TRF Summer Build Off: The Ellipse

Don't get discouraged...setbacks and snafus go along with scratchbuilding. Have you considered iron on fabric made for model planes? It conforms well, comes in large rolls, is lightweight and shrinks tight.
BTW- The frame looks fantastic!

So, I filled the really bad divots, applied a second layer of paper, applied thin CA like dope, and started sanding. I had resigned myself to the inward swoops between the structure pieces. The surface is much better, and then I noticed this:


It's bowed from end to end.

Well, this is why I had two sets made. I'll go back to the hobby store for more stringers and start over; perhaps I'll use basswood stringers this time. I'll also go to Staples and get heavier paper in a large enough sheet for the whole length of the tube in one piece. And finally, I'll try putting flue on the frame instead of wetting the whole piece of paper and see if that helps.

Yesterday I also filled and sanded the fins. The hobby store is closed on Sundays, so I'm done for the day, unless I try smoothing the nose cone somehow.

KenECoyote said:

Don't get discouraged...setbacks and snafus go along with scratchbuilding. Have you considered iron on fabric made for model planes? It conforms well, comes in large rolls, is lightweight and shrinks tight.
BTW- The frame looks fantastic!

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Looking at the frame more it looks too light to just apply a light skin, so nix my covering suggestion unless you have bigger stringers or supports. Now I see why you used heavy paper/card stock.

jqavins said:

So, I filled the really bad divots, applied a second layer of paper, applied thin CA like dope, and started sanding. I had resigned myself to the inward swoops between the structure pieces. The surface is much better, and then I noticed this:
View attachment 295107
View attachment 295106
It's bowed from end to end.

Well, this is why I had two sets made. I'll go back to the hobby store for more stringers and start over; perhaps I'll use basswood stringers this time. I'll also go to Staples and get heavier paper in a large enough sheet for the whole length of the tube in one piece. And finally, I'll try putting flue on the frame instead of wetting the whole piece of paper and see if that helps.

Yesterday I also filled and sanded the fins. The hobby store is closed on Sundays, so I'm done for the day, unless I try smoothing the nose cone somehow.

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Ouch! You may want to try lightly wetting/spraying water one side (top side when looking at the second picture...the side with the longer curve) and then using a hot hair dryer on it...you may be able to get it straightened out since the wet paper (and possibly wet wood) will shrink as it dries, but it will take finesse and may require adjusting the other side as well. Also I'm not sure how the paper will turn out...it may wrinkle up badly, so a test piece would be wise. YMMV.

*EDIT* I just reread that you used thin CA (which may have been the cause of the warping), so it's quite likely you can't wet the paper enough for this technique. Sorry!

It might be best to just harvest those nice bulkheads. Remove the paper and slowly cut out each slot while binge watching some show on-demand.

Not sure if it helps, but I saw this pringle-sized "can" of chips and saw the oval-shaped airframe... Something to consider.

rstaff3 said:

It might be best to just harvest those nice bulkheads. Remove the paper and slowly cut out each slot while binge watching some show on-demand.

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If my second attempt fails I may do that. But I originally ordered two sets of the laser cut parts - because failure is always an option - so I can just start over with the other set. I have three process changes (I hope improvements) in mind, and if I get it right with the second set than I'll just throw this one away.

I don't think I'll get out for more parts until Wednesday, so no work will be done for a little while.

Progress has been made. The frame is rebuilt, with some setbacks and modifications, and is ready to skin. Pictures and more description to come shortly.

OK, new pictures. First, the frame.

The sharp eyed will notice two things. First, it's shorter than the original. The new stringers are a smidge larger than the first ones and took a bit of forcing into the notches on the ribs. Some of the ribs cracked and were glued back together. The aft-most rib shattered irreperably as the fourth stringer was being inserted, after three stringers were already glued in place. So, the rocket got one rib, 2 inches, shorter. I'm sure it's tall enough for stability, so this is no big deal.

Second, the 45° stringers don't go all the way up. Another shopping blunder, I was two stringers short. But these are not really necessary up high; they're mainly there because the fins will be attached directly over them. So I cut the two I had in half and lived without them going all the way up.

There are two features installed at this point that I should have done the first time, but I thought of them this time.

The green is spectra fishing line, plenty strong enough for the parachute. In case the picture isn't clear enough, it comes into the parachute tube from the top, out through a hole in the side just below the top rib, around the tube, and is tied into a loop, with the know back on the inside. The edge of the hole has been wetted with CA.

Those piano wire engine "hooks" are something I saw in (I think) a Peak of Flight article on motor retention for min diameter rockets. In that case, the legs would go on the outside of the rocket since there is no centering ring, and can be hidden in fin fillets. But I decided I like it a lot better than traditional engine hooks and I've been using it as shown ever since.

And here it is wrapped.

The wrapper is poster board, so it isn't dimpling between the ribs. I used epoxy rather than wood glue so I would have some working time, and I applied it to the frame not the paper so the paper isn't all wet. I wanted to use rubber bands placed directly over the ribs to hold the paper in place, but couldn't get any that weren't too loose or tight enough to break something, so I settled for pulling the paper tight and applying the tape. The seam is far from perfect, but I hope enough filler and sanding will fix that. The seam is directly over one of the stringers, which should help. (Which is the main reason I put it over one of the stringers.)

So I will let it stand on the stand overnight, then take off the tape and start on seam fixing. There are a couple of wrinkles in the skin, but it's much better than the last one.

Nice!

Looks promising. You need to pose it with the nose cone just to give us a tease.

So, there is more than one way to skin a rocket!

Great thinking and kudos on the redux!

neil_w said:

Looks promising. You need to pose it with the nose cone just to give us a tease.

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I'll see that nose cone, and raise you a set of fins!

I knew that seam was going to need a lot of work filling and sanding before it will disappear. With the tape off I see now that it will take a whole ton of work. And it's not so well positioned over a stringer as I was trying for. So my next couple of evenings are going to be spent in the company of lots of sand paper add filler. And repeated cases of the fuzzies are going to be inevitable, so I may as well CA the edges before I even start. But in the end, it will work.

That's the SPIRIT!

I think it looks GREAT! :clap:

Looking good!

Uniquely and wonderfully weird! :cyclops:

The skin looks like it came out really good.

Excellent! Turned out really well.

Filling and sanding of that seam is going acceptably well. Pictures later.

Another sanding and filling over lunch. Also, finally got the nose cone shoulder reshaped. It had flared a little at the bottom during printing so was much too tight. (And a little tight the rest of the way up.) I tried sanding it down and made a discovery: PLA is hard. Even with 120 grit I was getting nowhere, so today I got out a file and that's done.

Incidentally, I've ascertained that it's PLA (as opposed to ABS) by chemical testing on the base of the shoulder. Acetone doesn't touch it and CA bonds well.

neil_w said:

I wonder if there's any reason someone couldn't actually roll proper spiral tubes in this shape, given a proper mandrel. Not at all suggesting you do this, just pondering theoretically. Does a rolled tube have to be round?

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I meant to reply to this a while ago. The geometry of wrapping a spiral strip around an elliptical mandrel is fine*. The problem is how to get or make the mandrel. If I had an answer to that, I would have gone that way (but parallel wound) rather than with the frame.

Opticians and optical labs use a machine that grinds the edge of a lens to the shape of the opening in a frame. (Here's an example.) The machine follows a form to duplicate its shape in the lens blank; think of a radial key grinder. What's needed is sort of a cross between that and a lathe, so it it can cut the outline of a form (which is just a shaped wafer) over the whole length of a work piece.

Or another way it might be done: put a dumbed down lathe on the bed of a CNC vertical mill. Add a position sensor to the chuck of the lathe (or its drive) and make sure the mill bed can move the whole length of the lathe bed, though it doesn't have to move in the other direction at all. Then with the rotary position available to the CNC system, the shape can be programmed for anything you want, even changing the cross-section profile along the way. We should probably replace the traditional lathe drive with a rotary position servo rather than just slowing it down.

I'm sure such a machine could be built. I've never heard of one, which doesn't mean it isn't out there, if it is out there it probably costs a fortune, and I'm sure not about to try to make one. (It would be useful for many more jobs than elliptical model rockets, and someone should make it, but I'm not that someone.)

* I'm pretty good with math, but at first I couldn't figure out if this would work. I didn't see any reason it shouldn't, but the circumference of an ellipse is an unsolvable integral, so things might be more complex than I could see. I thought it would work, but couldn't prove it to myself. (Which wouldn't have kept me from trying it if I could have made/obtained one.) Well, preparing the paper skin this time left me with a 2" x 14" strip of scrap, so I wrapped that around my present tube and can now confirm that the geometry is fine.

Well, I've done a lot of work on the seam area, and it's just never going to be perfect. After fill-sand-fill-sand-a little CA-fill-sand-more CA-sand-fill-sand-fill-sand, it's finally time to fish or get off the pot. I've applied a generous coat (partial soak) with Aerogloss, and moved on.

Henceforth, all photos will be taken of the good side.

