Compressed Air Rocket Launcher!

Inspired by an existing project I saw linked on Hackaday, I couldn’t resist having a go at building a compressed air rocket launcher. To keep the costs down, I tried to use commonly available plumbing fittings where possible (40mm solvent weld waste pipe). Originally I planned to use larger diameter pipe, but reducing fittings didn’t seem that easy to source locally.

Here’s my first prototype assembled, with the rocket (made of 15mm pipe insulation foam) in situ. The rocket is concealing a length of 15mm copper pipe which serves as the launch tube:

rocket_pipework

To get a reasonable volume of air in a small footprint, the pipework is folded into an S shape, rather than having one long length. The two parallel pipes at the left were intended to be closer together, but I accidentally welded in a longer pipe than I planned, and there was no going back 🙂

The 40mm pipe is solvent welded, and has two screw on access plugs at either end. These have been drilled through, and fitted with tank connectors, along with the various reducing fittings needed to connect the inlet and outlet valves.

This next shot shows a close-up of the outlet side which consists of a 12V solenoid valve (the black cylinder) which is normally closed, and fires a blast of compressed air when it opens. The other cylindrical component is an oil pressure sensor, which I plan to use to measure the air pressure. The pressure sensor has NPT thread, so an adaptor is needed for BSP thread. There’s also a 1/4 BSP female 3 way tee fitting used to connect the pressure sensor in circuit.

rocket_outlet

Various 1/4″ and 1/2″ brass hexagon busheshexagon nipples and other parts are needed to connect these together, with PTFE tape used on all joints. The final output is through a compression wallplate elbow, with a vertical 15mm copper pipe for the launch tube.

The inlet is a standard Schrader valve, so that a 12V car tyre pump compressor can be used to pressurise the system.

rocket_inlet

There was some guesswork in the design, because I didn’t bother to calculate what sort of volume of air was needed, and wasn’t sure if the solenoid valve would have a high enough flow rate (it has quite a narrow bore). I just decided to try it and see what happens.

Obligatory warning: if you build something like this, be aware that the plastic pipe could explode and shatter under pressure.

Having built it, I donned safety glasses, retired to a safe distance and pressurised it. At the first attempt I managed to get it up to 30psi, due to a slow air leak. The first launch was pretty impressive, and the foam rocket (lacking flights at this point) narrowly missed landing on the roof of the house! Adding some cardboard flights to the rocket improved the aim dramatically.

Some reassembly of the outlet joints cured the leak, and I’ve since had it up to 60psi. I’m going to box it up before pushing it to higher pressures. The compressor is rated for 100psi (just under 7 bar) so it will be interesting to see how high this thing can go 🙂

Next steps are to build a box for it, and to make the control unit. I’m planning to give it a mission control look with key switch, flip up covered toggle switch and big red button to launch… to be continued!

8 thoughts on “Compressed Air Rocket Launcher!”

  1. This is outstanding! Nice work!

    I’ll be really interested to hear how things pan out regarding your valve and its air flow rate. I got a little obsessive with my huge sprinkler valve and making sure there weren’t any corners or smaller pipe between it and the tank, but I really wonder if any of that actually matters.

    Up near 100psi, my launcher doesn’t have any trouble getting enough air in to rockets to blow them to pieces before they can get off the pad.

    Since I wrote my post, I have swapped the whole tank assembly out for black steel pipe. I got tired of sweating about the possibility of it exploding. Its very cheap and common stuff on this side of the pond, I expect your plumbing outlets will have it. It threads together easily, it has a working pressure up near 1k psi, and it splits instead of fragmenting if it fails. The only downside is weight, but it only added maybe 15kg. To me, that was a small price to pay for peace of mind. Maybe I’ll add an update to my post with some pictures.

    Good luck with your build! Looking forward to seeing how things progress!

    1. Hi Mike
      Thanks for the kind words!
      When I ordered the solenoid valve I went for one with 1/4″ BSP fittings, and was surprised to find it had a tiny bore inside the valve (maybe 2mm!). This worried me initially, but I was surprised to find it launches really well despite this. Most of the listings on eBay/Amazon don’t give much detail in the specification, so the only way to find out is to order one sometimes 🙂
      I’ve also been thinking of making a Version 2 at some stage with steel pipe. It’s pretty cheap here also. As you say, it would be worth it for peace of mind!
      I’ve got some parts to build the launch controls now (LCD, Arduino Nano, big red button, key switch and toggle switch with flip up cover) … can’t wait to put this thing together!
      Regards
      James

  2. Hi could you share any details of the steel pipe you used and where you sourced it from – dimensions etc

    Do you think its feasible to build it all in copper? (albeit lower volume)

    1. Hi Richard

      I’m still using the original 40mm (1.5″) solvent weld pipe pictured above, which is PVC-U. It’s commonly used in UK plumbing and easy to source. So far the PVC-U is still holding up, so I never replaced it. However, I did replace the valve since this article.

      At a very rough estimate (from the photo above which is on a 1cm grid), I reckon the pipework is about 20+10+20+10+10=70cm length. The internal diameter is actually 43mm (measured). That would make the cross sectional area = ∏(4.3/2)² = 14.522cm² and the volume = 70×14.522 = 1016.5cm³ which is 1016.5ml which makes it just over 1 litre in volume (please check my maths!)

      I’m not sure about using copper, it would be comparatively expensive versus steel. The pressure rating for the pipe looks sufficient: at 65°C the 15mm pipe rating is about 58 bar (841psi) and 22mm pipe rating is 51 bar (740psi). The joints are the weak point, but even those should be about 7 bar (100psi).

      Hope this helps?

      Best regards
      James

        1. No problem, thanks for the interesting link!

          I think if I was starting again, I would probably use Galvanised Steel Pipe, you can get 2″ (or even 4″) diameter lengths with BSP threads both ends fairly cheaply on eBay. You could screw a couple of female end caps on either end, then drill the end caps to fit the valves.

          Although made of PVC, mine is enclosed in quite a substantial chipboard box which is well braced, and I use a long control cable so I can retreat to a safe distance whilst wearing a pair of safety glasses, so I’m pretty comfortable using it!

          It’s not really rocket season though… the weather is a bit cold and wet 😉

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