CF Fin attachment
- Thread starter HyperSonic
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I was looking through different threads the other day when I came across a response written by Jim J. He was talking about something that happened to him a long time ago. Without going into any details here, it pertains to CF and it's conductivity. I read a lot of things from him, as it gets my wheels turning upstairs, and I love a beautiful CF rocket! PePe is actually a "want a be" of one of his rockets.
After thinking about this a little, I decided to look a little deeper into my setup, which is all CF, including the A/V lid. I will try to explain this the best I can, so bear with me here, ...................and away we go...........
I wanted to see just how conductive CF can be, so I took my A/V lid with a couple of powder wells mounted on and then I used an insulation tester ( or megger for short ) and measured just how many megs (mega ohms) I would get when testing my A/V lid. I took a couple of pics to help you understand what I am doing here. If you want to measure insulation strength, you need to crank up the voltage a little. This tester has ranges from 50 volts, 100 V, 250 V, 500 V, and 1000 V. Its a lot like an Ohm meter, just with more voltage behind it, and very little current. The top side of my lid has 2 or 3 coats of epoxy on it sanded down nice and smooth. I juiced it with the full 1000 volts with the leads only an 1/8 of an inch apart resting on the lid. It held the 1000 volts and gave me a reading of 2.2 G ohms. This means that the epoxy that I used is a very good insulator. Heres a pic
Now I flipped the lid over and measured the back side. I turned down the voltage to 50 for this test. The back side does not have any coats of epoxy on it, only the little left on that the breather/bleeder fabric did not suck out when the CF plate was made. I separated the leads a little more for this test. Here is a pic
You can see here that the 50 volts got sucked all the way down to 1 volt, and 0 megs. This means I have a dead short circuit between the leads. When I use an Ohm meter ( not the megger ) and do the same thing, I get around 25 ohms. This 25 ohms will vary up and down when I scratch the leads a little into the lid rubbing through the little epoxy that might be left on the fibers. This means there is a path for current to flow between my powder wells through the screw holding the well to the lid. There will be two more powder wells mounted on this lid for a total of 4, and they all will be electrically connected together. This can only mean one thing, and it is not good!
Two altimeters, each with its own battery V+bat1, V-bat1, and V+bat2 and V-bat2. The V+ from both battery's are sitting on one leg of the e-match inside each well waiting for the V- to fire. When the first drogue well fires at apogee, and the violent burning of the BP causes the V- wire to touch the inside of the well, which is electrically connected to all the other wells (all the wells are aluminum, a very good conductor) it might try to fire the main at the same time, if it can get enough current to flow through the CF. Then what if the V+bat1 wire touches the inside of the well during the BP burn, and now both V+bat1 and V+bat2 act like they are in parallel with each other. I see all kinds of weird things happening here, and none of them are good.
I will not give up my CF parts so easily. Its strength and beauty is equal to none. Sure, I could just use something other than CF for the lid that doesn't conduct, but PePe wouldn't like that, and neither would I. I think I'm going to electrically isolate each well from the lid by using nylon washers under the well and the head of the screw, and insulate the threads of the screw from touching the inside of the hole they pass through. Then I'll juice everything again with the full 1000 volts to make sure there are no paths where current can flow between any of the wells. I will also insulate between my double barrel shotgun configuration and keep it off the inside of the CF tube. Here are some pics
After thinking about this a little, I decided to look a little deeper into my setup, which is all CF, including the A/V lid. I will try to explain this the best I can, so bear with me here, ...................and away we go...........
