Compressed Air Rockets... Launched Up a Few Notches.
In a 2008 Make magazine, Rick Schertle wrote a great article how to make compressed air rockets. Like many of us Instructable-lovers, I've seen a great many articles, tutorials, Instructables, and How-To guides for all kinds of cool projects... But Schertle's article was different - for the first time ever, I actually get off my lazy butt and immediately went to the hardware store. And boy am I happy I did (btw, so are the kids, adults, and Hackerspace Charlotte fans that got to partake in all the rocket goodness with me).
But we makers are never happy with leaving well-enough alone, right?
From the very beginning mods were clearly required. For example: 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 this was a good thing. It got the wheels upstairs turning, thinking of all the possibilities, and their advantages and disadvantages of the various components. This 'ible shares the new design I've come up with and how you can build (or mod) it yourself.
Note for the slackers: If you don't want to be bothered or you don't like the design I have here, then you can always just buy a kit from the Maker Shed.
Shopping tip for the semi-slackers: Check your local Lowes or Home Depot - our nearby stores will let you order all the parts online and then pick them up in the store 20 minutes later (but give them an hour or two for all these parts)... much easier than digging through the mis-labeled bins yourself.
This instructable has been at the 90% finished mark for a couple years, so instead of waiting for me to all the way finish or put the latest revision in I'm publishing now "as is for" the benefit of all those that have asked through the years.
Step 1: Decide on a Design
What makes this Instructable any different from the others? Testing, redesign, more testing, and you guessed it more redesign. This led to stable and compact design with integrated base with easy aiming and interchangable parts. Hopefully you'll agree it's worth the writeup. There are several options to consider in the design of compressed air rocket launchers. You'll want to determine your size or volume approach, what connections you'll use, and then settle in on the right valve mechanism. For the most part we found that 1" PVC provides a good balance between volume of air, size of launcher and cost of parts.
High volume launchers don't require as high of a pressure to get good results. This is a great option for kids where 20-30 pounds is just about the limit of what you can do with a bike pump.
Low volume launchers are much more compact but require higher pressure to achieve the same altitude as the high volume launchers.
In the end all the designs require about the same number of pumps to achieve the same altitude. The amount of Work this system can do is after all the simple combination of Pressure times Volume ( W = pV ) with a little Volumetric Flow thrown in (which gives an advantage to the big high volume launchers).
Connections: Threaded vs Slip or a Mix?
Connecting the launcher pieces is another design consideration.
Threaded connections allow you to build an entirely glueless launcher. No messy primer or glue means you can assemble this launcher in the car on your way to the park (or even in the store to make sure you have all the parts ). Due to the availability of materials, threaded connections should be utilized with the low volume design, however threaded connections are more pricey. Sections of pipe with threads on both ends are called "nipples" (stop giggling), and I've only found then in Schedule 80 (grey) PVC in lengths up to 12 inches. Schedule 80 is stronger and has better high temperature properties (Schedule 80 is the only PVC version I recommend). If you're interested in the high volume design, then Slip, or Slip-Threaded connections are the way to go. Mixing in a couple of slip-threaded adapters to in few key locations will make modifying/reusing/repairing you launcher much easier. If you're going to do science experiments and test the pressure vs volume trade-off, I highly recommend using threaded connections where indicated. OK, I actually recommend using them no matter what, but the choice is yours.
Determining the best valve for your launcher is a not so obvious choice. Sprinkler valves make the whole project that much cooler - fast launches, electricity, a button to push... what could be better? Well if you're making your launcher with copper pipe for awesome yet expensive steampunk rocket launcher, a green plastic sprinkler valve is probably out of the question - brass quarter turn valves area must (or spray paint I guess). At close to $12 this valve is the single most expensive part of the launcher. And the sprinkler valve gives a much snappier (higher power) launch, but that may not be appropriate for all audiences. PVC quarter turn ball valves are much cheaper (less than a dollar online or $3 in a store), and they're sort of auto-regulating. Smaller kids can't open them as fast. And you're doing this as a class project, you can build one of these simple launchers with the PVC quarter turn valve for less than $10 for all your parts! But sprinkler valves make rockets go dramatically higher (~5-10x).
Step 2: Gather Your Materials
Save time and your sanity - leave the shopping to the professionals. Order your parts online at Ace/Lowes/Home Depot and then walk in to the customer service desk to pick up your parts a few minutes later. That way all you have to do is inventory, pay and go. The inventory is really important. It's just as easy for them to confuse a 3/4 x 1" adapter for a 1" x 3/4 adapter as it is for you to mistakenly order the wrong thing! To make it easy I've included the relevant portions from the receipt of my last Lowes order.
Please note that they do NOT have the 1/8" inlet/tank/schrader valve. You have to buy that elsewhere. Also no one store has the lowest price on everything. If you value cost over convenience, order for multiple sources. And if you have time as well, consider ordering everything but the big pipe online. And yes, I use more threaded connections than is absolutely necessary - feel free to glue parts together if you don't think you'll ever change them. Likewise if you don't like the way I change sizes, do what best suits your needs. A few of the size/thread decisions where dictated by what was locally available.
WARNING: I much prefer Schedule 80 PVC (not listed below), for personal use or Galvanized Steel Pipe for large scale/classroom use. Steel is obviously much stronger (and much, much heavier), but somewhat surprisingly - less expensive that Schedule 80 PVC!! Don't get the "Black Steel" pipe - it will rust, nearly overnight. Schedule 40 is not as strong as Schedule 80 and under no circumstances should you use Schedule 20 ( "thin wall" ) PVC. And DWV (Drain, Waste, Vent) or Foam Core PVC is NOT pressure rated at all. It's mostly available in the larger sized (3+ inches diameter), but avoid it like the plague all the same. And if you do opt for Schedule 40 PVC take care to wrap it in tape and pressure test. Also know that NASA pulled their Compressed Air Rocket Launcher instructions from their Rocketry Manual due to a failure that resulted in injury... (the reason wasn't specified, but they almost certainly weren't using the stronger Schedule 80 or Galvanized Steel Pipe).
These parts are for Schedule 40 PVC, because it's most plentiful, cheap, and what was recommended to me when I made my first dozen launchers (none of which have had any sort of catastrophic failure after many hundreds (maybe thousands) of launches each). Again - I recommend Schedule 80 or Galvanized Steel.
1 each - 1/2" x 10' Schedule 40 PVC Plain End Pipe
Item #: 23966 Model #: PVC 04005 0600 $1.78 Launch Rod - You only need about 12", so this is good for 9 more launchers!!
1 each - 3/4" x 1/2" PVC Pressure Sch 40 Adapter
Item #: 22698 Model #: 436101 $0.57 Launch rod threads you could go for 1/2" straight to 1" and skip the next part but it's harder to find
1 each - 1" x 3/4" PVC Pressure Sch 40 Bushing
Item #: 51348 Model #: 439131RMC $1.13 Launch Rod Adapter connects threaded launch rod to articulating arm
2 each - 1" PVC Pressure Sch 40 Elbow
Item #: 126824 Model #: 412010RMC $1.37 Articulating launch arm - Don't make these tight! They need to turn so you can aim
1 each - Orbit 1" Jar Top Valve
Item #: 249620 | Model #: 57661 10.22 Main Release Valve - This part can be replaced with a quarter turn ball valve... but why? Make sure the solenoid is off-center! (the off-center solenoid models seem to be the high pressure variety that we want).
1 each - 1" PVC Pressure Sch 40 Adapter
Item #: 23858 Model #: 436010RMC $0.48 Spinkler Valve Adapter - Your sprinkler valve _may_ come with two of these - check first
1 each - 1" x 10' Schedule 40 PVC Plain End Pipe
Item #: 23976 Model #: PVC 04010 0600 $3.50 Body Tube - You will need two 16" lengths,one 9" length, two 4" lengths, two 3" lengths of pipe
1 each - 1" X 1/2"PVC Pressure Sch 40 Tee
Item #: 23877 | Model #: 402130RMC $1.00 Inlet Tee - In with the good air
1 each - 1/2" x 1/8" Brass Bushings
Item #: 35094 Model #: A-826 $3.18 Inlet Adapter (Expensive!) - These can be found online in PVC for less, but shipping ends up spoiling it.
1 each - 1/8" Inlet/Tank/Schrader Valve ~$5.00 Bike Valve $1.49 from HD online only. $3 at Grainger. $5 at Ace Hardware in Stores Now!
2 each - 3/4" x 1" PVC Pressure Sch 40 Adapter
Item #: 22695 | Model #: 436102RMC $0.94 Air Tank Connector http://lowes.com/pd_22695-1815-436102RMC_?fromSto...
1 each - 1" x 3/4" PVC Pressure Sch 40 Tee
Item #: 23875 | Model #: 402131RMC $1.44 Air Tank Connector For "Q" Model Only - This is the tank end that connects to the launcher.
4 each - 1" ELBOW
Item #: 23870 Model #: 406010RMC $0.48 Q - Tank Corners For "Q" Model Only
You may also need a saw or knife to cut the pipe to size with. Primer and glue is also required. Duct tape makes it safer, and electrical tape over that makes it look cooler. Also a button (which is truly optional but so much more fun), wire, battery, and connectors are all available from Radio Shack, but they are covered later.
Step 3: Measure, Cut, and Dry Fit
Part of what makes this design so much better is the optimization / efficiency of the position and size of the components. The ability to twist/turn threaded connections either for assembly or aiming was maintained while still ensuring the smallest possible, yet highly effective launcher. Feel free to make adjustments as you see fit, but know that we didn't leave a lot of excess wiggle room on the table. When using schedule 80 or Galvanized Steel Pipe choose nipples of approximately the same dimensions or the nearest size you can find that fit. You will need to eliminate the back 9" piece and replace it with two caps on the 12" (cause they don't make 16" ) side pieces. Be sure to test fit in the store (for Schedule 40 or Schedule 80 slip fit buy the 10'x 1" PVC first, cut it to length and take it back to make sure you buy all the right pieces).
1" PVC pipe:
Sides - two pieces cut 16"
Back - one piece cut 9"
Front - two pieces cut 3.5"
Center - two pieces cut 3"
1/2" PVC pipe:
Launch tube - one piece cut 11" (as big as will fit when folded flat is the goal )
Dry fit everything to make sure it all fits properly with your connectors. But don't push too hard are the pieces will get stuck together. Glue will make the pieces easily fit all the way together. Just mark the loosely pieces with a pencil at whatever depth they go in nicely. You can gauge how much more they will fit together using the pencil mark. Do not dry fit the pieces with primer on them - they may never come apart!
Step 4: Prime and Cement
Insert your threaded connections all the way and glue them in place to ensure that the desired alignment/angles/seal is achievable once glued. Twist the parts as you insert them. This really does help to ensure that everything is evenly glued. Just make sure they're pointed in the right direction when you're done. The glue does not take long to dry. Hold the parts together for about 10-20 second - they will try to push apart! After that the parts should be pretty solid, but hold off on high pressure for a day.
This design does leave you with a situation where you will need to glue two connections at once and depending on where you do that, you either will need to bend your pipe a little to insert the parts or you will not be able to twist as you fit them together. I prefer the a little bending.
Step 5: Tape Threads
All threaded connections should be taped to prevent leaks. The teflon tape we use is either white or yellow. The yellow tape is most often used for gas and iron pipe while the white tape is often used for water and copper pipe fittings. Air being a gas you might be inclined towards the yellow tape. But I've used both and they both work just fine even at really high pressures, so you might want to let price dictate: white tape is about one third the price of yellow.
Tape in the direction of the threads so that the motion of threading the pieces together does not work to push the tape off, rather it tightens the tape on. This isn't terribly critical, but it does sometimes make life a bit easier. Go ahead and use a good thick layer of tape - enough that you can still see the threads underneath, but they're smoothed out by the tape.
Step 6: Make Launch Button
This step is optional, but totally worth it. If you've opted for the quarter turn ball valve, just skip ahead. I say this step is optional, because you can do a couple of things instead of a making a real button launcher.
Just turn it! The solenoid comes out by turn counter clockwise. Just give it a twist and the pressure will be released in an instant and your rocket will launch. This action happens much much faster that with a quarter turn ball value. If you're into fast pressure release you can actually make it even faster with an air hose blower adapter and have a totally manual (no electricity) trigger. If ever your batteries don't have enough juice to open the solenoid (it takes more power at higher pressures) this is the fall back method that always works.
