"Marvin Martian Jr."- Fin Recommendations/ Suggestions

I'm just starting the planning for a 1/2 scale version of the WAY COOL rocket that was launched at LDRS 38- "Space Oddity". My rocket will be about 5 1/2 feet tall, 24" at its widest, and will weight somewhere south of 45 pounds. I'm expecting it to fly to only 1,500 feet or so with an "L" 75mm reload.

The stability of the original rocket was a bit iffy, so I've designed my fins to exceed the maximum diameter of the rocket by about 8 1/8". As the attached pic shows, the portion of each of the three fins that will extend past the body diameter is 8 1/8" x 24" (right of the red vertical line). Not sure if I can mush the overall square inches of the exposed fin together, but it approximates to 8" x 12" of a rectangular fin in the airflow. Haven't simed the design yet, but does your gut tell you that these larger fins should solve the stability issue of the original rocket?

First decision is: 1/4" Baltic Birch plywood, or 3/8"? I'd love to keep the weight down, but don't want fin flutter. Your thoughts?

I plan to fiberglass the fins to the 3.9" Blue Tube. I also will place a 11.4" to 3.9" centering ring in top of the fins, and sandwich the fins with maybe 1" square wood stock on either side. I would screw that sandwich together, and also screw through the centering ring into the wood sandwich. I can expect no support from the external body of the rocket, so this fastening will need to be sturdy. Your thoughts/ suggestions?

On the big fella IIRC the forefins weren't flyfins.

AFAIK there was a rail binding issue which caused some of the initial tipover -- your rail solution will be pretty important.

I have a really simple rail solution, which I used on my McDonnell Douglas "Delta Clipper" successfully a couple of years ago. I epoxy a "rail" tube, which fits the 1515 rail, to the internal 4" body tube. Hole at the bottom of the rocket, as well as the top, to allow the rail to pass through. The whole rocket slides down the rail. Worked perfectly, with no chance of binding.

The ldrs project was great looking. How are you going to make yours fly a little better.

How about building a small lightweight model to confirm before commiting an L motor, and ditch the forward fins. building fin boxes internally for the fins to slide into is probably a good idea. I don't know with this heavy of a model on the male motor if you're going to have to worry about fin flutter that much. What construction are you using that is going to weigh 45 lbs?

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I've seen two versions of this rocket fly, both had similar flight profiles. I don't think rail binding was the issue.

-Kevin

I think that too little of the fins on the Big Marvin were actually in the airstream. When the body diameter reduced from it's maximum diameter, I don't think that the fin surfaces inside that maximum width were grabbing any air. Since I'll have in essence three 8" x 12" sized fins in the airstream, I think that will be enough to keep it stable. But I'll soon try out the design in RockSim to confirm. There were similar concerns voiced with my pyramid-shaped Delta Clipper, but that 45 pound rocket flew beautifully.

At this point, I really have no idea what the final weight will be. Most of the internal shape will be formed with 1" foam board, with a fiberglass skin. I'll have a 24" round plywood piece in the center on which to mount two dual deploy altimeters. Did something similar with my Delta Clipper- just have to figure out how to manage it with the round shape. Will keep the sampling holes in the airstream- not below the maximum 24" diameter, which is dead air space.

I don't plan on making the front mini fins. I have found some 5" hollow architectural hemispheres that i can load up with a lot of nose weight if necessary. I'm using 1/2" all-thread rod with a massive 1/2" lifting bolt. A 2.56" body tube, which will be inside the nose structure, can also take nose weight if needed. That's where i think my additional weight may be needed, if the fin end proves to be heavy because of the massive plywood fins. And that's why i was leaning toward the 1/4" Baltic Birch Plywood instead of the 3/8",

Have you considered using the 3/8" with some strategically placed holes to reduce weight and then skinning them?

Yes- looked at the build photos from the full sized one, and noticed that construction technique. Frankly, it's fun to consider all these possibilities, and to weight the pros and cons. Probably the biggest Con is that I'm a lousy fiberglasser....lol.

Keep the suggestions and ideas coming, Guys!

Now I need to figure out how to design this complex rounded fin in RockSim.......I'm stumped for now. Any direction would be appreciated.

Also looks like a LOT of nose weight will be a key for stability. For my initial and very rough RockSim design, with fins approximating the square inch area of the real fins exposed to the airflow, but not looking at all like the "Marvin" fins- I'm at about 15 pounds of nose weight for a 43 pound rocket, in order to nudge it into a stable state.

Still a LOT of refining work to do on the RockSim design.

Did a little math for the area of an arc segment of a circle, and the square inches of each fin exposed to the airflow would be 145. That equates to roughly a 12" x 12" fin. I may make the fins even a bit larger.

How are you modeling your version of this rocket?

Where does one get the notion that rail binding was the issue? The rail had 1/2" of slop in each of the guides - and onboard/pad footage does not indicate any rail binding. It simply wanted to go sideways.

I suppose one can argue that the desire to go sideways could be a rail binding issue, but it could not until it cleared the rails.

jmmome said:

Now I need to figure out how to design this complex rounded fin in RockSim.......I'm stumped for now. Any direction would be appreciated.

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Before we sent off the model for a true CP analysis using some supercomputers that none of us really have access to, I did a preliminary simulation in openrocket using 1" tall steps. I think I modified to 1/2" when the curvature became a bit too severe for the steps to be visible.

You can also just model using transitions in openrocket. Just because it's not 100% smooth doesn't mean you won't get somewhat close.

The simulations that were done on the supercomputers stated that we'd see stability around 100-250 fps. Above 250 or so, it became unstable again. We hit about 325, I think. But the reason we think it tipped was because there was too much weight to one side of the rocket (cargo parachutes are heavy! ~85 lbs of chute), causing it to tip. The onboard video would agree with this. I really need to post that.

mrwalsh85 said:

Above 250 or so, it became unstable again

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Quite fascinating! Above that range the air would separate and become turbulent, reducing the fin effectiveness?

mrwalsh85 said:

The onboard video would agree with this. I really need to post that.

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YES.

Yes, you need to post that video.

I did a quick & dirty design on Rocksim with 1" transitions. The curved fin shape still stumped me (I have NO idea how to create it in RockSim) so I used the same square inches of fin area that would be outside the 24" max. body diameter, with a square fin shape (a BIG assumption).

I have no parachute cannons in my 5 1/2 feet tall & 45 pound (approx.) version, so it was stable once I added enough nose weight to the two hollow metal hemispheres that will be joined together to replace the bowling ball on the Kaboom Krewe's rocket. If I underestimate the weight of the rocket as it is built, I'll simply add more nose weight to the hemispheres before epoxying and sealing them together. I can get a lot of BB's and epoxy into a 5" hollow sphere.

The nosecone section (all of the red nosecone portion) and rocket section will descend on their own chutes.

I'm using a 48" long internal Blue Tube epoxied to the 4" skeleton Blue Tube as the rail guide for a 1515 rail. That method worked perfectly for my pyramid-shaped McDonnell Douglas "Delta Clipper" in 2016.