|Rocket Electronics & Recovery|
|Launch & Static Tests|
|Motor Class Table|
|Iowa Amateur Rocketry Group|
|Don't Click Here!|
Prior to this test I built another power supply box for a pressure transducer. Since adding a N2O fill sensor to my remote launch controller, I needed another power supply. I now have data from chamber pressure, thrust and N2O pressure recorded on a PC at the test stand. And N2O supply pressure on my PC screen at the control site.
This was my fourth attempt to light the engine, and I'd used over 100 pounds of N2O in the past two days. I really wanted to save enough N2O for another static test from the bottle I just picked up. So I decided to fill the run tank for only 3.5 minutes. From past tests this should give me just under a full fill, maybe 28 pounds at the temperature we tested at. I really should build the test stand with a load cell to weigh the N2O tank as I fill it...
Bill was on hand to help set up, it went pretty quick since I'd done this three other times in the past two days. All of the ground support equipment has worked flawlessly in each attempt, and continued to work well for this test. Once Bill was at a safe distance, I opened the main N2O tank and retreated to the launch control table. The remote fill valve was opened and I started timing the fill. At 3.5 minutes I closed the fill valve, lit the pre-heater grain, then clicked the "Enable" box on the "Launch" button. I didn't have an extra mouse with me, so I was using the touch pad. As soon as I moved the pointer over the "Launch" button the touch pad clicked, and the PIRM2 fired and opened the N2O flight valve. If you watch the video you won't hear a count down and that's the reason. So the engine lit about 5 seconds sooner than I expected... Not that it was a problem, I don't enable the button until I'm ready to fire. But, I'll have to make sure to buy an extra mouse so that doesn't happen again.
Click Here for a short video of the burn.
Here's the engine at full thrust.
If you watch the video you can see a small puff of white vapor coming from the injector end of the engine. After the burn I inspected this area and found some of the thread tape blown off the pipe threads. So I had a small N2O leak at that point. Not enough to make much of a difference in the burn, but I'll have to use a better sealer next time.
A chart of the Thrust in pounds force (black), N2O tank pressure (blue) and chamber pressure (red).
One thing jumped right out at me when I looked at the data, the initial thrust was expected to be 1,500 pounds or more. But you can see the thrust started at about 1,000 pounds then dropped to around 900 before climbing to slightly over 1,100 pounds. This isn't just a sensor anomaly because the chamber pressure shows the same thing. It almost has to be blockage of the N2O somewhere between the run tank and the combustion chamber. I had three failed attempts to light the engine, I think either I had some injector blockage from the soot and particles of the previous three pre-heater grains, or I had some water condensation inside the run tank that froze and slightly blocked the injectors for a short time. From about the .9 second on, the thrust looks pretty close to what I'd expect. It's just the initial spike that's missing. I'm not sure about the dip in thrust at 4.75 seconds. That doesn't make sense and isn't apparent in the chamber pressure. So that one may be a sensor anomaly.
Here's a run down on the numbers:
Fuel Burned: 12.6 pounds start, 10.4 pounds end, 2.2 pounds burned
N2O load: 26 pounds (estimate)
N2O to Fuel Ratio: 92.2 to 7.8
Burn Time: 4.65 seconds liquid, 6.5 seconds overall
Peak Thrust: 1,068 lbf
Total Impulse: 5,267.9 pound seconds
Peak Regression: .007956"/s (.37" full burn)
This is the injector plate and retainer.
Here's the fuel grain coming out the injector end of the engine.
This is the nozzle end of the fuel grain.
Here's the combustion chamber side of the injector plate.
The injector end of the fuel grain.
The nozzle retainer and divergent end of the nozzle.
Here's all the hardware.
A cross section of the fuel grain at the very head end near the injector plate.
These are sections of the fuel grain near the nozzle.
The hardware all worked fine, the nozzle is in great condition as is the nozzle retainer. The injector plate is dirty (as usual) but undamaged. The casing looks great too.
The fuel grain does have a few issues. The upper 2/3 of the fuel grain was a mix of HDPE pellets and polyester resin. This area showed quite a few cracks going all the way to the casting tube. The good news is that there was no penetration of the casting tube. It would seem a crack in the fuel grain isn't a big deal when the pressure isn't flowing through the crack, but made static by the casting tube. On the other end of the grain where it was pure polyester resin, there are no cracks. Indicating the grain is stronger without the HDPE pellets. It did however shrink away from the casting tube. Without a good bond to the casting tube, during a run where most of the fuel is used, that could lead to slivers of fuel being ejected, or worse, block the nozzle causing a cato. I can live with a few cracks in the grain, so I think future casings will include the HDPE pellets in the entire fuel grain.
I cut the fuel grain in half every 4" to get an idea of the regression rate at different points on the grain. Below are the measurements:
Starting at the injector end...
at 1" Port Diameter was 3.72"
at 4" Port Diameter was 3.87"
at 8" Port Diameter was 3.82"
at 12" Port Diameter was 3.81"
at 16" Port Diameter was 3.78"
at 20" Port Diameter was 3.75"
at 24" Port Diameter was 3.80"
at Nozzle opening Port Diameter was 3.84"
As expected, you see sort of the Coke bottle effect on the port diameter, with a maximum port diameter of 3.87". The port started out at about 3.51", giving me a maximum regression of .36" during the burn. Since this test used just about exactly one third of the N2O the flight version is to use, I can assume .36" x 3 = 1.08" as a conservative minimum fuel web thickness for the full 84 pound N2O flight. In theory, as the port enlarges the regression rate should drop. Again, as expected, the casting tube I used in this test probably won't give me an adequate fuel web thickness. I'll either have to cast directly into the combustion chamber, or a thinner, wider casting tube.