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Modified Big Daddy Stability

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Spitfire222

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Hi everyone,

I'm just finishing up building an Estes Big Daddy. I know there are many threads about its stability, and I’ve read several of them. Though I know it would add some weight, I've made some minor modifications to improve resistance to wear and also practice some techniques for future higher-powered rockets.
The mods include:
1. Cutting off nosecone taper and replacing with 1/8" ply bulkhead with eye bolt
2. Replacing cardboard centering rings with 1/8" ply, with the fwd one having another eye bolt
3. Fiberglassing fins with 0.75 oz cloth

So far, the empty weight of the rocket is 257g, as measured by a digital kitchen scale. I know some may say these mods are pointless and these rockets should just be built stock, but I enjoy this process of minor modification, etc. In any case, the rocket is built now.

I'm also using these mods to get started learning how to use Open Rocket. I've downloaded a stock BD .ork file, and made tweaks to reflect the above changes. (I overrode the fin weights to reach the measured overall rocket weight since a don't actually know the individual fin weights). I've posted the modified .ork file below for reference. With an Estes E engine, I'm seeing 0.437 cal stability. Now, I've been reading other threads about cal ranges to aim for, and it sounds like less than 1 cal is usually undesirable, and I'm way below that. However, I've also read that short, stubby rockets like the Big Daddy can operate fine with lower cal values due to their base drag. Can anyone who has experience with Big Daddies chime in? Do I have an unstable and/or underpowered rocket on my hands? Simulations also show quite low off-the-rod velocities (9-12 m/s), so I'm also a bit concerned about that. Thanks for any advice or input!
 

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DuctTapeandRocketFuel

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Yes, short, stubby rockets can get away with less than 1 cal of stability. I modded a Big Daddy for 29mm motors, and had to add several ounces of BBs to achieve good stability. My approach was to take the finished rocket with the motor ( for you, the E motor ) and do a balance test. Your simulation will let you know the Center of Pressure (CP), and the balance test will help you determine your Center of Gravity (CG). For a 3 inch rocket, I would want about 0.5 cal or 1.5 inches forward of the CP for your rocket to be stable. Add nose weight to achieve this. Just my 2 cents.
 

JimJarvis50

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I have flown Big Daddy's for quite a while. I have several of them modified for 29mm motors and one for 54mm motors. I think they can fly on as little as 0.5 calibers, but I have typically flown them at around 0.7 to 0.8 calibers, and they fly fine.

Jim
 

Spitfire222

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Thank you for the input. I will double check the CG of the rocket loaded with an E engine and compare it to the simmed CP value. My remaining concern is the liftoff velocity being too low, since I'm using the stock 24mm motors. Both of you used 29mm motors, so presumably any extra weight was negligible, and the rocket was able to come up to sufficient speed off the pad that the four, large fins could provide adequate stabilization. The flying fields I have access to for now here in the mid-Atlantic region are not that large, so I don't want my Big Daddy pretending it's a scud!
 

AfterBurners

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Hi everyone,

I'm just finishing up building an Estes Big Daddy. I know there are many threads about its stability, and I’ve read several of them. Though I know it would add some weight, I've made some minor modifications to improve resistance to wear and also practice some techniques for future higher-powered rockets.
The mods include:
1. Cutting off nosecone taper and replacing with 1/8" ply bulkhead with eye bolt
2. Replacing cardboard centering rings with 1/8" ply, with the fwd one having another eye bolt
3. Fiberglassing fins with 0.75 oz cloth

So far, the empty weight of the rocket is 257g, as measured by a digital kitchen scale. I know some may say these mods are pointless and these rockets should just be built stock, but I enjoy this process of minor modification, etc. In any case, the rocket is built now.

I'm also using these mods to get started learning how to use Open Rocket. I've downloaded a stock BD .ork file, and made tweaks to reflect the above changes. (I overrode the fin weights to reach the measured overall rocket weight since a don't actually know the individual fin weights). I've posted the modified .ork file below for reference. With an Estes E engine, I'm seeing 0.437 cal stability. Now, I've been reading other threads about cal ranges to aim for, and it sounds like less than 1 cal is usually undesirable, and I'm way below that. However, I've also read that short, stubby rockets like the Big Daddy can operate fine with lower cal values due to their base drag. Can anyone who has experience with Big Daddies chime in? Do I have an unstable and/or underpowered rocket on my hands? Simulations also show quite low off-the-rod velocities (9-12 m/s), so I'm also a bit concerned about that. Thanks for any advice or input!
Nice looking rocket!
 

o1d_dude

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Adding nose weight is not the difficult task it appears to be if you have never done it.

I have added nose weight in the form of washers to the nose cone eyebolt. Not particularly efficient but it does work.

