Split rear fin rocket I may build

[Bill S]

Here is a rocket I am tinkering with. It has rear fins with large open areas - really its basically two fins put together with a large gap between them. Body diameter is an BT-55, with an BT-60 diameter nosecone. I'm working hard to keep the weight down so it goes higher and can use longer delay motors. The original design was kind of smack in the middle between 4 second delays being too short, and 6 second delays being too long. I solved that by adding an ounce of noseweight, but I decided to try the opposite approach, paring down weight, which seems to be working better. I will need a custom balsa nosecone made up, as my cobbled together plastic one is just too heavy (nearly an ounce).

Also, if anyone has ever tried anything with fins that have such large gaps, please let me know how it worked out. I had another rocket that long spars that extended from the fins to the midpoint of the rocket (Estes Longship), but this is a tad different than that.

Here is a preliminary picture (color is NOT the final paint job):


Its got an 24mm motor mount, and should fly decently on C6-5 or C18/D20 motors, as well as Estes D12-5/7.

 

[Sandy H.]

Here's a template from a fin set I cut for a BT-50 minimum diameter rocket. They were plain balsa and the rocket always flew fine, but the sharp point of the bottom of the fin would break if it landed hard. Papering or lite ply would have helped.

I think you could have good results using a 3-piece assembly, so you could optimize the balsa orientation. But, if you're using ply, it probably wouldn't matter much. I bet it will fly fine.

I also built an Apogee Diamondback years ago. It had a pretty big opening as well. Still fly it every now and then, as it is a good motor eater on a medium wind day.

Sandy.

 

[Bill S]

Sandy, I was planning on using 65lb cardstock to "paper" the balsa to give it more strength.

Wow, I would think the points on the fins would definitely be fragile and prone to landing damage.

 

[neil_w]

Here is a rocket I am tinkering with. It has rear fins with large open areas - really its basically two fins put together with a large gap between them. Body diameter is an BT-55, with an BT-60 diameter nosecone. I'm working hard to keep the weight down so it goes higher and can use longer delay motors. The original design was kind of smack in the middle between 4 second delays being too short, and 6 second delays being too long. I solved that by adding an ounce of noseweight, but I decided to try the opposite approach, paring down weight, which seems to be working better. I will need a custom balsa nosecone made up, as my cobbled together plastic one is just too heavy (nearly an ounce).

Also, if anyone has ever tried anything with fins that have such large gaps, please let me know how it worked out. I had another rocket that long spars that extended from the fins to the midpoint of the rocket (Estes Longship), but this is a tad different than that.

Here is a preliminary picture (color is NOT the final paint job):

View attachment 509648
Its got an 24mm motor mount, and should fly decently on C6-5 or C18/D20 motors, as well as Estes D12-5/7.

I think the fins should be fine. Perhaps you want just a touch more contact area between the front spar and the rear piece to anchor it better.

You could build the fins on the bench, but I'm more inclined to think you should mount the rear fins first, and then attach the front spars. Easier to ensure that the front spars make firm contact with the BT that way.

I had fins with a vaguely similar scheme in an old unfinished design, but curvier with made them more problematic. I think your design is very doable, and should look cool.

 

[Bill S]

Neil, that is a crazy cool design you did.

I tend to agree with you that the 2 fin pieces need a little more reinforcement at the tip where they join.

 

[Ez2cDave]

Neil,

I think you will be okay . . .

Frankly, if this were a rocket that would be going transonic ( 950 Ft/sec - just under Mach ), or going supersonic, I would say "no".

I think you will see increased drag from turbulence induced by the openings in the fins, resulting in lower altitude.

"Back in the day", I would have used Clear Monokote on the openings, masked off the openings, and painted it.

Please post pics, as you build and when she flies !

Dave F.

 

[GlenP]

Here is a rocket I am tinkering with. It has rear fins with large open areas - really its basically two fins put together with a large gap between them. Body diameter is an BT-55, with an BT-60 diameter nosecone. I'm working hard to keep the weight down so it goes higher and can use longer delay motors. The original design was kind of smack in the middle between 4 second delays being too short, and 6 second delays being too long. I solved that by adding an ounce of noseweight, but I decided to try the opposite approach, paring down weight, which seems to be working better. I will need a custom balsa nosecone made up, as my cobbled together plastic one is just too heavy (nearly an ounce).

Also, if anyone has ever tried anything with fins that have such large gaps, please let me know how it worked out. I had another rocket that long spars that extended from the fins to the midpoint of the rocket (Estes Longship), but this is a tad different than that.

Here is a preliminary picture (color is NOT the final paint job):

View attachment 509648
Its got an 24mm motor mount, and should fly decently on C6-5 or C18/D20 motors, as well as Estes D12-5/7.

What really looks cool is the gap between the fins is in the shape of a fin itself. Nice job!

 

[GlenP]

kind of reminds me of an Escher drawing where the fins are shaped like birds and the gap is shaped like a fish!

your design kind of makes you think: is it a wing or a fin? With that payload/warhead, it could be a sub-launched missile that does go from Water to Sky. Just some other artistic observations and inspirations that might help you come up with a theme for this one, if you don't already have something planned.

 

[Bill S]

GlenP, no I don't have a particular theme for this rocket; I was just doodling and came up with a fin configuration that I liked, and wanted something beside the usual nosecone.

 

[jqavins]

I'm working hard to keep the weight down so it goes higher and can use longer delay motors. The original design was kind of smack in the middle between 4 second delays being too short, and 6 second delays being too long. I solved that by adding an ounce of noseweight, but I decided to try the opposite approach, paring down weight, which seems to be working better.

The thing is...

I think you will see increased drag from turbulence induced by the openings in the fins, resulting in lower altitude.

I'd be concerned that the added drag from those gaps would do as much to shorten the coast as the reduction in mass helps.

Nice design.

 

[Bill S]

I guess I will see how it flies, drag-wise.

 

[GlenP]

The front fin gets most of the drag from the high pressure of the airflow impact. It creates a lower pressure wake behind it where the aft fin LE may benefit from considerably less drag than if nothing were in front it. Kind of like how a race car can draft behind another to gain momentum before passing. The lower pressure wake may even have a suction propulsion effect on the rear fin depending on spacing etc. I would not be concerned with any increased drag compared to a fin of the same outline and no cutout. If you make the aft portions slightly thinner, you can actually reduce drag quite a bit.

 

[Bill S]

That is interesting, thanks for the info.

 

[Ez2cDave]

I'd be concerned that the added drag from those gaps would do as much to shorten the coast as the reduction in mass helps.

Actually, that brings up an interesting point . . .

The "lightest" rocket does not always fly the highest.

There is an "optimum mass" for each different rocket design and with each different motor they are flown with.

Rockets below "optimum mass" and rockets above "optimum mass" will both achieve less altitude than a rocket with "optimum mass".

Dave

 

[boomtube-mk2]

 

[Bill S]

I started construction on this rocket. I'm using 65lb cardstock to paper the fins to give them some more strength. I ordered a custom balsa nosecone from Roachwerks Custom machining, so hopefully that will arrive soon.

Its a pretty basic rocket, not planning on decals at this time, so construction should proceed pretty quickly.

 

[GlenP]

You already have a good TE forward swept design that will allow the body tube and engine mount to take most of the landing impact load. You might consider reinforcing the body tube where the forward fins are attached, like with a coupler or maybe a baffle. Probably not needed, but just in case, to help prevent a body crumple zone if the fin takes an impact on landing and you get a concentrated impact at that LE fin-body joint, which I don't think is really likely. But if you have the weight and stability margin for it, maybe add an internal coupler there.

 

[Bill S]

Glen, I hadnt thought of that. Hmm... let me look at that.

 

[Bill S]

Glen, I was able to incorporate your suggest with no problem. Thanks.

The custom nosecone arrived from Roachwerks, looks good.

Construction is proceeding. Engine mount is pretty standard:


Fin glued together, without cardstock "papering":


Fin with cardstock "papering":


Custom nosecone from Roachwerks:


The fins are being glued on now, pics later on.

 

[Bill S]

The fins have been attached. They just need a little puttying of the edges to fill in the wood grain.



Rocket with the balsa nosecone on. It needs to be puttied up and sanded, which the weather has not been cooperative (either way too cold and windy or rainy cool and humid).



I have the launch lugs and standoffs to attach next as soon as they dry (later today).

 

[GlenP]

That looks even better than your rendering. One idea for when you paint it, not sure if you can mask off the fin surfaces after painting or can hand brush the inside edges of the fin cut out with a contrasting or highlight color, like a red or yellow or black. Just an idea.

Here's a rough sketch:

 

[Bill S]

That's an interesting idea, Glen. I haven't decided what color to paint the rocket yet, but I probably could pull off hand brushing the inside edges.

 

[Bill S]

I've got the rocket painted in an florescent purple color. Man, it was hard to get the paint coverage fairly even, as the paint recommended a white undercoat, and the spray paint never goes down perfectly evenly. There are a few faint blotches that I can't really fix - I ran out of the paint and its out of stock. In any case, continuing to spray it is resulting in the paintjob getting darker. Also, because it was a paint for RC car bodies (designed to be sprayed inside a clear polycarbonate body, it is VERY matte, and I have done a test of covering it with some UV-resistant acrylic gloss clear spray. That worked okay, not as shiny as I'd like but its an improvement.

To reduce my workload, I decided to not do decals nor paint the edges of the fin "inside hole".

Pics when its done (hopefully in a few days if the weather cooperates).

 

[Bill S]

Finished rocket:

 

[waltr]

That is bright. Should be easy to see anywhere.

 

[GlenP]

Very nice, it looks even better than the rendering.

(PS-Can you get a picture with the Sun lower in the sky from the side so you can see the shadow of the rocket better? Trust me, it will make a good photo of that rocket.)

 

[Bill S]

2 good launches today, both on C11-5s. About 450 feet high or so. Nice straight boosts, no damage upon landing.



Link to watch flight: