Build thread: Blackfish

[neil_w]

Transferred all the pieces to a piece of 1x2 that I've used for this in the past. Much lighter and easier to handle. Nose cone is on a piece of PVC that will plug into my paint stand.

Today was breezy but I was able to get primer down. I wouldn't do top coats in those condition but primer is non-critical.

This clears the path for final construction. *whew*

 

[neil_w]

FIRST FINSET

If there's one downside to papered fins that really bugs me, it's sanding the filler/primer. Once you expose the paper, it roughs up very quickly. This was my first time with glued paper; I was curious if it would behave the same as the label paper. Well indeed, it did rough up once I got through the primer (of course I stop as soon as I see the white showing through). It is possible that there's a *slight* difference here, though: after a finishing sanding with 1500 grit, everything felt pretty smooth again. We'll see how it all looks after paint.

Here's one sanded main fin:

That was a lot of very much un-fun nooks and crannies to sand. I decided that, rather than torture myself by trying to sand the whole set at once, I'd glue each fin after sanding it.

For the first time in a while, I broke out my old 3d-printed fin guide. It's an imperfect tool but I have fond feelings for it because it's the first thing I ever designed and 3d-printed (using a service, don't have my own printer). The fin guide is strapped to the tube, and then it holds (via clamps) two pieces of aluminum angle parallel to the body. First I got it in place, still very loosely clamped, so it was aligned over the fin slot (which you can barely see in this pic):

Then I double-glued the fin in place, sliding it between the two pieces of angle. Here's a back view:

And here it is after all the clamps are in place:

Frankly, given the TTW mounting, and the quick grab of the TBII, none of this was probably necessary.

After only a few minutes, I applied a quick fillet of TBII just to seal up the joint. Quick and Thick fillets will come later.

Next I mounted the front and rear fins. Originally I planned to use the popsicle stick trick to keep the fins aligned with each other, but these little guys are so small and stable that it was super easy to eyeball them. Double glue them with TBII, align them visually, and then 5 minutes later they're good to go.

Alignment is good:

Two more finsets to go. I won't be documenting them because they'll be exact repeats of this set. Might take me a while to get through it all, bench time is might scarce these days.

 

[mbeels]

Very nice, good to see some progress. This is the fun part where all the pieces come together, and it takes shape.

That was a lot of very much un-fun nooks and crannies to sand.

More or less than Plasma Dart ?

For the first time in a while, I broke out my old 3d-printed fin guide. It's an imperfect tool but I have fond feelings for it because it's the first thing I ever designed and 3d-printed (using a service, don't have my own printer). The fin guide is strapped to the tube, and then it holds (via clamps) two pieces of aluminum angle parallel to the body.

Interesting, does it only fit one body tube diameter? Or do you have a way of accommodating different sized body tubes?

 

[neil_w]

Very nice, good to see some progress. This is the fun part where all the pieces come together, and it takes shape.

Yes indeed

More or less than Plasma Dart ?

More. It didn’t take all that long but I find that particular task to be really annoying. Sanding filler/primer is my least favorite build task to begin with; anything that makes it harder...

Interesting, does it only fit one body tube diameter? Or do you have a way of accommodating different sized body tubes?

The bottom is an angle, so it’ll fit on any size tube, but probably loses effective above BT70 or so. And it doesn’t rest securely enough; there’s a bit of play in it which I don’t like. I’ve considered ways to make it more secure, most involving a set of interchangeable clips for different body tube sizes. Haven’t worked on it recently though.

I’ll get some pics up tomorrow to show it in more detail.

 

[neil_w]

I am having problems opening the design in TinkerCad which is making me Very Unhappy. In the meantime, here's a render of it, upside-down:

The body tube rests in the "V" on top. Straps go through the two slots to hold it to the body. Aluminum angle fits into the channel on each side of the main spine, and then clamps are fit over the top. Like this:

The problem, as mentioned above, is that the V channel doesn't hold its alignment on the BT firmly enough, especially for larger bodies (BT60 shown above). I think I needed to (a) make the wings wider, and (b) make the angle smaller, like 90 degrees. There are other ways to address it as well, but I haven't settled on anything. It would be much easier if I had my own printer and could experiment freely, rather than feeling like each print job needs to be "perfect" when sending it out to a service.

 

[mbeels]

The body tube rests in the "V" on top. Straps go through the two slots to hold it to the body. Aluminum angle fits into the channel on each side of the main spine, and then clamps are fit over the top. Like this:

Ahhh... I see. That picture makes it all clear. That is a really good use of Aluminum angle, I see a lot of 3d printed fin guides 100% 3d printed, but I like the use of straps and angle here.

 

[neil_w]

The benefit is that it is a flexible system that will work with most rockets regardless of fin configuration. The downside (as discussed) is that it doesn't rock-solid hold the fin in position. You need to make sure it's positioned properly before, during and after.

