New scratch build: Penumbra

[qquake2k]

I've been working on this design for a couple of weeks now. It started as a 3" version, that looked like an arrow. I finally decided to go with a 4"
design, and inverted the fins. I'm calling it "Penumbra".

View attachment Penumbra v3 4in 38mm v2.ork

 

[qquake2k]

I had a coupler, a 19" long piece of body tube, and a 19" long 38mm motor tube. I will repurpose a nose cone from a rocket I built several years ago, that broke a fin on landing. I just need to order a length of body tube. I decided to start on the coupler, to make sure it wouldn't be too tight. I've had that happen before. I flooded half of it with CA, and sanded it smooth on the lathe. It fits perfectly.

 

[qquake2k]

I bought a bunch of small u-bolts in bulk a while back. They didn't come with backing plates, so I made some from 1/8" x 1" aluminum bar.

 

[Incongruent]

Will the paint theme be styled after a penumbra?

 

[qquake2k]

Right now I'm leaning towards black and gray, but I'm open to suggestions.

 

[LithosphereRocketry]

Looking neat!

I personally think the design would be really set off by some small fins right at the front of the gray section, just to break up that right angle- just a thought.

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[qquake2k]

Hmmm...

Something like one of these, perhaps?

 

[qquake2k]

Or this? I made the main fins 10% smaller.

 

[LithosphereRocketry]

Hmmm...

Something like one of these, perhaps?

The first one is pretty much what I was thinking of.

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[cjp]

I like the design,how about a tail cone?As far as color red and grey?

 

[qquake2k]

I thought about a tail cone, but decided not to use one for a couple of reasons. One is cost. It would cost $30 with shipping to get a transition from LOC.
I could turn one, but it would cost about the same to get a basswood or balsa block big enough. The second reason is the weight. Even with a relatively light plastic tail cone, it would affect the stability. Right now I can get away with 4oz of nose weight. With a tail cone, that would increase to 8oz or more. The extra weight would seriously affect the altitude. So no tail cone.

As for color, I have to paint the forward body tube black because the eclipse decal will have a black background. Right now, I like black and a dark red. Since I prefer satin finish paint, I'll have to see what colors are available. There aren't as many satin colors as there are gloss.

 

[neil_w]

Looks good.

When LithosphereRocketry mentioned some small fins ahead of the mains, I was actually thinking to put them in the black part, to give a bit more space and contrast between them....

Aren't we helpful?

 

[qquake2k]

Putting them closer together gives a better chance that they'll whistle. LOL

 

[neil_w]

True!

 

[qquake2k]

Whistling fins is the holy grail of rocketry!

 

[viciouspeanut]

Really cool design! Your designs seem pretty closely aligned with my style, your build threads are always a good read.

 

[cjp]

I like the black and dark red,makes good sense not adding a tail cone.

 

[BABAR]

Love the name.
Rather than extra fins I was visualizing a slight concavity to the leading edges of the fins, sort of like the inside edge of a crescent moon.

 

[o1d_dude]

Dang, Jim. You've really come back to building rokcets with a passion.

FWIW I like the slotted fins. I cannot scientifically base my opinion on anything firm but I believe that anything spacing of one inch or less on a 3" or larger diameter rocket will whistle.

Oddly, tube fin rockets whistle all the way down to 2.6" diameter tubes.

No tailcone needed on your design.

 

[qquake2k]

I think I get back into building when A. I feel physically up to it. And B. when there are more chances that we'll have launches. I will say, I enjoy building them as much as I enjoy launching them.

 

[qquake2k]

I decided to start on the fins tonight. This is plywood that I got from Joann's. It was advertised as 1/4", but it's actually 6mm (0.236"). After printing the templates on cardstock, I traced them onto the plywood, and cut out rough "blanks". As usual, I pinned the blanks together to cut and sand them together.

 

[qquake2k]

Even though the small fins will be surface mounted, I put "sacrificial" tabs on them so I could pin them together. After I cut and sanded the leading and trailing edges, I cut the tabs off. Then just held them tight to sand the root edges. Finally, I rounded all the leading edges.

 

[Nytrunner]

For what it's worth, I liked the Gray backend more than red.

Has more of an umbral aesthetic in my eyes.

 

[qquake2k]

I agree, black and gray match the name. But the black and red is more appealing to my eyes.

 

[Incongruent]

It could be a lunar eclipse...

 

[qquake2k]

That is true. Lunar eclipses can be colorful.

 

[qquake2k]

One of my issues with hole saws has always been ejecting the piece of wood from the saw. I found this Spyder mandrel that seems to work pretty well.

 

[qquake2k]

With my 4-1/4" hole saw, I cut out the centering rings and bulkhead. I cut the center holes of the centering rings with a 1-9/16" hole saw, and sized them with the Dremel.

 

[gdjsky01]

Luv your builds QQ.

Just saying'. Always enjoy your builds.
Especially in the kitchen.

 

[qquake2k]

Thank you Jeff! Good to see you're still around, hope you're doing well.

It always annoys me when my son cooks on my workbench in the kitchen. :tongue: