Moonbug 2 Build

The scratch-built, tube-fin, goony Moonbug of my youth (the original in my current avatar) was lost recently so I'm building a modified replacement.

A lot of things seem backwards on this design. Some possible challenges:

• engine/motor mount axis alignment (2.75" with only one centering ring)
• engine/motor mount is pushing into the big side of the boattail
  
• tube fin axis alignment
• difficulty doing TTW tube fin supports?
• attaching the large ring fin
• limited recovery space

The long boattail is the main load-bearing structure. There is no internal support but there will be 3x strake-like external supports. Like an egglofter, is the tail cone strong enough to push everything forward?

This will be a glacial-speed build. Comments, questions, and/or suggestions welcome!



Starting the BT-60 tube cutting process with wrapped paper. The BT-80 ring fin could've gone better...

Nifty design! Do I gather that the boattail and ring fins are new features? (that's what it looks like from your avatar) That boattail should make for a very interesting build. I really like having the motor mount with only a front centering ring.

Two ideas for tube fin alignment:
1) Slot the tubes and put tabs on the strakes.
2) Pre-glue a very thin strip of wood to the tube fins, on-axis. Then glue that wood to the strakes. I've used this before and it works very well.

Subscribed.

Goonbug!

Is the nosecone bt-60?
If so I would run the mmt tube all the way up the boat tail with two bt60 CRs about a quarter inch apart.

Cutting off the tail of the cone leaves plenty of room for a chute.

May have been used before but at least I haven't seen such a combo of

boat tail

Ring fins

Tube fins

And strakes

And it all flows together well.

Really cool!

neil_w said:

Nifty design! Do I gather that the boattail and ring fins are new features? [...]
Two ideas for tube fin alignment:
1) Slot the tubes and put tabs on the strakes.
2) Pre-glue a very thin strip of wood to the tube fins, on-axis. Then glue that wood to the strakes. I've used this before and it works very well.

Click to expand...

Good eyes! You win a cookie! Yes, the boattail & ring fin are new and making things challenging. The single centering ring is only half-way up the boattail.

Thanks for the ideas on tube fin alignment - I especially like idea #1. I'm also worried about tube fin axis alignment in-towards or out-away-from the motor. I suspect I'll do TTW supports to try to line it up.

dhbarr said:

Goonbug!

Click to expand...

D'oh! How'd I miss that?!???

Amsterdam said:

Is the nosecone bt-60?
If so I would run the mmt tube all the way up the boat tail with two bt60 CRs about a quarter inch apart.

Cutting off the tail of the cone leaves plenty of room for a chute.

Click to expand...

Yes, BT-60. 1/4" between CRs seems very short. Great suggestion. I'll keep playing with it to see what works.

BABAR said:

May have been used before but at least I haven't seen such a combo of [...] And it all flows together well. Really cool!

Click to expand...

Thanks! Here's an alternate color scheme (Hi Neil_W and Starship Avalon! )



I'm playing around with the boattail setup. If I do the single CR halfway up, I'll need to double-up the boattail thickness; if I extend the engine tube all the way up, the thickness seems fine. There isn't much room for recovery.


Mock up:



Tube fin support pattern cut out with the strake separated. I'll try to do TTW after the boattail is in place.

The 2nd boattail layer is cut. Note that I've marked the fin support & launch lug positions while flat.




I'm still debating the ring fin size, arrangment, and attachment:

1) size? the ring fin diameter should ideally be 2x BT-60 (3.274") to loop through the center of each tube fin. BT-80 is a little smaller at 2.6" but is passable.

2) whole ring fin or just the 3 portions that connect the tube fins?
2a) if the whole ring fin --> slot the back of the tube fins and then front of the ring, and slide from the back. Are there better ways?
2b) if partial ring fin (just the sides like a colonial viper), I could cut the BT-80 to the approx size for 3.274" dia or keep it at 2.6". There's the issue that the cut portions could flatten out and squish the tube fins upward.

Thanks for the comments & suggestions. Keep 'em coming while I muddle through this!

Idea for tube fin alignment, but tolerances will be close.

First, your tube fin cuts must be perfect cylinders, so when they are all resting on a flat surface they are all in perfect fore-aft alignment.

