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I want to build an easily portable tennis ball launcher for my young son to practice cricket and tennis with similar basic abilities as this www.zooka.com/ without the fancy computer extras and would like advice or be redirected where I can find help with its design.
Hoping to be able to have ball speed between 50-100kmh over 10-20metres I'm unsure how much pressure I would need to fire a piston to launch the balls. I'm assuming barrel and chamber sizes are important along with other things, but as I've only just started researching, I'm having trouble finding specific information for my requirements.
As the above commercial product is battery operated using compressed air, how are they able to have such quiet performance as most small air compressors I am aware of are usually fairly noisy? Or have they edited that part of the audio from their promo?
Apologies if my knowledge seems vague but I am keen to find resources that will help me understand my requirements for this project.
Welcome to the forum, and great motivation for a project
Your requirements are fairly basic, you can achieve that with a fairly small chamber and a commerically available sprinkler valve (that can be modified for manual actuation for better performance, but if you want electrical actuation that can be worked on.)
Here's a good example of what might be sufficient for your needs: http://www.spudfiles.com/forums/second- ... 14735.html
There are quiet compressors out there, a fridge compressor for example is virtually silent but unfortunately flow is quite low.
The FAQ for the "zooka" was clearly written by someone in marketing...
Here's an even simpler idea that will get good performance for very little pressure due to the efficiency of the "valve":
The idea is to have some kind of restrictor in a length of suitable barrel that will hold the the tennis ball until enough pressure builds up behind it, at which point it pops out like a champage cork
Thanks for the extensive advice. I'm hoping to make the unit as light, compact and uncomplicated to use as possible so it is enjoyable for my son to use. Also being able to have a portable supply of air (eg portable 12-24v compressor) sufficient enough for a couple of hours use at the park is definitely on the wish list, unless a portable air storage tank I could fill at home is feasible. Also variable launch speed would be great.
Possibly some of the project specific details I think I need to formulate are:
-Minimum/maximum length and diameter of barrel for a standard tennis ball to be reasonably accurate(within 1-2 feet) over 10-20 metres
-Air volume/pressure required to obtain 50-100kmh on exiting the barrel
Also, I'm still a little unclear whether the modded sprinkler valve activates the piston valve or replaces it.
And showing more of my noobiness...
Is it more 'air-efficient' to use the compressed air to propel the ball directly, or use the compressed air to activate something inside the barrel that strikes the ball to propel it? The reason I ask is I would like to be able to just drop a ball into the open end of the barrel, let it roll to the closed end ready for firing rather than forcing it into position to eliminate air loss around the projectile. Which way is more efficient?
You can see my exploits in these two videos here:
I'm in NZ myself but you should be able to find the same valves and stuff I used; if you so require.
From memory, I was getting a maximum speed of 260kph averaged over the length of the nets (~25m), so muzzle velocity would be quite a bit higher than that while the velocity of the tennis ball when it reaches the batsman is much less. I've faced 160kph from a bowling machine in the nets before and this cannon definitely fired tennis ball quicker. I would have got 260kph from using about 120PSI in the chamber, when dialled down to about 35-50PSI the ball is actually hittable although still difficult because tennis balls don't react the same way as cricket balls and the method of delivery is quite sudden (no run up to judge when the ball will be delivered).
Although my cannon uses a 2" sprinkler valve as the main valve, you should be able to get 160kph muzzle velocities from a 1" sprinkler valve with a 50mm x 40cm chamber, 6.75cm x 100cm barrel and 80-100PSI. I think this would be adequate for your needs.
If you want to use it for tennis, you'll need a ladder if you want to fire it over the net and have it bounce on the other side. It may be possible to stand at the net and fire it directly over the net in to the ground of your son's court, though I never tried this method.
Edit: The only problem I see with JSR's suggested method is that you lose the timing for when the ball will be delivered, which I found to be key to being able to hit the ball at high speeds (we used a count down before firing). If you're only looking for 50-100kph out of the muzzle, that can be easily achieved but a tennis ball coming out at 50kph from 15-20m away will slow down a heck of a lot (depends how old your son is whether it will still be fast enough for him).
If you wanted to get really technical, you could set up a timing circuit, air compressor and magazine loader which would allow you to fill a mag with 10 balls or so and have the valve fired at intervals with the air compressor filling the chamber between intervals.
A sprinkler valve is typically a diaphragm valve which operates on the same exhaust principle as a piston valve. Modding it removes the low flow electric solenoid and replaces it with a higher flow ball valve or blowgun.
If you want to mod while keeping electric actuation, replace the solenoid pilot with another unmodded sprinkler valve
You can conceivable make a ball "kicker" where a valve dumps air into a piston which strikes the ball. I had modded a small nailgun to fire coins, maybe it will work for tennisballs too
Thanks for your valuable input MrCrowley. I came across your awesome YT videos while researching this idea which fortunately led me directly to this forum. My boy is only 9 so I'll wait till he's a bit older before we mimick some of your wilder exploits LOL
I'm assuming your listed muzzle speeds are based on a very snug fit in the barrel, (in one of your videos you force the ball into the barrel with a broomstick) is this correct? Potentially I was hoping to include an auto magazine loader as you suggested which I expect would require the ball to roll unassisted to its pre-launch position. This is partly why I asked about the "kicker" type option JSR referred to as I am concerned there would be substantial air loss around the ball during launch if the barrel was even only marginally wide enough for the ball to roll in. I'm keen to hear any advice or suggestions regarding how much affect this air loss will have and how much air pressure is likely needed to increase to compensate. Also where is the best place for info on designing a "kicker" mechanism if this is the best method to overcome barrel air loss.
As this chamber holds just under 1 litre of air, is it viable to use a plastic bottle for testing until I am happy with a final overall design or will I not be able to achieve enough pressure? Is there any special reason for this barrel length or can it be reduced without loss of performance, and if so by how much? Reducing overall size of the unit is highly desirable. Regarding sprinkler valves, is size based on inlet/outlet connections?
Is there somewhere I can read more about this as electric actuation is definitely high on my wish list.
I understand a lot of this is going to be trial and error so mostly my queries and everyones answers are likely to be general but I'm hoping not to reinvent the wheel and use as much of the advice from experienced members here to make my first project as successful and "tweakable" as possible.
The tennis ball itself isn't compressed when forced down the barrel, the broomstick is required because it simply has too much friction to roll down by gravity alone. I believe my barrel is PN12 65mm PVC pipe but the inside diameter of this pipe will vary depending on the pressure rating (PNXX) of the pipe you buy. For example, PN9 (rated to 9 bar) 65mm PVC will have a larger inside diameter whereas PN15 65mm PVC will have a smaller inside diameter. Perhaps PN9 65mm PVC will allow the ball to roll freely. Best take a tennis ball with you when you go buy it, though tennis balls do vary slightly in size.
A plastic bottle could be used but you will have difficulty attaching it to a valve. You will have to epoxy a 1" threaded fitting to the PET bottle threads so that it can be threaded to a 1" sprinkler valve. I would suggest starting with a chamber at least 2 litres in volume, also be careful using PET bottles and never exceed 120PSI with them.
A sprinkler valve should come with its own solenoid ready for electric actuation, all you do is hook up a battery. The down side is that solenoids have poor flow and actuate the valve a lot slower than if you mod it with a blowgun. For your purposes, this ~15% reduction in performance may not be a bad thing.
I would suggest familiarising yourself with GGDT. It is a cannon simulation program that allows you to test the hypothetical performance of designs before actually building them. There should be adequate information and examples on the website to get you started, it's not all that difficult once you get the hang of it and learn the terminology.
Thanks for directing me to GGDT. It looks like that should answer a lot of questions for me. And probably create just as many more LOL.
http://www.spudfiles.com/forums/ggdt-hg ... 23256.html
After spending most of a sunny Sunday researching my new project (instead of attending to the many needed jobs around the home = angry wife) I'm getting a pretty good idea on the use of GGDT and how it affects my design. There are still a few things that I need guidance on though.
Considering I am trying to keep my unit as compact as possible, I have considered a co-axial setup and using GGDT there appears to be minimal difference in muzzle velocity for the values I have entered compared to an in-line setup. In real-life, is there much MV difference between co-axial and in-line pneumatic launchers with similar barrels and chamber volumes?
