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
 
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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.
 
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