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  1. #31
    Join Date
    11th November 2014
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    690
    Quote Originally Posted by Steve Shannon View Post
    Weathercocking shortens the height of apogee, but more importantly you have horizontal velocity which remains fairly high throughout the flight. Any significant velocity when a parachute opens can cause problems.


    Steve Shannon
    Right, while the y-component of velocity may have reached zero, the x-compenent would still be quite high. Coupled with a cell phone in the payload bay, the amount of energy that the payload bay would have exerted on the shock cord/bolt & nut would have been *quite* high, since kinetic energy is mass * velocity squared. Ok, maybe not that high, since the two pieces were flying together, and the separation would make the KE velocity relative to each other.

    But it still would have been quite a bit of force. If the shock cord was still attached to the lower body (not the payload section), still had some metal bits, and was nylon, could it have rubber-banded back after separation and caused the damage above the fin?


  2. #32
    Join Date
    11th February 2017
    Location
    south Florida
    Posts
    571
    What do you mean by "metal bits" in the shock cord? The cord is still attached to the bottom section and it is made of tubular nylon. Your rubber band theory makes some sense -- at ejection, the eyebolt was loose but not completely out, the cord extended to full length then snapped back when the eyebolt popped free. Only problem is that I have not been able to duplicate any snap effect on the ground. I've pulled the shock cord as tight as I dare, but when I let go the eyebolt drops straight to the ground, no rubber band effect at all.


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

  3. #33
    Join Date
    15th October 2016
    Location
    Huntsville AL
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    1,877
    You're standing in one place with the cord fixed I take it?
    That's not a representation of what happens at ejection.

    The rocket is free falling, the charge goes off, both pieces are kicked opposite each other, the cord reaches full extension, the heavier piece will be slowed, the lighter piece will be jerked back towards the heavier piece.
    (Try getting a slightly lighter weight friend to hold the other end of the shock cord, then run full speed away from each other! please share the results and video)

    Or your aft section hit a rogue piece of dark matter.

  4. #34
    Join Date
    11th February 2017
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    south Florida
    Posts
    571
    I see your point, but isn't it more like the two friends run away from each other and at full extension one lets go of the cord? It does appear that either the eyebolt or the NC hit the aft section, probably the eye bolt since there's no mark on the plastic NC.

    The charge appears to have gone off prematurely, based on the video. I can't see any sign of the chute coming out but someone clearly says "chute already" at the 16 second mark, just 11 seconds after liftoff and 4 seconds after burnout. At 21 seconds, my son yells, "Something's falling!" The motor comes with a 9 second delay and I used the tool to reduce it to 6 seconds. Just rechecked the tool and it was definitely set for 3 second reduction. I know that the motor delay is sometimes imprecise, but every other CTI motor I've flown has been spot on. Guess this one fired 2 seconds early. Not sure if this was the prime cause of the problem or if it was a combination of unfortunate events.

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

  5. #35
    Join Date
    11th January 2013
    Location
    Old Bridge,,,,,,,,,,,,, New Jersey
    Posts
    5,201
    12 ft is not long enough for a harness on a 3" rocket..

    Use a tiny drop of blue loctite on every thread you make on a rocket..

    Use welded closed eye bolts,, bent wire eye's will eventually fail / open..

    Teddy




    www.Onebadhawk.com

    Ted Chernok
    Old Bridge New Jersey
    METRA- BoD- New York..
    URRG- Way upstate New York..
    MDRA- Maryland..
    QCRS- Illinois- Best politicians money can buy ( Tim L. )..
    Kloudbusters- Kansas..
    NAR # 87063
    Tripoli # 14443
    Level 3

  6. #36
    Join Date
    11th February 2017
    Location
    south Florida
    Posts
    571
    Thanks for the replies. Just reviewed this video from an earlier flight - same rocket, same shock cord, same phone, almost same apogee, same flightpath (slight cocking near apogee), but longer delay:


    Unless someone says otherwise, I'm now pretty convinced that the primary problem Saturday was that the delay was too short. This rocket has flown several times with no issues until this last flight, but I'll now repair the damage, lengthen the shock cord, and seal all hardware with loctite.

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

  7. #37
    Join Date
    18th March 2009
    Location
    Wisconsin
    Posts
    3,611
    I have to ask this- what did this rocket weigh with motor on board? Secondly, how was the wind?


    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

  8. #38
    Join Date
    11th February 2017
    Location
    south Florida
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    571
    2212g, 10mph

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

  9. #39
    Join Date
    13th June 2014
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    Cocoa Beach, FL
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    Quote Originally Posted by billdz View Post
    2212g, 10mph
    Were those ground winds or winds at apogee?
    Tim
    L3 NAR 98225

  10. #40
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    11th February 2017
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    south Florida
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    Ground. How do you determine wind speed at apogee?