I'm a klutz. Specifically, while my manual dexterity and balance are pretty good, my hand-eye coordination stinks, my depth perception is practically non-existent, and my vision in general is pretty bad. So, my motto is "Measure twice, cut once, hope I can fix it." This is why I use fixtures and other aids everywhere I can. I love my Estes grey fin alignment jig, but it obviously won't do me any good on this build.

I created a pattern for the positioning/alignment aids below in MS Visio so that I could make sure it is set up by the numbers. I glued two copies to a Coco Puffs box and cut them out using a straight edge wherever possible. They are fastened to the tube with blue painters' tape and hold the fins aligned and positioned just so.

The glue will dry over night. Tomorrow I will apply glue fillets, and I should be able to start painting on Monday.

(Prime-sand-prime-sand-prime-sand...)

It's coming together nicely. I often only photograph my rockets on their good side

Fins and launch lug attached and filleted. Nose, fins, and half the body tube covered in first primer coat.

Yes, half. I was spraying along fine with my rattle can of Rust-oleum Painter's Touch Ultra Cover 2x yesterday, half way up the tube, when the spray switched off abruptly. Not faded, not sputtered, just switched off. I shook the heck out of it in multiple orientations to no avail. I pulled the nozzle and found this can is of the nipple-on-the-can type, so I pressed the naked nipple down (control yourselves, boys) and still nothing. I let it sit overnight and tried again this morning, because some things just magically get better over night, but this one didn't.

Any idea why this happens? More importantly, any idea how to fix it?

Update: I chucked the can in my lathe and set it spinning on its lowest speed. I'll check it in an hour.

In the mean time, I finished spraying the tube with a different can. It's not as good for priming, but it'll do for the moment.

jqavins said:

Fins and launch lug attached and filleted. Nose, fins, and half the body tube covered in first primer coat.

Yes, half. I was spraying along fine with my rattle can of Rust-oleum Painter's Touch Ultra Cover 2x yesterday, half way up the tube, when the spray switched off abruptly. Not faded, not sputtered, just switched off. I shook the heck out of it in multiple orientations to no avail. I pulled the nozzle and found this can is of the nipple-on-the-can type, so I pressed the naked nipple down (control yourselves, boys) and still nothing. I let it sit overnight and tried again this morning, because some things just magically get better over night, but this one didn't.

Any idea why this happens? More importantly, any idea how to fix it?

Update: I chucked the can in my lathe and set it spinning on its lowest speed. I'll check it in an hour.

In the mean time, I finished spraying the tube with a different can. It's not as good for priming, but it'll do for the moment.

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That has happened to be and I never got the can working reliably. I tried soaking the cap in thinner and working thinner down into the nipple. That worked very temporarily and was not worth the cost of a new can. If paint is flowing but not coming out evenly, cleaning the cap has worked.

rstaff3 said:

That has happened to be and I never got the can working reliably. I tried soaking the cap in thinner and working thinner down into the nipple. That worked very temporarily and was not worth the cost of a new can. If paint is flowing but not coming out evenly, cleaning the cap has worked.

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Thanks. Yeah, the nozzle (cap) is fine; it is clear both visually and blowing through. The problem is all in the can.

Assuming it isn't the nozzle, what has worked for me with similar scenarios is first shaking it up and down briskly/with verve and then trying to spray upside-down. Usually it will spray a bit wimpy or fizzy at first and then clear. It may be an odd clog inside the can at the straw end and usually they can be cleared. Note you may need another nozzle in case the current one clogs as you're trying to unclog the can. Good luck!

Good to know, hopefully I remember your suggestion the next time.

rstaff3 said:

Good to know, hopefully I remember your suggestion the next time.

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Whatever you do, don't forget the verve.

Tried that already, but I'll give it another shot. I looked through my two boxes of rattle cans and found another one of the same stuff. <GRUMBLE>Of course, this one feels less than half full and the dead one is nearly full.</GRUMBLE>

neil_w said:

Whatever you do, don't forget the verve.

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jqavins said:

Tried that already, but I'll give it another shot. I looked through my two boxes of rattle cans and found another one of the same stuff. <GRUMBLE>Of course, this one feels less than half full and the dead one is nearly full.</GRUMBLE>

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Did you shake it with verve? :wink:

Also I just recalled that when I have this issue, I'll grab one of my old nozzles (I keep them on the side before tossing the empty spray can...never throw anything away! ) and use that for clearing since I don't mind throwing it away. You can also try putting the can in warm/slightly hot water to increase the pressure before you try clearing. Good luck!

'bout time I got out of round to looking at this thread...

Looks a lot better!