I have also added nose weight by mixing lead shot into epoxy and pouring it into the nose cone...scuffing the nose cone interior to provide a “grabby” surface is recommended. I’ve even drilled holes and inserted bamboo skewers as anchors (very work intensive).

A simple way of determining the amount of ballast needed is to load up the rocket into launch configuration (motor, recovery, chute, etc.) and then take small/tiny “filmy” plastic bag filled with ballast BB’s and tape it to the location on the nosecone where it will reside. Check your new CG and adjust as needed. No epoxy is involved at this step but the permanent mounting with epoxy will add additional stability.

Another thought: Pour the BB’s into a plastic cup and weigh it. Add the epoxy and weigh it again. Remove mix from the cup until it weighs the same as the initial measurement. Now you’re good to go.

Edit: Don’t use 5 minute epoxy. 15 minute may work, 30 minute is better.

Don’t overthink this.

There are many paths to rocket nirvana.
 

Spitfire222

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Adding nose weight is not the difficult task it appears to be if you have never done it.

I have added nose weight in the form of washers to the nose cone eyebolt. Not particularly efficient but it does work.

I have also added nose weight by mixing lead shot into epoxy and pouring it into the nose cone...scuffing the nose cone interior to provide a “grabby” surface is recommended. I’ve even drilled holes and inserted bamboo skewers as anchors (very work intensive).

A simple way of determining the amount of ballast needed is to load up the rocket into launch configuration (motor, recovery, chute, etc.) and then take small/tiny “filmy” plastic bag filled with ballast BB’s and tape it to the location on the nosecone where it will reside. Check your new CG and adjust as needed. No epoxy is involved at this step but the permanent mounting with epoxy will add additional stability.

Another thought: Pour the BB’s into a plastic cup and weigh it. Add the epoxy and weigh it again. Remove mix from the cup until it weighs the same as the initial measurement. Now you’re good to go.

Edit: Don’t use 5 minute epoxy. 15 minute may work, 30 minute is better.

Don’t overthink this.

There are many paths to rocket nirvana.
Thanks for the input! I will check the CG and try to make sure any nose weight required is as little as necessary. I'll have to drill a sufficiently large hole in the bulkhead to get the ballast in and pour epoxy into. I pretty much only use 30 min epoxy these days, I'm not in any rush. We'll see how it goes
 

Flyfalcons

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CG something like 1/2" forward of the leading edge of the fin root, with motor installed, should be stable up to mach 1.
 

TonyL

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I have not been able to fit anything larger than a K in mine, but the fins fluttered at Mach 1.4 and ultimately had issues. They survived through burn out but the dwell on the coast down was apparently too much and they came off. Around a J may be the practical limit for plain balsa fins.

Keep in mind that a mixture of epoxy and BBs may exotherm depending on quantity of ballast and the epoxy type.

Also a reason short fat rockets are happy with lower static margin is that the CP changes more slowly with angle of attack than very long skinny rockets. I believe there is a newsletter article out there on this.

br/

Tony
 

jrap330

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Thank you for the input. I will double check the CG of the rocket loaded with an E engine and compare it to the simmed CP value. My remaining concern is the liftoff velocity being too low, since I'm using the stock 24mm motors. Both of you used 29mm motors, so presumably any extra weight was negligible, and the rocket was able to come up to sufficient speed off the pad that the four, large fins could provide adequate stabilization. The flying fields I have access to for now here in the mid-Atlantic region are not that large, so I don't want my Big Daddy pretending it's a scud!
You took a rocket that is not that heavy and made a 9 oz rocket, Estes Saturn V stock is 14 oz. Big Daddy is stable, reports were nose cone not coming off.......I lost my BD due to weak ejection charge. So glad you learn something but you took a great D flyer and are now limited to maybe E's and F's...
 

Spitfire222

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You took a rocket that is not that heavy and made a 9 oz rocket, Estes Saturn V stock is 14 oz. Big Daddy is stable, reports were nose cone not coming off.......I lost my BD due to weak ejection charge. So glad you learn something but you took a great D flyer and are now limited to maybe E's and F's...
Thanks for the input. Luckily, if this one doesn't work out well, I can/will build another Big Daddy and keep it stock. 👍

You can check CP with the base drag by adding a zero mass cone that is pi x dia. long and the same diameter behind the rocket. Check this thread.
https://www.rocketryforum.com/threads/short-fat-rockets.157644/post-1966108
Thank you for this, I'll read through the thread! This should be good practice for OR as well.
 

Spitfire222

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I finally received the video that someone recorded of my Big Daddy's first flight on an E12-4. The arcing over isn't actually as bad as I remember, but not vertical enough for this launch site (the rocket barely stayed out of the trees at field edge). The second flight was on an Aerotech E20-4W, and it was terrible: crazy skywriting until it hit the ground near the launch pad. Can anyone please help me troubleshoot this rocket's behavior? Does this sound like instability that can be solved with nose weight? Thanks for any help!
 

