use of pseudo-fin for stabilizing small-finned scale model

[hetzer]

I am trying to build a 3d printed v2 but the scale size of fins require too much nose weight. In order to reduce nose weight I plan to place the engine recessed and use holes at the tail section to decrease Krushnic Effect.
Orange area is the engine mount.
I read the apogee newsletter but confused about design of holes to allow ambient air to enter the body tube.
Attached images show the design idea and asking help of experts to make it work.

 

[dhbarr]

This design will absolutely melt / burn. What size / weight are we looking at here?

 

[hetzer]

I did a couple of tries with 3d printed rocket and rocket parts. So far, even PLA resisted the abuse. Initial design is 350mm long to be powered with a 24mm engine. Total weight will be around 130-160 grams.
Below is the link for pla printed cluster tail for sky eagle.

 

[RocketT.Coyote]

Can clear plastic flight fins be added? These could be made from plastic packaging or cannibalized from a cheap picture frame.

 

[Nytrunner]

I did a couple of tries with 3d printed rocket and rocket parts. So far, even PLA resisted the abuse. Initial design is 350mm long to be powered with a 24mm engine. Total weight will be around 130-160 grams.

But have you tested a part where the hot exhaust flow is streaming past the plastic (as in the holes on your boattail)?
Very different thermal environment than two motors sticking out of a printed part.

Not saying it won't work, but it's something to consider (plus the use of a better plastic like PETG)

 

[BABAR]

To avoid Krushnic effect I think your holes are going to have to be well ANTERIOR to the nozzle of the engine, not simply lateral to the engine jet. Looking at your design, I don't think you have nearly enough "hole-age" nor adequate forward positioning of the holes themselves to rescue your engine thrust.

Take a look at Lil Augie, a chad staged rocket with a large lower outer tube.

https://www.rocketryforum.com/threa...-i-got-8-so-where-are-the-first-seven.141208/

 

[hetzer]

First try with a 30cm model was half success.
Model was lost due to wind and could not inspect the tail.

 

[BABAR]

Soooooo, was it stable off the pad?

Did you see a chute deploy?

 

[burkefj]

Where are you assuming your cp is located? You may be assuming too much nose weight due to conservative cp location.

 

[hetzer]

I have identified cp location both using flowsim and silhoutte and center of mass method.
I have tested the first prototype in a windy day. Result was half success. Minimum thrust loss but could not recover the model to see effect of heat.

 

[burkefj]

So I'm just poking a bit on your thought that moving the motor forward is really saving significant nose weight. It is a 24 mm motor, you've moved it forward maybe an inch and a half or so, that motor weighs maybe 45 grams, given where the CG should be which should be just slightly forward of where the front of your motor mount currently is, that means you only reduced your nose weight requirement around 10-15 grams due to the longer moment arm of the nose weight location, did you really save enough to make it worth the trouble?

 

[hetzer]

I used a d12 weighing around 70 grams. Total length of rocket was 300mm and I did not have chance to benefit from a longer moment. I agree with the trouble part but wanted to experiment a recessed setup.

Also added calculated CP Location

 

[burkefj]

Understood, but a d12 from Estes weighs 39-45 grams not 70, you show a cg symbol and call this cog.png, if this is cg that looks about right where i said it should be. I still think your nose weight reduction is not that significant, in this case but if you get this to work as an experiment what the heck. I do a lot of tradeoffs like this on my rc rocket glider kits and tend to simplicity if the gain isnt significant. Im not sure if you were using a non estes motor.

 

[hetzer]

I have access only to a local manufacturer and D12-6 engine he produces weighs 70grams. Cog.png shows the center of mass in a solid made using silhoutte of actual model which happens to be also CP of actual model.

 

[burkefj]

Ok, if the motor weighs 70 grams then than will help reduce an ounce or so, I still think the CP is conservative based on my other modeling of V-2's, your location is closer to my flight CG's.