I can understand that those with their own 3D printers would never use such a thing; too easy to just print out a custom jig for each rocket based on any of the various parametrized designs on Thingaverse.

 

[jqavins]

I like the design. Off hand I think you're right that longer wings at 90° would work better. It might also be helpful to attach something grippy like jar openers to the inside of the V.

What about the web thickness on the vertical? That makes it fit just one fin thickness, doesn't it?

 

[neil_w]

The one I'm using is 3/32", and I've found it can accommodate a bit of variation in fin thickness. Putting a layer of tape on the inside of the angle pieces can enlarge it to 1/8". I also printed on with a 1/16" vertical, but I rarely use that because my fins are almost always 3/32" or 1/8".

 

[neil_w]

Second set of fins is on. Just for yucks I decided to see if I could clamp all three at once. Answer:

Note that I let the main fin dry, and then applied the clamp around the still-wet front and back fins.

Also, to wrap up the discussion of the fin jig: here it is sitting on the BT60:

The "wings" could stand to be a tad longer, perhaps, but the main problem seems to be the overly obtuse angle. It also occurred to me the other day that I don't need the wings to extend the whole length of the jig; I just need a pair of narrow wings at each end. That might work even better.

 

[Ted Cochran]

A little late to the thread, but I have usually found that papering fins with glossy photo paper eliminates the need for sanding, albeit at a cost in weight. I’ve even printed wraps with them before putting them on, like my Maryland flag-themed Nike Orion.

 

[neil_w]

A little late to the thread, but I have usually found that papering fins with glossy photo paper eliminates the need for sanding, albeit at a cost in weight. I’ve even printed wraps with them before putting them on, like my Maryland flag-themed Nike Orion.

Well that's certainly one I haven't heard before. I'm not sure how perfect smoothness would achieved around the edges of the fin using that kind of paper. That's where my papered fins benefit most from the filler/primer, although the copy-papered fins definitely benefit as well, since the paper conforms to the grain a bit.

Fillets finished on fin set #2, only one more to go.

Sometime soon I'm going to need to figure out where the launch lug(s) go. I deferred that decision in the past and it just occurred to me the other day that I need to get to that else I forget entirely.

The tricky bit here is staying out of the way of the decals.

 

[Ted Cochran]

You are correct, airfoiling fins is difficult, and even if you taper the leading and trailing edges to a point, you still have the fin tips to muck with. For slab fins it works well, and it can be a timesaver for a challenging paint job.

Thumbnails (The Der Red Max was spray painted; the Terrier Orion is straight ink jet on photo paper)

 

[neil_w]

All fins are on. The third main fin feels a little less rigid than the others; I fear I didn't get a great bond (for whatever reason) on the fin root/motor mount. No way to either know or do anything about it, so I'll move on. It doesn't feel loose by any stretch, just not as rock-solid as the other two.

Here's the whole thing, still minus the launch lug(s):

Just a plain old 9F7NC1T3R.

 

[mbeels]

And minus the ringy thingies? (the "3R" I presume?)

By the way, I'm trying to decide if you have a rocket shadow on your ceiling, you're standing close to a wall, or you have a very clean work bench and the photo is rotated 180 degrees.

 

[neil_w]

And minus the ringy thingies? (the "3R" I presume?)

Yeah, those go on at the very very *very* end, so I kind of forget about them. Further, I don't plan to cut the slot in them until after paint, so I can't even dry fit them in for now. If I could I surely would; I've never seen what the whole thing looks like with the mitered rings and would very much like to. Gonna be a (depressingly long) while before paint is complete.

By the way, I'm trying to decide if you have a rocket shadow on your ceiling, you're standing close to a wall, or you have a very clean work bench and the photo is rotated 180 degrees.

Now that you mention it, that is kind of a weird picture. But no rotation and no ceiling; it's just my holding it in front of a large white backdrop. The shadow is cast from my workbench light, which is behind me to the right. It seems to be above the rocket because I'm holding the camera somewhat higher than the rocket, looking down at it.

Ultra-wide lens on my phone came in handy; this rocket is just long enough that I can't quite get the whole thing into the picture at arms length with the 1x lens.

 

[afadeev]

Nice.
Very Interceptor-ish (that's a compliment)!

Are you still going with an all-black color scheme, or has that evolved?
Any metal flakes over black?
Color shifting paint?

 

[neil_w]

Color scheme is unchanged up until now, but I might fiddle just a bit with the lettering. Color is POB (plain old black, and needs to be so I can match the decals to the background. The design requires light-on-dark decals, so given that I don't have an ALPS printer I must print them on white decal paper, with black background to match the paint.

