Fat Man Atomic Bomb Semi-Scale Model Rocket (3D printed + internal tubes)

[BigMacDaddy]

I needed another challenge so decided to take a crack at a 3D printed Fat Man Atomic Bomb design. This is REALLY short and fat. BT-80 tube for size comparison. Really difficult to figure out the aerodynamics of this since the boxtail is mostly closed with some angled internal fins. Do you all think it is appropriate to use the base drag hack to model this in Open Rocket?





Base drag cone has a diameter of the small diameter of box tail and is 3x the length of that diameter (per Apogee rules of thumb). You can basically delete all the fins and CP stays in about the same place - actually moves back even further.

Without base drag hack moves forward quite a bit (as you would expect)...

 

[neil_w]

I would say no. Base drag will be very low in that rocket due to the tapered tail.

 

[BigMacDaddy]

I would say no. Base drag will be very low in that rocket due to the tapered tail.

Did you see how small the hole is in the boxtail? Opening is about 6cm across in middle (with conical boattail inside it) while full boxtail is 12cm across. Just wondering if that factors into your mindsim...

I actually wonder if the tapered internal fins will draw in more air and force it through that small hole (with motor pulling it through and pushing it out that back).

 

[neil_w]

Did you see how small the hole is in the boxtail? Opening is about 6cm across in middle (with conical boattail inside it) while full boxtail is 12cm across. Just wondering if that factors into your mindsim...

It did not. Now that I look at the pictures more closely (man that fin box is weird) I conclude that I have no idea how it'll behave. There will be plenty of base drag, though, as you correctly point out. That whole thing is going to be unbelievably draggy.

One possible way to handle the base drag is to calculate (or measure) the total area of the back of the fin box that blocks the air, and then calculate an equivalent circle with the same area. Then use the diameter of that circle as the basis for your base drag CP correction. But really that's just a guess.

I actually wonder if the tapered internal fins will draw in more air and force it through that small hole (with motor pulling it through and pushing it out that back).

Those are the sorts of things that I don't even try to reason out, because I have no reason to trust my intuition. Short of a wind tunnel or CFD analysis, it's a complete unknown as far as I'm concerned.

 

[BigMacDaddy]

It did not. Now that I look at the pictures more closely (man that fin box is weird) I conclude that I have no idea how it'll behave. There will be plenty of base drag, though, as you correctly point out. That whole thing is going to be unbelievably draggy.

One possible way to handle the base drag is to calculate (or measure) the total area of the back of the fin box that blocks the air, and then calculate an equivalent circle with the same area. Then use the diameter of that circle as the basis for your base drag CP correction. But really that's just a guess.

Those are the sorts of things that I don't even try to reason out, because I have no reason to trust my intuition. Short of a wind tunnel or CFD analysis, it's a complete unknown as far as I'm concerned.

Thanks - I was thinking something similar about the total area of blocked boxtail for base drag hack.

 

[aerostadt]

The 3-D printing looks really cool. However, the proof is the flying.

 

[Hobie1dog]

just fly it

 

[terryg]

I would think that it would just pinwheel before it could get up to speed.

 

[teepot]

Go for it. I think it will work fine.

 

[Cape Byron]

Shedloads of base drag. The original was designed to orient itself chubby side down.

 

[ghostfather]

I flew something similar, also 3D printed. It was the mini-nuke from Fallout. It had a 2 grain 38mm motor tube, and a 98mm interior airframe to hold the parachute and such. I had to add a lot of nose weight. I painted it dark green with lots of rust spots and peeling paint, like someone might find in the game Fallout 4.
I made an old ramshackle wooden crate for it, with military looking stencils and a big radioactive symbol. and left it out on my table when I went to a launch. People were curious of course, but got a good laugh when I would pretend to pry off the lid, and they see the rocket.

I found the 3D files somewhere on line, and scaled them to fit the 38mm motor tube.

 

[lakeroadster]

Nice! Very cool rocket, err uhm, bomb.

Should be woefully underpowered on a D motor since it's plowing through a lot of air. 29mm composite would likely be a more energetic choice.
The "California Parachute" boxed tail should make it air worthy.

Looking forward to the video.

 

[terryg]

View attachment 623863

I flew something similar, also 3D printed. It was the mini-nuke from Fallout. It had a 2 grain 38mm motor tube, and a 98mm interior airframe to hold the parachute and such. I had to add a lot of nose weight. I painted it dark green with lots of rust spots and peeling paint, like someone might find in the game Fallout 4.
I made an old ramshackle wooden crate for it, with military looking stencils and a big radioactive symbol. and left it out on my table when I went to a launch. People were curious of course, but got a good laugh when I would pretend to pry off the lid, and they see the rocket.

I found the 3D files somewhere on line, and scaled them to fit the 38mm motor tube.

View attachment 623864

How well did it fly?