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

  11. #41
    Join Date
    18th March 2009
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    Wisconsin
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    Ok, for the weight of the rocket and the wind you were substantially underpowered.

    You were flying in a 10 mph wind with about a 3:1 thrust to weight ratio. This a grossly underpowered for the wind. This is the biggest reason it weathercocked so much. You should have had a motor with at least double the thrust or more in this wind.

    This rocket/motor combination should not have been allowed to fly in this wind. This is a matter of poor motor selection, and a questionable RSO job at the launch.


    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

  12. #42
    Join Date
    11th February 2017
    Location
    south Florida
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    571
    The wind was light, maybe my estimate of 10mph was high. I'm no expert but it does not seem like this launch was grossly underpowered.
    "A minimum ratio of 5:1 is typically required. This can be as low as 3:1 at the RSO’s discretion. This decision will require low ground level winds and stable design (Cg more than 1 caliber ahead of Cp). A 5:1 ratio is determined simply by taking the rocket weight (loaded with motors) in pounds and multiplying by 22.25 to get minimum required motor thrust in Newtons (N). Note this is the motor thrust off the pad, not the motor total thrust."
    http://metrarocketclub.org/thrust-to-weight-charts/
    I55 thrust off the pad = 94.5N http://www.pro38.com/products/pro38/...odid=395I55-9A
    weight of my rocket = 2212g = 4.8 pounds
    This gives a 4.38:1 ratio with low ground level winds and stable design (Cg 4.32 cal ahead of Cp). This rocket had 3 prior great flights with the same motor and substantially higher winds, and I think this flight would have been OK except that for some reason the ejection charge fired early.

    NAR Level 2 Exam Question Pool
    A10) What is the maximum allowable weight for a high power rocket permitted per NFPA 1127?A) 100 poundsB) 400 poundsC) 3069 poundsD) There is no limit provided the rocket weighs less than 1/3 of the average certified thrust of the motorsintended to be ignited at launch-----------------The answer is “D“.

    I55 average thrust = 55.2N
    1/3 of 55.2 = 18.4
    weight of my rocket = 4.8 pounds

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

  13. #43
    Join Date
    13th October 2014
    Location
    SouthEastern, WA
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    5,708
    Just a heads up 18.4N is 4.136 lb , and 55.2N is 12.4lb so the 4.8lb rocket was only a 2.58:1 Thrust to weight (someone please check my math it maybe/probably is wrong). The maximum thrust however is 21.2 lbs max thrust giving a 4.41:1 thrust to weight.
    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/

  14. #44
    Join Date
    18th March 2009
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    Wisconsin
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    Eyebolt came loose and rocket separated, what caused this damage?

    The motor has a slight peak as the op mentioned that lasts for about 0.1 second. Underpowered...trust me, I look at a lot of motor files.

    Wind, it is there. You can try to say this was ok, but it is at best questionable.

    A three to one, unless it is almost windless, is trouble waiting to happen. You can quote the rules all you want- the proof is in the flight video. I have flown for a long time, and RSO'd a bunch, and I am telling you under the conditions I do not think it should have been allowed.

    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

  15. #45
    Join Date
    15th July 2015
    Location
    Tidewater area of Virginia
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    1,397
    Quote Originally Posted by billdz View Post
    Ground. How do you determine wind speed at apogee?
    This web site, also has an app for your phone. Can be adjusted to see winds at altitude for your location.

    https://www.windy.com/?36.675,-75.916,5
    ATCS(AW) Tom Keith, USN, ret. _____NAR 99781 L1_____MDRA 212
    SEVRA, NAR 621http://www.sevra.org/ Tripoli East North Carolina (Bayboro), TRA #65, http://ncrockets.org/, MDRA http://www.mdrocketry.org/
    LVL 1 24 October 2015, Leviathan, CTI H133, 2469 ft, Bayboro
    LVL 2 Soon, Super DX3, AT J420 Redline, est 3500 ft, Bayboro or MDRA

  16. #46
    Join Date
    23rd July 2011
    Location
    Butte, MT
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    Quote Originally Posted by billdz View Post
    The wind was light, maybe my estimate of 10mph was high. I'm no expert but it does not seem like this launch was grossly underpowered.
    "A minimum ratio of 5:1 is typically required. This can be as low as 3:1 at the RSOís discretion. This decision will require low ground level winds and stable design (Cg more than 1 caliber ahead of Cp). A 5:1 ratio is determined simply by taking the rocket weight (loaded with motors) in pounds and multiplying by 22.25 to get minimum required motor thrust in Newtons (N). Note this is the motor thrust off the pad, not the motor total thrust."
    http://metrarocketclub.org/thrust-to-weight-charts/
    I55 thrust off the pad = 94.5N http://www.pro38.com/products/pro38/...odid=395I55-9A
    weight of my rocket = 2212g = 4.8 pounds
    This gives a 4.38:1 ratio with low ground level winds and stable design (Cg 4.32 cal ahead of Cp). This rocket had 3 prior great flights with the same motor and substantially higher winds, and I think this flight would have been OK except that for some reason the ejection charge fired early.

