Celestial Jet

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mbeels

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I had a sudden urge to build a rocket (that happens to me on occasion) and I wanted it to look like a jet. I especially like designs by Shrox and borrowed some inspiration from him while perusing this thread. The result was this:

Terrestrial_Jet.png

For a starting point, I used the BT-80 body tube with 29mm motor mount that I originally assembled for the Plasma Dart II XL (before changing my mind and building another with a 38mm mount). I also added four 24mm motor mounts for more air starting fun.

With up to 5 motors in the rear, I'll need to be very mindful of weight and balance, so I'm going to try and pay attention to the real weights of the parts as I build this.
 
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mbeels

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I decided to build the major fins out of 1/4" plywood. The "wings" and vertical fin are fairly long, and I didn't want them to twist or flutter. But solid 1/4" would be too heavy, exacerbating a tail heavy problem. I decided to cut lightening holes and fill the internal voids with 1/4" balsa. I weighed them before and after, and in the end, they are about 1/3 lighter. So a 3 oz fin is down to 2.0 oz.

PA161485.JPG


PA161486.JPG

PA201487.JPG

The top and bottom 24mm motor mounts are canted, and the left and right side mounts are straight. The canted motor mounts can fit a CTI 1 grain case, and the straight mounts can hold a 3 grain case.
 

BABAR

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So the 29mm gets it off the pad and going whether the outer motors air start or not?
I like it!
 

neil_w

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Wow those fins are amazing
 

mbeels

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So the 29mm gets it off the pad and going whether the outer motors air start or not?
I like it!
Thanks, that's the idea. Something like an H128 in the center gets it off the pad, and then the 24 mm motors are lit like afterburners.
 

DAllen

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Wait wait wait...You gotta back up here lol. How did you get such good looking cuts on those fins??? I would normally look at something like that and think laser cuts but that's not what it looks like.
 

mbeels

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Wait wait wait...You gotta back up here lol. How did you get such good looking cuts on those fins??? I would normally look at something like that and think laser cuts but that's not what it looks like.
Thanks, all done on my scroll saw. I drilled a hole and threaded the blade through. It's a great tool.
 

mbeels

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And routered out the voids?
I used the scroll saw for that as well. After cutting out the fin, I drilled a 3/8" hole in the middle of the void, removed one end of the scroll saw blade, pushed it through the 3/8" hole, and then reattached the scroll saw blade. Then after cutting out the void, remove the blade again to get the fin out.
 

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Nice. Do you think leaving the voids empty and papering the fins would’ve saved more weight?
 

mbeels

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Nice. Do you think leaving the voids empty and papering the fins would’ve saved more weight?
Probably, but I don't think very much. With the balsa in there the whole fin is very solid. I'd be worried that if I left the voids empty I'd pick up the rocket wrong and poke a hole in the fins.

I also considered framing out the fins with internal braces and sheeting the top and bottom with 1/32 plywood. I think that would have been lighter then what I did as well.
 

mbeels

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I fiberglassed the central BT-80 with 5 oz cloth. I don't have a lot of experience with fiberglass, but it came out alright. It sure is a great deal stronger.

PB021535.JPG
 

mbeels

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(some pictures out of order).

My idea is to have the fins and central BT-80 take all the flight loads, and the outside tubes be just decorative. I glued down some strips of balsa in order to have something solid to glue the BT-60 and BT-80 tubes to. The fins have forward extensions to support the sliced up tubes.

PA271522.JPG
 

mbeels

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Egads, the back of the rocket looks busy. I'll clean this up later.

PB021538.JPG

I also sliced up more sections of tube and installed them upside to radius out the gaps between the tubes on top. This will take some filling and finishing work.

PB021539.JPG
 

neil_w

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Wow that is a lot of action back there. I can see why you expended effort to lighten up the fins.

Looking good so far.
 

mbeels

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That is a lot of joinery in the back
Yes, it'll take some clean up. There are a few more internal fillets and joints to finish.

Emboldened by how the 5 oz fiberglass turned out, I decided to do a veil layer of 3/4 oz fiberglass. Parts of it turned out well, other parts not so well. The problem I had was the cloth not laying down in the concave parts of the tubes. I guess I had too much tension on the cloth, so it pulled up. Tuck down one corner, and another pops up. Like whack-a-mole, but I lost.

The ok:

PB031624.JPG

And the ugly:

PB031625.JPG

I can work with it. Because it is just cosmetic, it means more trimming, filling, and sanding, but I'd like to learn how to do this better. In most places the cloth lay down nicely and really smoothed out the heavy 5 oz cloth. Perhaps someone more experienced has some tips for this kind of situation. Most of the examples and videos I see cover fiberglassing a single round tube, not odd shapes like this.
 

neil_w

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That is some ambitious glassing (note to self: good album title idea). Of course, never having glassed anything before, it *all* looks ambitious to me. :)
 

mbeels

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That is some ambitious glassing (note to self: good album title idea).
I'm not sure what genre that would be. I could imagine a bluegrass band by that name, or punk metal. But I think it definitely has at least one string instrument.

Now I'm getting down to the details. I added some fairings between the cockpit area and side engines, and sliced a conical nose cone in half. After sanding it on a piece of BT-80, it fits.

PB051661.JPG

I also tried bending some wood with windex to stiffen up the front of this opening. It worked alright, not as well as steam, but more convenient. Bending balsa is fun.

PB051662.JPG

Everything needs lots of filling and sanding. Buy stock in CWF (NWL).
 

neil_w

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Nice, bend that balsa!

Now, why does that look like a launch lug inside the bottom scoop?
 

mbeels

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Now, why does that look like a launch lug inside the bottom scoop?
Because there is a 1/4" launch lug inside the bottom scoop :). I glued a pair of lugs on the central tube, before adding the bottom scoop. I'm still planning on adding rail buttons (I think I have a plan now), but this gives me options.
 

BABAR

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Because there is a 1/4" launch lug inside the bottom scoop :). I glued a pair of lugs on the central tube, before adding the bottom scoop. I'm still planning on adding rail buttons (I think I have a plan now), but this gives me options.
Options are good!
 

mbeels

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"The Ugly" is getting epoxy and phenolic micro-balloons fillets.

PB061663.JPG

Then everything is getting a lot of CWF.
 

mbeels

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I think that your fiberglassing job may have been easier if you had done your fillets first.
That is a very good suggestion, and in hindsight, seems like it should have been obvious. Next time...
 

mbeels

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This will take several rounds of CWF and sanding, but it is starting to get there. I need to clean up a few spots yet, but nearly ready for primer.

PB071665.JPG

Some of the seams will be hard to hide completely. When it comes time to paint the rocket, I think I'll cheat and do some stripes in some strategic places.

PB071664.JPG
 

mbeels

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First coat of primer is on, it will likely need a second. Then painting, electronics, and 'chute cannon in the nose cone.

PB101670.JPG PB101669.JPG
 
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