Android controlled t-shirt cannon

Every year, Qualcomm hosts the “Battle of the Schools.” This year the goal was to build homemade contraptions that would be judged on how cool they are. [Doug DeCarme], [Shaver Deyerle], and [Zach Rattner] – three Qualcomm employees at Virginia Tech – built an Android controlled t-shirt cannon for this event and ended up tying with Michigan State for first place.

The cannon is built around an Arduino Uno and a BlueSMiRF Bluetooth modem. [Doug DeCarme], the Android developer of the group, put together an app that would fire each barrel independently. The valves for the cannon are just 150 PSI inline sprinkler valves, bought from the local Home Depot. From the project breakdown, the team spent less than $150 on entire project.

From the video, we see that they’re getting some pretty good distance firing t-shirts at 160 PSI. Although we question the wisdom of using PVC as a pressure vessel holding 160 PSI, changing the PVC to a proper air reservoir wouldn’t be that hard.

Comments

  1. zigzagjoe says:

    It looks like all they are doing is using the Arduino/android linkage is to fire the mess. Why not put a push button (optionally with relay) on some decent length wires and have done? About, what, 500$ cheaper?

  2. Hackerspacer says:

    STOP using PVC for compressed air. It is an extremely unsafe use of the material for that application.

    Yes, you can “get away with it” – until something goes wrong and it explodes and throws razor sharp shrapnel all over. PVC is NOT appropriate for compressed air use.

    • Davis says:

      It is completely safe to use PVC as a pressure chamber for a t-shirt cannon. PVC can be bought pressurized with a ratingg up to about 210 psi. All that is necessary for safe use would be a pressure release.

  3. Hackerspacer says:

    Why is PVC unsafe for storing air when it is used all the time for safely transporting water and other liquids?

    Water, like most liquids, is not compressible, therefore it cannot store energy. When a hydrostatic failure occurs, water is projected, but the shrapnel is not projected very far.

    On the other hand, air and other gases are compressible. This can result in large amounts of stored energy. System failure could lead to a disastrous situation when this energy is released, sending shrapnel outward. Severe injury and damage can result.

    PVC is susceptible to breaking – through physical damage, crushing, sudden impacts, etc. Small scratches on the outside of the pipe can turn into stress points that when combined with air pressure behind them, can lead to sudden and rapid failure – and injury.

  4. Slanderer says:

    Slightly dangerous? Yes!

    ..but no one is going to make a spud gun with anything else, so stop preaching this. 99% of people don’t have the capabilities or equipment to work with other materials. The danger is negligible when these things are only used a handful of times.

    Hell, it’s even remote controlled, which means that they won’t need to be standing next to it if it goes off.

  5. Hackerspacer says:

    How hard is it to weld on an NPT fitting and attach a steel pressure tank that you can get for $20 – $40?

    There is no excuse. Make the barrel out of PVC if you must. Just don’t make the pressure containment out of PVC. Can you get away with it for a while? Maybe. Should you? No. This isn’t rocket science, we know how PVC behaves under pressure. It is wholly unsuitable for use as a containment vessel for air, especially when used in close proximity to people.

    The danger is not negligible – it becomes dangerous the very first time you fill it with air. The fact that you only use it a few times might mean the total risk is lower than if you used it thousands of times but the risk is not negligible.

    You are correct that is able to be remotely fired but the only way to minimize the risk is to fill it remotely to maintain distance from it. The danger, as you seem to indicate, isn’t when it goes off. The danger is if any kind of physical damage takes place to the PVC while it is under pressure, causing a sudden and catastrophic failure of the PVC vessel.

  6. rusty says:

    you need a life^

  7. pinky says:

    @Hackerspacer you’re argument is unsound. Whether it be liquid, compressed air, or whatever, PVC pipe is rated to a certain pounds per square inch. Beyond that point, it will fail. Granted, there is a factor of safety, but it doesn’t matter how much energy, if it is beyond the ultimate stress (which is in lbs/sqin) it will FAIL. Whether you want to be around 300psi releasing in a .25 square inch rupture is up to you. But, sir, I would do the math before you attach your paintball gun to some drainage pipes.

  8. Dave says:

    @pinky- no, he’s right. When liquid cracks the pipe it just leaks, the liquid doesn’t expand. When air does it it explodes and you see catastrophic failure.

    I have seen both failures occur first hand, and I fortunately still have both eyes. I now take extra precautions- like if I DO store compressed air in a PVC pipe, I keep that pressure vessel in something that will contain the shrapnel, like a wooden box with enough air holes to vent the gas but contain the plastic.

    What would help more in a forum like this though is a link to instructions on how to build a metal pressure vessel with available materials. Not just the vessel, but a large output orifice (by large I mean 3/4″) as well. Most of your metal pressure vessels are limited by 1/4″ output somewhere.