Second, need a drawing on paper to space them out properly

Third, attach your connecting ring in three parts and glue in place.
Now all three tube fins should be in perfect alignment. This is your tube fin construct.

Fourth, build your core rocket with the radial fins/strakes. OVERSIZE the radial fins a bit (wider span) than needed, this gives you latitude to “finess” them so you can drop them down the center of your three tube fins.

Fifth, shave/finess the radial fins until the core slips perfectly inside the tube fin construct.

Really like this design I am so planning to build something similar.

BABAR said:

Idea for tube fin alignment, but tolerances will be close. [...]

Click to expand...

Thanks for the great suggestion! I'll try that, or possibly to glue the strakes onto the tail assembly and then fit _that_ to the boattail.

rharshberger said:

Really like this design I am so planning to build something similar.

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Thanks, much appreciated! and I'd love to see that! If this one works out, I'd like to try a BT-80 version.
Also, I can posts the OpenRocket file if anyone's interested.

Thanks for the comments & suggestions!

Any progress on this bird?

Unfortunately, no. Life has thrown some real curve balls but thanks for the reminder!

I tried adding a 2nd layer to the boattail but offset the two cones too much; the small end of the cone has to split and flatten in order for the tube to pass. It might be good enough but I may try making another.

Adding a 2nd transition layer is tricky. Gluing two premade cones didn't work well when I tried it on the Babylon 5 starfury pods; the glue grabbed in the wrong place. This one is much larger so I tried wrapping the 2nd layer around the first (line up & tack the edge, spread thin glue, and wrap/roll up). It seems like it would've worked had I kept them closer together at the top. It is significantly stronger than a single layer.

Balsa supports and strakes have been cut but haven't been pieced together yet. No pics yet.

Thanks for the continued interest!

I built a OR file last night loosely based on the Moonbug. It appears to be a BT60 and Big Bertha nosecone, with a 8" transition, for the tube fins I used 3xBT60 @ 1 caliber lenth, and the ring fin is a 1" long section of BMS T300 airframe. Thinking about kit bashing a Moonbug from a Baby Bertha kit.

Cool! The actual dimensions/parts:

Big/Baby Bertha nosecone; original is Centuri's PNC-160 (lost alas )
1.250" BT-60 front section
5.250" BT-60 -> BT-20 transition
1.875" BT-60 tube fins, set 1/4" up front the end of the motor/engine mount
0.875" BT-80 ring fin; ideally 2x BT-60 (3.274" dia)

The original is even easier. Looking forward to seeing your take on this. Let's get more in the air!

grapetang said:

Unfortunately, no. Life has thrown some real curve balls but thanks for the reminder!

I tried adding a 2nd layer to the boattail but offset the two cones too much; the small end of the cone has to split and flatten in order for the tube to pass. It might be good enough but I may try making another.

Adding a 2nd transition layer is tricky. Gluing two premade cones didn't work well when I tried it on the Babylon 5 starfury pods; the glue grabbed in the wrong place. This one is much larger so I tried wrapping the 2nd layer around the first (line up & tack the edge, spread thin glue, and wrap/roll up). It seems like it would've worked had I kept them closer together at the top. It is significantly stronger than a single layer.

Balsa supports and strakes have been cut but haven't been pieced together yet. No pics yet.

Thanks for the continued interest!

View attachment 367220
View attachment 367221 View attachment 367222 View attachment 367223

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From the PICS I first thought this might be a missing South Florida ballot, but then I saw you were from California.

I did a bit on my version of the Moonbug2. The BT20 has ribs for under the transition and the shroud was made from copier paper hardened with CA then mounted on the motor tube, the shroud was then wrapped with a single layer of 4oz FG which makes it pretty solid. The fin standoffs are nothing more the ribs reversed, and epoxied to the shroud directly over the ribs.

Cool. Will the grain direction of the fin standoffs be a problem?

How did you get those body tube cuts so perfectly straight?

BABAR said:

Cool. Will the grain direction of the fin standoffs be a problem?

How did you get those body tube cuts so perfectly straight?

