Neil_W's half-baked design thread

[Nytrunner]

This one might be within my build skills.

BoatGoon!

 

[neil_w]

This one might be within my build skills.

I'm not actually sure why it would be any easier than the normal one.

 

[neil_w]

Having completed the solipsistic exercise of creating goonies of my own designs, I will now be focusing on two brand new designs, one sci-fi (I have ideas) and one just a plain sport rocket (nothing in mind yet). Of course I will post any other stupid idea that comes to mind in the meantime.

 

[jqavins]

What, before building (at least) one of the goonies? Or the upscale pencil?

 

[neil_w]

My build activities are constrained for a while, probably for another month, although I have been working on the ferrule for the pencil rocket. Either way, design activity proceeds in parallel. I don't know if I will build any of the goonies, although they are cute.

 

[mbeels]

Having completed the solipsistic exercise of creating goonies of my own designs, I will now be focusing on two brand new designs, one sci-fi (I have ideas) and one just a plain sport rocket (nothing in mind yet). Of course I will post any other stupid idea that comes to mind in the meantime.

Looking forward to all of the above, including the "stupid" ideas!

 

[jqavins]

I'm rooting for Goonyhazard!

 

[neil_w]

I decided the wrinkling label paper was not a good idea, so I tried CWF-ing a piece of balsa and painting that. I did two coats plus a bit of sanding to knock down the brush marks.


It was so easy to paint and the results are more than good enough. If I did another coat and sanded a bit more I could probably have it looking as good as spray. Interestingly, it's also quite smooth after a 400 grit sanding. Overall a nice finish. 220 roughs it up enough to make it *extremely* matte and emulate the surface feel of an eraser a bit better, but I think the smoother finish will be more durable. We'll see. In any case, that leaves me with no more questions on the ferrule. My next big obstacle is figuring out how to cut the Lexan fins cleanly.

 

[jqavins]

220 roughs it up enough to make it *extremely* matte and emulate the surface feel of an eraser a bit better, but I think the smoother finish will be more durable.

How about the 220 grit matte finish for the paint followed by satin clear coat?

 

[neil_w]

Maybe. I don't know the ins and outs of clear-coating this sort of paint; some learnin' to do.

 

[kuririn]

My next big obstacle is figuring out how to cut the Lexan fins cleanly.

A power tool of some sort is the way to go. Ditch score and snap.
An oscillating tool or jig saw will give you more control than a Dremel.
Lesson learned from many botched fins.

 

[neil_w]

Yeah, I've already given up on score and snap.

 

[boatgeek]

I used a jigsaw to cut Lexan. It wasn't too hard to get them pretty close to even. Be aware that the chips will be electrostatically charged and will stick to everything.

 

[neil_w]

I guess I'll give it ago with a jigsaw. I don't have one but my Dad does. What do you do with the protective film? Can the saw cut through it without getting fouled? Pre-cutting the film in perfect alignment on both sides seems difficult.

Also I guess you really need to clamp down the sheet.

 

[lakeroadster]

I guess I'll give it ago with a jigsaw. I don't have one but my Dad does. What do you do with the protective film? Can the saw cut through it without getting fouled? Pre-cutting the film in perfect alignment on both sides seems difficult.

Also I guess you really need to clamp down the sheet.

IMHO.... a scroll saw works much better for intricate cutting.... cut them slightly oversized and then sand to fit.

 

[boatgeek]

I guess I'll give it ago with a jigsaw. I don't have one but my Dad does. What do you do with the protective film? Can the saw cut through it without getting fouled? Pre-cutting the film in perfect alignment on both sides seems difficult.

Also I guess you really need to clamp down the sheet.

My saw went right though the film with no trouble. I wasn’t as worried about clamping, but my fins were pretty thick, too. It can’t hurt to clamp it down well and go slow.

 

[neil_w]

How do I determine if 1/16" or 3/32" Lexan is needed? There's a useful weight savings with the 1/16".

 

[SDramstad]

Fly it with the 1/16th. If it breaks you need the 3/32nds........



Yes, I am a smart @ss......

 

[BABAR]

Fly it with the 1/16th. If it breaks you need the 3/32nds........



Yes, I am a smart @ss......

How to decide how long to microwave a potato.

Buy two potatoes of similar size.

Put one in microwave on high, set for 20 minutes.
Wait for it to explode.
Clean the oven.
Put the second one in for one minute less.

 

[jqavins]

But seriously, folks. When in doubt, test. And please test responsibly.

Neil, you normally would make fins for rocket of this size from balsa, would you not? How thick, 1/8", 3/16", 3/32"? Cut a square piece of that, about the size of your fins. Support it between two blocks, the grain running across the gap, and place weights in the middle until it breaks.

