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  1. #1
    Join Date
    13th November 2009
    Location
    Greater Chicago Area
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    4,640

    Question Mini Fat Boy - 18mm Upgrade - CG Question?

    I am working on an Estes Mini Fat Boy with one of my children. I have made a mod to convert it from 13mm to 18mm, so it can fly on B6's.

    However, I am not sure what the CG should be, since I am certain that it has change due to adding more mass to the aft.

    Does anyone know where the CG should be?

    Greg


  2. #2
    Join Date
    21st March 2011
    Location
    central America
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    1,966
    Before the "official" kit came out, I made one using a Baby Bertha kit, and an 18mm mount. With 0.25 oz nose weight, OR says it has around 0.5 calibers stability. It flies fine.


  3. #3
    Join Date
    13th November 2009
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    Thanks for the data point.

    I forgot about using OpenRocket to check the stability.

    Greg

  4. #4
    Join Date
    28th December 2011
    Location
    Dallas, TX
    Posts
    294
    Keep in mind that the stability assumptions used in sims assume a relatively long body to the diameter. And in this rocket you will have a lot of base drag. This is why one might be fine with only a half caliber stability.

  5. #5
    Join Date
    10th January 2012
    Location
    Athens-- No, the other one, in SE Ohio
    Posts
    1,318
    I didn't know you could do this... modify a mini fat boy to accept A-B-C size motors?
    If I knew that, I wouldn't have repaired it so quickly, but tried to tear out the motor mount
    and insert something larger after my CATO last club launch.
    (Darn A10-3T motors! Went up like a roman candle! No delay at all!)
    "What else am I going to do with all this trivia I have stored up in my head?" -- Mark Evanier, Jack Kirby biographer, circa 1996

  6. #6
    Join Date
    21st April 2010
    Location
    So central WI, USA
    Posts
    5,417
    when I bought mine, all the tubes were more than a tad oval, so I cut a bt60 an inch longer than stock (6.75"), gave it an 18mm mmt, and added 10g of nose weight. flies well on B6-4s, almost out of sight on a 'Big C6-5'. I also use a top flight thin mil 12" chute for recovery.
    Rex
    L2-competitor 3, AT J350W, 8/27/2016, Bong, 2557'
    my youtube channel http://www.youtube.com/channel/UC0gB...?feature=watch

  7. #7
    Join Date
    13th November 2009
    Location
    Greater Chicago Area
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    4,640
    Quote Originally Posted by 1tree View Post
    Keep in mind that the stability assumptions used in sims assume a relatively long body to the diameter. And in this rocket you will have a lot of base drag. This is why one might be fine with only a half caliber stability.
    Thanks for the reminder. I don't usually fly "squatesque" rockets so this is a little new for me.

    Quote Originally Posted by Kirk G View Post
    I didn't know you could do this... modify a mini fat boy to accept A-B-C size motors?
    If I knew that, I wouldn't have repaired it so quickly, but tried to tear out the motor mount
    and insert something larger after my CATO last club launch.
    (Darn A10-3T motors! Went up like a roman candle! No delay at all!)
    All you need are new centering rings and an 18mm motor tube. It's not a difficult mod, you just don't have a lot of room in the airframe for recovery once it's done.

    Quote Originally Posted by Rex R View Post
    when I bought mine, all the tubes were more than a tad oval, so I cut a bt60 an inch longer than stock (6.75"), gave it an 18mm mmt, and added 10g of nose weight. flies well on B6-4s, almost out of sight on a 'Big C6-5'. I also use a top flight thin mil 12" chute for recovery.
    Rex
    Thanks for the data point Rex. I was thinking that a C6 would give it quite a ride!

    Greg

  8. #8
    Join Date
    21st April 2010
    Location
    So central WI, USA
    Posts
    5,417
    OR says about 800', I suppose if one wanted to lose it, that a D10 might do the trick .
    Rex
    L2-competitor 3, AT J350W, 8/27/2016, Bong, 2557'
    my youtube channel http://www.youtube.com/channel/UC0gB...?feature=watch

  9. #9
    Join Date
    13th February 2009
    Posts
    4,273
    Fat Boys are cool!
    The process is continuous...

  10. #10
    Join Date
    10th January 2012
    Location
    Athens-- No, the other one, in SE Ohio
    Posts
    1,318
    Quote Originally Posted by bradycros View Post
    Fat Boys are cool!
    I'm not sure how I should feel about that statement.... LOL!
    "What else am I going to do with all this trivia I have stored up in my head?" -- Mark Evanier, Jack Kirby biographer, circa 1996

  11. #11
    Join Date
    25th January 2009
    Location
    Glennville, GA
    Posts
    17,900
    D21 would make it fast.
    -----------------------
    Chuck Haislip
    NAR/Tripoli Level 3
    Formerly a Prefect of ICBM - TRA #60

    Level 1 - LOC Minie Magg; Level 2 - PR Broken Arrow;
    Level 3 - 10 inch Nike Smoke
    2015 Ns for Year: 1315 Newtons
    My rockets usually fly naked. If they survive, they earn their paint.

    Come fly with ROSCo or ICBM in Camden SC => http://rocketrysouthcarolina.org

  12. #12
    Join Date
    20th October 2013
    Location
    Tucson,Az
    Posts
    495
    I modified an original fat boy to 29mm flys great on a H-225.Next one is going to be 54mm with soda can motor.

    Gary
    TRA 1639
    Wille E. Coyote super genius

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