I got around to testing my 3D printed spark torch igniter again and this time with great success.
To give some background leading up to this test period, the igniter was printed with the intention of using the orifices within the body itself for propellant injection, the fuel was set up to swirl and the oxygen straight in. If you saw my first tests, this did not work so well. Since then I fitted precision orifices within the fitting that screws onto the igniter, drilled out the oxygen orifice within the igniter body and plugged and redrilled the fuel orifice in the body, so now the propellants impinge at 90° to each other.
I carried out 14 tests as shown in the video above, varying the inlet pressures of the oxygen and fuel, trying to achieve the most optimum performance. Of note, I tested with a methanol/water mixture (75/25%) instead of ethanol, as I was at a different test location and only had access to methanol. With that in mind, the testing was tuned towards this fuel mixture.
All tests used a 1mm orifice on the oxygen side and a 0.35mm orifice on the fuel side.
I aimed for a mixture ratio of 1.279, tests 11 and 12 came closest to this and as can be seen the plume looks pretty nice. You can see the earlier tests are oxygen rich and the later tests become more fuel rich. The design chamber pressure is 70 psi, but as can be seen I only ever really achieved mid-30s (avg). I put this down to the lower overall mass flow rate with the new orifices and hence the larger than needed throat area now. I had previously burnt through a few spark plugs (I use an NGK ME-8) and had purchased a few more just in case, but this was not meant to be with no burning of the plug in all tests.
For all the tests I had to deal with a dodgy oxygen inlet sensor, I had not had any previous trouble with this before but all of a sudden it would read between 10-20psi out for each test. This meant I had to change the offset in the code to achieve a zero starting value, because of this I think there would be a fair bit of error in the ox inlet pressure readings. I am using cheap pressure sensors so only have myself to blame, they are a bit noisy but have been working good enough for this project up until now.
I tested again the following day with mixed results. I set up as I had for test 11, 170psi on the ox side and 220psi on the fuel side, upon ignition it was clear that not all the propellants had combusted. Mach diamonds were still visible in the exhaust but propellant still also came out. I then tried upping the feed pressures on both sides to try and increase the combustion pressure but this did not work too well and I started to reach the limit of my oxygen regulator. I then reverted back to a 180psi fuel feed and 160psi ox feed, and the igniter ran as good as it had done the previous day.
In all, I had 26 successful test fires of the igniter, where a mixture ratio of ~1.3 ended up being a good fit. I really need to run the tests with ethanol, but I imagine there would not be too much change. I’ll probably keep tinkering and if time permits print a new version to achieve my design chamber pressure, but as it is in its current state, I would be happy to bolt it onto my main chamber.
Where too next?
The hireage of oxygen and nitrogen really kills it for me on this project. What I would ideally like to do for the next iteration is switch the fuel to LPG, this I can steal from my BBQ bottle, is super cheap, self-pressurizing and is always around the home. This would eliminate one cylinder of gas at least and make it easier on my pocket. The whole point of a spark torch igniter is to bleed off some propellant for the main engine, so am I going to build a LOX/LPG rocket, no. But as I am just playing around I do have a few ideas for this little rocket propellant combination.