Build thread: Blackfish

[jqavins]

with the exception that papering also in most cases takes care of the filling of the grain.

For the record, I didn't say it has the same benefits, just that it uses the same process. Papering the balsa ply for finishing purposes would still make sense.

 

[neil_w]

My experimentation with balsa laminating suggests that two-ply doesn't buy you that much. What you mostly get is equivalent strength in all directions, but it's still less than you'd get from a single solid piece in the "strong" direction (perpendicular to the grain). More plys would probably improve things but at some point it's not worth the effort except under special circumstances.

As for basswood, it is very strong against the grain but surprisingly weak with the grain. After glued paper is applied it feels extremely rigid in both directions. My papered 3/32" basswood feels 10 times stronger than the 1/8" two-ply balsa laminate I experimented with.

Of course, this type of strength is not often needed in LPR applications. But when it is, this will be my go-to.

I still need to do some more experiments with glued-paper on balsa to see how it compares. My adhesive-papared balsa fins feel pretty good (and are, for the most part), but APRO Lander II did break a fin on its first landing, before I reinforced the fins with leading-edge cuffs. In hindsight, those would have been good candidates for glue-paper balsa; I probably wouldn't have wanted the extra weight of basswood there.

 

[neil_w]

FINISHING UP THE FIN ASSEMBLIES (MORE OF THOSE $*%@& RINGS)

Next up are the little additional fins that are needed to attach the rings to the pods. I decided a while back that the rings would be sufficiently supported at only two points 180 degrees apart: we've already covered one of those, now it's time for the other.

OK, "depressed" isn't the right word, but I couldn't resist another Marvin GIF. But this is very fiddly and frustrating work. It would be oh so much better in a kit with laser-cut parts. But anyway, let's get to it.

Early on I thought I would cut a strip out of the rings to make them fit around the fin. Fortunately I tried it with a test piece, because it was a mistake; the slot ended up way too wide. So I made another test piece, and this time made a straight cut with some scissors. The piece of BT naturally opened up a bit, in fact almost exactly the thickness of the fin.

Perfect.

Next I went to cut the little piece. Oy, so small and fiddly, so hard to sand, so hard to test fit, so hard to place correctly. Eventually I got it on OK.

Once again, a view from the end confirmed correct alignment (well, *almost* correct)...

Next two pics are me holding the test ring in place, making sure everything will fit correctly:


And that, as they say, is that, at least for the first one the I completed there. Still two to go.

The rings will be painted separately and attached to the fins only after the rocket is completely painted, right at the end. I love rings but damn they are a pain in the butt.

You might ask: is that enough attachment for the rings? To which I would answer: it had better be.

 

[K'Tesh]

Took me a while to get around to looking at this... Very Cool!!!

I remember once I was walking though a market in Oxford, England with my (at the time fiancee (now ex-fiancee)) I saw mackerel for the first time, and I thought to myself that their patterns would look absolutely fabulous on a SR-71A Blackbird. I still have that feeling, and I want to make a 1/48th scale version with the mackerel paint scheme.

 

[BABAR]

Maybe a little late on this section of the party.

If these are cut from body tube segments, any advantage to papering them rather than filling the spirals? Since they have an open segment anyway, you would have no paper “seam” to cover.

 

[mbeels]

Nice, is that extra support piece basswood, or balsa?

I hadn't thought about cutting the tubes with scissors, that is a distinct advantage of the thin wall tubing.

You might ask: is that enough attachment for the rings? To which I would answer: it had better be.

Looks waaaay strong to me....

 

[neil_w]

Nice, is that extra support piece basswood, or balsa?

Basswood, mainly because... well no reason, although it turned out to be a good decision as it was able to withstand the manhandling required to do the proper fitting of the piece.

I hadn't thought about cutting the tubes with scissors, that is a distinct advantage of the thin wall tubing.

Scissors are awesome. I've also experimented a bit with using them to cut balsa. Works well across the grain at least up to 1/16".

Looks waaaay strong to me....

They shouldn't be subject to too much stress, so hopefully OK.

 

[neil_w]

If these are cut from body tube segments, any advantage to papering them rather than filling the spirals?

Honestly it never occurred to me. Probably would have worked well, at least for the outside. I would have papered a whole section of BT50 and then cut the pieces from it. Maybe next time!

 

[jqavins]

You might ask: is that enough attachment for the rings? To which I would answer: it had better be.

I'm sure it's plenty strong. It bugs me seeing a ring supported at two points because that allows it to rock, whereas three points define a plane and so do not allow rocking; the same reason I'd never secure a coupler with two sheer pins, but rather use three even it two are strong enough. In this case, however, you don't have two point support; you have two line support, and it won't rock, and the part of my brain saying it needs three can just shut the hell up!

