Compressed Air Rocket Launcher

A while ago I read a neat article that detailed how to make a compressed air rocket launcher. ┬áThe thought of making rockets from scrap paper and launching them several hundred feet high with nothing but compressed air sounded pretty neat (and economical). So over the last several months we gathered materials and purchased a few necessary tools in preparation for building this great little device. Of course, I wasn’t satisfied with a few aspects of the original design, so I made some small modifications to the plans.

The Maker Shed sells a kit with all of the parts, precut and ready to go, for about $50 (but be prepared to spend upwards of $20 on shipping). I’m glad we got the kit because shortly after ordering it I found that I can’t make a straight, clean cut in PVC to save my life. Other than tools, the only thing you really need to get that doesn’t come in the kit is the PVC primer and cement. Notice my sloppy application of the purple primer in the photo below.

Following the plans to the letter, I assembled the compression chamber last week, but a friend of mine from work wanted to sit in on the electronics portion, so I held off on that until today. Tools required to get to this point were slip jaw pliers, Channellock Griplock pliers, a rubber mallet, and a few other things I didn’t have. I wished I had some kind of sand paper (something other than a metal file) to get the rest of the rubber off the tire valve (at the end of the hose in the photo). Since I couldn’t get it completely clean, the valve didn’t make a great seal with the tube. Also, the little metal ring that you crimp on there to seal it tight requires side cutters (or something similar) to make the crimp. I didn’t have tools to adequately crimp it. So the end of my tube leaked a lot on the first pressure test. I fixed it with some good, old-fashioned super glue and wrapped the mess in electrical tape pulled tight to add a little more seal.

Oh, and make sure you have or get some rubber gloves to use when working with the PVC primer and cement. You need to keep that stuff off your skin, but it will get all over your hands and everything else (no matter how careful you think you can be). I used leather and canvass work gloves and worked inside of a cardboard box, but I think some of it still leaked through to my hands a couple of times and a few drops somehow ended up staining my computer mouse. The warning on the cans said to wear rubber gloves, and I should have just bought some rubber gloves. Oh, and work in a VERY well ventilated area.

Another note: above 60 psi I noticed that the hose started leaking from the hose attachment closer to the chamber. Jiggling the zip tie fixed it, but this kit’s main problems all seem to be with the sealing methods used at each end of the hose. If I worked more with airtight, pressurized systems I might be able to fix those issues, but for now I’ll just keep jiggling things to keep them in line.

To pressurize the chamber you need a bicycle pump. We found a cheap one (under $20) with a pressure gauge built in (DO NOT surpass 90 psi) at Target, but we had been planning on getting the pump on Amazon before we found the one at Target.I can’t tell you how nice it is to have the pressure gauge built in. Knowing exactly how charged the chamber is can be essential, and taking the pump off to use a tire pressure gauge would be annoying.

As of this morning, that’s as far as I had gotten. All of that work (assembling the pressure chamber, testing it and cleaning up my mess) took about four hours. The rest of the work, which I did very slowly today to prevent errors, took about six hours. That’s about ten hours to fully complete my version of the project. Following the stock instructions and moving with more confidence would probably cut that time in half.

As for today’s work…

The pressure chamber will eventually fail, and when it does you don’t want a bunch of PVC shards flying at your face. So it is recommended that you wrap it up in a few layers of duct tape.

Once that was finished, the only structural element remaining was the stand (which looked easy enough, so I put it off because I couldn’t wait to get to the electrical portion). I didn’t want to install the stand until we had put a layer of colored duct tape on the chamber, which was my wife’s job since it had to be pretty (and I stink at that).

Since I would be doing some electrical work, and I wanted to solder the leads (much more secure than just taping them), I needed to get some supplies. So a week and a half ago I turned to my new favorite online electronics store and bought the following:

The electrical portion in the provided plans was simple. There is a small pipe with end fittings, you drill holes in the ends, stick a button in one hole and pull two twisted wires out the other end, wire a couple of nine volt batteries into the circuit, zip tie them to the launcher, and hook the valve motor up to the whole circuit. Push the button, and the valve releases the air (which is pumped in with the bicycle pump).

I didn’t like the weak look of the two little wires coming out of the launch button tube, and I didn’t want to zip tie the batteries to the launcher. So I came up with an alternate circuit design. I wanted something modular – I wanted to house the batteries in a box, I wanted to plug the launch button into the battery box, and have the motor plug in as well. That way you can unplug everything, wrap the cords up, and store everything without worrying about stuff tangling up too bad. Plus, if I used better wires, I figured it’d be tougher.

Then I got the idea to put a power indicator light on the battery box with a switch. Then I was browsing around a neat electronics parts store and found this missile switch cover and I knew that my project needed that part.

That’s when I got excited and ordered the following parts to make my vision for improving the device a reality.

From Sparkfun:

From RadioShack:

From Parts-Express:

From Amazon:

So one night I thought long and hard about how I could make this circuit work (especially since I already knew exactly what parts were going into it), and I came up with the following hybrid, nearly technical drawing of how it could work (after several hours of research).

The toggle switch, two jacks (RCA, not drawn well), batteries and little red power light are all pretty clear. That zig zag line is a mystery resistor.

Since the light I found was a 12v light, and the circuit was running off two 9v batteries (that 18 volts!), I knew that I would need a resistor to be in series with the light (which, incidentally, is in parallel with the push button and valve motor). Finding the proper values for that resistor took another few hours of research. I finally found the ratings for the lamp I chose, and using those I was able to calculate that I needed a 100 ohm resistor rated at around .5 Watts. So I added to my list of things to buy from RadioShack:

I also decided to get some cheap 9 volt batteries there.

