Big Daddy Mods

[hossinarizona]

I have a Big Daddy that I will be upgrading a bit. I will be using a Aerotech F20-4W, and swapping out the stock 24mm tube and centering rings, with a 29mm tube and centering rings. I also am changing from balsa fins to plywood for better durability. I know my CG will be affected, I think the CG is supposed to be about 2" from the from of the fins, does that sound right? Anyone have any Idea about how much weight I might have to add? Any additional information would be greatly appreciated.
Thanks,
Mike

 

[Voyager1]

Sim it in OpenRocket. Change the standard Big Daddy sim model to reflect your material mods and check its CG and CP. The Big Daddy is susceptible to added masses, particularly if they shift the CG further aft. Changing to a 29mm MMT and ply fins will most likely affect the stability.

You can add the appropriate mass into the NC in your sims before committing to the hardware mods. I've just simmed it with your mods and 110g in the NC to get the cal > 1.0.

Additionally, you better profile the fins for better performance.

 

[RobertH3]

Vanderburn had a nice kit with plywood fins and a 29 mount. - I did mine with it. G77R and ker-whoosh.
It's been a lucky model. Many overpowered dropkick launches and he's always come back : )
G80NT will be the next big launch on a very calm day.

 

[cichlidgoob]

I agree with simulating in Open Rocket, and here is my experience with a similar project.

I built a Big Daddy with a 29mm motor mount, papered the balsa fins, and modified the nose cone by cutting off the sloped part and installing a plywood bulkhead. I stuck a long shanked eyebolt in the nose cone bulkhead with a few extra washers and bolts on it, which equates to about 3 extra ounces total in the nose. It has flown successfully on a G76, G79 and F67. The papered balsa fins have been fine so far and the higher thrust motors were better flights.

 

[K'Tesh]

Having a sim to start with might help... https://www.rocketryforum.com/threads/kteshs-openrocket-files.123564/post-1604745

 

[bjphoenix]

You can add the appropriate mass into the NC in your sims before committing to the hardware mods. I've just simmed it with your mods and 110g in the NC to get the cal > 1.0.

I thought we had discussed that the stock BD had less than 1.0 caliber of stability, when simmed without the base cone hack. Did you sim from scratch or start with a standard BD sim?

I just opened a BD OR sim that I had on my hard drive, turns out I already had the ones linked in post #5. The sim only had one profile, containing an E9, and it shows stability of 0.59. I changed it to a D12 and it showed 0.72.

 

[Voyager1]

I thought we had discussed that the stock BD had less than 1.0 caliber of stability, when simmed without the base cone hack. Did you sim from scratch or start with a standard BD sim?

I just opened a BD OR sim that I had on my hard drive, turns out I already had the ones linked in post #5. The sim only had one profile, containing an E9, and it shows stability of 0.59. I changed it to a D12 and it showed 0.72.

I simmed the stock BD with the OP’s 29mm MMT, ply fins and an F20-4W motor added. It still had a cal of <<1.0, but with 110g mass added to the NC, it had a cal > 1.0.

 

[hossinarizona]

I simmed the stock BD with the OP’s 29mm MMT, ply fins and an F20-4W motor added. It still had a cal of <<1.0, but with 110g mass added to the NC, it had a cal > 1.0.

So with what I plan to mod on the BD, plywood fins, 29mm motor tube, and 29mm motor retainer using a 24mm to 29mm motor adapter for the E20-4W, I will still need to add 3.88 ounces to the nose?
I really do appreciate all of everyone's help, I normally just build them stock, but I am building this with my grandson and I want it to be a big success to keep him interested.
Thanks again,
Mike

 

[Voyager1]

Most added masses will affect the CG, and consequently the stability margin. With the additional mass of a motor retainer and an adapter, you will likely move the CG closer to the CP. So, you will need to add the appropriate amount of mass to the NC to bring the CG back to a point where the stability margin is at least 1, or more. Remember, you can easily test the CG position with a simple balance test with the rocket loaded with chute and motor, etc, ready for flight. You add extra mass in the NC until your CG has moved forward to the point that your stability >= 1.0.

Are you able to perform a simulation? This will help you with these questions.

 

[hossinarizona]

Most added masses will affect the CG, and consequently the stability margin. With the additional mass of a motor retainer and an adapter, you will likely move the CG closer to the CP. So, you will need to add the appropriate amount of mass to the NC to bring the CG back to a point where the stability margin is at least 1, or more. Remember, you can easily test the CG position with a simple balance test with the rocket loaded with chute and motor, etc, ready for flight. You add extra mass in the NC until your CG has moved forward to the point that your stability >= 1.0.

