Clear HPR Fins?

I've seen fins made of acrylic used in LPR and MPR but never in HPR.

I apparently do more designing than building now days. I've got two designs I'm working on that would visually benefit from clear fins. The only material that I'm aware of that would be strong enough is Lexan (polycarbonate).

It's definitely strong enough. But is it too flexible? And if it's viable, what would be strong enough to bond it to blue tube or fiberglass airframes?

First off how fast do you plan to push this rocket? If its less than Mach 1. I'd say anything ranging from hobby epoxy to rocketpoxy or even Aeropoxy.

If your going fast, you can bet they will melt and get destroyed.

I have little experience with building either clear fins or anything meant to fly on more than an I, but I do know Lexan is heavy. As I understand it, both polycarbonate and acrylic have glues that will weld two pieces into one effective piece, so I suggest testing out building polycarb or acrylic fins much like one would build model airplane wings - rib and skin. I've built plywood fins for MPR/HPR this way on a 4" Astrobee 1500 and they are incredibly stiff for their weight and the materials used. So stiff and strong that the next version gets thinner wood throughout. You may need to use something besides the plastic to mount the fins, as any clear tabs may not take flutter well, however.

I have used 3/16" polycarbonate clear fins on my Gemini-Titan and my 1/12 scale Atlas Mercury. For the Titan the fins were through the wall and epoxied in place. The model has flown on two H-motors and two G-motors. For the Atlas the fins are bolted in place in fin slots. The model has flown on a single K-motor.

Details for the Gemini-Titan can be found here:

https://www.rocketreviews.com/scratch-gemini-titan-5in-by-bob-morstadt.html

The Titan also appeared in the May/June 2012 issue of Sport Rocketry. Details for the Atlas-Mercury can be found here:

https://www.rocketryforum.com/showthread.php?50629-Atlas-Mercury-1-12-scale

Others have also done this, too, but I am not familiar with the details. I remember years ago someone had a huge Long March II (Chinese) scale model with clear plastic fins. Also, Andy Woerner had clear plastic fins on his huge Space Shuttle model that I think was K-powered.

Well one design is definitely going to be moving fast. Not sure if it'll break mach, but I'd say there's a fair chance. I'm still working up the design, but it's a 38mm min diam ring fin. There will be three "stand-off" fins to support the ring. Blue Tube airframe and ring. Hopefully polycarbonate fins. Pinnacle plastic nose cone. So obviously the fins will be surface mounted. Choosing the fin material and bonding adhesive are the last two steps before I sim it out.

Try this...https://www.jcrocket.com/crayon-bank.shtml.
Great website.

I like Lexan as a fin material and will use it again. The last clear-finned rocket that I flew was my Tikva.

See:

If I use it again, I will make the fins replaceable. Not so much out of fear of one breaking, the Tikva fins were too large, but because they get scratched easily.

I used bolts and nuts to attach wood strips to the root edges of the Lexan fins which allowed them to be easily glued to the rest of the "fin can." Next time I will do something similar without glue so that a broken or scratched fin can be removed and replaced.

-- Roger

aerostadt said:

Others have also done this, too, but I am not familiar with the details. I remember years ago someone had a huge Long March II (Chinese) scale model with clear plastic fins.

Click to expand...

That would be Rick Boyette.

In reading about these fins, and some of the problems with them, I'm wondering if a rear ejection for a crayon would be a good idea.

Suggestions Welcomed!

If (big) Lexan fins worked on my 15lb+ Green Alien night launch rocket, it'll work for most anything...

https://www.rocketryforum.com/showthread.php?t=26333

My Gemini-Titan weighed 10 lbs and the Atlas-Mercury weighed 40 lbs. I would consider clear plastic fins for a scale model or some kind of demonstration model. I would not use it for a high performance like going Mach 1 or higher. The speed on the Atlas was on the order of 250 fps. I'm sure the Long March II scale model (Rick Boyette) was very heavy.

Kruegon said:

It's definitely strong enough. But is it too flexible? And if it's viable, what would be strong enough to bond it to blue tube or fiberglass airframes?

Click to expand...

The problem I had with the Tikva is that I made the ratio of the length of the root edge of the fin to the span of the fin too small. This gave the fin less rigidity and allowed it to flex so much that it broke during flight. A more reasonable shaped Lexan fin should hold up to most HPR flights. I will say, though, that Lexan fins are heavier than fiberglass, wood, or a composite of the two. If you don't need or want the coolness factor of clear fins, I would recommend wood or fiberglass.

Having said that, I found Lexan easy to cut and sand to shape. I cut the fins from the Lexan with a jigsaw. I tacked them together (still in their protective paper covers) using a spray adhesive and cut them all at once. Then I used a belt sander to ensure they were all the same shape. Finally, I used finer sandpaper on the edges to polish them.

To avoid the problem of what adhesive to use with Lexan, I bolted strips of wood to the root edges of the fins which I glued to the body tube and centering rings.

-- Roger

Well one design at least is a min diam. So that brings me back to an adhesive. It may not be viable to build it my way, but it would be very cool. If I were doing TTW fins, I know a couple of ways to bond them. None of them are viable for surface mount though.

I'm looking at a root edge of about 5.5 inches and a fin span of about 3 inches. Give or take. Angled leading and trailing edges. Think solar flare fins with a ring. That's more or less the same effect.