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  1. #1
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    Shear pins - when to use?

    When is it a good idea to use shear pins to hold a nose cone or payload? I've never used them but they are mentioned all the time, and some people say it is better to use shear tape instead of pins. Are these mainly used with larger, dual deploy rockets? Apparently in some cases a good friction fit is not enough.


    L1 3/25/17 H135
    L2 8/12/17 J180

  2. #2
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    Quote Originally Posted by billdz View Post
    When is it a good idea to use shear pins to hold a nose cone or payload? I've never used them but they are mentioned all the time, and some people say it is better to use shear tape instead of pins. Are these mainly used with larger, dual deploy rockets? Apparently in some cases a good friction fit is not enough.
    Yes for larger DD rockets.
    If you think though the DD sequence, the first ejection charge fires at apogee to separate the booster from the "avionics bay + payload bay + nose cone", aka rest of rocket (ROR). When the shock cords extends fully, it will recoil and yank the two pieces of the rocket back towards each other. If your "avionics bay + payload bay + nose cone" package are not held together securely, the pieces are liable to separate prematurely. That could lead to main chute deployment at apogee, and a very long recovery walk, or drive.

    You don't want that, thus the shear pins that only allow ROR to come apart when the second ejection charge fires to deploy the main chute.

    More here:
    https://www.apogeerockets.com/Buildi...r-Pins-20-pack

    a

    P.S.: Then there is the whole issue of air pressure change if you go high enough, but that can be mitigated by drilling breathing holes in the airframe.

    Last edited by afadeev; 5th November 2017 at 04:41 PM. Reason: PS
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  3. #3
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    Are shear pins considered the best method of dealing with this issue? I heard tape was better. Any other options?

    L1 3/25/17 H135
    L2 8/12/17 J180

  4. #4
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    Quote Originally Posted by billdz View Post
    Are shear pins considered the best method of dealing with this issue? I heard tape was better. Any other options?
    I would favor the pins/rivets/screws, simply due to the difficulties inherent in reproducing a particular piece of tape with a specific amount of stretch, precisely adhered.

    Tape is neither precise nor accurate. Nylon pins can be both, with careful prep.

  5. #5
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    Thanks for the replies. Guess this is one of those times where the medicine is potentially worse than the cure. If the NC comes out prematurely, it just means a long walk. If the pins are too strong and the NC is not blown off, it means a hard landing. I assume the pins are quite reliable?

    L1 3/25/17 H135
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  6. #6
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    Quote Originally Posted by billdz View Post
    Are shear pins considered the best method of dealing with this issue? I heard tape was better. Any other options?
    Where’d you hear tape was better?
    Shear pins is all I use on NC. Even using them on booster on big rockets and any high G flight. Drag separation is not pretty!
    Tim
    L3 NAR 98225

  7. #7
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    If the NC comes off premature, sometimes it a short walk..

    This is my Madcow Seawolf that drag separated. I thought the NC - BT was tight enough with tape, but a punchy motor gave the mass of nose weight some extra momentum.. NC peeled off as it was ascending & started to topple. Everything then just came 'undone' with a 'chute being shredded on it's final moments of 'upwardness'. Shear pins, at least one or two when she's repaired.. Lost a JLCR in the process.. That hurt the most..
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    -paul

    NAR# 101258 - L1
    www.CRMRC.org
    I don't know the same things you don't know..

  8. #8
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    Quote Originally Posted by billdz View Post
    Thanks for the replies. Guess this is one of those times where the medicine is potentially worse than the cure. If the NC comes out prematurely, it just means a long walk. If the pins are too strong and the NC is not blown off, it means a hard landing. I assume the pins are quite reliable?
    Well, the pins are reliably flimsy, which is what you want them to be.
    Strong enough to prevent premature separation, but flimsy enough so that you can shear them easily with ejection charge. If they are too strong, they will hold, and your ejection charge will blow up the rocket body instead of separating the nose cone. Could be fun, though probably not what you want to achieve.



    Quote Originally Posted by billdz View Post
    Are shear pins considered the best method of dealing with this issue? I heard tape was better. Any other options?

