Work has kept me busy, and after work, doing maintenance on my microlathe. New variable speed motor, cleaning, and some mods. OT though!
I looked at some pictures that James Fields took at Potter. One of the pictures shows something very interesting. In the vent bay, I have the main exit hole for the cryovalve vent. That is jetting out a stream of vapor at a pretty good clip. Really that valve should have a flow restrictor on it, but oh well, something for the next design motor.
What is interesting is the vapor stream from a couple inches lower down. That's the drain hole for the ventbay. Little should be coming out there, but clearly that's not the case. That is leading me to think the leak I had is not the cryovalve misbehaving, but instead that I might have not had the cryovalve screwed into the upper bulkhead solidly enough - leading to leakage at the base of the valve - or the upper bulkhead O-rings were failing. With the age of the latter, and all the handling the cryovalve got while I was building the rocket, either is quite possible.
I think the venting was sufficient that when I pull it all apart, there will be evidence of where the gas was coming from.
As an aside, seeing venting in a chilled hybrid is super easy. There's no need for a sensor with this sort of system! You can also easily hear the vent cycle from a thousand feet or more away. The vent generates enough thrust to push the rocket sideways a couple inches on the rail. It could use a flow restrictor to reduce the vent rate when the valve is open.
However, watching it push the rocket sideways (a few pounds of thrust) makes me think a cold flow RCS could work for our scale rockets, at a weight penalty of course.
I've attached two pics from the flight, when the motor was up to full burn. That's an 8' plume... which is a bit bigger than it should have been. And a bit bigger than it was in the static test. I think I know why the motor went unstable halfway through the boost. One of the injectors burned out or otherwise lost the internal tube. That opened that injector up larger than design diameter.
The larger total injector area that resulted decreased the pressure drop between the flight tank and the combustion chamber. I didn't design with much margin there. If one has too little pressure drop, combustion will be unstable.
On my static test it was very stable. On this boost it was very stable, until suddenly it was not. At that same time, the acceleration suddenly jumped up roughly an additional 2G. That would be fully explained by the tube core letting go in one injector giving a sudden increase in oxidizer flow. The combustion chamber pressure would have gone up a little, and the pressure drop across the injector would have decreased a little.
The plume size in flight would have been slightly larger than from a static test anyway, but not a couple feet larger! At least not at that point in the boost. The increase would have been from acceleration-induced pressure increase on the supply side of the injectors, and aero effects at flight speeds.
Assuming a half full tank that would have been about four pounds of nitrous, under 12G acceleration, so roughly 50 pounds over a surface area of about 6 square inches. So it only adds about 8PSI to the effective tank pressure. Or, about 1.5%. As in, not much.
I looked at some pictures that James Fields took at Potter. One of the pictures shows something very interesting. In the vent bay, I have the main exit hole for the cryovalve vent. That is jetting out a stream of vapor at a pretty good clip. Really that valve should have a flow restrictor on it, but oh well, something for the next design motor.
What is interesting is the vapor stream from a couple inches lower down. That's the drain hole for the ventbay. Little should be coming out there, but clearly that's not the case. That is leading me to think the leak I had is not the cryovalve misbehaving, but instead that I might have not had the cryovalve screwed into the upper bulkhead solidly enough - leading to leakage at the base of the valve - or the upper bulkhead O-rings were failing. With the age of the latter, and all the handling the cryovalve got while I was building the rocket, either is quite possible.
I think the venting was sufficient that when I pull it all apart, there will be evidence of where the gas was coming from.
As an aside, seeing venting in a chilled hybrid is super easy. There's no need for a sensor with this sort of system! You can also easily hear the vent cycle from a thousand feet or more away. The vent generates enough thrust to push the rocket sideways a couple inches on the rail. It could use a flow restrictor to reduce the vent rate when the valve is open.
However, watching it push the rocket sideways (a few pounds of thrust) makes me think a cold flow RCS could work for our scale rockets, at a weight penalty of course.
I've attached two pics from the flight, when the motor was up to full burn. That's an 8' plume... which is a bit bigger than it should have been. And a bit bigger than it was in the static test. I think I know why the motor went unstable halfway through the boost. One of the injectors burned out or otherwise lost the internal tube. That opened that injector up larger than design diameter.
The larger total injector area that resulted decreased the pressure drop between the flight tank and the combustion chamber. I didn't design with much margin there. If one has too little pressure drop, combustion will be unstable.
On my static test it was very stable. On this boost it was very stable, until suddenly it was not. At that same time, the acceleration suddenly jumped up roughly an additional 2G. That would be fully explained by the tube core letting go in one injector giving a sudden increase in oxidizer flow. The combustion chamber pressure would have gone up a little, and the pressure drop across the injector would have decreased a little.
The plume size in flight would have been slightly larger than from a static test anyway, but not a couple feet larger! At least not at that point in the boost. The increase would have been from acceleration-induced pressure increase on the supply side of the injectors, and aero effects at flight speeds.
Assuming a half full tank that would have been about four pounds of nitrous, under 12G acceleration, so roughly 50 pounds over a surface area of about 6 square inches. So it only adds about 8PSI to the effective tank pressure. Or, about 1.5%. As in, not much.
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