Help Support RocketryForum by donating using the link above or becoming a Supporting Member.


Results 1 to 14 of 14
  1. #1
    Join Date
    30th December 2015
    Location
    Las Vegas!
    Posts
    666

    Problem- HPR fin root/trailing edge cracking, G10 , multiple examples. Help?

    Having this reoccurring theme and not sure where to go from here. I have 5 HPR that all have the same crack. It seems like it occurs in flight.

    Fins are all kit supplied G10, G12.
    RocketPoxy, kevlar for bonding.
    Internal fillets.
    Rounded or routed leading edges.

    Pics below.






    TRA Las Vegas, L2

  2. #2
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    2,556
    That fillet crack propagation from the aft end of the fin is what Iíve seen on some of my rockets with fin flutter. A stiffer fin (stiffer material or thicker g10) and/or larger fillets can prevent that.
    It could also be that your fins sweep back and the impact of landing is causing this. Same solution.

    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  3. #3
    Join Date
    23rd January 2009
    Location
    New York
    Posts
    5,460
    Make sure that you properly bond prep. Sand wherever the epoxy will reside and then clean thoroughly with alcohol. Do not touch the area after prepping. Even with that, my Dynacom Scorpion developed such a crack when one fin landed firmly on a rock. The fin had a large ding in it and the root of the fin developed a crack. I ground the fillet away around the crack, bond prepped the area, then re-applied the damaged fillet. Flew last month on a 76-3500 L700 to mach without issue.

    I think that it's due to the flexibility of the G10. The fin flexes when it takes a shot but the epoxy does not. The hard and brittle epoxy cracks as a result.

  4. #4
    Join Date
    30th December 2015
    Location
    Las Vegas!
    Posts
    666
    Thanks for the replies. I would imagine this is fin flutter OR from landings... I can say for sure that the 2 Blue rockets did that in flight, as it spiraled half up. The others are suspect on the landings maybe. Had you not mentioned it I would have never considered it. Also bonding and composites all done by the book.

    Just short of making my own plate (materials on hand) or Laminating (wrap and vacuum) the existing fins with carbon/Kevlar, then using them as usual?
    TRA Las Vegas, L2

  5. #5
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    2,556
    Either of those should work fine. I donít know that they even need vacuum bagging. Just a wide strip on each side and a slightly narrower strip, overlapping the bottom third or so of the fin and an inch of the body tube should be sufficient.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  6. #6
    Join Date
    30th December 2015
    Location
    Las Vegas!
    Posts
    666
    Kinda like a full tip to tip, but just on the root area?
    TRA Las Vegas, L2

  7. #7
    Join Date
    13th February 2016
    Location
    Gilroy, CA
    Posts
    580
    Quote Originally Posted by Rob702Martinez View Post
    Having this reoccurring theme and not sure where to go from here. I have 5 HPR that all have the same crack. It seems like it occurs in flight.

    Fins are all kit supplied G10, G12.
    RocketPoxy, kevlar for bonding.
    Internal fillets.
    Rounded or routed leading edges.
    I agree with the previous replies questioning possible fin flutter and/or landing impact on a fin.
    A few questions:
    What kind of velocity do your rockets see?
    What are your descent rates under the main chute? Any chance you're touching down faster than ~20 ft/sec?
    What size radius are fillets? (for each tube diameter)

  8. #8
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    2,556
    Quote Originally Posted by Rob702Martinez View Post
    Kinda like a full tip to tip, but just on the root area?
    Exactly! You got it! Then see what size motor it takes to crack that!!
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  9. #9
    Join Date
    18th March 2009
    Location
    Wisconsin
    Posts
    3,760
    Quote Originally Posted by Rob702Martinez View Post
    Kinda like a full tip to tip, but just on the root area?
    That said, if the problem is fin flutter here is what I would do.

    Large filets with Kevlar pulp or chopped carbon fiber. Then a series of layers similar to a tip to tip. Start with a layer that is smaller than the fin then another larger layer, and then a full sized tip to tip.


    Sent from my iPhone using Rocketry Forum
    Mark Koelsch
    Tripoli 6155 L3
    Owner of http://www.rocketryfiles.com/
    Editor of http://www.thrustcurve.org/
    Member of the Tripoli Motor Test Committee, and keeper of the motor file

  10. #10
    Join Date
    20th February 2009
    Location
    Cayuga, Indiana
    Posts
    12,882
    It would be nice to get the specs on each rocket/fin combo and what motor you flew when first noticing the crack. Learned early on how the G10 fin thickness typically supplied with kits today are far below the advertised motor range when it comes to flutter.

    From the Ether...
    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

  11. #11
    Join Date
    30th April 2012
    Location
    St. Louis, MO.
    Posts
    676
    My 4" bird has the same problem from landings. I've flown her six times, and two of the six cracked a fin root on the trailing edge. The first time not so bad... Landed under main chute. The last time I flew it, pretty bad... I had a piece of tape holding the Kevlar blanket burrito'd around the main chute... Forgot to remove the tape. But I was careful enough to not wrap it, and literally just put enough tape to let it rip off under descent (I made sure I didn't even push the tape into the blanket...). Well, guess what? Tape stayed put. Main popped, but no chute... Just a burrito coming down.

    From what I can gather, the upper section "staked" into the ground vertically, then the motor section was still horizontal, came down right on top of the upper section -- The damage indicates that the upper section collided with the motor section pretty aggressively. I don't even think I'll bother fixing this one. If I do, I'll just cut off the body tube above the motor tube (assuming there's enough space) and splice a new body tube over it. We'll see.

    I should take pics sometime.

    -Mike
    Mike Walsh
    NAR L3 - 07/27/2013

  12. #12
    Join Date
    21st March 2011
    Location
    central America
    Posts
    2,088
    This might be a stupid question, but here goes; does anyone ever see (hear) fin flutter with tube fins?

  13. #13
    Join Date
    23rd July 2011
    Location
    Butte, MT
    Posts
    2,556
    Quote Originally Posted by sooner.boomer View Post
    This might be a stupid question, but here goes; does anyone ever see (hear) fin flutter with tube fins?
    I donít think itís a stupid question, but I donít recall seeing a supersonic tube fin rocket.
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  14. #14
    Join Date
    30th January 2016
    Location
    US > OK > NE
    Posts
    3,740
    Quote Originally Posted by Steve Shannon View Post
    I donít think itís a stupid question, but I donít recall seeing a supersonic tube fin rocket.
    Paralyzer, Bluefin Tuba. And yes, you can blow them apart.


Similar Threads

  1. Replies: 3
    Last Post: 27th June 2017, 10:18 PM
  2. Eggfinder Bay Examples
    By Dad Man Walking in forum Rocketry Electronics and Software
    Replies: 45
    Last Post: 16th August 2015, 03:37 PM
  3. custom router bits for trailing edge
    By pythonrock in forum Techniques
    Replies: 16
    Last Post: 20th March 2014, 12:50 AM
  4. Atlantis trailing ISS and Earthshine
    By snede2112 in forum The Watering Hole
    Replies: 16
    Last Post: 22nd June 2007, 07:38 PM
  5. Root Canal
    By cliveroonie in forum The Watering Hole
    Replies: 10
    Last Post: 20th January 2007, 01:46 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •