Big Daddy on a F35

Discussion in 'Mid Power Rocketry (MPR)' started by byoungblood, Oct 21, 2019.

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  1. Oct 21, 2019 #1

    byoungblood

    byoungblood

    byoungblood

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    I built and launched a Big Daddy on a F35 this past weekend and almost immediately after burnout experienced a drag separation of the nose cone. Happy to report there was no damage to the rocket.

    I had added about 2oz of nose weight to aid with stability with the larger motor. In addition, reading the thread linked below, I think leaving the aft centering ring out (I intend on adding a screw on motor retainer, I just didn’t have any with me) probably increased the base drag on the body of the rocket and the combination of the two led to the unintended separation.

    My question, how much extra drag did leaving that centering ring off impart on the rocket? I added the nose weight thinking I’d need it for the F motor, but apparently the base drag of these stubby rockets aids stability. I’m curious now if I could just fly with an unweighted nose and leave the aft ring out permanently and still have stable flights on the F35.

    Boat Tails =advanced=
     
  2. Oct 21, 2019 #2

    Bat-mite

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    Something to know about short, stubby rockets like the Big Daddy -- they are stable with a lot less than 1 cal CG/CP ratio. In fact, you can probably get away with less than 0.5 cal. I don't know how the base drag was affected, but you can always put some tape on the NC shoulder. Big Daddies have a known blow-by problem due to the diagonal cut away where the harness attaches. Search the forums and you'll find lots of posts about BDs failing to get the NC out due to blow-by.

    Taping around the NC shoulder helps mitigate blow-by, but does not prevent it. It will, however, significantly reduce the likelihood of drag separation.

    Best practice, however, is to get a new NC with a flat base. And I'd put the CR back in, but others may disagree.
     
  3. Oct 21, 2019 #3

    beantownJPL

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    I also built and maidened a Big Daddy this past weekend. The largest motor I launched it on was a BP E12. Like you I added a screw-on motor retainer, but I did not omit any centering rings. I just moved the forward CR to the end of the motor tube, and the aft CR forward about 1/8". This nicely accommodated the retainer, and didn't require any modification of the TTW fin tabs. I also attached the shock cord to the motor mount ... more on this later...

    My kit only came with two centering rings ... so does that mean you were flying with only 1 CR? Perhaps I'm missing something, but that clearly wouldn't hold the motor in proper alignment.

    I got two launches with this rocket on Saturday ... first was with a D11, second on the E12. The NC on my kit was "fall out" loose, so I added a little blue tape to make it "shake it out" loose (also taped over the hole in the NC to reduce the volume necessary to presurize on ejection. I had no "nose cone" related issues. However, my second launch resulted in a separation on ejection. Fortunately the nose cone is heavy enough that it came down for an in-field landing, and the airframe tumbled home quite nicely and with no damage.

    Upon inspection, I discovered that the separation was a result of my Kevlar shock cord burning through! The ejection charge even torched a few holes in my 'chute protector!

    There was fortunately no damage to the nylon chute though, so I've got that going for me, which is nice...

    I'm guessing that there was something "unusual" about this ejection charge ... crazy to burn through a Kevlar shock cord after only two launches. Admittedly the chute protector was a heavy-duty nylon one, rather than Nomex ... but it's an old one that's been through many launches and never had a burn-through.

    Here's mine:
    IMG_20191017_215300.jpg IMG_20191019_114430.jpg

     
  4. Oct 21, 2019 #4

    Bat-mite

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    A little anecdote -- not meaning to hijack the thread ... but my first flight of a Big Daddy was pretty cool. I took my kids to a local school sports field and set up to launch on an E9. We didn't know it, but there was a summer camp being hosted at the school that week. Right before we could launch, the campers and teacher came out for recess. We debated whether or not to push the button, but my kids' pleading won out. I called over to the campers that we were going to launch. Everyone stopped and waited. I half-expected the teacher to ixnay us, but he watched, too.

