CF Fin attachment
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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.
Do you see a problem if the load was say 25% top, 75% bottom, or vise versa? Here is a pic of where my top of casing contacts the glued in coupler. It's about .150 from the bottom of the rocket. If I put a spacer on top of casing around .145, this will put me just shy of bottom of rocket. I will snug up the top of casing to the coupler/spacer with the threaded rod that's used for motor retention. Then I will screw on boat tail until it is snug against bottom of airframe, since the bottom of casing is about .005 from the bottom of airframe. Now both ends of motor are making contact with their push points.
The boat tail is mainly there for an increase in speed, since one of my goals here is to break Mach 3. It might move the CP a little forward, but I will just have to adjust my CG to compensate.
Heck if you tighten the boattail enough you'll have tension on your MD retainer rod
Hey Nytrunner,
Flew a rocket (Penelope) at Michiana Mayhem on 4/10/21. Check out the on board video on you tube. It got cloudy at altitude, but you can still see Lake Michigan from 15K. The name of the file is "M1230_15K_4/10/21"
I don't see a problem with either load distribution if the rocket is well constructed. The "problem" is that you will have a really hard time controlling your load distribution. Both your airframe and your motor are rather stiff. This means changing loads by a lot will only result in a rather small compression or expansion. Looking at this from the other way, very small length changes are equivalent to significant load shifts. This means if your motor is only slightly longer than you need, basically all the loads will be transferred through the forward end and vice versa. It will be tough, if not impossible to get both lengths matched up well enough. This is before you even consider the fact that your tubes have different coefficients of thermal expansion and that your motor will expand as soon as it becomes pressurized. At the end of the day, you will probably transfer 90%+ of the load via one end, so you still need to make sure that both ends can handle the full thrust.
If you really want to spread the load in a controlled manner, you'd probably need to put a spring or something equivalent in there. But that's not necessary. Both your motor and your tube will easily handle the thrust of an N5800. Just use the end that you prefer.
Edit: If I didn't mess up my calculations, the full thrust of an N5800 should compress the casing by about 0.13mm (5 thousands of an inch). So for any thou that you're off from the ideal value, the load shifts by about 20%.
Reinhard
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Thanks for the explanation. I always appreciate your input very much!
rocketace
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Just curious on this thought. Is the internal pressure of the motor considered since it will basically be pushing the closures apart?
This is ignoring internal pressure. If you put two identical motors on the test stand, one with forward retention and another one with aft retention, their length difference should be about 5 thou.
The motor will actually expand, because the internal loads are significantly higher than the thrust (guestimate by a factor of about 5).
BTW: I was sloppy and lazy with the above calculation and completely ignored the stiffness of the airframe (elastic modulus of aluminum is easily googleable but without additional data the composite tubing is an unknown). So an length error of 1 thou will generate quite less load mismatch than 20%, but it would be a non trivial problem to distribute the loads none the less.
Reinhard
I had considered such load distribution when getting into HPR, but decided that it was not necessary. I just push at the front. I do normally leave a little gap at the back with the intent that if the front mount lets go the motor is almost home before the rear takes the thrust anyway. Never had it happen yet.
The reason for both front and rear is mainly for backup, incase one end failed for some reason, the other would only be a few thou from taking over. Like Reinhard was telling me, one end will eat most if not all the load here. And also like OTT was saying, it most likely isn't necessary.
This flight will be pushing things to their limit, including me, and I just want to have a backup plan for some of the most critical phases in the flight. Getting your M3 (Mach 3) is a lot more stressful than getting your L3. But it is a challenge that I can't wait to take! ( I mean PePe can't wait to take )
This flight will be pushing things to their limit, including me, and I just want to have a backup plan for some of the most critical phases in the flight. Getting your M3 (Mach 3) is a lot more stressful than getting your L3. But it is a challenge that I can't wait to take! ( I mean PePe can't wait to take )
I finally got everything put together to get some idea of where the CG will be for two different motors in PePe. I want to fly the CTI M2245 first, then tweek the sim (RASAero II) to match the actual data from the flight as close as I can. Then resim the CTI N5800 flight to make sure I won't go by their 50K wavier. Right now the sim says around 46K.
PePe fully loaded with the M2245 is 33 pounds 2.5 oz. The CP is 90.65 inches. The CG is 76.625 inches. Stability Margin of 3.46. The motor with SCPrecision thrust plate and Aerotech flange mount retainer weighs 18 pounds 15.1 oz. (Including Ti threaded rod which holds everything in place)
PePe weighs 14.21 pounds without motor.
PePe fully loaded with the N5800 is 46 pounds 13 oz. The CP is 89.35 inches. (including Gen 2 boat tail) The CG is 78 inches. Stability Margin of 2.8. The motor weighs 32 pounds 10 oz.
The main chute will be an 84 inch rocketman ultra lite and the drogue a 3 foot pro experimental also from rocketman.
The sim for the 2245 flight is around 23K at M1.8 and 46K at M3.1 for the 5800.
I will take some pics of everything in the near future and post them here.
Notice the Gen 2 boat tail moved the CP forward about an inch and a quarter. (the CP number comes from the sim)
PePe fully loaded with the M2245 is 33 pounds 2.5 oz. The CP is 90.65 inches. The CG is 76.625 inches. Stability Margin of 3.46. The motor with SCPrecision thrust plate and Aerotech flange mount retainer weighs 18 pounds 15.1 oz. (Including Ti threaded rod which holds everything in place)
PePe weighs 14.21 pounds without motor.
PePe fully loaded with the N5800 is 46 pounds 13 oz. The CP is 89.35 inches. (including Gen 2 boat tail) The CG is 78 inches. Stability Margin of 2.8. The motor weighs 32 pounds 10 oz.
The main chute will be an 84 inch rocketman ultra lite and the drogue a 3 foot pro experimental also from rocketman.
The sim for the 2245 flight is around 23K at M1.8 and 46K at M3.1 for the 5800.
I will take some pics of everything in the near future and post them here.
Notice the Gen 2 boat tail moved the CP forward about an inch and a quarter. (the CP number comes from the sim)
I'm hoping for a nice and straight smoke trail at just under M2. Then if everything looks good everywhere else, and he tracks straight and true (no coning) he will fly the 5800. Just have to keep him under 50K. PePe is the only one I know who does a shake down flight with a M2245. This is "low and slow" for him.
In case anyone is wondering about my 98 to 75 adapter, here are some pics....
The Aeropack retainer doesn't do anything here, but since the thrust plate was drilled and taped for it.... The threaded rod is for motor retention and to keep it snug against the bottom of airframe.
The Aeropack retainer doesn't do anything here, but since the thrust plate was drilled and taped for it.... The threaded rod is for motor retention and to keep it snug against the bottom of airframe.