Cut a matching piece of 3/32" lexan, place it in the same test rig, and subject it to the same weight. I bet it takes it, even though it's likely to flex more than the balsa. Flex? Are you worried about flutter? If the flex is enough to scare you then go up to 1/8", if not go with 3/32".

Unless you get an answer from someone who already knows, in which case I'll just shut my mouth and go away.

 

[kuririn]

For low power, I think you'll be fine with 1/16".
Lexan and acrylic are much more rigid than the equivalent thickness of balsa.
Plus no grain.

 

[neil_w]

For low power, I think you'll be fine with 1/16".
Lexan and acrylic are much more rigid than the equivalent thickness of balsa.
Plus no grain.

That is my hope. I'll be looking at D and E motors for this thing, probably, LPR-like velocities (this is not the rocket I'll be sticking an F44 into).

But seriously, folks. When in doubt, test. And please test responsibly.

Fair. In this case though I'm hoping to sponge off the experience of others if possible. At the moment I don't have a piece of 1/16" lexan to test with (anyone know where I could find some at retail?)

 

[boatgeek]

How do I determine if 1/16" or 3/32" Lexan is needed? There's a useful weight savings with the 1/16".

... At the moment I don't have a piece of 1/16" lexan to test with (anyone know where I could find some at retail?)

I was tempted to be a smartass but I guess I can be helpful for once. Out here, Lowe's and Home Depot both have Lexan sheet, sometimes under the generic name polycarbonate. My highly scientific method is to go to that aisle, hold the sheet firmly so the exposed part is the size of my fin, and flexing the sheet. If it seems too flexible, go to the next size up.

 

[kuririn]

Space X Falcon 9 kit, 1/16" clear fins.
24mm power, flew straight as an arrow on it's maiden flight.
Fin flex is not a big concern on LPR.

 

[neil_w]

Space X Falcon 9 kit, 1/16" clear fins.
24mm power

Sold!

 

[SDramstad]

Only one in my fleet with lexan fins. Flew great on a quest "D".



Measured them at .075 inches thick. About 2mm. Lost 2 on landing but it was super glue on paint so they didnt stand much of a chance...

 

[BABAR]

For low power, I think you'll be fine with 1/16".
Lexan and acrylic are much more rigid than the equivalent thickness of balsa.
Plus no grain.

Although I went to an engineering school, my degree is in biology.
Is rigidity equivalent to durability, in this case?

 

[kuririn]

Although I went to an engineering school, my degree is in biology.
Is rigidity equivalent to durability, in this case?

Gee, now you got me thinking. I'm not an engineer either, but I would have to amend my statement to say that plastic fins may be more elastic than balsa depending on the direction of the force (i.e. with or against the grain).
But plastic fins can take the same force without snapping like wood fins.
So in that sense it is more durable.
Perhaps an engineer can explain this in terms of coefficient of elasticity and modulus of rigidity for us lay people?
I need to lay down, I've got a headache.
Cheers.

 

[boatgeek]

I'm an engineer, so I may as well dive in.

Rigidity/flexibility isn't exactly the same as durability/strength, but they can be related. For rigidity, you're looking at how much the material moves when you push or pull on it. Steel is more rigid than aluminum, which is more rigid than plywood. That's pretty much the flexibility test I recommended above. If you want to get all technical, it's measured by the Young's Modulus (E), which is in units of pressure (usually kips [aka 1000 lbs]/square inch for people who use God's units, and MPa for people who use rational ones ).

You can look up Young's modulus for most materials you like--steel is around 29000 ksi, and Lexan is around 300 ksi. That indicates that for a given thickness, fin shape, and force, a Lexan fin will deform about 100 times as much as a steel one.

Durability is entirely another kettle of fish. Materials can be very flexible and return to their original shape, or very rigid and prone to breaking under the slightest load. A balloon is a great example of the former, or of the latter if it's been dipped in liquid nitrogen. A little bit of flexibility tends to make things more durable because the flex absorbs shock loads. Fatigue can also play a part, where a material is fairly strong until it starts cracking under repeated loads. There's a few ways to measure durability for metals, but it's harder to give a measurement for a generic material because there are all kinds of different failure modes.

Here's a fun trick if you have a spare bobby pin or two. First, try bending it straight. It'll be pretty hard. Now, take a lighter and heat the bend until it glows. Let it cool and try again. It should be a lot easier. Finally, heat it until it glows and dunk it into water. It'll be harder to bend at first and then it will suddenly break.

 

[kuririn]

You can look up Young's modulus for most materials you like--steel is around 29000 ksi, and Lexan is around 300 ksi.

So what is Young's modulus for balsa wood?