I wanted to see just how conductive CF can be, so I took my A/V lid with a couple of powder wells mounted on and then I used an insulation tester ( or megger for short ) and measured just how many megs (mega ohms) I would get when testing my A/V lid. I took a couple of pics to help you understand what I am doing here. If you want to measure insulation strength, you need to crank up the voltage a little. This tester has ranges from 50 volts, 100 V, 250 V, 500 V, and 1000 V. Its a lot like an Ohm meter, just with more voltage behind it, and very little current. The top side of my lid has 2 or 3 coats of epoxy on it sanded down nice and smooth. I juiced it with the full 1000 volts with the leads only an 1/8 of an inch apart resting on the lid. It held the 1000 volts and gave me a reading of 2.2 G ohms. This means that the epoxy that I used is a very good insulator. Heres a pic
Now I flipped the lid over and measured the back side. I turned down the voltage to 50 for this test. The back side does not have any coats of epoxy on it, only the little left on that the breather/bleeder fabric did not suck out when the CF plate was made. I separated the leads a little more for this test. Here is a pic
You can see here that the 50 volts got sucked all the way down to 1 volt, and 0 megs. This means I have a dead short circuit between the leads. When I use an Ohm meter ( not the megger ) and do the same thing, I get around 25 ohms. This 25 ohms will vary up and down when I scratch the leads a little into the lid rubbing through the little epoxy that might be left on the fibers. This means there is a path for current to flow between my powder wells through the screw holding the well to the lid. There will be two more powder wells mounted on this lid for a total of 4, and they all will be electrically connected together. This can only mean one thing, and it is not good!
Two altimeters, each with its own battery V+bat1, V-bat1, and V+bat2 and V-bat2. The V+ from both battery's are sitting on one leg of the e-match inside each well waiting for the V- to fire. When the first drogue well fires at apogee, and the violent burning of the BP causes the V- wire to touch the inside of the well, which is electrically connected to all the other wells (all the wells are aluminum, a very good conductor) it might try to fire the main at the same time, if it can get enough current to flow through the CF. Then what if the V+bat1 wire touches the inside of the well during the BP burn, and now both V+bat1 and V+bat2 act like they are in parallel with each other. I see all kinds of weird things happening here, and none of them are good.
I will not give up my CF parts so easily. Its strength and beauty is equal to none. Sure, I could just use something other than CF for the lid that doesn't conduct, but PePe wouldn't like that, and neither would I. I think I'm going to electrically isolate each well from the lid by using nylon washers under the well and the head of the screw, and insulate the threads of the screw from touching the inside of the hole they pass through. Then I'll juice everything again with the full 1000 volts to make sure there are no paths where current can flow between any of the wells. I will also insulate between my double barrel shotgun configuration and keep it off the inside of the CF tube. Here are some pics
Have a Safe and Happy Thanksgiving to all!
Just wanted to say Pepe is looking beautiful and please do keep the updates coming. A couple of questions:
1. Can you post a pic of your camera mount?
2. Do I see charge wells next to the eyebolt? Would those be for a chute cannon? So 2 charges to pop the shear pins and another 2 to push out the chute?
Also, the links Jim posted on leveling no longer work. If you still have the files would you mind sharing them? I would love to read them.
1. Can you post a pic of your camera mount?
2. Do I see charge wells next to the eyebolt? Would those be for a chute cannon? So 2 charges to pop the shear pins and another 2 to push out the chute?
Also, the links Jim posted on leveling no longer work. If you still have the files would you mind sharing them? I would love to read them.
Thanks for the compliment.
The two longer charge wells are for the drogue chute. They fire one at a time, the second one will fire about 1.5 to 2 seconds behind the first one. The reason for two is one for the primary charge ( only takes one to separate nose cone) and the second one is a backup, just in case something went wrong with the primary charge. The same thing goes for the two smaller wells in the chute cannon. One will fire at around 1000 feet, and the backup charge will fire around 700 feet.
The chute cannon will use a piston to push the main out when at the right altitude. This keeps the main tucked away in the cannon as not to get tangled up with the drogue chute. This flight will be under drogue for over 7 minutes or so before it gets close to the ground. The only separation point is the nose cone.
I have gotten a new desktop PC recently and the files for Jim's laser leveling instructions are on my old PC. When I have time I will hook up the old PC and look for them and post them here.
Here are some pics of the camera setup.
I forgot to mention that there will be a little hole in the airframe that lines up with the camera lens, to look out of. Then when the nose separates the camera will have a full field of view.
Here is the sim file for PePe on the N5800 for those of you who have RASAero II. And for those of you who don't, here is a look into the windows of RASAero II.
Once again I forgot to mention the rocket itself is 14 pounds. The motor is 33 pounds for a total of 47 pounds on the pad.
