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  1. #31
    Join Date
    31st August 2016
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    83
    Quote Originally Posted by Steve Shannon View Post
    I would put the knot right at the top. Why deliberately put stress on one side or the other?
    You're doing a nice build and I don't want you to think the Kevlar will snap easily. It'll take many flights and may never fail.
    Thanks! The theory is that when new, even a single side of the kevlar is far stronger than needed (I believe it is rated to nearly 5000lbs, and I guarantee other things will break before I hit that, not to mention that would take a 300G+ loading to hit). Under that theory, loading one side will be completely fine either forever or until the wear and tear from the loading breaks it down. And if it breaks down then I'll know about it and the rocket will still come down safely connected to the fresher side.

    Also as a general rule odds are when I do something different than the modern conventions for hpr it probably isn't an improvement, so that certainly nudges me towards leaving it symmetrical :-)


  2. #32
    Join Date
    31st August 2016
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    Quote Originally Posted by kevinkal View Post
    Thanks for all the details and pictures. This is an excellent build thread with excellent discussion in just a few days already.

    It is my understanding that we need to completely saturate the Kevlar strap through each of the bond lengths to the motor tube. This epoxy is the "matrix" for the Kevlar fiber reinforcement.. together forming the composite material. To get the saturation, a thinner epoxy can be used, or perhaps heat will initially thin the epoxy sufficiently to get improved saturation/flow into the Kevlar weave.
    From Wikipedia:
    "Composites are made up of individual materials referred to as constituent materials. There are two main categories of constituent materials: matrix (binder) and reinforcement. At least one portion of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials, while the wide variety of matrix and strengthening materials allows the designer of the product or structure to choose an optimum combination."
    Without completely saturating the Kevlar, the load distribution will be nearly all at the top of the bonded length with very little load distribution down the bonded length. Only the Kevlar fibers that are inside the epoxy you used to bond the underside of the strap will be in a matrix that will allow the Kevlar fiber to distribute load.
    This is an interesting topic. You're absolutely right that if we want/need to have a kevlar/epoxy composite it should be saturated. On the other hand, we know that the kevlar by itself is plenty strong enough (since once you're past the epoxy it supports the load on it's own), so we don't need the composite for strength of the strap alone. The question then is what's the strongest and most wear resistant way to join the kevlar to the tube - a surface bond or making a full composite section.

    I think you may well be right, since we agree that we want this part of the kevlar to be as stiff as possible relative to the tube, and clearly a kevlar+epoxy composite is the stiffest option. Also, your way is the way everyone else does it which carries a lot of weight :-) However, it comes at the cost of having the entirety of the kevlar+epoxy to raw kevlar interface happen at a single spot where the epoxy ends. My understanding is that this interface is the weakest spot over time. By not saturating the strap, we let that interface instead be spread out over the entire surface area of the mmt bond, and further since it is a woven fabric and I laid it under load, I believe that the full depth of the kevlar will be under load the whole way down. Also, it feels like having the interface along the axis of the rocket and spread down the length means that it will be less exposed to movement.

    Thoughts?

    Quote Originally Posted by kevinkal View Post
    I like the idea of a section of the Kevlar strap just above the epoxy bond being "strain releaved" using an outer sleeve and/or a wrap around the motor tube. Generally, however, the tensile load direction in the Kevlar strap should be ~straight up the booster tube.. there *should* be little force acting in any other direction.
    Sure almost the entirety of the load is straight up, but there is some lateral force both on the initial tug and as the whole thing sways. My understanding is that small amount of lateral movement/force is a big contributor to the kevlar breaking down at the end of the epoxy bond.


  3. #33
    Join Date
    26th October 2014
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    North of Seattle
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    436
    Over thinking it. Not that there's anything wrong with that.

    Will the joint outlast the rest of the rocket? Will it outlast your desire to fly this particular rocket? Will it outlast you?

    Big strap. Good epoxy joint at least partially protected from being cut. Big, hairy motors, rocky terrain and no tracker? No problem.
    Kyndel
    Bradford’s Law – restated for dummies: There are exponentially diminishing returns on doubling down on stupid.

  4. #34
    Join Date
    31st August 2016
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    83
    Quote Originally Posted by Igotnothing View Post
    Over thinking it. Not that there's anything wrong with that.

    Will the joint outlast the rest of the rocket? Will it outlast your desire to fly this particular rocket? Will it outlast you?

    Big strap. Good epoxy joint at least partially protected from being cut. Big, hairy motors, rocky terrain and no tracker? No problem.
    Agreed on probably overthinking :-) I'll lose this rocket to a tree before anything else I'm pretty sure. It's fun to think about these things though, and recovery failures do happen...

