 will be when the membrane breaks.  Part of the length will have vacuum, and the rest will have the same initial pressure.  The ball is in the vacuum portion.  However, there will be one or two diameter changes along the length.  I also hope to include flow resistance along the pipe (turbulence), at least some crude approximation, as a refinement after initial runs.
David Knierim

Offline johnf

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Re: QCW with replaceable ferrite-core primary
« Reply #5 on: October 22, 2020, 09:14:24 AM »
Vacuum ie no air
What turbulence???
we are at the crossover between laminar flow and molecular flow--two very different sets of equations / mismatch of the two

good luck Dave

Offline klugesmith

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Re: QCW with replaceable ferrite-core primary
« Reply #6 on: October 22, 2020, 05:33:39 PM »
Carrying on off-topic,  professional hypervelocity guns generally use hydrogen instead of helium.
https://en.wikipedia.org/wiki/Light-gas_gun
Works better, and the fire hazard can be managed.  Just like hydrogen that cools electric generators in most large power plants.

Glad to hear about simulation and modeling, and that a contest has been going on.
Am reminded of the Punkin Chunkin' contests, which came to my attention before anyone had brought pneumatic cannons. In those days the categories were things like elastic slingshots, trebuchets, and centrifugal hurlers.
/>At the time I had been launching small pumpkins (150 mm; 6 inch) from a steel mortar, propelled by 2 liter soda bottles popped with carbon dioxide.  A representative air time is 9 seconds.  Barely pre-Internet, it took a lot of work to find thermodynamic charts for CO2, to get numbers for simulation. 
Could tell about testing gases with different gamma values, but let's have a discussion thread with appropriate title.
« Last Edit: October 22, 2020, 05:53:22 PM by klugesmith »

Offline Uspring

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Re: QCW with replaceable ferrite-core primary
« Reply #7 on: October 23, 2020, 03:03:28 PM »
When you change diameter, you'll have a 2-dimensional problem, e.g  r and z for cylindrical symmetry. Once you include turbulence, you'll have to go 3D. Turbulence likely occurs, the Reynolds number being way above the limit at your gas velocities. Numerical solutions of these kind of 3D problems are _very_ involved.

I feel a bit queasy about the recommendation using hydrogen. When it exits the muzzle it is quickly mixed with air and you have lots of it. The ball might heat up by friction in the barrel and the impact of it on the target board can also liberate some heat.

Offline klugesmith

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #8 on: October 23, 2020, 05:34:39 PM »
Thanks for moderator attention to separate this thread, Hydron.

Re: hydrogen as the working fluid, and fire hazard.

Why not take steps to make sure the vented H2 always ignites?  Not many grams of fuel, not pre-mixed with air, so we aren't talking about blast pressure or other destructive effects comparable to those of hypersonic ping-pong balls.

If any "fire" would disqualify a shot for the contest, then just be prepared in case it happens.
Outdoors, or indoors with a fan moving air around, I bet released H2 would diffuse away to non-explosive concentrations within seconds.

Offline Hydron

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #9 on: October 23, 2020, 07:10:22 PM »
Why not take steps to make sure the vented H2 always ignites?  Not many grams of fuel, not pre-mixed with air, so we aren't talking about blast pressure or other destructive effects comparable to those of hypersonic ping-pong balls.
Brings to mind a Delta IV Heavy launch

Offline Twospoons

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #10 on: October 23, 2020, 11:39:35 PM »
If you are concerned about hydrogen ignition, why not put the entire target assembly in a large box flooded with CO2 or nitrogen? There's no chance of ignition from the target impact, and the resulting dilution of the hydrogen with inert gas greatly reduces the chance of a later ignition when everything is disassembled.

Another benefit for using hydrogen is much lower cost.
Helium is a limited resource we should be conserving for industrial and scientific use ( i.e. not filling kids party balloons with it)

Offline davekni

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #11 on: October 24, 2020, 04:07:24 AM »
Hydron,

Thank you for moving this topic!  I'd wondered how to do that, or if such was even possible for non-administrators.