Are you able to perform a simulation? This will help you with these questions.

Thanks for the quick reply. I tried to download the application to do a simulation however, it failed 3 times? Not really sure why, my computer is a bit old but I have downloaded other applications with out issues.
Is the CG about 2" in from of the fins, I could load it up with everything and go from there.

 

[hossinarizona]

Most added masses will affect the CG, and consequently the stability margin. With the additional mass of a motor retainer and an adapter, you will likely move the CG closer to the CP. So, you will need to add the appropriate amount of mass to the NC to bring the CG back to a point where the stability margin is at least 1, or more. Remember, you can easily test the CG position with a simple balance test with the rocket loaded with chute and motor, etc, ready for flight. You add extra mass in the NC until your CG has moved forward to the point that your stability >= 1.0.

Are you able to perform a simulation? This will help you with these questions.

I think you also mentioned that you did a sim with my mod's and came up with 110g or 3.88 ounces of needed nose weight, does that still sound about right?

 

[Capt. Eric]

On my last BD (which blew up on an E12), I cut off the NC shoulder, then sanded the NC edge a little, and slipped the shoulder back into it with some glue. Gave me a little over an inch more room for chute and cord. That payload area can be tight!

 

[Voyager1]

I think you also mentioned that you did a sim with my mod's and came up with 110g or 3.88 ounces of needed nose weight, does that still sound about right?

To answer your previous question: the CG will be closer to 3" from the leading edge of the fin root. Remember that 1 caliber for the Big Daddy is 3" (it has a body diameter of 3"). So, from my simulation of you modded BD, the CP is just behind the fin root. So your CG is 3" forward of that point. In this case I did add 110g into the NC tip.

 

[SolarYellow]

There are multiple in-depth discussions in recent weeks about stability of short/fat rockets. The conclusion that seems to be tentatively (OK, adamantly by some, and others open to it) accepted in them at this point is that a more appropriate denominator than airframe diameter is overall rocket length. Targeting somewhere in the range of 8 to 15 percent of overall rocket length works well. It is especially more useful and safer than "calibers" on both short/fat rockets and long/skinny rockets.

 

[Donnager]

Don't stop at 29mm.

You can actually build them with a 38 or 54. An H-550 in a Big Daddy goes pretty fast.

The 54 is a little sketchy, though.

 

[Back_at_it]

I can't tell you how much nose weight to add but I can say that my BD with similar mods flies great with the CG right at the 12" mark when measured from the tip of the nose cone. According to OR my stability is .726 without the cone hack.

Never had a bad flight and it's been up on E12's, F44's etc. All perfectly stable. If you're CG is at of forward of the 12" mark you will have a nice flying rocket. Also, the F20 isn't a hard hitting motor so no worries there.

 

[hossinarizona]

I can't tell you how much nose weight to add but I can say that my BD with similar mods flies great with the CG right at the 12" mark when measured from the tip of the nose cone. According to OR my stability is .726 without the cone hack.

Never had a bad flight and it's been up on E12's, F44's etc. All perfectly stable. If you're CG is at of forward of the 12" mark you will have a nice flying rocket. Also, the F20 isn't a hard hitting motor so no worries there.

Thanks for you reply, I think my CG will be OK, I did have to add about 2 ounces to get it to balance, so we will see. What would you suggest as to my next motor to try, after the F20-4W?

 

[Back_at_it]

Thanks for you reply, I think my CG will be OK, I did have to add about 2 ounces to get it to balance, so we will see. What would you suggest as to my next motor to try, after the F20-4W?

F50 or F44.

 

[krislhull]

The 54 is a little sketchy, though.

Go big or go home!

@Flyfalcons did it!

 

[tsmith1315]

Go big or go home!

@Flyfalcons did it!

It’s funny to hear the LCO announce an Estes rocket going up on a J-motor!

 

[RobertH3]

And if you are enamored of the design, Dragon has the kit upscaled to:

4" diameter: https://dragonrocketry.com/products/4-big-daddy

Probably has the parts for 5.4 LOC tubing looking at the other kits too.

 

[Donnager]

Go big or go home!

@Flyfalcons did it!

Yeah,

My 54 has all the recovery in the nose, or at least 80% of it. The 29 is mostly stock, manually cut down fins, but still factory balsa with CA. The 38 has plywood fins and some internal Airframe tube support. The 54 is fully glassed with the Estes balsa fins.

29mm/38mm/54mm.