This was the second static test of the 2"x15" motor. You can see details of the motor design and construction here. This was also the first static test using digital data acquisition for both thrust and chamber pressure. The motor was the same as used in static test 25, the only change was to use spacers between the grains.
The previous test raised some questions, a rather abnormal pressure/time curve, and higher than expected chamber pressure. It was thought the grains were not spaced far enough apart to permit the ends to burn, leading to a progressive burn, with high chamber pressure the end result. So the grains were trimmed a little shorter than usual, the grain length was about 4.75" per grain, as opposed to 5" per grain as designed. The grains again used 2" PVC thin walled vacuum tubing as an inhibitor/casting tube.
Fuel Weight: 2.08 lbs.
Grains: 3x4.75"
Spacers: 2) Between the inside grains.
Core: .75" diameter
Throat: .625" diameter
Here is the pressure/thrust/time trace of the test.
The first thing to note, the load cell for some reason maxed out at 347.58 pounds of thrust. I'm working on the reason for that, but don't have an answer yet. So the flat line from about the 4 mark to the 11 mark is in error. It would have followed the pressure curve above it had it functioned properly. Update:April 21, 2004: I did a little thinking on the load cell maxing out at 350 pounds, and came up with the reason. The Data aquisition board reads 10 volts, only it's not 0 to 10 volts, it's -5 volts to +5 volts. And I had my load cell amplifier was set with too much gain and at 350 pounds of force it was going over +5 volts. So all I needed to do was adjust the amplifier to a lower gain setting. The load cell now reads full scale.
Next is the chamber pressure, I was hoping the grain spacing would lower the max pressure, it didn't. As you can see the pressure peaked at 1,472.8 psi. That's even higher than in the previous test. As you can see by the chart, for all practical purposes, the motor burned most of it's fuel between the 1 mark and 16 mark, that's a real time of .53 seconds. While the rest of the chart doesn't look like much, there is chamber pressure and thrust, so there is still fuel burning at the tail end.
There is also a little hump on the downward slope. I'm not sure what caused that, but it's there in both pressure and thrust, so I'm certain it's not an anomaly.
So, what's up with this strange profile?
Could it be erosive burning? It's possible, but I don't think it would account for all of what I see here.
Could the grain be cracking, or fracturing? That's possible, and would account for very rapid burn rates and high chamber pressure. It could also explain the low thrust at the tail end of the burn, propellant pieces sticking to the casting tube still burning at more or less normal rates. When I cast the "prilled" version of the propellant it does seem to be weaker than the powdered version of the propellant.
Could the propellant have higher burn rates than the powdered variant. I doubt it. I really think there has to be some sort of physical reason.
My PVC casting tubes could be a cause too. I've never used them before, it's possible they are imparting a stress on the grain as it cools. Or perhaps they are sealing against the chamber wall, leading to grain stress.
I think the next test I do will use grains using powder KNO3, and also try using a paper inhibitor.
I'm adding this several hours after the fact. I should have watched the video of the test sooner. It shows and you can hear very clearly something was wrong with the burn. You can see a belch of darker smoke at the beginning of the burn, then a lesser more normal ending burn with white smoke. This reinforces my theory of grain fracture. I'm 90% sure that's what happened. The question remains why.
I think the PVC is a little too tight of a fit in the motor casing for one thing. I also use RTV silicone to seal the nozzle, it may well be sealing the bottom grain to the casing. There probably isn't enough clearance for the pressurized gases to get behind the grains quickly enough. Causing the grain(s) to be pressurized from the inside, and cracking them.
There's one more factor I've been considering. The prilled form of the propellant doesn't burn as cleanly, and tends to leave a heavy residue inside the motor. It is possible, the residue itself is sealing the grains, causing a pressure imbalance.
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Here is the motor just after ignition. |
Here you can see the beginning of a belch of dark smoke. |
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Here is the belch, then the smoke went white again. |
Here is the start of another belch of dark smoke. |
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Now the smoke is white again. |
Then one last belch of dark smoke. |
The last .5 second looked like a normal burn, albeit, with diminished thrust.
That's one advantage of using an all steel motor, there is enough of a safety margin that even with an event such as this the motor remains undamaged. Post test inspection of the motor again revealed no blow by or damage in any way, and it's maximum pressure was near 1,500 psi.
On a construction note. I really like the way I attached the nozzle on this motor. Only tapping threads into the casing and letting the nozzle press against the screws. It is much easier to assemble the motor, and the threads on both the casing and screws don't get dinged up. The welded forward bulkhead is not a problem either, and makes construction easier and cheaper.
An interesting note. At a chamber pressure of 1,487 psi, there is a force acting on the bulkhead of over 5,000 pounds, the nozzle would be somewhat less, but still, those 12) 10-32 screws held.