Uspring,

I'm trying to make a very-simple simulation, not details of 3D effects.  Something closer to the impedance change in a coax if the diameter ratio changes.  My goal isn't to be precise, but rather to see rough relative performance of different options.

Concerning hydrogen,

I'm too paranoid to use compressed hydrogen.  Something like operator (myself) mistake that mixes air and hydrogen in the pressure chamber and some bit of ESD or whatever ignites it while confined.  Mike (the guy in Pennsylvania that is the likely next record holder) has tried hydrogen.  It ignites on exit every time.  My guess is ESD, as the ball charges going down the tube.  Also, if there's a target, the ball always ignites momentarily as it hits the target.  Mike says it sounds like a howitzer, so he's concerned about noise for neighbors.

More puzzling, Mike said that the ball breaks during launch at much lower pressure using hydrogen, just above 40PSI rather than 120PSI.  I can't think of a reason for hydrogen breaking the ball at so much lower pressure.  Perhaps there's some detail about Mike's setup that would explain such.  My best guess is that Mike has some air contamination that is igniting before the ball exits.

I have several ideas for improving speed over our 2016 runs.  Most I've already shared with Mike, but don't know which if any he's tried, or what other ideas he's had.  Mike said he'll share details after his record runs on November 21st.  My simulation goal is to compare different ideas for improvement without the work of building each idea and variations and combinations.  As long as simulation gives me some relative comparison of variations, I'll waste less time experimenting in non-useful directions.

All this conversation (here and with Mike) is getting me too excited about ping pong ball launching again.  I still have a couple years of DRSSTC and QCW experiments to try before spending time on another major project. :)
David Knierim

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #12 on: October 24, 2020, 04:16:06 AM »
Per Hydron's suggestion, here's background copied from the old thread.  It wasn't moved here because some posts were mixed with content that belongs in the original "QCW with replaceable ferrite-core primary" thread.


BTW, completely unrelated except for modeling, but I'm hoping to do some simple one-dimensional gas pressure/flow modeling for a  supersonic ping pong ball launcher.  Had the Guinness speed record for a while, and plan eventually to reclaim that record.

/>
/>
/>


John,  (In response to his comment "Always wondered about doing it with golf balls??"):

Golf balls could be launched with gun powder and a bit of packing to insulate the ball.  Ping pong balls take more finesse, so a more fun challenge.  Also more impressive to see what a 2.7g ball can do, going through 3/4" plywood (nominal 3/4, actually 17mm).  Our (a father-son project) official record is 806m/s.  We since hit 873m/s (2764f/s), shown in the later videos.  (The "smarter every day" youtube channel has a video of supersonic baseball launching.)
David Knierim

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #13 on: October 29, 2020, 05:41:13 PM »
Here are a few simulation results for Davids ping pong launcher. In order to reduce complexity, I've stayed with a simple one dimensional model. That implies:

1. No viscosity or turbulence is taken into account. For this a radial dependency of velocity and pressure would be needed.
2. No changes in tube radius are possible along its length. For the barrel, that isn't needed. But for the gas reservoir it would be a nice to have. So in this simulation, the pressurised gas is kept in long piece of tube (6m) upstream of the ball.

The diagram below shows the velocity of gas molecules at different times. The curves are at times 1 ms, 2 ms, 3 ms, ... after release of the ball. The value on the x axis is the position of the gas molecule at the start of the experiment and the value on the y axis shows the speed of this molecule e.g. after 1 ms (blue curve). Units are m and m/s for x and y.



The gas molecules at 6 m are close to the ball and the velocity of the ball can be seen to reach 800 m/s after 6 ms. At that time the ball has travelled almost 3 m, i.e. it is end the end of the barrel. Also it can be seen that the acceleration is quite non uniform. The speed increase for each additional ms decreases a lot.
The gas expands also quite non uniformly. The begin of expansion travels backwards in the tube at about 1 km/s, which is the speed of sound in He.
Below is the same diagram for using air instead of He. I've diagrammed only the last 3 m of the tube and increased total time to 8 ms, which is, when the ball has travelled 3 m. Max speed comes out to about 500 m/s.



Interesting are also the pressures in the tube. The diagram below show the pressure along the tube at 2, 4 and 6 ms. The curve ends at the right, where the ball is located. The vertical unit is Pa, the gas He.



The agreement between measured and calculated velocities is good, but perhaps coincidental. There are 2 opposing effects: One is turbulence and ball friction, which aren't included. The other is the "thin" gas supply, which prevents gas being delivered  rapidly into the tube. If you look at the last diagram, the pressure at 6 m (i.e. the starting point of the ball) is less than half of the starting pressure of 650 kPa. A spacious gas supply vessel should be able to keep the pressure up at that point.
« Last Edit: October 29, 2020, 05:43:36 PM by Uspring »

Offline davekni

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #14 on: October 30, 2020, 04:42:16 AM »
Thank you for the simulations.  I'm considering using a smaller-diameter and longer pressure chamber, taking advantage of sound velocity not bouncing off the back end and up to the ball in time to further reduce pressure.  All our optimization was around getting maximum ball speed for a fixed initial pressure.  What actually matters isn't initial pressure in the chamber, but rather initial pressure at the ball.  Higher initial pressure is fine if it drops before hitting the ball so as to not break the ball.

So, one of my ideas for improvement is to have at least the final portion of pressure chamber at the same ~40mm diameter as the ball, and place the starting ball position farther from the bursting membrane, perhaps 20-30cm away.  Then the helium pressure immediately drops, splitting from 40mm diameter pressure section with 40mm diameter vacuum section.  Hoping that might allow close to twice initial pressure, so maintain higher pressure for longer time.  Likely want the pressure chamber to taper to larger diameter away from the membrane, but would approximate that with a couple steps (~45 degree reducer bushing) changes in diameter.

Another thought is to use a weighted flapper valve to slow helium flow initially, but increase flow as time progresses.

For simulating different diameters and turbulence, my thought is to use an interpolated look-up table of pipe flow pressure drop as a function of density and flow rate and diameter.  That way the simulation is still 1-dimensional.
David Knierim

Offline Uspring

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #15 on: November 05, 2020, 05:08:38 PM »
In my simulations I've found, that the length of the pressure chamber should be between 1.5 and 2 m to avoid bounce back reaching the ball.
The analogy to a transmission line is pretty but has to be taken with care, since the system is non linear due to the high pressure changes involved. I presume, you have a funnel shaped section in front of the pressure chamber. Reflection of the sound wave due to the widening of diameter can possibly cause a reflection there of opposite sign, sending back a high pressure wave. If properly designed it might give an extra push somewhere down the barrel. Sadly this is out of the scope of my simple model.

Putting in some space between the first foil and the ball is an interesting idea to flatten the initial pressure peak. My model in its present implementation does not allow for an extra gas filled volume between the chamber and the ball. A simulation with a vacuum space between chamber and ball which I have tried, leads to a higher acceleration due to very fast Helium (>2 km/s) hitting the ball. The latter simulation is somewhat kludged due to technical problems involving the violent action, when fast He is hitting the ball.

Offline davekni

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #16 on: November 05, 2020, 08:00:43 PM »
"A simulation with a vacuum space between chamber and ball which I have tried, leads to a higher acceleration due to very fast Helium (>2 km/s) hitting the ball."

That's interesting result.  I'd thought the helium would spread out, with only a few of the fastest atoms hitting the ball first, then more over time.  Higher acceleration is likely going to break the ball at lower pressure, so the wrong direction for me to try.  (I wasn't contemplating initial gas between the pressure membrane and ball, as there's no way to maintain that gas and vacuum past the ball.  Other than my thought about a weighted flapper valve that allows some gas by initially and more as time progresses.)
David Knierim

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Re: Ping pong (and pumpkin?) cannons - split from ferrite-core QCW topic
« Reply #16 on: November 05, 2020, 08:00:43 PM »

 


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