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jrap330

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I will repeat what every implies. You added weight. You use a stock file and made changes. Did you compared the stock file vs your modified one and see how far the CP/CG relationship is. Measured you NEW CG and see if it is real close to the original CP.? And does open rocket simulation for your modified version show a good flight profile.

In the end you took a great D flyer and change it for High Power modeling skills. So look at your fligt profile and see if it is too slow off the pad....otherwise add a little weight. I love the Big Daddy until weak ejection charge cause it to be destroyed. It really flies great, straight and true on an Estes D12-3.

Sorry, after the video I say you had a stable flight...so a more punchier engine should do the job......
 

jrap330

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I will repeat what every implies. You added weight. You use a stock file and made changes. Did you compared the stock file vs your modified one and see how far the CP/CG relationship is. Measured you NEW CG and see if it is real close to the original CP.? And does open rocket simulation for your modified version show a good flight profile.

In the end you took a great D flyer and change it for High Power modeling skills. So look at your fligt profile and see if it is too slow off the pad....otherwise add a little weight. I love the Big Daddy until weak ejection charge cause it to be destroyed. It really flies great, straight and true on an Estes D12-3.

Sorry, after the video I say you had a stable flight...so a more punchier engine should do the job......
Now you say the E20-4W was crazy...how much heavier is the E20-4W than an E12-4.....if the next "punchier" engines are heavier than YES add some weight to account for it.
 

neil_w

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E20 is about .2 oz heavier than the D12, and .4 oz lighter than the E12. I am pretty surprised that the E20 would skywrite.
 

Spitfire222

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First off, I've mentioned before that I've learned my lesson about modifications and that weight addition and distribution has more of a practical effect than I initially believed.

After the first flight with the E12-4, I thought the rocket was stable enough, but a bit underpowered due to its heavier weight. This is why I tried an E20-4 next: I figured it needed a bit more "oomph" to get it going straight up. The skywriting was a surprise and disappointment; I didn't think a slightly heavier motor would throw the stability off that much! I'm tempted to think that another attempt with an E12-4 would work if I used a longer launch rod on a completely calm day.

I did indeed modify the stock Big Daddy Open Rocket file to mimic the modifications I made to my model. Surprisingly, the calculated stability is higher (see attached images), probably due to the trimming of the nose cone shoulder and the new bulkhead + eye bolt. I'm going to re-measure the CG in a few configurations: empty, E12, and E20 at least.

In the end, I just want a rocket that flies safely and reliably. If that means I'm cornered into only using only one specific motor (i.e. E12) then so be it. I've already cut a hole in the bulkhead to add nose-weight, but I wanted some advice before I did. I have 7g lead weights that are used to balance RC aircraft. Two of them (14g) brings the stability to the ~0.9 range. But then I wonder if it would shift the rocket to be a good flier with E20s only, and no longer work with E12s.

Sorry, I'm still learning! Thanks for the help/replies so far!
 

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neil_w

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After the first flight with the E12-4, I thought the rocket was stable enough, but a bit underpowered due to its heavier weight. This is why I tried an E20-4 next: I figured it needed a bit more "oomph" to get it going straight up. The skywriting was a surprise and disappointment; I didn't think a slightly heavier motor would throw the stability off that much!
Again, the E20 is considerably *lighter* than the E12. That's why your result is so puzzling. It should have gone faster and straighter than the E12.
 

Spitfire222

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Again, the E20 is considerably *lighter* than the E12. That's why your result is so puzzling. It should have gone faster and straighter than the E12.
Ah yes, my mistake, I misread your post. With the E20, it did indeed launch fast off the pad, but almost instantly went crazy. The first flight on the E12 at least showed that it can fly relatively straight. Maybe that second flight was an anomaly....?
 

Mike Haberer

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I flew a stock Big Daddy w/o any extra nose weight many times with no issues. Flew it only on D-5s. The only reason I'm still not flying it is because I lost it in a tree....
 

jrap330

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Sorry for being so hard on you., since I never modified a rocket( except Sizzler) and have not gotten beyond a D12 engine. As Neil states puzzling. In the end whatever engine works w/o losing the Big Daddy is the end result.
 

Philip Tiberius D.

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I have flown Big Daddy's for quite a while. I have several of them modified for 29mm motors and one for 54mm motors. I think they can fly on as little as 0.5 calibers, but I have typically flown them at around 0.7 to 0.8 calibers, and they fly fine.

Jim
Estes Big Daddy?!? On a 54mm !!!!
 
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