 

[neil_w]

Yeah, normally I would want three supports, but these guys are so small and so well protected from impact, I figured that the dual-supports should be OK. It's not a big worry for me at this point.

 

[BABAR]

Yeah, normally I would want three supports, but these guys are so small and so well protected from impact, I figured that the dual-supports should be OK. It's not a big worry for me at this point.

Gee, I thought you'd be fin with just one attachment site on the main fin (technically two, since you are on both sides of the fin) and the infamous "gusset". So I would say technically you currently HAVE 3 attachment points, although they are not equidistant.

Plus the rings are small.

Plus I don't think these rings are likely to be an initial impact site on recovery.

I haven't had problems with attachment sites at a single point with much larger rings.

https://www.rocketryforum.com/threads/more-half-baked-designs-thread.155195/#post-1927921 post 25

 

[neil_w]

It's possible I could have lived with just the attachment point at the gusset, but I'd have been waaaaaay too nervous about it. I really don't know how strong the split joint is going to be, so I don't want to be completely dependent on it.

 

[jqavins]

And it looks better this way. My opinoon, of course.

 

[neil_w]

Fins two and three went somewhat faster.

I'm not totally happy with my filler job on the pod cones; I think I sanded all the way through the filler in some places. Might to another round (or ever some CA) before filler/primer on all the fins.

 

[BABAR]

Those look really cool even without the rings!

 

[neil_w]

I have to admit I am feeling extremely unmotivated now, while stuck at home and drowning in coronavirus news. I do however have some accumulated build progress to document, which I suppose I ought to do. Further actual progress on the build itself is TBD.

 

[Greg Furtman]

I have to admit I am feeling extremely unmotivated now, while stuck at home and drowning in coronavirus news. I do however have some accumulated build progress to document, which I suppose I ought to do. Further actual progress on the build itself is TBD.

@neil_w Don't get cabin fever.

 

[neil_w]

The tail cone ends at BT55 size. I chose to use a split 5055 rolled paper ring that I had lying around. I cut a sliver of BT55 to go around it: this would hold it to shape and also provide a 360 degree solid anchor for the small end of the tail cone:

I glued the split ring into the bit of tube, and then had the idea to soak some CA into the aft end to seal up the paper and allow me to sand it smooth and flat:

Sharp-eyed readers will not that the inside of the ring looks odd. Well, only after I glued the ring into the tube did I remember that I'm using heavy-walled BT50H tubing for my motor mount, as is my custom. The paper ring was too small and wouldn't go over it. So I peeled out a layer of paper, but as I learned, each layer or paper on those things is *thick*. So then it was too back and I needed to glue in *2* layers of copy paper to bring the ring back down to the correct size. Oy.

Anyway it's all good but definitely more effort than a stupid centering ring should consume.

 

[neil_w]

TAIL CONE

The tail cone shroud is pretty straightforward, as far as shrouds go. BT55 to BT60, 1.25" long. But it will have fins mounted on it, which adds an extra wrinkle or two. Also, *every* shroud I make is a technique exploration where I try to get better at it for the future. Apologies I didn't take enough pictures on this one to really show everything I did. More detail on request.

Since this one will have fins on it, I needed fin placement marks, which are surprisingly hard to add yourself after-the-fact. After an endless amount of fussing, I finally got my Perl installation fully working on my laptop so I was able to use my custom shroud generator to create this template, which is more of an adventure in trigonometry than you might think:

I wish I had had this script when I did Diamond Cutter; getting precise placement of the canards onto the forward transition was quite difficult (and I did not succeed TBH).

Anyway, I made the first shroud on 110 lb cardstock normally. I put the tab on the top of the template shown above, so the top fin line was "below" the other end of the paper. Then I cut out a second template to go on the inside, and here's where my technique deviates from typical practice. I do not form a second complete shroud, because I when I do this I always end up with bulges where the seam is on both the inner and outer shrouds. Instead, I cut off a piece from one end of the template about the size of the overlap tap on the outer shroud. Then I glue it in place like so:

Things to note here (click to enlarge):

1. One end of the inner shroud just about butts up against the glue tab on the outer shroud. Normally I want to get it touching, but I didn't quite hit the mark here. Close enough.
2. The other end comes just about to where the seam is on the outer shroud (not visible in above picture).
3. The inner shroud is still offset a bit from the narrow end of the outer shroud. If this were not true than the inner diameter of the shroud would be too small.

When glued like this, there is (almost) exactly two layers of paper around the entire shroud. There are *no* spots where there is an extra layer that will tend to bulge. Also, there is never a problem where the inner shroud is not *quite* the same shape as the outer, and they don't glue perfectly surface-to-surface. The inner shroud conforms perfectly to the outer. Downside is there is a *slight* weak spot in that gap between the inner shroud and the outer glue tab. It also is very difficult to do this on long skinny shrouds (will explore this further in a future build). Frankly it is a bit tricky to pull off just in general, but I really like the results.