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As short as the fins are, and with the large ring interlocking with the smaller fins it shouldn't be an issue with the grain. I will also be soaking the fin standoffs with CA.
The cuts were easy, I have a homemade Kuhn style cutter and inside the tube I use a coupler to stiffen the tube being cut when available. I will take a picture of my setup when I get home, but its pretty much like most other Kuhn style cutters. For a blade I use a single edge safety razor blade ( like paint scrapers use).

Heres my cutter BABAR

Made with 3/4" thick scrap material, the slot for the razor blade was cut with a Zona thin kerf razor saw. I will probably redo it someday soon, mainly to improve the looks, I like my commonly used tools to look like a true craftsman made them, not a wood butcher...

Very nice! I like the jig - definitely easier and more precise than wrapping with paper and cutting by hand.
Slotting the tube fins to get the ring fin through is a great idea!

Re: ballots: Ha! No, it's just a source of cheap, thin, medium-grade cardstock. Hope people voted!

Need to get back to my Moonbug 2 when life settles...

The other reason for slotting the tube fins is that I want to paint them and the ring fin before assembly since they will be two different colors. I am planning on Testors Sublime green and gloss black, with some hombrew black decals.

rharshberger said:

Heres my cutter BABAR

Click to expand...

That is nice. Are you able to use that setup for cutting small (LPR) tubes? It looks like the razor blade won't go in far enough to access a small tube at the bottom of the "V".

I don't know what it is about this design but I really like it...may have to do a 4" version myself.

neil_w said:

That is nice. Are you able to use that setup for cutting small (LPR) tubes? It looks like the razor blade won't go in far enough to access a small tube at the bottom of the "V".

Click to expand...

I have cut 1/4" launch lug in it, the blade isnt fixed so it took a bit of practice to learn to use right.

rharshberger said:

Heres my cutter BABAR

Made with 3/4" thick scrap material, the slot for the razor blade was cut with a Zona thin kerf razor saw. I will probably redo it someday soon, mainly to improve the looks, I like my commonly used tools to look like a true craftsman made them, not a wood butcher...View attachment 367799View attachment 367800

Click to expand...

very cool. I was thinking however of the longitudinal cuts for the fins. How did you get THOSE so straight?

BABAR said:

very cool. I was thinking however of the longitudinal cuts for the fins. How did you get THOSE so straight?

Click to expand...

I used a long coupler though all three tube fins with some tape to keep them from shifting. Then using a aluminum angle (3/4" x 3/4") as a straight edge and a new #11 Xacto blade made several light cuts, to cut the ends of the slots the fin was slid just just off the end of the coupler and the end plunge cut. The couple makes the tubing much stiffer and easier to cut cleanly.

grapetang said:

The scratch-built, tube-fin, goony Moonbug of my youth (the original in my current avatar) was lost recently so I'm building a modified replacement.

A lot of things seem backwards on this design. Some possible challenges:

• engine/motor mount axis alignment (2.75" with only one centering ring)
• engine/motor mount is pushing into the big side of the boattail
  
• tube fin axis alignment
• difficulty doing TTW tube fin supports?
• attaching the large ring fin
• limited recovery space

The long boattail is the main load-bearing structure. There is no internal support but there will be 3x strake-like external supports. Like an egglofter, is the tail cone strong enough to push everything forward?

This will be a glacial-speed build. Comments, questions, and/or suggestions welcome!

View attachment 363116

Starting the BT-60 tube cutting process with wrapped paper. The BT-80 ring fin could've gone better...
View attachment 363117

Click to expand...

Go with the green. Very cool design.

I just stumbled upon this thread. Man, I really dig your mods of the original design.

Subscribed!

Infamous said:

Go with the green. Very cool design.

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Thanks! I assume the mostly green one? or the mostly black one with green trim?

Tobor said:

I just stumbled upon this thread. Man, I really dig your mods of the original design. Subscribed!

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Thanks for your interest too! I really need to get back on this!!!

grapetang said:

I really need to get back on this!!!

Click to expand...

Yes, you do!
I am looking forward to seeing how this goes!

Finished my version of grapetang's Moon Bug 2 today. Actually still need to put launch lugs on it but its basically finished. Thanks grapetang for a cool design.