    Quote Originally Posted by timbucktoo View Post
    Where’d you hear tape was better?
    Shear pins is all I use on NC. Even using them on booster on big rockets and any high G flight. Drag separation is not pretty!
    +1.
    The other problem with the tape is that if will get peeled off if you are going fast enough past Mach.
    I can't tell you precisely at what speed it will peel off, since I don't know what tape we are talking about here, and what adhesive it uses, but it will come off.
    Pins will stay in place.

    a
    Last edited by afadeev; 5th November 2017 at 10:00 PM.
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    Radrocketeers.org NAR L2

  9. #9
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    Wow, those pics convinced me. Where's the best place to buy some?

    L1 3/25/17 H135
    L2 8/12/17 J180

  10. #10
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    Quote Originally Posted by billdz View Post
    Thanks for the replies. Guess this is one of those times where the medicine is potentially worse than the cure. If the NC comes out prematurely, it just means a long walk. If the pins are too strong and the NC is not blown off, it means a hard landing. I assume the pins are quite reliable?
    If the pins are too strong and don’t “shear”, you probably didn’t do any ground testing.

    I use these on airframes 4" & smaller:
    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    Last edited by timbucktoo; 5th November 2017 at 10:16 PM.
    Tim
    L3 NAR 98225

  11. #11
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    Quote Originally Posted by billdz View Post
    Wow, those pics convinced me. Where's the best place to buy some?
    I've ordered mine from McMaster-Carr. https://www.mcmaster.com/#catalog/123/3056/=1a4ytul

    At the moment, I use 2-56 shear pins for all of my birds, since I haven't built anything over 4" diameter yet. I know that people like to use the 4-40 size pins for larger rockets.
    Kris Hull
    www.hullaeroimages.com
    L1 - 05/26/2017 Mansfield WA. RIP Binder Design Excel Back in the Saddle - I245G; 1,281ft
    L2 - 11/11/2017 Pasco, WA.MAC 4" ARCAS - J400SS; 1,888ft

  12. #12
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    Quote Originally Posted by timbucktoo View Post
    If the pins are too strong and don’t “shear”, you probably didn’t do any ground testing.

    I use these on airframes 4" & smaller:
    https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
    As mentioned, you need to test on the ground to find the right amount of powder to get pin breakage and separation without stressing your harness. Online calculators help you find a starting value, but testing until you get consistent results is the only way to avoid learning through rocket loss.

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
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  13. #13
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    Consistent for a given value of "consistent" Nylon 2-56 screws shear at ~25-30 pounds, and 4-40s can be up around 50.

    Important for cardboard airframes: Reinforce the holes, so they don't get worn out or deflect. Some just harden the area with superglue, I like to peal the internal layer of the tube and epoxy a small brass strip (shear plate) where my hole will be.

    And use a black powder calculator/formula to determine if your charge is sufficient.
    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

    NAR 101195
    Level 1: Big SAM, 9/10/16

  14. #14
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    Good advice above.

    Steer clear of tape. It has been the downfall of many flights because it sometimes develops superhuman (supertape?) properties and becomes very tenacious.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  15. #15
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    Quote Originally Posted by billdz View Post
    Are shear pins considered the best method of dealing with this issue? I heard tape was better. Any other options?

    I'm one of the "shear tape" guys. I use it all the time for specific needs such as when I stick a motor too long in a rocket to get payload or nose cone on.

    Click image for larger version. 

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    It functions perfectly when used correctly. You really need someone adept at using it, to show you how.

    I like to use it with cardboard rockets also. Pins are a pain to use/install. As mentioned they will deform the airframe after several uses and need CA hardening or brass shear shims installed.

    2 ....1/4in. wide strips placed 180 from each other is preferred method. It's just so simple on 3-4in paper rockets. Yeah it may look funky on the pad , but so what.....simple, won't screw up your paint or tubes and will work up to Mach 1.5-1.9 this is from experience, not guesswork. Just burnish the edges flat with thumbnail & good to go.

    Pins are the way to go with glass/carbon. drill holes & done.
    They will always shear clean.
    ON cardboard I have seen way too many , bend, pull free, not shear and jam between parts, when not properly installed.

    So both have a place, knowing how to use each technique, is the thing.
    Jim Hendricksen
    L-3 Tripoli 9693
    [ICBM, Orangeburg,SC R.I.P.] - QCRS ,Princeton ILL - MDRA , Price Maryland - Woosh, Bong Wisconsin- ROCC, Charlotte NC , ICBM Camden SC
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    Rocketry...........an exact science.......but not exactly !!!