Also, has anyone got some data on comparisons chambers/reservoirs where diameter vs length vary but still maintain similar volume. GGDT appears to show minimal difference for the values I enter but at any size is this likely to change? Is there an ultimate ratio (diameter:length) for the chamber? And in a co-axial setup is there additional variables due to sizes of voids in the chamber or does the circular shape of both the chamber and barrel negate this?
And another general GGDT data input question, if I'm using a 1" solenoid sprinkler valve from my local hardware store, what valve type and valve data should I use if none of this info is supplied with the product I purchase?
If any of these questions should be directed in other areas of this forum or presented as new threads, please suggest alternatives to my questions
following your post here:
Most cheap portable 12v compressors tend to be quite shìt.
In the long run it would be much cheaper to invest in something higher spec like this: http://www.ebay.com/itm/VIAIR-92c-AIR-C ... 37&vxp=mtr
If you're sticking to electric actuation, I would go for the biggest diameter sprinkler valve available - you can get a 2" one for a reasonable price: http://www.ebay.com/itm/IRRITROL-2-SPRI ... 33691b6a94
This would give you better performance for less chamber volume and pressure, thereby not taxing the pump too much.
Nothing you would notice without a chronograph.
In general, a short fat chamber is more efficient, but again it's only a small percentage difference, nothing you would notice in practice.
This is a tricky one, people usually work backwards - build the launcher, measure performance then tweak the valve specs until GGDT matches up A sprinkler valve in particular is difficult because there might be internal flow restrictions, you need to know the dead volume, how far the diaphragm moves back... I would stick to a generic valve with fixed parameters and play with the other variables.
Absolutely correct. But another member posted his launcher...search "LOKI"
He found what appears to be an off-the-shelf decent 12v compressor that filled his chamber requirements within 30 secs. Obviously this is intended to be run off a continuous car power supply such as a cig outlet but possibly there are ways to run a similar unit from a portable battery source.
That's very counter-productive even for hobbyists. There is so much valuable experience and computed data available on this site and throughout the internet relating to pneumatic launchers! But wait a minute!! I just found a big rock in my garden. I wonder what would happen if i chisel it into a circular shape? OHHHH you guys call it a WHEEL? Seriously, why are hobbyists working old data backwards when scientists are using old data to move forwards???
I think JSR is implying that people get accurate results out of GGDT by building a design, testing performance and then modelling it in GGDT until GGDT matches real world performance. GGDT is a sophisticated program but it can't account for everything, so the results can be about 10% off. For example, to model the range of golf balls you find you have to enter a lower drag coefficient than expected (because the range simulation built in to GGDT is more of a convenience to get a rough idea of projected range) because in real life you measure the golf ball to a distance of 420m where GGDT will say about 350m with the 'correct' drag coefficient.
One may also chrony a cannon and then model it and GGDT and find that the entered valve opening time or seat diameter might be too conservative and GGDT is giving results slightly lower than the chronographed readings.
The GGDT website provides the variables for a standard 1" sprinkler valve but you may find that yours is slightly different in design and has a larger seat, more pilot volume and a different type of blow gun for the pilot valve.
From the same source, this seems like a better idea and not much more expensive: http://www.harborfreight.com/12-volt-10 ... 96068.html
Fair point but on the other hand, electrically actuated sprinkler valves aren't exactly on the cutting edge of spudgun development, most of the experienced builders on this website only use them for low spec "fun" projects. Well, they're all fun, but you get my point
If you want some hard data, Mr. Hall has kindly put some up on his GGDT site:
http://thehalls-in-bfe.com/GGDT/library ... rbit1.html
http://thehalls-in-bfe.com/GGDT/library ... rbit2.html
These are specs for modified valves though, for an unmodified valve you would have to reduce the vent diameter significantly, the best is to take the thing apart and get your vernier out
Thanks for putting the GGDT inputs into perspective. My previous post may have been a little facetious due to me being pedantic about specs that probably aren't so critical for my intended project. The generic input figures provided on the "hall" website will more than likely be sufficient for me. I thought I had been thorough scouring the site but I managed to overlook that info.
Hopefully I can maintain some control and not launch the first tennis ball at my son at 200+kph, even if the video of it will score some big bucks on "funniest home videos".
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