    When I did the countdown and hit the button, the think shot up to about 900 feet, and there was a chorus of "wow--" from the campers. The teacher then said, "Wow, wasn't that great?" And he led the campers in giving us a round of applause.

    Turned out to be a perfect flight and recovery.
     
  5. Oct 21, 2019 #5

    NOLA_BAR

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    The Big Daddy really hauls butt on an F35. This is mine on an F35-5 last year at the Bama Blastoff. It had an early chute deployment, which I attributed to a faulty delay grain, but I don’t really know for sure.
    IMG_1713.jpg
     
    beantownJPL likes this.
  6. Oct 22, 2019 #6

    byoungblood

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    I used the lower CR to align the motor mount and then used generous fillets on the fins where they attached to the motor mount. It is pretty solid and it flew well...until the nosecone popped off early! It will get the rear CR after I add the screw on retainer. I just don't have the means of procuring one for another few weeks.

    I thought about adding some tape to the shoulder of the nosecone, but after reading the threads about people having ejection issues with this rocket, I decided against it. It was kind of loose, but I could invert the rocket and it wouldn't just pop off (sans nose weight) so I decided to leave it be.

    I had a kevlar shock cord get singed pretty badly on a Vagabond I built about 5-6 years ago, but it probably took 2 years of flying for it to get that way. Probably a victim of the notorious "hibachi effect" of AT motors. Found it when I was fixing some damage after it suffered a bad zipper from a delayed ejection. There was enough undamaged kevlar left that I just spliced some more on and still fly it today.



    Nice!
     
  7. Oct 22, 2019 #7

    beantownJPL

    beantownJPL

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    Duh ... Totally forgot about the fin tabs. Now I get it.

    In my defense, it WAS 5 whole days ago that I glued mine on, who can remember that far back?

    As for the Kevlar, I used the trick where you put a straw between the two CRs, and punch a hole in each end. Replacing it too all of two minutes. I'm going to see if I come up with something to protect it better on the hot side .
     
    Last edited: Oct 22, 2019
  8. Oct 23, 2019 #8

    John Taylor

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    They also fly OK on a F24W.
     
  9. Oct 23, 2019 #9

    byoungblood

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    Out of curiosity, I went into OR to see how much weight you'd have to add to a Big Daddy to get (close) to 1 cal stability....

    Over 200 lbs! It actually stopped changing the stability calculation at 200# at .95 cal, I guess I found a limit of the application.

    I think I am going to cut off the tapered part of the shoulder and put in a bulkhead in the nose to eliminate the issue with the nose cone. Only other replacement that I can find that is said to fit that body tube also has a taper.

    I just got the 24/60 case and wanted to fly it. Out of the rockets I have with me the Big Daddy seemed like the best candidate for one that would stay recoverable. OR said it should get up to about 1300' on the F35. I try my best to keep anything I fly under 2000' just to avoid long walks or having one land somewhere rather inconvenient.
     
  10. Oct 23, 2019 #10

    Bat-mite

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    This thread explains a way to "fake out" simulation programs to account for base drag. Enjoy.
     
  11. Oct 23, 2019 #11

    jqavins

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    Good on the fake out simulation of base drag stabilization. And it gets better for Daddy; this is separate from the fact that less than 1 cal margin is necessary.
    • Lesser margin is acceptable when the rocket is short.
    • Base drag helps stability noticeably when the rocket is fat.
    • Big Daddy is both.
     
  12. Oct 28, 2019 #12

    Jozef

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    IMG_6036.JPG IMG_6036.JPG I fly my BD on CTI F50 skids. May I suggest you cut the base of your nosecone at the ramp and and cut a ply bulkhead as shown in the pic below. Gives more room for recovery gear as a bonus. Never had a deployment issue with this mod. Other mods include an Estes screw on retainer and mini rail buttons and papered fins.
     
  13. Oct 28, 2019 #13

    Flyfalcons

    Flyfalcons

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    Leaving a centering ring out did nothing for drag. You need to have a snug fitting nose cone when adding nose weight and/or flying faster speeds than you typically get on stock black powder motors.
     

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