  9. loans says:

    If it’s rated to 300 psi, is that -after- any kind of stress? PVC tubing is fairly brittle, and scuffs/scratches/dings easily to boot. I think concerns about using such a material to contain pressure are valid.

  10. John says:

    As someone who has seen PVC fail under pressure and is very thankful that no one was around it when it blew I can assure you its not a good material for storing compressed air. I think if you go on some manufactures websites they will even say not to use it for compressed air. It can and will go boom and little pieces of plastic end up everywhere. (150 psi air compresser on it by the way)

    That said, I replaced the PVC line that blew with pex which isn’t rated for 150 psi either. Heck, at least when that fails it will just split.

  11. John says:

    http://www.lni.wa.gov/Safety/Basics/HazAlerts/902.asp

    There. Even a government somewhere says don’t do it.

  12. Andrew says:

    BRB while I build more PVC air cannons to piss off Hackerspacer.

    My life, if I’m willing to take the risk then you have no right to tell me not to. It won’t hurt you.

    Quit your whining.

  13. pogyhauler says:

    Hey Andrew;
    Your right. it’s not a crime to be an idiot.
    It’s not even a crime to be an idiot in public.

    However, There is such a thing as Gross Negligence with Criminal Consequence.

    Build it if you like. That’s what the Darwin award is for.
    But, Hurt somebody else, Or Make statements contrary to facts as you know them, encouraging others to engage in dangerous behaviors,

    And I will be there to cheer when they flip the switch on your ignorant ass.

  14. Hirudinea says:

    That’s a pretty neat little Maude killer (Simpsons referencde) and as for PVC storing compressed air and shrapnel, what about wraping in in carbon fiber or its cousin, duct tape.

  15. pogyhauler says:

    Or, How’s ’bout you don’t futz with compressed air without a reasonable container?

    Home Depot will sell you a few thousand ways to kill, Maim, and be Generally Dastardly.
    That don’t mean you gotta go try them.

    Stick with paper airplanes, and arduino shields.

    You -can- hack to your little heart’s desire without dancin with the devil.

  16. Steve-O-Rama says:

    According to the data on this site, its pressure limit is dependent upon the pipe’s length, as well: http://www.harvel.com/tech-specs-pvc-pipe-40.asp

    It’s probably safe to assume that the danger level increases in some proportion to pipe length, wall thickness, damage (if present), and pressure level.

    That said, the Schedule 30 stuff works freaking GREAT as air inlet tubing! And as a vacuum reservoir when you use a pipe with caps on both ends.

  17. aEx155 says:

    While it is important to keep in mind the very real danger that the PVC/compressed air combination can be, think about the reason why this hasn’t stopped happening.

    As a readily available and cheap material, many inexperienced people who are starting out may choose PVC as a building material without knowledge of the risks it poses.

    As these people progress, they should become educated on its various failure modes and then move away from PVC if they want to continue their new hobby (both life-wise and progress-wise).

    So, rather than indicate various dangers and ratings and such on a site like Hackaday, go to Instructables or contact the original author if they don’t have an appropriate warning, because it would make much more sense to have those precautions where people are reading about the actual build.

    Pressure vessel choices aside, I think the group had a nice execution for the contest and they did a good job.

  18. aEx155 says:

    Also, if you say that inexperience isn’t a reason, and that people should read up before attempting a build, that’s all the more reason to have a warning with every build-log or instruction.

  19. CRJEEA says:

    Maybe the adition of one on thoughs portable cigaret lighter powered air compressors to refill the compressed air at a distance (maybe with a pressure sensor to provid feedback via Bluetooth) (a big stack of salvos to auto reload would be nice too) maybe a doubble buffer sort of arangment were one barrel is attached to two vesels one being loaded and charged whilst the other is fired? And maybe some way to aim the system. Maybe with face tracking :D
    Oh and as numours people have already pointed out… A safer means of storing the compressed air.

  20. password says:

    this might make a lot of people here cringe , but a good pressure vessel for compressed gas is 2l coke bottles

  21. Jerry_J says:

    That fact that someone is giving awards for unsafe designs bothers me more than anything. These are, presumably, engineering students.

  22. Dave says:

    @Jerry_J: Safety may not have been a factor in the contest, although I agree, it should not be overlooked. I don’t see a presence sensor on the business end either. At 160psi, I’m betting the muzzle velocity of a tshirt could easily put out an eye. I think that if you’re going to enforce safety, you’ve really got to lay it out in the requirements.

    @password: It’s better, but still not perfect. When a coke bottle explodes (at about 150psi) it doesn’t toss much shrapnel, but if you wrap a tshirt around it it’ll shred it pretty nicely. And anybody nearby will lose their hearing temporarily.

  23. Otacon2k says:

    Meanwhile there’s a comment on their website about pvc being no safe choice for this kind of project, which should keep people from just copying their design. Thanks!

