Compressed Air Rocket

Design and Construction of a small compressed-air rocket

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Introduction

Whilst on holiday in Sunny Cornwall, we had a couple of days of heavy rain. Being house-bound gave us the opportunity to build a small compressed-air rocket launcher from parts (relatively) easy to come by in a holday town. The rockets are made from paper so although primarily for the children, we found out that all the adults were soon constructing rockets to see who could produce the highest flying specimen. Yes, and my wife made the winner, something that she has reminded me of of a few occassions :-)

My expectaions on performance were not high. I was hoping to maybe get a 5 metre maximum height from the launch but when we launched the first rocket, it easily cleared a two story house and reached a hight of nearer 20 meters, very impressive for a mornings work.

The idea is simple. Make a reservoir that can be pressurized with a bicycle or foot pump. The pressure can be released by opening a valve and forcing the air into a tube-shaped rocket which is then free to fly given its initial compressed-air kick. In order to source all the parts, we needed to visit a plumbing/hardware shop, a washing machine shop and a bicycle shop.

The key component to the design is an electrically operated pressure valve. The proptotype used a washing machine water inlet valve which worked admirably but had the disadvantage of being mains-voltage powered. Once home, I replaced this with a 12V valve so it was more child-friendly but when on holiday, we had to use what we could find!

Goal

Build a low-pressure compressed air rocket launcher (preferabley 2 bar)

Warning

Anything that contains gas under pressure can potentially be dangerous. The components here are repurposed from their original use so may fail under pressure. This could range from a seal going bang to a pipe shattering and firing plastic shards everywhere. The rocket launcher could be housed in a further enclosure to contain any such event. Take appropriate precautions for compressed air systems. If you don't know what these are, find out first. This is all done at your own risk.

Design

The main storage reservoir for the compressed air was made from 40mm PCVC pipe. A Tee-piece was mounted near one end to allow the attachment of a pump whilst still allowing the laucher to sit on the ground without fouling the pump valve. The valve itself was a car tyre valve making it easy to find a variety of pumps to fit it. A car type foot-pump was at hand which did the job admirably.

The electrically operated launch-valve was fitted at the other end of the 40mm pipe and a smaller 15mm pipe fitted to the output of the valve. Paper rockets were made to fit the 15mm diameter pipe and the decrease in diameter gives a further pressure increase to help for a more explosive launch. The diagram below shows the general design.

Construction

Making the launcher body

The 40mm pipe was cut to length and glued to the tee-piece using pvc pipe cement. It may be possible to assemble everything using self-sealing joints but these are more expensive and do not give as much leeway for sealing as doing it the 'old' way. The cement is very similar to the glue we used to use to assemble plastic models as children. It is very strong and dries clearish. It has a very strong smell and is best done in the garden if you want to avoid headaches and complaints. There were now three open-ends to the construction and pipe-collars were also glued to each of the three ends. It was left overnight to ensure it had set while the other parts were prepared.

The very bottom end of the structure was fitted with a screwcap. Jointing compound and PTFE tape were pushed into the screwthread to make it airtight.

Making the Pump Valve Attachment

To create the valve attachment, an old bicycle tyre was used that had a car-valve attchment. The inner tube was cut off 5cm either side of the valve. A hole was drilled through a pipe end screwcap, just large enough to accommodate the body of the tyre valve. The valve was pushed through from the inside and then the extra rubber from the type trimmed into a circular shape so it fitted into the screwcap. Two circular pieces of cork were cut from an old floor tile (with a central hole) to sandwich the valve insert. It was then pushed through the endcap, slathered with jointing compound and screwed tightly onto the tee-piece. A 10mm nut was threaded over the valve and glued to the outside of the structure. This served only as a mechanical strengthener and was not part of the valve seal.

Fitting the Launch Valve

The electrical valve used to trigger the launch had a 'standard' 3/4 inch thread at one end and a half-inch plastic barb at the other. For this design, a straight-through (sometimes refered to as 180 degree valve) design was used, rather than a 90 degree valve. This keeps everything in a straight line which makes things easier to mount. The pipe endcap was drilled with a 3/4 inch hole, a large cork 'washer' on the inside and cork washer on the outside. It was held in place with 3/4 inch plastic nut which was cut off a washing-machine inlet pipe. Lots of jointing compound and the nut was tightened to squash it all together. More jointing compound was put in the screw threads and the endcap screwed into place. At this stage the presure chamber should be complete.

The foot pump was attached to the launcher and pumped up to 2 bar to see if it held pressure. It did not so more jointing compound was added and the launcher valve assembly tighted (as that is where the leak was). The chamber could be then be pumped up to 4 bar and it held its pressure. The pipe should be rated up to 8 bar and the jointing compound is rated to 5 bar so 4 bar seemed a good maximum value. My goal was to try and keep to pressure down to 2 bar if possible, no more than a car tyre so that if anything did go bang, it would not be too dramatic.

Mounting the Rocket Launcher Tube

The rocket launcher tube needed connecting to the output barb of the launcher valve. When on holiday, while scratting around for parts, the pole from a rock-pool crabbing net was an excellent fit over the output port of the launcher valve (with a little drilling). This was a 1/2 diameter plastic pipe. When I returned home, after replacing the launcher valve, I also replaced the launcher tube for a length of 15mm diameter copper pipe as that fitted the valve better and was a more standard size. As it turned out, I found that the plastic crabbing pole allowed me to get the same launch performance at lower chamber pressure. This was because the walls of the tube were thicker, meaning the inner diameter was 10mm rather than nearly 14mm for the copper tube. The pressure inside the launch tube is greater for the same chamber pressure. I therefore reverted back to using the plastic tube and I built the paper rockets using the 15mm copper tube.

Wiring the Launch Valve

This is fairly trivial as it just requires a 12V battery and a push-switch. I did consider making an ACME plunger switch like wiley coyotte uses to detonate dynamite but thought that was going a bit far and it would rather be lost on my 3 year old. So I resorted to mounting a simple push-switch in an old vitamin container and ran a wire back to the launcher valve and battery. This gave something that you grip in your hand and push the button with your thumb (after a suitable countdown of course!).

Building a Launchpad

Rather than propping the launcher tube up against an object to get a suitable elevation, we wanted a more stable launchpad. This was easily done using scraps of old wood and and a couple of hinges. The most important part was the painting. It had to be pink and I had lots of willing help painting it. It is not really necessary to make a lauchpad but it does tidy-away a lot of rubbish and gives a consistent lauch elevation.

Launching Rockets

Find a suitable site to do some test launches. An open field is ideal. Connect the pump and pump it up to 2 bar. For my pump, this is 15 compressions. Fit the rocket to the launcher tube. A well made rocket should be able to be pushed its whole length onto the tube. You may find some tighter-fitting specimens require pushing down a shorter distance, otherwise they do not leave the launch tube. Higher pressures should give better results but I have chosen to keep my launch pressure to 2 bar which seems more than adequate. With this we can fire rockets from the back garden, over the house and over the tree in the front garden. The less powerful rockets get stuck in the tree but usually come down in a day or two.

My children (and their friends) love playing with the launcher. Some of them are too light to press the pump but this means I can keep an eye on what is going on because if left unattended, you will find the thing being used as a shoulder-mounted RPG launcher before you know it !