  16. #16
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    Do you use pins/tape only on larger rockets or on all dual deploy rockets? Someone said only necessary at near mach speeds.

    L1 3/25/17 H135
    L2 8/12/17 J180

  17. #17
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    Quote Originally Posted by billdz View Post
    Do you use pins/tape only on larger rockets or on all dual deploy rockets? Someone said only necessary at near mach speeds.
    Obviously "someone" doesn't quite understand how things work.

    Think of traveling in your car, going up & down in high hills or mountains. Your ears "pop" trying to equalize the pressure trapped inside, with changing pressure outside. Same goes with your rocket.

    You need pressure relief vent in each section...fincan & payload ...to bleed of the differential as rocket goes higher.[not same as altimeter vents]

    Faster it travels up...the faster you need to equalize internal pressure with exterior.[larger vent for extreme speeds]
    If you don't, pressure inside, will cause premature separation of fincan/payload or payload/nosecone. This usually happens at Max-Q [ motor burn out]. No longer is there a force acting on front of rocket pushing parts together. Things pop apart...chute comes out, while moving at high speed and wham-o ....shred or zipper.

    Also using to short of an apogee shock cord will snap hard/stop abrupt and forces will continue on parts, causing NC to come off, opening main at apogee.

    Some cases the drag on lower section can be higher than upper, once again you get parts separating when they are not supposed to.

    SO.....it just happens faster when using hard hitting motors or ones that get you to mach quicker.

    Shear pins/tape, alleviate all this.

    So...I use them on all flight where DD is used. I only do main pins, some folks use them for apogee,to hold fincan to payload . My recovery designs never need them. It's a delicate dance between ejection charge sizes, length of shock cord, number of pins.

    Now you're going to hear some folks telling you "always use 2-3 pins" or risk the parts cocking/wedging from off center thrust of charge.....pure BS...urban legend passed from one to another reading something and repeating it.
    May have had some validity years ago when part tolerances were nothing like today's or with very loose fitting plastic NC's having "ribs" on them used with sloppy tolerance cardboard tubes. I'm talking fiberglass here.

    So...I [and many, many of my experienced friends]use just 1 pin on 38mm airframe
    2 pins on 54 & 3in. airframes
    3 pins on 4 & 5in. airframe

    6& 7.5 & 8 in. use 4-40 pins [4 of them] due to how much more those NC's weigh.

    12 in. airframe 6 pins 6-60.

    I have used shear tape on 54mm to 4in airframes. Same amount of BP used for both pins/tape. Experience has shown...for me..it works, same.

    How you build/fit parts...tight or loose determines how you use them.
    Jim Hendricksen
    L-3 Tripoli 9693
    [ICBM, Orangeburg,SC R.I.P.] - QCRS ,Princeton ILL - MDRA , Price Maryland - Woosh, Bong Wisconsin- ROCC, Charlotte NC , ICBM Camden SC
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    Rocketry...........an exact science.......but not exactly !!!

  18. #18
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    There are three things you are trying to avoid. Crazy Jim has covered separation die to pressure build-up. There is also drag separation, where part of the rocket continues upward after burnout, but the more draggy bottom of the rocket does not. This is desirable for stage separation, but not for booster/payload separation.

    Finally, there is premature ejaculation of the nose cone at apogee when the force of the apogee event stretches the harness and the momentum is transferred to the NC.

    All of these can be mitigated by tape or pins. I just think most people find pins easier and more straightforward.

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
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  19. #19
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    Quote Originally Posted by Bat-mite View Post
    There are three things you are trying to avoid. Crazy Jim has covered separation die to pressure build-up. There is also drag separation, where part of the rocket continues upward after burnout, but the more draggy bottom of the rocket does not. This is desirable for stage separation, but not for booster/payload separation.

    Finally, there is premature ejaculation of the nose cone at apogee when the force of the apogee event stretches the harness and the momentum is transferred to the NC.

    All of these can be mitigated by tape or pins. I just think most people find pins easier and more straightforward.
    Are you sure about the PE Batmite?
    Tim
    L3 NAR 98225

  20. #20
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    Quote Originally Posted by timbucktoo View Post
    Are you sure about the PE Batmite?
    I knew I'd get some comments on that one ....

