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-   -   Interceptors and siblings - Gooblinterceptor (http://www.oldrocketforum.com/showthread.php?t=18925)

DavidQ 10-15-2020 06:09 PM

Interceptors and siblings - Gooblinterceptor
 
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I decided to take up tbzep's suggestion that the Goblin would be a goony Interceptor.



I was going to go with a standard goony conversion, but of course the Goblin looks harder.

So start with a rocksim. To fit the really squat design, I went with a 1.75" body tube, and a 1.75" rear transition. The nose is a 60MS, so has a longer aspect ratio than the goblin. I had to limit it to 13mm engines to avoid the extra mass.

Oh yeah, I decided to call it a Gooblinterceptor.

I'll add build, etc., photos as I build this one.

Cape Byron 10-15-2020 07:11 PM

We really need a 'Like' button here. That looks amazing, David.

neil_w 10-15-2020 07:57 PM

What does Rocksim say about CP on that thing?

DavidQ 10-15-2020 09:01 PM

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About a .5 caliber stability margin. But, that is with an 11g nose cone. The one I am casting will be closer to 18g, so the Cg should move further toward the nose.

DavidQ 10-15-2020 10:03 PM

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Parts are rounded up.

Laser cut the fins.

Laser cut the transition. Made 2, so I could put one inside the other for double thickness.

The nose is one I cast from resin, using a mold I made a while ago. It was for a goony Xarconian Cruiser I was planning. It came out at 18.6 grams.

DavidQ 10-16-2020 09:08 PM

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Gluing the rocket together.

The transition is made from two transition sections, glued inside each other. They are the same size as each other, which causes the inner one to stick out. When the glue dried a bit, I just trimmed the overhang with scissors.

The vertical stabilizers have a 45 degree (or 135 degree) bent. My fin guides don't have 45 degree measurements, but I figured how to do it anyway. The inner slots are 30 degrees apart. The outer slots are also 30 degrees apart. But, they are centered with the inner slots. So, I laid an emery board along the outer slots (an emery board because it was there). Then, I put one piece if the fin in the inner slots. and taped it to the other piece of the fin. This was held along the emery board, for a 135 degree angle. In the photo, they are sitting on a roll of blue tape.

Then, the dried vertical stabilizers were mounted at 45 degrees. I used the fin guide to eyeball the placement, while the guide held the wings.

I used another fin guide at the same time, held between the swept back fins, to align the skids.

Once those dried enough in place, I applied fillets of yellow glue. I felt it would dry stiffer than the white glue, which I wanted because these fins have a small root edge against a handmade cardstock transition.

ghrocketman 10-17-2020 03:11 AM

Make it fly in a scale manner: LOUSY !

DavidQ 10-17-2020 10:53 PM

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Quote:
Originally Posted by ghrocketman
Make it fly in a scale manner: LOUSY !

It might... it just might.

But, to help avoid that, I changed my mind about the vertical stabilizers. They have a 135 degree bend in them in the Goblin, and I wanted to carry that into the Goony. But, I also wanted to have the VStabs slope forward like an interceptor.

After I assembled it, I didn't like the look. It also looked like it didn't add much to the stability.

So, I cut them off carefully. I drew up some similar VStabs with a rearward slope, also with the 135 degree bend. Just to help them look like an interceptor VStab, I also included that pokey thing.

I like this look much better. Rocksim says it moves the Cp backwards by just under a half inch, while moving the Cg hardly at all. I think it also fits the look of the Goblin better.

DavidQ 10-18-2020 11:03 PM

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Added pods to the wingtip. I want to keep the weight as low as is possible, so I did a minimal finishing job.

Put on a little of the red body filler, and sanded it off to fill small gaps.

Put on only one layer of white sandable primer, that is not a filler kind like the thick gray stuff.

Sanded it when it dried, and discovered just how much I depend on the gray sandable filler primer.

ghrocketman 10-19-2020 02:30 AM

Actually, this rascal most likely will fly far BETTER than it's full-scale counterpart.
I suggest the newly re-released C5-3 as a great test motor.


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