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.
As is customary during study week, pre-semester 1 exams, I find myself getting distracted by rockets. With winter coming out in full force, the garage is not so enticing so I found some computer work I could do for my spark torch igniter. I am gearing up to test it again soon (I finally have some time) and thought I would sort out my data analysis spreadsheet.
The igniter GUI will log all the data to a CSV file from which I can post process. To make this even easier I wrote a MATLAB script to import the data, graph it, display the performance specifications and resave all this new information and graphs into the second sheet of the raw data file. Thus having the whole test info in one place.
While doing this, I found a mathematical error in one of my post processing equations which in turn gave some pretty good performance specifications (I left the π out of the orifice area calculation!). The old data was within reason and hence is why I never thought to double check, it did not help I was gearing up to leave town for the summer after the last test as well. If I had analysed the video better I may also have picked up on this.
Now fixed, the corrected test data was as follows,
|Fuel mass flow rate||0.022918||kg/s|
|Oxygen mass flow rate||0.009416||kg/s|
The flow rates are well and truly off the total expected, of 0.0099kg/s, whereas the mixture ratio was correct but from the video, it looked very oxygen-rich, not sure about this. The chamber pressure was also about 15 PSI higher than what it should have been. I am going to essentially write this test off, as discussed earlier I now have some pretty accurate orifices and have verified these with water testing. Along with this improvement, I will also remove the swirl injection of the fuel and run the optimum mixture ratio for the propellant, and hopefully, then I will be in the ballpark of the calculated specs.
This update is pretty late, as this occurred at the end of November 2017, before I headed out of town for my summer internship. I have not been able to do any testing since but am just now revisiting the igniter as I am back in town and at University again.
After my first initial hot fire semi-success I was on a good high and happy everything worked. In order to show some friends how cool this was, I set up in the exact same manner as before and attempted another hot fire, this time I was unable to light the igniter.
The inlet pressures and valve timing were all exactly the same, the only difference I noted was the spark plug had eroded, this was replaced and tried again. No ignition.
After talking to others over the summer I have a rough idea on how to proceed,
- Switch from swirl fuel injection to 90deg with ox
- 3D print (with SLS) smaller and more precise orifices that will press fit in my 1/8 nipple fittings
- Future version, move inlets further away from spark plug to help prevent erosion or lengthen hole so does not protrude into chamber as much
I’ll start with the orifice fix and go from there. One other thing is that for each test I need to hire an O2 and N2 bottle which is a bit costly for me as a poor student, I am seriously thinking switching fuels from Ethanol to a gas like methane for example. This would alleviate the N2 and only require buying a mini disposable gas canister from the hardware store. Thus enabling me to test more often and gain experience much quicker.
I also have borrowed a NI Labview DAQ module, so I will also work my way towards converting the avionic system over to this. Labview for students here I come!
Although I set up this blog/news site to concentrate and have in one place all the amateur experimental rocketry activities happening around the world, I do have to make an exception for what is happening in my own backyard.
Rocket Lab are due to launch their Electron rocket on its maiden flight soon and a recent video posted today shows it being transported to Launch Complex 1 on the Mahia Peninsula. After fit outs and tests, one could predict the launch is just weeks away all going to plan.
Good luck to them and I look forward to watching the launch.
Not quite amateur/experimental but since it is in my backyard, I thought it worth the mention : ).
Rocket Lab today announced its Rutherford Engine has been qualified for flight after the completion of a rigorous test program.
A day late sorry!
This weeks pic goes to me! Busy patching a few tears in a 21ft diameter main parachute for my liquid fuelled rocket project (thanks mum for doing the sewing!).