It's too bad, this would look great in black metallic, but it is not to be. I'm still trying to fine-tune the decal strategy, but only radical changes would enable me to diverge from the black, and I don't plan radical changes because I quite like what I have right now.

Illustration:

All the orange fade-to-black must be printed black to match the paint. Were it not for the fade, I could laboriously cut out each chevron and wouldn't need to match the background. I still haven't decided if I'm going to print the decals in one large piece, or cut out the chevrons separately. Separately would make for a miserable placement task, but would enable me to eliminate rather large areas of solid black decal that may show slightly.

 

[jqavins]

The large decal patch would show only very, very slightly, I suspect. Any difference in surface texture will be washed out by a clear coat. Save yourself the headaches and consequent large aspirin bill.

 

[neil_w]

Save yourself the headaches and consequent large aspirin bill.

Undoubtedly.

 

[lakeroadster]

Decals are a love hate thing. Love the graphics... hate the texture / sheen variances.

Maybe for the next rocket go modular: design slip in fins... and then wrap the body tube.

If only we all had Big Daddy Ed Roth's air brushing skills..

 

[neil_w]

Back to launch lug placement. I could put a nice 2" lug alongside one of the front fins. However, that'd be about 2-3" behind CG. Does this matter? I've never come to any solid conclusion about lug placement.

I could put it up around CG but of course that would be a lot uglier.

 

[mbeels]

Back to launch lug placement. I could put a nice 2" lug alongside one of the front fins. However, that'd be about 2-3" behind CG. Does this matter? I've never come to any solid conclusion about lug placement.

I think that is fine, I have several LPR with the launch lugs tucked into the fin roots. No problems.

 

[Nytrunner]

I like to put the lug or forward button at around the lateral CP. Since that's where the resultant wind forces will be acting while its on the rail. That way the wind is just putting a side load on the lug/button, instead of a torque

Also, for rockets this size, there's a lot of leeway.
I call it the Babar principle: You can get away with a lot at low power.

 

[boatgeek]

Back to launch lug placement. I could put a nice 2" lug alongside one of the front fins. However, that'd be about 2-3" behind CG. Does this matter? I've never come to any solid conclusion about lug placement.

I could put it up around CG but of course that would be a lot uglier.

In my humble opinion, lug length (or spacing of rail buttons) is far more important than the exact location. Further aft gets you more speed off the rail before you lose guidance. I am happy to be educated otherwise, though.

 

[jqavins]

I'm in the same fix. I keep going back and forth on the physics, whether it should be at the CG, anywhere forward of the CG, [EDIT] at the CP (thanks to being ninja'd by Nytrunner and boatgeek)[/EDIT] or anywhere you feel like. I currently think it doesn't matter. And it's definitely better to have two that are spread apart, so you could put one short lug alongside a forward fin and another alongside the corresponding aft fin. And that way they can be smaller - an inch or even less each - as the separated pair should be as good as a single one as long as the full length of the two plus the gap all combined. (Think of it as one very long lug; taking away most of it from the middle makes no real difference.) Two lugs each a half to three quarters long could be made quite unobtrusive.

 

[neil_w]

I like to put the lug or forward button at around the lateral CP. Since that's where the resultant wind forces will be acting while its on the rail. That way the wind is just putting a side load on the lug/button, instead of a torque

That makes good sense. Not sure why I would care about CG when it's on the rod. Conveniently, CP is exactly on that front fin.

In my humble opinion, lug length (or spacing of rail buttons) is far more important than the exact location. Further aft gets you more speed off the rail before you lose guidance. I am happy to be educated otherwise, though.

I normally use 2", 3/16" lugs on my 24mm rockets. But I've always been putting them close to CG.

And that way they can be smaller - an inch or even less each - as the separated pair should be as good as a single one as long as the full length of the two plus the gap all combined. (Think of it as one very long lug; taking away most of it from the middle makes no real difference.)

Well, at least until the front lug clears the rod, and then you're left with only the rear one. Although I've seen it done I wouldn't be inclined to put the front small lug way far up, especially on a longer rocket.

I think a 2" lug at the CP (front fin) should work well enough here, and be comparatively unobtrusive.

 

[mbeels]

I call it the Babar principle: You can get away with a lot at low power.

That's a good one. I'm going to quote you/him on that.

 

[neil_w]

I dunno if this is the best or smartest or best-looking I could have done, but I think it'll work OK. As it turned out, I couldn't really get a 2" lug next to the front fin without it looking horrible, so I went with two smaller ones, each about 3/4" long plus or minus.


We're almost ready for paint.

 

[jqavins]

[Y]ou could put one short lug alongside a forward fin and another alongside the corresponding aft fin.

View attachment 419116

I'd forgotten that the aft fins are on the boat tail, so lugs alongside them obviously wouldn't work. Looks good, and I"m sure it'll work great too.