9 Volt Battery - The leads on the solenoid should already be stripped. If you stick one terminal of a 9V battery to each lead the solenoid should actuate, releasing the pressure and firing your rocket. Really simple, and all you need is the 9V battery. But it leaves you a little too close to the action. I like to tape one wire to one side of the battery with electrical tape and then tape the other wire near the other terminal. Then I can still "push" the wire kinda like a button.
Lengthen the wires - add some speaker wire (or any other suitable wire) to the leads (just twist the one speaker wire to one solenoid wire and repeat) from the solenoid and apply the battery as above. This puts you at a safe distance where you can better observe the launch.
But there's nothing that says press me like a red button! For my dedicated launch buttons I use two 9V batteries or six 3V watch batteries (CR2032). The watch batteries can be much cheaper if you order them online. It's important to note that the solenoid expects to be wire to a 24V AC circuit. We're using DC and a solenoid is basically a short circuit when it comes to DC. So do NOT hold the switch closed or you could burn out your solenoid (and heat up you batteries quite a bit). Also if you get the "wrong" sprinkler valve it will require more than 9 volts to operate the solenoid if you're over 40 psi or so. Two batteries usually does the trick and three might put your solenoid at risk.
On to the wiring. You will have three pairs of wires. One pair from the battery. One pair from the solenoid. And one pair from the button. Connect one wire from each pair to one wire from each of the other two pairs. The length of the wire doesn't matter as long as each part (button, battery, solenoid) is connected to the other parts with one of its wires. Sometimes I put the button on battery holder so the wire from the battery to the button is really short but the wire from the battery to the solenoid is long and the wire from the battery to the solenoid is also long. Other times I strap the battery to the sprinkler valve so the wire from the battery to the solenoid is short, but the wire to the button from the battery is long and the wire from the solenoid to button is long, as well.
Step 7: Wrap It Up
Pressure rated Schedule 40 PVC is reasonably strong stuff. The 1" variety can hold around 400 psi before it bursts... under ideal conditions. As the temperature rises, that burst pressure falls off fast. On a hot summer day you have a little more that half that pressure rating. But your high pressure sprinkler valve is only rate for 150 psi and the low pressure valves are rated for a mere 75 psi. If you get above that the valve won't open when you apply voltage (but you can always turn the value to manually fire the rockets). And most bicycle pumps or compressors top out below 200 psi.
But that's for new well care for pipe, not something that is getting bumped around constantly, pressurized and depressurized, and worst of all - left out in the sun to deteriorate the PVC. Sadly, as with all things, the integrity of your launcher will eventually deteriorate until it fails. So we take steps to lengthen the time to failure and reduce the hazard when that failure occurs.
Tape will stop a bit of the damage caused by the invisible light of the day star. And it will [hopefully] prevent nasty shards of PVC from flying about in the event of catastrophic failure. The as yet untested idea being that good reinforced duct tape can provide enough give to let the PVC break and release pressure, and enough strength and grip to keep tiny shards from completely separating.
Use a couple of layers of tape here. Cover everything plastic, especially the straight pipe (the elbows and such are a bit stronger, but no guarantees here). Don't skimp - think of the kids!
Duct tape may be strong but it looks terrible. Electrical tape can provide a nice finished look for your launcher. And who doesn't love stripes? Nothing says danger like black and yellow stripes!
Step 8: Test It... Safely
Before you launch at the park, give your launcher a pressure test... safely. Stick the whole launcher in a trash can or something else filled with enough water that you feel will adequately contain the chaos if it does decide to burst. Use a pump with a long hose so you're not in the blast/soak zone. Start with low pressure! Make sure basic operation works. Increase the pressure gradually test firing "blanks" along the way. When you get to your max intended operating pressure, repeat the pressurization/de-pressurization test several times. Then over pressurize your launcher by some unreasonable, amount. You don't want to discover at the park, with kids gathered near, that a few extra pumps when you're not looking is all it takes to turn your launcher into countless high speed projectiles.