AGM-12C Bullpup B

BigMacDaddy

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I posted about this in the what I did today rocket-wise thread but decided to post a bit more of my process since I had not designed a model by measuring pictures in a while. I could not find a good blueprint, plan, or dimensions for this rocket so needed to measure parts.

I first search extensively for prototype pictures. Looking for one that is square to camera (and has fins square to camera). I prefer to find a clear picture of an actual rocket but the measurements will be distorted if the picture is not square (or if the camera person was too close to the rocket). Given this I sometimes need to rely on pictures of the model or diagrams. Some examples:

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I use Inkscape to measure one picture. It has a convenient ruler tool that returns results in pixels. You can also measure multiple pictures if different parts are clear in various pictures but need to anchor your measurements on something common so that you can adjust and compute sizes. I usually anchor on main body diameter.

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I enter all my measurements in an Excel spreadsheet. You basically need to set the dimensions to the scale appropriate for the model you are building. In this case I wanted to make the main body tube a BT-80 tube so I enter the OD for that body tube into the table next to all the measurements. I then take a proportion of the measured dimension to the actual BT-80 dimension. 66.04 / 75.3 = .877% So basically I multiply all my measured dimensions by that ratio/percentage. This gives me the numbers in the right column. Note: I measure fins the way I design them (a square with triangle cutouts) but you can also measure fins with points that help you enter them into OR or RockSim.
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I then use these dimensions (rounded to reasonable numbers) to mock up the main elements to see how realistic the rocket is to make. I replace any cylinders with standard body tubes (I am willing to make compromise to use a cheap standard body tube rather than custom ordering something that is more exact), design simple 3D parts, replace fins with 2mm elements that represent the plywood fins I will use, etc...

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BigMacDaddy

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Once it seems realistic to build, I make the main parts hollow (I use cutouts that are 2.5 or 3.5mm smaller than the main body to create a 3 or 4 wall thick side around the object) and I design in shoulders/coupler parts, shock chord attachments, motor mounts, etc... I also have to break up parts that are either too large to print or that will not print well (or without a ton of supports).

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I also look for a number of good quality pictures of the detailed parts so that I can add some details to the parts that will be 3D printed. These are some of the better ones I found.

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I also found a ton of pictures of this rocket but it seems to be modified, rebuilt, or parts were recreated (or it has a ton of coats of paint on it). Not sure why this rocket is so rough. There are actually not many good pictures of this variant of the ever-popular Bullpup.

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Some great pictures of a 3D design are out there but I usually rely on these last since I am not sure how much creative license was taken.

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Same thing goes for models -- some beautiful models out there (although OOP and not too many versions).

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Anyway, then I start doing test prints. Testing fit, look for print issues, etc...Still need to design the detailed parts for the main fin mounting brackets.

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Finally, I setup fins for cutting on the CNC - not done with the main fins here yet but I have to go teach...

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afadeev

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Let me know if you need any extra measurements - I have two Launch Pad kits sitting in my build pile (3.75" K006 and 2.6" / BT-80 K005 versions).

I don't know how accurate those kits are, but they are way more available then the source model ;-).

a
 

BigMacDaddy

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Let me know if you need any extra measurements - I have two Launch Pad kits sitting in my build pile (3.75" K006 and 2.6" / BT-80 K005 versions).

I don't know how accurate those kits are, but they are way more available then the source model ;-).

Thanks so much for the offer. Those kits seem to be pretty high demand from what I was reading. Seems like this is still a pretty uncommon model rocket! In the end, I wonder how much more the 3D printed version will weigh than the BT-80 kit version.
 

afadeev

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Those kits seem to be pretty high demand from what I was reading. Seems like this is still a pretty uncommon model rocket!

I can only speak for myself, but I'm a big fan of BullPup's excessive number of fins, transitions, and the tail cone. Funky shape of the aft fins only adds to the uniqueness of the shape.

In the end, I wonder how much more the 3D printed version will weigh than the BT-80 kit version.

It will certainly require a lot more nose weight, especially if you will be 3D-printing the tail cone.
I would consider modeling everything in OR to figure out the required nose cone weight, and then printing attachment points for ballast placement in the NC.

Anything you could do to reduce the tail cone weight (re-use parts of butchered plastic nose cone for main shape) would aid stability of the model with minimum additional ballast and overall weight.

a
 

BigMacDaddy

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I can only speak for myself, but I'm a big fan of BullPup's excessive number of fins, transitions, and the tail cone. Funky shape of the aft fins only adds to the uniqueness of the shape.

It will certainly require a lot more nose weight, especially if you will be 3D-printing the tail cone.
I would consider modeling everything in OR to figure out the required nose cone weight, and then printing attachment points for ballast placement in the NC.

Anything you could do to reduce the tail cone weight (re-use parts of butchered plastic nose cone for main shape) would aid stability of the model with minimum additional ballast and overall weight.

Thanks for comment -- made me think about tail cone before printing. I did lighten things up a bit but also moved engine forward around 15mm - should move CG forward a bit and give a little GDS boost to stability if it does not melt the tail cone (I don't think it will). Although looking at this again I worry that it might restrict the engines ability to fire if the tail hole is too small..

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EDIT: I am probably going to go with the safer motor setup with motor sticking out the rear. Other than possible GDS effect the placement of motor has surprisingly little impact on CG / stability.

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BigMacDaddy

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I would normally (and I recommend) modeling in OpenRocket (or Rocksim if your hobby budget is larger than mine) earlier in the process. However, I thought this rocket was pretty safe and would be stable. Mostly true - however, this needs a surprising amount of weight in nose cone to be stable -- around 100-115g. I guess those forward placed reverse swept fins push CP forward. I am definitely going to need to put something inside to be sure the parachute does not slide to the rear during launch.

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p.s., made a couple of small compromises above. Body tube is around 1.6cm longer than my measurements (that is a full-size Estes BT-80 tube and nice to not have to cut it). Also moved fins back 1.5 or so cm. Both of these reduce the nose weight needed by a fair amount (20-30 or so grams less).

Would be nice if I knew I could recess to motor to have a bit of GDS help and reduce nose weight...
 

rharshberger

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I would normally (and I recommend) modeling in OpenRocket (or Rocksim if your hobby budget is larger than mine) earlier in the process. However, I thought this rocket was pretty safe and would be stable. Mostly true - however, this needs a surprising amount of weight in nose cone to be stable -- around 100-115g. I guess those forward placed reverse swept fins push CP forward. I am definitely going to need to put something inside to be sure the parachute does not slide to the rear during launch.

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p.s., made a couple of small compromises above. Body tube is around 1.6cm longer than my measurements (that is a full-size Estes BT-80 tube and nice to not have to cut it). Also moved fins back 1.5 or so cm. Both of these reduce the nose weight needed by a fair amount (20-30 or so grams less).

Would be nice if I knew I could recess to motor to have a bit of GDS help and reduce nose weight...
in order to get Gas Dynamic Stabilization benefits, there needs to be ports for air to flow through in the sides of the rocket. Pushing the motor deeper into the airframe simply moves the CG forward normally, but pushing it too far forward can lead to bernoulli lock, krushnic effect, and or "hibachi effect" where the inside of the airframe gets BBQ'd by the exhaust, so lots of tradeoffs to consider.
 

BigMacDaddy

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in order to get Gas Dynamic Stabilization benefits, there needs to be ports for air to flow through in the sides of the rocket. Pushing the motor deeper into the airframe simply moves the CG forward normally, but pushing it too far forward can lead to bernoulli lock, krushnic effect, and or "hibachi effect" where the inside of the airframe gets BBQ'd by the exhaust, so lots of tradeoffs to consider.

Thanks you -- I definitely think the boattail is more likely to restrict airflow and thus reduce the power of the motor. However, and I am totally new / learning naively about GDS, the argument is that a recessed motor does create some GDS impact.


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CTimm

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Some cruddy pics.
I have dimensions somewhere but have to dig them out.
 

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CTimm

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More cruddy pics
 

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CTimm

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Last batch.
 

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BigMacDaddy

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Some cruddy pics.
I have dimensions somewhere but have to dig them out.

Wow -- these are fantastic. These are the best and most detailed pictures I have seen. Thank you so much for sharing. I am going to update a couple of my 3D printed parts before I print them!!!
 

BigMacDaddy

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Amazing how quickly a rocket can come together when it does not break too many assumptions of model rocketry (i.e., a model that can be accurately simulated in Open Rocket). Some glue still needs to dry, motor mount as well as nose cone sections still need to be glued together, and nose weight needs to get added but pretty close to done. Can I say again how much I love the CNC for cutting out fins.

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