Compressed Air Rocket Mk II

Design and Construction of a Larger Compressed Air Rocket

Rocket Launcher MkII

Introduction

The original rocket launcher was built from PVC pipe. There has always been a niggling doubt in my mind that this will one day shatter under pressure and send shards of PVC everywhere. My children are much older now and rediscovered the rocket launcher recently, being a good source of entertainment during our COVID-19 lockdown. I felt it was about time for an upgrade and it was a good exercise to show an application of Boyles law and pressure/force/area relationships. We also have an on-going game of tag with a couple of other families and we needed to fire 'tag-rockets' over some trees to get it on their back deck. We need a more powerful rocket launcher.

Goal

Build a steel-walled low-pressure compressed air rocket launcher capable of withstandung up to 10 bar pressure.

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

I wanted the new launcher to be steel walled so we could use it at a pressure of about 6 bar so over-engineering is the name of the game. It had to be able cope with as much pressure as could be applied with a stirrup pump, so overinflation would not worry me.

Originally, I was going to make a steel reservoir by welding two plates over the ends of some large steel tubing. Although my welding has improved, making the welds air-tight was beyond my skill level so I needed another plan. I decided instead to use steel gas pipe. This was ideal as it has a very high pressure threshold, is threaded and there are lots of adapters that can be utilized for making bends etc. It comes in three diameters so I selected the largest 25mm pipe. It would have been better if I could find a larger diameter pipe but you work with what you have.

The problem with a steel launcher is the weight. I had worked out that about 3 metres of this pipe would give me same volume as the PVC launcher. I could make this something that is easier to manhandle by using 3x 1 metre lengths joined with adapters (like a 'S' shape), but felt it was too heavy for the kids. I knocked up a protoype using a single 1 metre length to test how the seals held and provided you use PTFE tape and jointing compound, it seals well.

So my compromise was to make a launcher that was used two 1 meter length 25mm pipes as the reservoir, with the capacity to add a 3rd pipe for expansion if I needed to pimp it later.

Construction

The main reservoir pipe is 25mm diameter.

The irrigation value used for launching has 3/4 inch threads.

The launch tube is a 15mm copper pipe.

The bicycle valve clearance hole is 6mm in diameter

So there are few adaptors and reducers required and I sat on the floor of Mitre 10 with pipes and reducers until I had worked out everything I needed.

Assembly was pretty straightforward, I used a Tee-piece at one end with one end capped. If we ever need an extra chamber, it is just necessary to flip the tee-piece round. A few pictures are worth a thousand words.

The whole world is metric. Unfortunately, America still uses imperial measurements and my valve is American. The 12 volt irrigation value I am using has 3/4 inch threads which is a pain in the ass. Fortunately, 20mm is pretty close so by copious use of PTFE tape and jointing compound, you can join the 3/4 inch with 20mm screw threads and it holds the seal. Note the wooden cross-member at the far end of the tubes. This was to ensure the two pipes were attached to each other both ends to prevent the weight of the pipe lossening a connection. A connector block was mounted on the wooden piece to help tie all the electrical connections together.

As you see the design is different to the PVC version, I wanted to keep all the weight near the floor as the launcher is heavy. Aiming is achieved by tilting the pipes on their side rather than their end. I was going to build a wooded platform to aim the launcher but this adds more weight. Since it is very low to the floor and is very stable, a simple wooden wedge proved to be more than adequate.

To charge the the reservoir, I use a bicycle stirrup pump. I deally, I wanted a screw-in schrader valve but could not source one anywhere. In the end I found a rubber based presta valve in a bike shop. This was intended for use for a mountain bike rim conversion. It simply meant drilling a 6mm clearance hole and pushing it in there. As the pressure builds up, it gets forced further into the hole and the seal becomes better.

The electrics are the same as the previous launcher, using a small 12V battery and a firing button which is mounted in a plastic container.

Testing

After putting everything together, the device held pressure well. 6 bar seemed a good compromise on pressure otherwise it will take too long to charge the reservoir, and it should still give up to 5000N of launch force at the rocket. A quick test from the back deck suppassed my expectations. The rocket went very high, cleared the end of the garden and ended up in the reserve. Here is a video of a test launch:

May 2020