2 Jul

Compressed air rockets

Rick Schertle wrote a great article in Make magazine on how to make compressed air rockets. I’ve seen many great articles, tutorials, instructables, and how to guides for all kinds of cool projects, but this one, this one made me get off my slacking butt and head immediately to the hardware store. I am so happy I did, and so are all kids and adults that got to partake in all the rocket goodness with me.  Hackerspace Charlotte wouldn’t be the same without them.

But from the very beginning mods were required. I didn’t have an old tire from which to salvage a tire stem and my first store didn’t stock a decent substitute (but, Ace Hardware does have them). But this was a good thing. It got the wheels upstairs turning, thinking of all the possibilities, and their advantages and disadvantages. It led to the current design, which I’m very pleased with.  Of course if you don’t wank to be bothered or you don’t like the design we’ve come up with here you can always just buy a kit from the Maker Shed.  But here’s a pro tip: Lowes and Home Depot will let you order all the parts online and then pick them up in the store 20 minutes later… way better than you going blind looking in mislabeled bins for 3 hours trying to find all the parts yourself

The current design is as efficient as possible. It has the smallest volume, for less pumping, that still allows for high altitude launches. If the tube were larger you could eek out slightly high altitudes at higher pressures but it’s just not that big of difference with lightweight paper rockets.

Heavy payloads are however significantly impacted by a smaller “storage tank”.  So the design allows for easy switching of storage tank options (although our small tank is quite capable… it just requires a tad more pressure 😉

One of the more significant consequences of this particular layout is that elimination of the stand.  The pressure tank wraps sufficiently around to provide a stable base and the two threaded elbows on the “non-pressure side” side of the value allow for a fully articulating launch tube.

3/4″ pipe is just a bit too small (in this shape/size) to hold sufficient air for really high launches but 150-200′ launches are possible with moderate pressure (75psi).  But with 3/4″ pipe you can actually make a launcher with no gluing required!  Just walk into Ace Hardware with $30 and walk out with all the parts you need to assemble the launcher in the car on the way to the park!!  And with a mix of schedule 40 and 80 it looks pretty cool too.
3/4″ threaded connections were established as a bit of a standard as far as swapping.  3/4 PVC is pretty cheap, but because all the connections are threaded and you’re using schedule 80 nipples (threaded pipe sections), the cost ($30) is higher than expected, but still pretty darn reasonable compared to something from a toy store.

So 1″ PVC is our standard size, but we use a 3/4″ threaded connector to join the expensive launcher parts to the relatively cheap storage tank parts.  In part this is due to the all around lack of 1″ threaded tee connectors (otherwise the glueless solution would be 1″ instead of 3/4″).  It’s also due in part to the first few launchers used 3/4″ sprinkler valves and hence the previously built tanks all had 3/4″ threaded connections.  And we had a few straight tanks for testing the effects of storage tanks size on rocket performance (more on that in another post).

The standard sizing and having both sets of expensive parts (sprinkler value and schrader valve plus brass bushing) grouped together is nice because it does allow quick switching from the smaller “J” models to the larger “Q” models and even “I” launchers (only used for “scientific” testing).

As I mentioned, we quickly determined that only a short length of 1″ pipe was required to achieve nearly full height launches.  So why bother with bigger storage tanks?  Well to launch glow sticks mortar style out of a much larger diameter (1-1/4″ ) pipe of course.  And as as rockets go most of the pressure is wasted.  You tend to leave the value open long after the rocket has left the building (so to speak).  But how long should you leave the value open, and how can you control it precisely enough to matter?

An Arduino of course.  And a potentiometer to dial in the time to keep the value open.  And some LEDs to let you know what the heck is supposed to be happening (nice for debugging).  And a transistor to keep you from smoking your Arduino (apparently it ain’t for power).  It’s a very simple circuit (follow that example, but connect your sprinkler in place of the motor) and even easier code to get working (combine the two in the demo – until i get around to posting mine ;).  And while it’s not much to look at… if you put it in an enclosure with a missle launch button (though not shown online, Radio Shack version I got in store has a translucent cover with an led on the tip of the switch) and that great big red button with a light that SparkFun sells that you’ve been trying to find a good use for (now I have to figure out where I left it collection dust)… Well you’ve got yourself something no kid (of any age) will be able to keep his hands off whethere there is a rocket attached or not!!  Oh and it’s even functionally helpful to the cause.

Anywho, I have a few more pictures up on google plus and picasa that I’ll be incorporating into an instructable on the laucher(s), the controller, and even the rockets (we make that fast, strong, and light), as well as some of the other things we do with our launchers.

It’s not a bomb!
Duct tape below, two colors electrical tape on top
Sadly Missing Big Red Button 

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