Thursday, 2 February 2012

I had been getting a fit frustrated with the slow progress (have been waiting 2 weeks for parts) I have been making with the new test stand so I decided to make a simple thruster to test the silica gel consumable permanganate catalyst I want to use in the 100N engines for the hovering platform. It is pretty simple,  just consists of a vertical pipe which is filled with peroxide, and pressured with compressed air from a fitting at the top. There is a relief valve in the top also for if the peroxide in the pipe started decomposing. There is then just a right angle, check and ball valve then a shorter length of pipe where the catlyist is. I made a crude nozzle out of a hole in a end cap. All the fittings are brass or gal, which is not ideal but the peroxide is only there for a short period of time.

 I really enjoyed building it, I just sat down in the hardware store and went through a few design revisions. the first design I put together was made all of PVC pipe and fittings, which I figured would be good for peroxide compatability.  Since I was only planing on using it for low concentration peroxide I thought the PVC probably wouldn't melt. The PVC ball valve was quite stiff and would have been hard to trigger remotely (just by pulling a piece of string) so I went with a brass ball valve which was much smoother. Eventually I also decided to go with all metal fittings so I could use it with higher concentrations of peroxide (without it melting). I really enjoyed just sitting in the hardware store trying to figure out the most efficient way to make the thing, it was like building something out of lego... The whole thing ended up being about $100, although I think I was undercharged a bit. Most of the cost was in the relief and check valve, which were both designed for hot water systems, but siuted my application exactly. I was debating weather to  bother having a relief and check valve in such a small thing, but I decided to head caution.

The thruster worked really well and I learnt allot about the silica catylist. I was quite surprised that the exhaust appeared to be all gas even with cold %55 peroxide. The catalyst tube just consisted of  granular catlyist with steel wool at the end to stop it getting blown out. The first time I fired it by opening the ball valve all the peroxide just squirted out covering everything. The nozzle hole was too big, and the peroxide flowed through the catalyst too quickly to react. For the first run the peroxide was also about %55 and was chilled as it had been in the fridge. We obviously triggered it remotely (with string), and watched it via camera. One thing I had not thought of till now was how to clean up if things get covered in peroxide. I ended up just hosing the set-up down, which worked because there were not any electronics, but I am not sure what I would have done if there were. Will have to give that one more thought. For the first run the catlyist tube was about half full, but for the second run I changed the catlyist and filled it all the way.

For a second run I changed the nozzle for another with a smaller hole, and instead of triggering it by pressurising then opening the valve, I opened the valve first then slowly increased the pressure. It worked beautifully and there was a nice jet of steam. For the second run the peroxide had also warmed up. I also put more peroxide (about 200ml) in this time.

I emptied the catlyist pack to see what condition it was in and this is what it looked like - second test closest, third farthest and some new catlyist on the paper.:

The first 2cm had turned into a fine white powder, but the rest of the catlyist looked pretty good. I think that the reason the first bit of the pack turned to powder is because the peroxide hits it at high speed which causes it to rub with the other unrestrained grains, pulverising them. The next time I do a test I plan on putting some steel wool at the beginning of the pack to compress it and diffuse the incoming stream. I was quite impressed that apart from the powder there was not much stripping of the catalyst, epically with such a long skinny tube, in which the flow of gas would be quite high, and considering that the thing ran for about two minutes. I am keen to see how it holds up to %90.

I did a third test in which I reduced the nozzle again and experimented with pulsing the flow, whihc workd quite well. I pressurised at 60PSI but I think that because there was no much line between the compressor and the the tank the pressure increased too slowly. There was quite a delay between when I would open the air valve and I would see a result. Here is a fairly uneventfully video of the third test. You can see the result from pulsing the pressure.

We found a really nice area out the back of the shed with is mostly protected on its sides by bessa brick. I plan on clearing the grass and laying bricks when I have some free time.

I plan on concentrating some peroxide to %85 this weekend and repeat the experiment. I would also like to add in a solenoid because the ball valve is a pain to operate with a string. I also realised that I need to give more thought to test logistics with peroxide because although I at no point felt unsafe (was wearing full PPE) there were times when I thought I could have done things better.


  1. Hi John,
    Did you have some sort of injector after the ball valve here ? In a real motor, besides regulating your flow rate that helps turning the peroxide into droplets that react more easily with the catalyst.

  2. Hey Damien.

    Thanks for the advice. There wasn't any injector/restriction in the flow upstream of the catalyst which is probably why, as you suggested the flow rate was too high. With a smaller exit restriction the flow was low enough that the catalyst could decompose the flow, but it probably would have been better with an injector.