    NAR Level 2 Exam Question Pool
    A10) What is the maximum allowable weight for a high power rocket permitted per NFPA 1127?A) 100 poundsB) 400 poundsC) 3069 poundsD) There is no limit provided the rocket weighs less than 1/3 of the average certified thrust of the motorsintended to be ignited at launch-----------------The answer is ďDď.

    I55 average thrust = 55.2N
    1/3 of 55.2 = 18.4
    weight of my rocket = 4.8 pounds
    You mentioned your stability margin of 4.32 calibers. In combination with these conditions (low average thrust to weight, possible 10 mph winds) that is an overstable configuration and very prone to weathercocking.
    Yes, peak initial thrust might appear to be technically sufficient, but as Mark said, that is a very brief peak of thrust that immediately drops back to an insufficient thrust to weight ratio. Throughout the flight weathercocking happens and winds aloft are frequently much higher than surface winds.
    Donít feel bad; we have all been there.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  17. #47
    Join Date
    13th June 2014
    Location
    Cocoa Beach, FL
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    3,555
    Quote Originally Posted by billdz View Post
    Ground. How do you determine wind speed at apogee?
    Quote Originally Posted by Banzai88 View Post
    This web site, also has an app for your phone. Can be adjusted to see winds at altitude for your location.

    https://www.windy.com/?36.675,-75.916,5
    Here is another:
    http://www.usairnet.com/cgi-bin/Wind...course=azimuth
    Tim
    L3 NAR 98225

  18. #48
    Join Date
    11th February 2017
    Location
    south Florida
    Posts
    571
    @Banzai88 - Thanks, that is a helpful map
    @Mark - I believe you but do not fully understand. In the video in post #36 of this thread, we can hear the wind gusting, and at the 45 second mark we can see a large American flag held taught by the wind. Same rocket, same motor, same phone, great flight. In the flight at issue in post #22, we hear no wind and see no movement of the long grass. Is there some connection between thrust to weight ratio and the ejection charge firing prematurely?

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

  19. #49
    Join Date
    23rd July 2011
    Location
    Butte, MT
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    2,266
    Quote Originally Posted by billdz View Post
    @Banzai88 - Thanks, that is a helpful map
    @Mark - I believe you but do not fully understand. In the video in post #36 of this thread, we can hear the wind gusting, and at the 45 second mark we can see a large American flag held taught by the wind. Same rocket, same motor, same phone, great flight. In the flight at issue in post #22, we hear no wind and see no movement of the long grass. Is there some connection between thrust to weight ratio and the ejection charge firing prematurely?
    Thrust to weight has no effect at all on the length of time the delay grain burns and could not cause the ejection charge to fire early.
    The best way to determine actual wind speeds at different altitudes at your launch site is to observe several flights before you launch. Itís amazing how much variation there can be.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  20. #50
    Join Date
    26th November 2009
    Posts
    4,915
    Run your sims and if your venue allows, point rocket a few degrees downwind. Will provide a curving path to a lower
    energy apogee deployment. Don't believe me? Do multiple Sim runs.
    If launch site isn't large enough, you should only be flying model rockets at the site.
    A low energy apogee deployment is more desirable for high mass Rockets with an apogee main parachute opening.
    Kurt

  21. #51
    Join Date
    6th September 2009
    Posts
    1,648
    Quote Originally Posted by Steve Shannon View Post
    You mentioned your stability margin of 4.32 calibers. In combination with these conditions (low average thrust to weight, possible 10 mph winds) that is an overstable configuration and very prone to weathercocking.
    Yes, peak initial thrust might appear to be technically sufficient, but as Mark said, that is a very brief peak of thrust that immediately drops back to an insufficient thrust to weight ratio. Throughout the flight weathercocking happens and winds aloft are frequently much higher than surface winds.
    Donít feel bad; we have all been there.
    Steve Shannon
    I really hate TTW discussions, as the correct thrust to use is open to interpretation, and simple force/mass conversions get people discombobulated. Also, old rules of thumb are easily replaced with more accurate simulations.

    Steve's conclusion is correct. I did a quick thrustcurve motor guide check, assuming the dry weight is 1775g (by subtracting the mass of the motor from billdz's 2212g loaded figure). The CTI I55 is horribly underpowered for this rocket, giving a launch guide velocity of just 29 ft/s. Throw in the wind and over-stability on top of this, and the I55 is not a good choice at all.


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