  5. #35
    Join Date
    31st August 2016
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    83
    Cleaned (soap + water then lacquer thinner) the motor tube and centering rings. Attached the 1st, 3rd and 5th with rocketpoxy, making fillets with a popsicle stick, then added a couple of spots of superglue since they were sliding all over the place. They were placed so that the aft edge of the fins are 41.5mm and 195mm from the rear of the MMT respectively. I was tired and things were being fiddly, so I'll attach the other two another day using the fins to offset them from the 1st and 3rd.
    Click image for larger version. 

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  6. #36
    Join Date
    31st August 2016
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    83
    Bonded the 2nd and 4th centering rings, using the fins to place them the exactly the right spot. I decided to split the difference and soaked one of the arms of kevlar in epoxy and left the other just surface bonded. Finally, I rounded the end of the MMT to minimize any cutting effect on the kevlar.
    Click image for larger version. 

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    I had meant to encase the kevlar in a 60" tube of nomex, but too late for that... Instead I'm going to wrap the 6-12" closest to the MMT with aluminum dryer duct tape as a poor man's heat shield/ablative/sacrificial surface. I saw someone else do that on TRF and it seems like a good idea. Up next the foam barriers for a dam!

    Finally, as a side note, on both this and my previous FG rocket, I'm having a hard time convincingly passing the "water break test" when using 80 grit sandpaper. Either this requires *way* more sanding than I'm doing (and I'm sanding quite intensely) or I'm misunderstanding the test. The initial drop still beads up to some degree, though it very easily "breaks" if I blow on it or shake things rather than running away as a bead. On my last rocket I sanded the dickens out of the find tabs and never got it to pass as well as I assumed it should.

  7. #37
    Join Date
    26th July 2009
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    Huntersville, NC
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    588
    Thanks for sharing this with us, and all the pictures too.
    Earth: The Insane Asylum of the Universe, nowhere else could things be this screwed up.
    I prefer rockets with 4 fins.
    Tripoli# 14243
    Level 1

  8. #38
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    20th April 2012
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    Fredericksburg VA
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    Quote Originally Posted by Igotnothing View Post
    Over thinking it. . .
    +1
    There are probably 50 ways for the recovery system to fail that are more probable than the epoxy kevlar bond on the motor mount failing.
    NAR Level 2 #96210 🚀 Tripoli Central VA #17 🚀 MDRA #113

  9. #39
    Join Date
    13th February 2016
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    Gilroy, CA
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    403
    Quote Originally Posted by rocketsam2016 View Post
    Cleaned (soap + water then lacquer thinner) the motor tube and centering rings.
    Quote Originally Posted by rocketsam2016 View Post
    Finally, as a side note, on both this and my previous FG rocket, I'm having a hard time convincingly passing the "water break test" when using 80 grit sandpaper. Either this requires *way* more sanding than I'm doing (and I'm sanding quite intensely) or I'm misunderstanding the test. The initial drop still beads up to some degree, though it very easily "breaks" if I blow on it or shake things rather than running away as a bead. On my last rocket I sanded the dickens out of the find tabs and never got it to pass as well as I assumed it should.
    This seemed like a good video describing the break test:

    I've not used Lacquer Thinner to clean Fiberglass before bonding. I've always used either 99.9% Isopropyl Alcohol, or Straight Acetone before sanding and immediately before bonding. To be honest, I didn't bother with the water break test after cleaning. I think you're wise to being checking it. Is there any chance the Lacquer Thinner could be leaving a film?
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  10. #40
    Join Date
    17th January 2011
    Location
    Spring Green WI
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    2,305
    I have done a good number of Kevlar to MMT with no problem in rockets L1-L3
    TFRLLC
    TRA 2225
    TWA
    QCRS
    WOOSH

  11. #41
    Join Date
    31st August 2016
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    83
    Quote Originally Posted by kevinkal View Post
    This seemed like a good video describing the break test:

    I've not used Lacquer Thinner to clean Fiberglass before bonding. I've always used either 99.9% Isopropyl Alcohol, or Straight Acetone before sanding and immediately before bonding. To be honest, I didn't bother with the water break test after cleaning. I think you're wise to being checking it. Is there any chance the Lacquer Thinner could be leaving a film?
    The sticky at the top of the sub says to use lacquer thinner.... I suppose it is possible though. I was doing the water drop test a little differently too: putting a single drop on and seeing if it stayed as a drop. Might have passed the way the video does it thanks for the link

  12. #42
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    13th February 2016
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    Gilroy, CA
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    Quote Originally Posted by rocketsam2016 View Post
    The sticky at the top of the sub
    I don't understand what you mean by this.. could you please elaborate?
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  13. #43
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    31st August 2016
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    Quote Originally Posted by kevinkal View Post
    I don't understand what you mean by this.. could you please elaborate?
    First post that is permanently at the top of the high power rocketry sub:
    http://www.rocketryforum.com/showthr...nt-you-to-know

  14. #44
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    13th February 2016
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    Gilroy, CA
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    Thanks. I'll have to peruse that thread.

    I just ordered some Aeropoxy ES6209.. just in case I need a thinner epoxy when I do the internal fillets and Kevlar2mmt bond. I also threw in a bag of milled glass fibers just in case I decided to thicken it. I now have too much epoxy.. I'll have to buy more kits. =]
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  15. #45
    Join Date
    29th December 2013
    Location
    Florida
    Posts
    53
    I'm building the same rocket!

    What parachute did/are you getting for it?

    I got a 80" angel chute and I can't get it to fit into the upper airframe.
    Current Fleet (Since keeping track)
    AT Mustang - 6 flights
    AT G-Force - 3 flights
    Meltdown (Custom) - 5 flights
    Madcow Patriot - 2 flights
    Minie-Magg - 1 flight
    Madcow Torrent - Painting
    Biohazard (Custom) - Building
    Level 2 - Building

    RIP To all of the lost rockets

  16. #46
    Join Date
    31st August 2016
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    83
    Bought a 60" fruity chutes iris. Left in package though so I can return if weight ends up different than I expect. Using a 15" fruity elliptical for drogue. And a 24" elliptical as a pilot on the main since I'm using a deployment bag.

  17. #47
    Join Date
    29th December 2013
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    Florida
    Posts
    53
    Quote Originally Posted by rocketsam2016 View Post
    Bought a 60" fruity chutes iris. Left in package though so I can return if weight ends up different than I expect. Using a 15" fruity elliptical for drogue. And a 24" elliptical as a pilot on the main since I'm using a deployment bag.
    I see, hopefully you can get it in there, my 80" is just too big
    Current Fleet (Since keeping track)
    AT Mustang - 6 flights
    AT G-Force - 3 flights
    Meltdown (Custom) - 5 flights
    Madcow Patriot - 2 flights
    Minie-Magg - 1 flight
    Madcow Torrent - Painting
    Biohazard (Custom) - Building
    Level 2 - Building

    RIP To all of the lost rockets

  18. #48
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    31st August 2016
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    83
    Quote Originally Posted by Sevian View Post
    I see, hopefully you can get it in there, my 80" is just too big
    Yeah it fits pretty easily, especially with the deployment bag. Thanks for the warning though!

  19. #49
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    31st August 2016
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    83
    Well I'm a bit frustrated. Somehow the centering rings have small (not much more than 1mm) discrepancies in height that are enough to prevent the assembly from sliding into the body tube. The distance between the MMT and the outer frame should be 10mm, and the height of the rings over the MMT varies between 9mm and 10mm that I've found so far.

    My current plan is to verify that the first and last rings are true, and if so then just sand the interior ones down enough to let everything slide in. Gonna make fillet injection entertaining but I'm not sure what else to do.

    Oh, and the two extra rings that I used (kit came with 3) are from Apogee - I had ordered them for a different project and not used them, so I have no idea if the problematic rings are the ones that came with the kit or the ones that I got from Apogee.

    Aaaaaarrrgh. It's a much bigger pain than I would've thought figuring out which rings are the culprits and where to sand. I did notice that a few were a (tiny!) bit loose when I slid them on, so I'm guessing they were looser than I thought and ended up hanging askew. It's either that or they came with the center hole drilled asymmetrically....

  20. #50
    Join Date
    13th October 2014
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    SouthEastern, WA
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    4,430
    Get your forward and aft rings where you want them and the use a straight edge beyween them to find the high spots.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  21. #51
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    31st August 2016
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    83
    Quote Originally Posted by rharshberger View Post
    Get your forward and aft rings where you want them and the use a straight edge beyween them to find the high spots.
    That was kinda the plan but you stated it much more succinctly and simply. I'll do exactly what you suggest, thanks :-) and come back if the 1st or last is hopelessly asymmetric

  22. #52
    Join Date
    23rd July 2011
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    Butte, MT
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    808
    Quote Originally Posted by rharshberger View Post
    Get your forward and aft rings where you want them and the use a straight edge beyween them to find the high spots.
    If they're not already glued in place put all your centering rings tight against each other on your motor mount tube. You should be able to see the highs and lows. You can sand them more easily this way too.
    If the central holes are cut off-center you should be able to turn them to match more closely. Then, draw a line along all the centering rings to keep them lined up.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  23. #53
    Join Date
    31st August 2016
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    83
    Quote Originally Posted by Steve Shannon View Post
    If they're not already glued in place put all your centering rings tight against each other on your motor mount tube. You should be able to see the highs and lows. You can sand them more easily this way too.
    If the central holes are cut off-center you should be able to turn them to match more closely. Then, draw a line along all the centering rings to keep them lined up.
    See earlier posts... Already epoxied in with fillets. All 5 of them. I had stacked them before gluing but clearly didn't look close enough. I wasn't thinking that differences in the 0.5mm to 1mm were likely and would be so aggravating. You can be sure I won't make this mistake again!! - I'll get them perfectly lined up and matched as you describe before gluing them on.

  24. #54
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    13th February 2016
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    Gilroy, CA
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    Quote Originally Posted by rocketsam2016 View Post
    Well I'm a bit frustrated. Somehow the centering rings have small (not much more than 1mm) discrepancies in height that are enough to prevent the assembly from sliding into the body tube. The distance between the MMT and the outer frame should be 10mm, and the height of the rings over the MMT varies between 9mm and 10mm that I've found so far.

    My current plan is to verify that the first and last rings are true, and if so then just sand the interior ones down enough to let everything slide in. Gonna make fillet injection entertaining but I'm not sure what else to do.

    Oh, and the two extra rings that I used (kit came with 3) are from Apogee - I had ordered them for a different project and not used them, so I have no idea if the problematic rings are the ones that came with the kit or the ones that I got from Apogee.

    Aaaaaarrrgh. It's a much bigger pain than I would've thought figuring out which rings are the culprits and where to sand. I did notice that a few were a (tiny!) bit loose when I slid them on, so I'm guessing they were looser than I thought and ended up hanging askew. It's either that or they came with the center hole drilled asymmetrically....
    Thanks for posting your findings on this issue.
    I think I have 8 of these centering rings... I'm going to take a close look at their symmetry tonight. I did notice that the units that I purchased separately from Madcow were relatively loose on the motor mount, whereas the rings that came with each of my kits were very snug/tight fits. I hope the center holes are concentric to the outer diameter!
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  25. #55
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    13th February 2016
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    Gilroy, CA
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    Quote Originally Posted by Sevian View Post
    I'm building the same rocket!

    What parachute did/are you getting for it?

    I got a 80" angel chute and I can't get it to fit into the upper airframe.
    Quote Originally Posted by rocketsam2016 View Post
    Bought a 60" fruity chutes iris. Left in package though so I can return if weight ends up different than I expect. Using a 15" fruity elliptical for drogue. And a 24" elliptical as a pilot on the main since I'm using a deployment bag.
    I am currently planning to use a Top Flight PAR70 standard parachute. I have used this exact chute in my Madcow Level2 4" FG rocket, and it comes down extremely slow with an empty 54/1706 case in it. This rocket's empty weight is about 4 lbs lighter than the 4" FG Frenzy XL. I modified the Parachute Cd in Openrocket to match my measured descent rates (@ 15 ft/sec).. and then used this parachute model with my Frenzy XL 4" FG Openrocket model.. and the descent rate is simming at 19.2 ft/sec after burning the M1297W propellant. The Cd I'm using is 1.4.

    My Madcow Level2 Parachute "Payload" compartment is about 1" longer than the 4" FG Frenzy XL's compartment. In my Level2 rocket, the PAR70 chute is wrapped in an 18x18 Madcow Chute Protector along with 15 feet of TN shock cord. It's a nice fit.. not too tight, and not too loose. I suspect that if I were to use the next size up Top Flight PAR84 parachute, that it would be a tight fit.

    As I plan to fly smaller RMS 54/1706 motors at first before the RMS 75/5120 M1297W, I should be able to get a feel if the PAR70 chute will be sufficient with the added weight of the 75mm 5120 motor. If I need to, I'll be talking with Gene to pick up an Iris Parachute as well... since they will definitely fit in the available volume and have an excellent Cd.
    Kevin NAR, Section #534 LUNAR / TRA
    L2- Madcow Level-2 4" Fiberglass, Redundant DD
    Work on the Drawing Board: Madcow 4-inch Fiberglass Frenzy XL

  26. #56
    Join Date
    20th February 2009
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    Cayuga, Indiana
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    12,187
    Small gaps between the CRs and airframe are not that big a deal as long as you are using a thicker 1:1 filled epoxy and not a low viscosity laminating epoxy.

    L3, TRA #11847
    Tripoli Indiana #132
    Tripoli Central Illinois #59
    Central Illinois Aerospace (NAR) #527
    Chicago Rocket Mafia, "Big Bucks" Dixon
    ___________________________________

    Quiet little voices creep into my head. -- We Were Promised Jetpacks

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