  24. aEx155 says:

    @Otacon2k Exactly what I mean.

    Regarding the mention of soda bottles as air reserviors:

    Most bottles that hold carbonated drinks are usually good to about 120 PSI. The people at aircommandrockets.com have used fiberglass and epoxy resin to reinforce 2L coke bottles and they’ve used them in rockets up to 250 PSI. If you were planning to use plastic bottles as an air tank, maybe fiberglassing them would make them much safer for use at 100 PSI. A sort-of hybrid air tank. It would also be much lighter than any other tank of the same volume. Just be careful with it.

  25. BacklitVillain says:

    With regrads to the saftey of using pvc pipe, could the risk of danger from explosion be over come by slipping a wider tube (say 10-30mm) over the pipe contaiing the compeesed air?

    If the compressed air does explode the shrapnel fired of will be contained in the larger tube rather than flying of into the surrounding area, minimising the risk whilst keeping material costs down.

    Im not an expert and this is just something that came to me reading the same argument about compressed air and pvc again.

  26. aEx155 says:

    @BacklitVillian

    You would need sufficient vent area to allow the expanding gas to escape while maintaining the structural integrity of covering pipe, which might be hard to do.

    Maybe there are other protective coverings that can be used? Chainmail? Kevlar?

  27. aEx155 says:

    But then again, you’d have to weigh the cost of a protective covering versus a proper pressure vessel, too.

  28. Steve-O-Rama says:

    I’d think a blow-off valve would be better than a containment vessel (think of water heaters and turbochargers), but those can sometimes fail as well. Another type of container would be the best choice (obviously), but this is merely prototype work in its bare naked form, IMO. I’m sure that many HaD’ers have made dangerous prototypes over the years, myself included.

    Grabbing an idea off of the Internet and running with it, without a fundamental understanding of what’s really going on, is plain stupid, let alone dangerous, and I think that’s what most of us are saying… ???

  29. Alan says:

    Am I the first one to suggest ABS? I’ve used it in spud guns in the past because it is known to split rather than fragment. I learned a lot from burntlatke.com

  30. KillerBug says:

    Who cares about safety on something like this? The only reason they used an arduino was so that they could get out of the blast radius…if it was shoulder fired like that fireworks RPG thing a few days back it would be a different story…but no one can get injured if no one is nearby!

  31. Hacksaw says:

    Pressure ratings are pressure ratings.Schedule 40 PVC 3″ in diameter is rated at 260 psi with a 2:1 safety factor. Someone suggested “welding on an npt fitting”…What kind of pipe are you suggesting there? Black iron? Galvanized? you go ahead breathe the fumes from welding galvanized i’ll live a lot longer than you even building pressure vessels out of pvc.

  32. willow says:

    Aside from all of the comments, nobody else noticed they’re running 160psi through a 150psi valve? Bueller?

    @Andrew: You can be stupid all you want, but if the chamber-turned-potential-bomb erupts with anyone else nearby.. then you’re involving others. Shrapnel from a small vessel like this will fly a good 100′ in all directions. The soundwave will temporarily deafen anyone within half that range.

    @Hacksaw: Pretty sure he was referencing steel vessels which can be had for cheap $20-$40 locally or online, which usually only come with 1/8″ or 1/4″NPT fittings.

    Pressure-rating aside you still have to calculate the derating figure. Most PVC has a specific maximum operating pressure for a set temperature (usually 72*F). Anything above or below this will change the properties of the pipe, and you have to use a derating calculation.

    If the ambient air temperature, or the PVC temperature, hops to 90*F, a general derating figure of .75 for SCH40 PVC should be used. This immediately drops 160PSI rated 2″ pipe down to just 120PSI. With a fast-inflation (recharge) of the PVC chamber, the air temperature easily shoots over 100psi instantly and this can be a dangerous situation. Moreso with warm air.

    If it’s cooler the PVC becomes brittle, so the slightest jar or sharp strike at just the right time can lead to failure.

    Something else to remember are the fittings. Threaded fittings are more likely to introduce failure, than properly-glued fittings.

    They’re pushing the PVC to the limit, and they don’t have to. They could spend $3 and get pressure gauges also, which would be wise, and another $5 for a safety-pop valve.

    Also, they could’ve easily gone to 4″ PVC, double the chamber volume, drop the operating pressure to about 60psi (or even less), run a 1″ Rainbird valve and ended up with an even more efficient launcher, using less air, far below the de-rated PSI limit, which is far safer.

    One of my frequent toys uses a ~200ci steel chamber with a 3/4″NPT outlet, 1″ valve, 28×2″ barrel, operates at no more than 40psi and can launch 30g projectiles more than a football field at ~215fps. And that’s a pretty inefficient setup (the 3/4″NPT is the chokehold).

    Do the research folks. Hackerspace is right on target with this.

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