Madcow FG 4" Frenzy XL build thread

Discussion in 'High Power Rocketry (HPR)' started by rocketsam2016, Jan 9, 2017.

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  1. Jan 12, 2017 #31

    rocketsam2016

    rocketsam2016

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    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. Jan 12, 2017 #32

    rocketsam2016

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    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?

    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. Jan 12, 2017 #33

    Igotnothing

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    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.
     
  4. Jan 12, 2017 #34

    rocketsam2016

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    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. Jan 13, 2017 #35

    rocketsam2016

    rocketsam2016

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    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.
    IMG_20170113_075010.jpg
     
  6. Jan 14, 2017 #36

    rocketsam2016

    rocketsam2016

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    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.
    IMG_20170113_203755.jpg IMG_20170113_203745.jpg

    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. Jan 14, 2017 #37

    hobie1dog

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    Thanks for sharing this with us, and all the pictures too.
     
  8. Jan 14, 2017 #38

    Nathan

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    +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.
     
  9. Jan 14, 2017 #39

    kevinkal

    kevinkal

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    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?
     
  10. Jan 14, 2017 #40

    crossfire

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    I have done a good number of Kevlar to MMT with no problem in rockets L1-L3
     
  11. Jan 14, 2017 #41

    rocketsam2016

    rocketsam2016

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    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. Jan 15, 2017 #42

    kevinkal

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    I don't understand what you mean by this.. could you please elaborate?
     
  13. Jan 15, 2017 #43

    rocketsam2016

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  14. Jan 15, 2017 #44

    kevinkal

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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. =]
     
  15. Jan 18, 2017 #45

    Sevian

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    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.
     
  16. Jan 18, 2017 #46

    rocketsam2016

    rocketsam2016

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    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. Jan 18, 2017 #47

    Sevian

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    I see, hopefully you can get it in there, my 80" is just too big :mad:
     
  18. Jan 18, 2017 #48

    rocketsam2016

    rocketsam2016

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    Yeah it fits pretty easily, especially with the deployment bag. Thanks for the warning though!
     
  19. Jan 18, 2017 #49

    rocketsam2016

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    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. Jan 18, 2017 #50

    rharshberger

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    Get your forward and aft rings where you want them and the use a straight edge beyween them to find the high spots.
     
  21. Jan 18, 2017 #51

    rocketsam2016

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    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. Jan 18, 2017 #52

    Steve Shannon

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    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.
     
  23. Jan 18, 2017 #53

    rocketsam2016

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    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. Jan 18, 2017 #54

    kevinkal

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    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!
     
  25. Jan 18, 2017 #55

    kevinkal

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    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.
     
  26. Jan 18, 2017 #56

    dixontj93060

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    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.
     
  27. Jan 19, 2017 #57

    kevinkal

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    I just checked 5 of my centering rings to see how well the inner motor mount hole was centered. The worst of the 5 had a variation of 0.6 mm in the 10mm width, suggesting that the 75mm cut-out was 0.3mm off center. One side was 10.1mm and the opposite side was 9.5mm. Three of the rings are within plus/minus 0.1 mm, and the other two are significantly worse up to the plus/minus 0.3 mm difference. The edges seem to have brown burns and 2 layered cuts, which might imply a dull cutting tool.
     
  28. Jan 19, 2017 #58

    rocketsam2016

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    OK so yeah yours are better than mine. I was able to sand a few down selectively and the assembly now fits with a bit of effort.

    I'm assuming you used a pair of calipers? I don't have one. I looked for something relatively cheap and large enough to measure a big body tube from the side but didn't really find anything.
     
  29. Jan 19, 2017 #59

    timbucktoo

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    Dude you will be fine! Like Dixon said, it's not a big deal with the right epoxy!
     
  30. Jan 19, 2017 #60

    rocketsam2016

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    Oh yeah I know I'm fine, the sanding worked well. I just want a pair of calipers if I can find the right ones
     

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