3/25/2013

After a long delay, I had two good runs this weekend on the 250 lbf regen motor.

This is the one that had the hard start a couple of years ago so I switched from using direct spark ignition to an augmented spark igniter which seemed to work great. The first run was a 5 second run to check the mixture ratio and the run the next day was an full-duration 18-second run.

There doesn't appear to be any damage or erosion in the chamber and you can still see the tooling marks from the lathe. I was worried that the Viton o-ring seals wouldn't stand up to the chamber temperature but they seemed to work OK. Note this motor is designed to run fuel rich (r=1.8) and have a lower chamber temperature to help with cooling. Also the soot deposits from kerosene make a good insulator and help with cooling.

The performance was a bit lower than I had hoped - based on the measured mass flow, I should have seen about 300 lbf but the load cell only measured 243 lbf. I suspect poor mixing in the injector. The good news is the chamber pressure was right where it should be (~270 psia) and it seemed to have plenty of thermal margin.

On both runs, there was a bit too much LOX lead so there was a bit of hesitation for about a half second until the fuel flow picked up. The extra LOX really enhanced the effectiveness of the igniter though. On the first run, the igniter ran on GOX from a cylinder but the 2nd run used a heat exchanger to generate GOX from the LOX tank. The LOX and kerosene tanks were pressurized with helium. Although I didn't touch the pressure regulators in between the runs, there was more mass flow recorded on the 2nd run. I cleaned up the injector exit orifices with a bit of sandpaper in between runs and the change in mass flow corresponds to changing the discharge coefficient from about 0.72 to 0.77. I measured 0.65 during the cold water tests but I had cleaned up the inlet holes a bit after that test and the hot fire. I plan to run another set of cold test (without touching the orifices!) to validate this theory.

I was hoping for 20 seconds on the 2nd run (I filled enough for 30) but it ran out of LOX at 18.5 seconds due to a LOX leak, probably at the LOX manifold at the top of the chamber. The video, still photos, and flowmeter data show it in both runs although in both cases, it seemed to have sealed itself up by the end of the run. After the 2nd run, when I was inspecting the motor, the nut that secures the LOX manifold to the igniter body seemed to be a bit looser than I remember so that's the likely place for the leak. Other than tightening the nut more, a better fix would be to use a spring loaded face seal instead of a Teflon o-ring ($85/ea!).

The really interesting data was from the embedded thermocouples in the chamber walls for the 2nd run. I wouldn't have expected the temperature data to rise and then fall in the middle of the run. I believe what happened is that there was a clogged fuel orifice for a couple of seconds that corrected itself. A clogged fuel orifice would have caused the oxidizer to spray directly on the hot wall. On the 5 second run, the two chamber TCs are pretty close but on the 18 second run, one of them shoots up really high, then drops down near the other one. The only other time I've seen a jump in conditions during a run like this was when I had a Grafoil gasket flaking away during the run and clogging up fuel holes on my 100 lbf motor.

Aside from the LOX leak, I had a problem with a sticky LOX vent ball valve prior to the first run. I had to use a heat gun on it and loosen the body bolts to get it to unstick before the test. I cleaned out all the Krytox and loosened the bolts which seemed to work well for the 2nd run. I don't think this was the source of the leak however.

The next goal for me is to fly a liquid rocket. There are a few issues with this motor but I think it will work for what I want it to do.

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