Finally after the glue is totally dry the extra is trimmed off the large end and here's the finished piece:

Final notes:

1. I *always* set my shrouds on a body tube and centering ring to keep both ends in perfect round while drying. And usually I store them that way (if necessary) until it's time to use them. That is why the shroud looks so perfect in the above pictures.
2. This finished shroud is almost certainly stronger than it needs to be. I have no problem with that.

An there is Too Much Writing About A Paper Shroud. You're welcome! (Seriously though: do folks actually want to read this much about little stuff like this?)

 

[kuririn]

(Seriously though: do folks actually want to read this much about little stuff like this?)

I do. Wondering if you plan to CA cardstock shrouds/nozzles after final assembly?
I tend to do that on all cardstock components, even double shrouds, but I might be a bit OCD on that point.

 

[neil_w]

I do. Wondering if you plan to CA cardstock shrouds/nozzles after final assembly?
I tend to do that on all cardstock components, even double shrouds, but I might be a bit OCD on that point.

Usually I do, with Q-tip and thin CA. And I think I did it on this one but I can't remember!

Actually I usually do my CA before final assembly because... well I'm not sure why. I do keep it away from attachment points of course.

Even without the CA this one was plenty strong, and with normal finishing procedures it'd be smooth. If I wasn't attaching fins to it, a single 110 lb cardstock shroud with good CA application is more than strong enough.

 

[jqavins]

I have to admit I am feeling extremely unmotivated now, while stuck at home and drowning in coronavirus news.

That's a shame and ironically not surprising. Building would be a great way to fill the empty time and keep your attention off the flood of news. And depression, clinical or otherwise, all too often takes away the motivation to do the very things that would be good against it, like socializing (in other times) and engaging in hobbies. I hope writing about stuff already done helps to get the juices flowing and restore your motivation to build. Failing that, just start doing some despite the lack of motivation and maybe it will catch. My sister-in-law gave me some really good advice once: if you can't be happy and productive, pick one. And I'll add that sometimes the other miraculously follows.

An there is Too Much Writing About A Paper Shroud. You're welcome! (Seriously though: do folks actually want to read this much about little stuff like this?)

Absolutely! What's special about your build threads is that they are instructional as well as entertaining; most are only the latter, which is fie, but both is better.

 

[neil_w]

if you can't be happy and productive, pick one.

That's quality life advice right there (no sarcasm).

 

[neil_w]

MOTOR MOUNT/TAIL CONE ASSEMBLY

The split ring was glue flush with the end of the motor mount tube, a standard hook was inserted, and then a centering ring was glue in just to hold in the ring:

The reason I used a ring there rather than my usual wrap of electrical tape was two-fold:

1. I have lots of spare 50H55 rings. I've found that the 50H lite ply rings tend to be a bit sloppy on the inside fit, and so nowadays I only use regular BT50 rings and sand them out with the trusty sanding stick. That way I can get exactly the fit I want.
2. Putting tape in the location I wanted would have probably conflicted with the rings to be installed in front of it.

Then the shroud was slid on to the end ring and glued...

...and then a fiber ring was pushed into the end of the ring from the other side and glued, because I have a bunch of spare 50H60 rings as well:

Finally glued two more rings to provide anchor for the main BT60 airframe:

Note that I often find the wood rings to be a bit small on the outside as well, especially after I sand off the laser charring. So I often find myself adding a wrap of office paper to the outside of the ring to get a nice snug fit (which I definitely wanted here, to ensure perfect alignment). Despite the extra effort I still really like working with the wooden rings due to their rigidity.

Oh yeah, the usual Kevlar is attached at the forward ring.

 

[jqavins]

Why sand off the laser char?

 

[neil_w]

Why sand off the laser char?

Want the glue to adhere to fresh wood, not char.

However, I'd do it even if there were no material benefit (pardon the pun), just on aesthetic grounds.

 

[BABAR]

However, I'd do it even if there were no material benefit (pardon the pun), just on aesthetic grounds.

What goes (unseen) under the body tube, STAYS (unseen) beneath the body tube.

Kinda like the back side of a cross-stitch.

 

[neil_w]

What goes (unseen) under the body tube, STAYS (unseen) beneath the body tube.

Kinda like the back side of a cross-stitch.

I prefer to think of it as the back side of the cabinet.

 

[kuririn]

What goes (unseen) under the body tube, STAYS (unseen) beneath the body tube.

Kinda like the back side of a cross-stitch.

Statement: If it's inside a body tube nobody's gonna know.
My OCD response: Yeah, but I'M gonna know.

 

[BABAR]

Statement: If it's inside a body tube nobody's gonna know.
My OCD response: Yeah, but I'M gonna know.

I have enough trouble making the OUTside of my rockets look decent, I don’t sweat the innards