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
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    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  21. #21
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    You need shear pins to prevent the main chute payload bay from shaking loose due to deceleration when the drogue fires. The heavier the payload section (and nose cone), the more likely this is to occur. I've used tape on cardboard rockets, but you have to be very careful how much you use. Generally, two 1/4" wide pieces of Scotch tape is enough. For FG rockets, use 2/56 nylon screws. And of course, GROUND TEST FIRST!

  22. #22
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    Quote Originally Posted by timbucktoo View Post
    Are you sure about the PE Batmite?
    Quote Originally Posted by Bat-mite View Post
    I knew I'd get some comments on that one ....
    This problem can be overcome by use of 1 Viagra pin........no matter what size.....
    Jim Hendricksen
    L-3 Tripoli 9693
    [ICBM, Orangeburg,SC R.I.P.] - QCRS ,Princeton ILL - MDRA , Price Maryland - Woosh, Bong Wisconsin- ROCC, Charlotte NC , ICBM Camden SC
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    Rocketry...........an exact science.......but not exactly !!!

  23. #23
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    Quote Originally Posted by Nytrunner View Post
    Consistent for a given value of "consistent" Nylon 2-56 screws shear at ~25-30 pounds, and 4-40s can be up around 50.

    Important for cardboard airframes: Reinforce the holes, so they don't get worn out or deflect. Some just harden the area with superglue, I like to peal the internal layer of the tube and epoxy a small brass strip (shear plate) where my hole will be.

    And use a black powder calculator/formula to determine if your charge is sufficient.
    The above is all correct. Also comments from Blackjack/CJ in this thead apply. The real goal in using shear pins is to achieve consistent separation force that you can measure/guard against opposing gee force in a normal recovery event or flight anomaly. Tape is less predictable and although it might work, in general, it becomes a crap shoot as you scale (up or down) your designs.
    L3, TRA #11847
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  24. #24
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    This problem can be overcome by use of 1 Viagra pin........no matter what size.....
    I think this is a case where size does matter...
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  25. #25
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    I'll throw my $0.02 in here although it might not be worth that much.

    I only use shear pins on fiberglass rockets. The wear and tear on cardboard tubing isn't worth the trouble to me. I've flown up to 4" 12 lb DD rockets with nothing but tape on the nose cone shoulder to provide friction to hold the nose cone in.
    There are two things I do to make sure this works.
    One, make sure I can lift the rocket by the nose cone and give it a "gentle" shake without the nose cone coming off. It may separate a little, but it shouldn't come off. That is also my test to prevent drag separation and has worked fine so far.
    Two, I make sure I don't over do the apogee charge. You shouldn't be blowing the two halves apart so hard that they snap back. All you need is enough separation to get the drogue chute into the air flow. The drag of the drogue chute will pull all of the shock cord out if it isn't already and position the parts of the rocket during free fall for a successful main chute deploy.

    I know many will disagree with this, but my method is based on my personal experience and works for me. I've flown DD for 10 years and hundreds of flights with cardboard tube rockets and tape on the nose cone shoulders. The only main chute at apogee deployment I had was my fault. I had put a new payload tube on the rocket and didn't retest the friction fit in my rush to fly the biggest motor the vendor had that would fit my rocket on a day with zero winds above about 300 ft. The main deployed at 6400 ft and took over 5 minutes to descend. Fortunately it was that perfect day and the rocket was over the pads when the low level winds caught it and it landed about 300 ft from the pads. YMMV
    Handeman

    TRA #09903 L3 3/29/2015

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  26. #26
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    CF and canvas phenolic handle shear pins very well, too. Cardboard works if you shim it with a little thin piece of metal.

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  27. #27
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    ATCS(AW) Tom Keith, USN, ret. _____NAR 99781 L1_____MDRA 212
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  28. #28
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    Should the shear pins be used if the flight is single deploy?

    L1 3/25/17 H135
    L2 8/12/17 J180

  29. #29
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    Should the shear pins be used if the flight is single deploy?
    Similar considerations to those listed above apply to both single and dual-deploy. As the flyer, you get to choose
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  30. #30
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    Quote Originally Posted by billdz View Post
    Should the shear pins be used if the flight is single deploy?
    Quote Originally Posted by OverTheTop View Post
    Similar considerations to those listed above apply to both single and dual-deploy. As the flyer, you get to choose
    Like OTT says - you get to choose! It really has a lot to do with motor you choose! Is it a high G motor? You are going to need some retention else drag separation. Really depends!

    Tim
    L3 NAR 98225

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