 

[BigMacDaddy]

View attachment 623863

I flew something similar, also 3D printed. It was the mini-nuke from Fallout. It had a 2 grain 38mm motor tube, and a 98mm interior airframe to hold the parachute and such. I had to add a lot of nose weight. I painted it dark green with lots of rust spots and peeling paint, like someone might find in the game Fallout 4.
I made an old ramshackle wooden crate for it, with military looking stencils and a big radioactive symbol. and left it out on my table when I went to a launch. People were curious of course, but got a good laugh when I would pretend to pry off the lid, and they see the rocket.

I found the 3D files somewhere on line, and scaled them to fit the 38mm motor tube.

View attachment 623864

Looks very cool!

I'd like to be able to launch on 24mm engine but there is a chance that a 29mm engine would be better and allow more nose weight.

 

[BigMacDaddy]

I would think that it would just pinwheel before it could get up to speed.

I think I will use 1010 rail guides so I can launch on my 8' long 1010 rail. OR suggests this gets it almost 50% more speed on launch rod exit.

 

[neil_w]

Where are you going to mount the guides?

 

[BigMacDaddy]

Where are you going to mount the guides?

I was thinking of mounting 1010 rail guides on outside of boxtail and outside of bomb body. Bit off the centerline so not sure if that will be an issue but will give me a longer runway.

Alternatively, I could do a rod-style hole closer to center-line, outside BT-80 tube... Would just need to drill these in my prototype.

 

[jmasterj]

Looks very cool!

I'd like to be able to launch on 24mm engine but there is a chance that a 29mm engine would be better and allow more nose weight.

AT 24/60 motors might be your best friend for this bad boy.

 

[BigMacDaddy]

Successful flight yesterday for the Fat Man A-bomb Model Rocket on a E12-4. I forgot to take a picture of this Big Chungis on the launch rail.

It flew pretty well - although one mid-air 30 degree or so angle adjustment and then continued to fly straight (that was into the wind so maybe it windcocked a bit???). Anyway, flew quite well for a bowling ball-shaped rocket...

Here are a couple of angles on the flight:
View #1: https://youtube.com/shorts/Zjg2uELT5hU
View #2: https://youtube.com/shorts/FoiNbsTQh6Q

 

[BigMacDaddy]

I was thinking of mounting 1010 rail guides on outside of boxtail and outside of bomb body. Bit off the centerline so not sure if that will be an issue but will give me a longer runway.

Alternatively, I could do a rod-style hole closer to center-line, outside BT-80 tube... Would just need to drill these in my prototype.

I wound up doing the 1010 rail guides on the outside of the model and did not see any issue with smooth movement on the rail.

 

[Hobie1dog]

Thanks for the videos. Are you going to try a little bit bigger motor Next time?

 

[BigMacDaddy]

Thanks for the videos. Are you going to try a little bit bigger motor Next time?

Maybe -- E12 is about the largest motor I usually launch with but I do have some bigger composite ones on the way.

 

[BigMacDaddy]

I only put 2oz of weight in the nose of this guy. It is so short/stubby that it seemed like it would take a ton of weight to really move the CG so just decided to try it with a little weight (basically enough to offset an E engine I guess) - kinda winging it, I guess I have had decent luck with 2oz of nose weight in lots of marginally stable rockets.

I also assume, based on these results, that the base drag hack was appropriate -- at least this seems stable to me (not sure what caused the turn mid-flight). I did revise the simulation to use a smaller base-drag cone based on the actual closed part of the boxtail. However, I think that the motor is pulling air through the hole in the boxtail so there should be some induction stability as well.

 

[aerostadt]

Can you show the simulation stability, if you turn off the base-drag?

 

[BigMacDaddy]

Can you show the simulation stability, if you turn off the base-drag?

Sure, actually it was hidden there as the last simulation in the original post -

 

[aerostadt]

So, that is interesting. Even without the simulated base drag the model could be considered to be stable, although it is hard to say what one caliber of stability is in this case. The simulated base drag creates about another 2.3 cm of aft movement for the CP. What, again, is your simulated base drag? (I see the triangle.)

 

[BigMacDaddy]

So, that is interesting. Even without the simulated base drag the model could be considered to be stable, although it is hard to say what one caliber of stability is in this case. The simulated base drag creates about another 2.3 cm of aft movement for the CP. What, again, is your simulated base drag? (I see the triangle.)

I subtracted the square air inlet hole from the total base of the boxtail (I may have added back the actual ring of the motor mount but I cannot remember). Basically I used the blocked area as a guide and created a base-drag cone based on that. The base drag cone in the final model is 7cm across and 21cm long (I usually do 3x length to be conservative instead of 3.14x).

 

[BigMacDaddy]

Someone reminded me that the diagonal internal fins were called a "California Parachute" (this also came up in my Little Boy Atomic Bomb thread) and were designed to slow down the model. I bet it would fly as well or better without them but wanted to stay closer to scale and I have never seen a picture of the Fat Man without those internal "vanes" / diagonal fins.