2011-09-29

I've completed the design of my Augmented Spark Igniter (ASI). It is essentially a miniature rocket motor that exhausts into the main chamber and provides a reliable ignition source. The Igniter Design Spreadsheet lists the details but it will be made of 304 stainless steel and run very fuel rich (O/F ratio = 1.0) since it will be uncooled. The L* is quite a bit lower than normal to allow it to keep burning (after burn) when it enters the main chamber. It uses an NGK ME-8 spark plug from Sparkplugs.com and an RCEXL A-01 CDI ignition system from Paragon RC. Three #10-32 compression fittings from Beswick Engineering will be used for the GOX, fuel, and pressure tap connections. I've previously used these fittings for pressure taps on my previous motors and they seem to seal good enough even though they use an o-ring. Since the mass flow is so small in the igniter, you need really tiny orifices (0.010 fuel, 0.026 GOX) to limit the flow rate. Beswick also sells miniature #10-32 orifice fittings and filter assemblies. The igniter has a 9/16-18 external thread that will screw into the body of the new injector and a Viton o-ring will help to seal between the igniter and injector. The injector required quite a bit of redesign since the LOX manifold was previously in the center space now occupied by the igniter.

An ASI should significantly reduce the chance of a main chamber hard start since you can verify operation with the igniter chamber pressure before opening the main propellant valves. Once the main chamber is up and running, the igniter will be turned off. Some people have successfully reused the igniter pressure port to measure the main chamber pressure but I may keep separate measurements since it will be interesting to see the startup transients between the igniter and main chamber. I haven't decided yet whether to include a purge gas flow through the igniter to help with cooling after things have started up and the main chamber is running.

To avoid chill down and 2-phase liquid/gas flow issues, the igniter will use GOX instead of LOX. For igniter testing, I'll just use a GOX cylinder but for testing of the 250 lbf motor, a small tap off the LOX tank with a coiled heat exchanger will be used to generate GOX. For the GOX and fuel valves, I'm using A2012 valves from Gems Sensors. I had to buy a minimum of 5 valves at $53/ea but I'm sure I'll find a use for the others.

It shouldn't take more than a day to fabricate the igniter and once the rest of the equipment arrives (valves, fittings, etc.), I should be able to test the igniter. However, the test stand still needs some work to repair the damage done from the previous hard start so that will take most of the time getting ready for the test. Then, I need to fabricate the new injector and I can re-test the 250 lbf motor.

2011-05-10

After a 2 1/2 year delay, I finally got a chance to test the new 250 lbf motor last weekend. Unfortunately it experienced a hard start which split open the motor and damaged some of the test stand. The big pit I recently dug for safety and noise abatement definitely helped in this situation.

From my initial analysis of data and video, I believe two issues led to the hard start: 1) the direct ignition system had enough energy to start my previous 100 lbf motor but probably not enough to start this one, and 2) I need a real augmented spark igniter (ASI) instead of the external system with wires running up into the chamber through the exit. I had known this was the weakest part of the design but since it worked so well on the smaller version, I was reluctant to commit to the extra complexity of an integrated igniter. There were also issues in getting an ASI to fit geometrically with the particular injector design I chose.

I switched to thread inserts for the injector/chamber fasteners which was probably a good idea since the thread inserts just pulled out of the parent material in the chamber body. Actually it was a fairly clean separation and the chamber looks in pretty good shape so I might be able to clean it up and reuse it. The injector and load cell mount are a different story. It looks like there was a secondary explosion inside the injector where the overpressure from the chamber forced kerosene up into the LOX manifold of the injector.

I had cracked open the LOX main valve during tank filling so the plumbing was chilled down good but I think the pre-test purge contributed to the problem in two ways: 1) it warmed up the feed line which caused it to take longer for the LOX to arrive when the main valve opened, and 2) pushed the igniter wires farther down toward the exit of the chamber which made the igniter less effective. I'm pretty sure the igniter was still in the chamber because the data shows a much higher response from the ignition monitor, probably due to the spark being enhanced in the oxygen-enriched environment. I also think I need to slow down the opening speed of both propellant valves to create a more gradual buildup of thrust. This was actually the second run - the first run auto-terminated due to the flowmeters going above a preset limit from gas/liquid fluctuations in the LOX feed. However, the purge didn't work as expected because there was a detectable kerosene odor in the LOX flex hose after that first run. I don't think the hard start was caused by anything left over from the first run because I cleaned out the flex hose and injector with IPA followed by a soap/water rinse.