All of that planning and researching was accomplished last week. Today all I did was wire it all together (thinking in reverse order to prevent myself from making mistakes), solder everything and put heat shrink over bare wire whenever I remembered, and stuff it all the the box. Amazingly, without any testing along the way, after everything was soldered and screwed inside the box, I flipped the switch and the power light went on. Even more amazingly, when I plugged the launch button and the motor into the battery box, I was able to activate the motor with the launch button! I was amazed that it all worked so perfectly right away. I guess all that research really paid off.

The great thing about the circuit is that it doesn’t matter which jack you plug which item into. The switch and motor are interchangeable. Also, if you are planning on attempting this and you’ve never done anything like it, keep in mind that the lamp and resistor can go in any order along their little parallel circuit. I actually reversed them from the drawing, but they would work either way. I’m not going to go into a detailed explanation of why or how, but that’s just the way DC works with those particular parts.

So, to get started putting it all together, I cut the RCA cable into three parts: I measured off a few feet from one end and made a cut, then I measured about 20 feet from the other end and made a cut. That left a bunch of spare cable that I didn’t need (but could use for another project in the future). The longer cable is for the launcher button, and the shorter one is for the motor. Since I didn’t have a nice cable stripper, I had to use my Leatherman blade to prepare it for attachment to the components. If you’ve never done this, practice on the extra cable – you need to make sure you don’t cut the little tiny wires under the surface of the black jacket. These wires cut really easily. After removing the jacket I grabbed them all up and twisted them tightly into a single wire bunch. Then I stripped the cladding from the center wire. Again, don’t nick that center wire too badly or it will break later.

Note: I ended up stripping the rest of the white cladding off on both of these cables. I originally thought I might want it on there, but I didn’t need it.

Afterward I drilled holes in the end caps for the launch button handle and threaded everything onto the longer cable.

Finding just the right spot to put that knot was impossible before soldering the wire onto the button. Put the knot in anyhow right away and you can move it later. Also, on top of that knot don’t forget to thread on the nut and washer for the button that goes in the other cap. Then thread the cable through the top cap’s hole and solder the button on there.

Perhaps one of the hardest things was figuring out how to screw the nut onto the bottom of the button after it was inserted into the cap since I used a thicker cable than was recommended by the project designers. I don’t have a picture of my solution, but in the above photo you can see some of the black jacket from the coaxial cable. I cut about a half an inch off of that and slipped it over the end of a small (but NOT a precision) flat head screwdriver. This created a tiny finger with a little more grip against the metal nut than I had with the screwdriver alone. After a few minutes of jiggling, wiggling and┬ámaneuvering, I was able to get it screwed together pretty securely in there.

After drilling all the holes in my project case I began wiring it all together. This part was simple. I just followed my diagram, thought in reverse (what do I need to thread through where to make sure it ends up in the right place after everything is attached?), and went very, very slowly. The one part of the project that isn’t yet complete is anchoring down those battery holders inside the enclosure. I’m still debating on whether I want to hot glue them in, super glue them in, or use some other fancy bonding technique or chemical.

Here are some pictures of everything after it was completed:

The finished battery box and launcher button handle.

The valve motor and it’s cable with an RCA plug on the end.

The back of the battery box with its RCA plugs. You can plug either the launcher or the motor into either jack.

Here’s everything completed. Notice the new coloring on the pressure chamber. Another few layers of duct tape certainly can’t hurt.

That’s pretty much the whole build process. In all, making the silly little paper rockets was probably the most difficult thing. Those nose cones and fins are nearly impossible to mount properly. Also, put several more layers of tape around the body of the rocket than you believe necessary unless you are going to launch at low psi (below 40, we think). At 80 psi none of our rockets survived and none of them flew very high with gaping gashes in their sides (one of them split in two across the middle). When we get the rocket building process right, I’ll let you know.

A silly note though: If you shop at Costco and you buy their Kirkland Spring Water in bulk, the normal 16.9 OZ bottles fit perfectly over the launch tube and fly off spectacularly in an explosion of mist. It’s pretty cool, but they aren’t aerodynamic enough to go very high (even with fins and a nosecone). Oh, and dropping Nerf darts (the screamers work well) into the launch tube is a riot. A Nerf dart shot out at 80 psi will completely disappear, but if it’s a screamer you can hear it squealing as it leaves the atmosphere and enters orbit around the Earth.

To see more of the family aspect of this project, head over to our family album and YouTube channel to see pictures and video. If you have any questions or comments, or if you use any of my ideas when you build your project, please share in the comments below. Thanks for reading.

**EDIT**

We resolved many of the issues we had here. I think the tube is more secure now, I’ve attached the 9v battery clips to the inside of the battery box, and we finally built rockets that don’t rupture when launched. Oh, and the launcher is now named CARL. For details on all of these improvements, see the follow-up article.

4 Responses to “Compressed Air Rocket Launcher”


  1. 1 Larry September 23, 2013 at 8:18 pm

    Any problems with your 9v batteries overheating/

    • 2 Mediocre Renaissance Man September 24, 2013 at 3:02 am

      No, but we never did it for any extended period of time. I’m certainly not an expert at all the electrical stuff, so if there is a danger of overheating the batteries in my design I wouldn’t really know!


  1. 1 Compressed Air Rocket Launcher Update « The Mediocre Renaissance Man Trackback on August 29, 2010 at 8:20 am
  2. 2 Really, Really Late « Haddad Family News Trackback on October 17, 2010 at 12:10 am

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