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Discussion in 'Scratch Built' started by neil_w, May 8, 2017.
Hmm, those diagrams show balsa having a *higher* modulus with the grain than Lexan, but values I've looked up elsewhere show Lexan having 3-4x the modulus.
This website says 3 GPa for balsa, vs. 13-23 GPa for Lexan.
It certainly feels to me like the 3/32" Lexan I have from Home Depot is much stiffer than 3/32" balsa.
I had read those charts as "polymers have lower variance than natural products" i.e. odds are better than half that a random sample of polycarb will be stiffer than an equal thickness of a random sample of balsa.
Ooh, goody. Looks like it's time for another test. Shear test of Lexan vs balsa, same thicknesses.
Just to amplify boatgeek's statement about durability being harder to determine due to multiple failure modes: "durability" is a qualitative thing and so inherently impossible to quantify. Resistance to many failure modes can be quantified, and then some of the failure modes are either themselves qualitative or dependent on environmental variables. And then one must consider the application, i.e. which sorts of failure inducing forces the item is likely to be subjected to. So all in all, not only is there no single answer, there fundamentally cannot be a single answer.
It is for such reasons that I mentioned two different things: a breaking strength test, if a rather crude one - supportable load for a given span and width - and a flexibility observation - deflection at the weaker material's breaking load. These are the two "failure inducing forces [a fin] is likely to be subjected to" as far as I could estimate. And, as noted, the bending load, i.e. fin flutter, is unlikely to be an issue here in LPR so stiffness is only an issue if it is scary low.
On the other hand, practical, experiential answers have since been given, and I did promise to shut and go away if such should occur, so [sound of a zipper closing and footsteps receding].
In-flight testing is king, is it not?
My Falcon 9 flew fine with the stock fins, but those occasional rougher-than-normal landings is what cracked them *every time*
I would personally go with one step thicker than that.
Hey, long time no post!
That is valuable input.
It does look like the F9 has those swept-back pointy-tipped fins which would seem to have a lot more vulnerability to landing impact. My tail will be more like this:
In theory the fins should be subjected to much less landing impact shock. I'm still fiddling around with the final fin shape. Because they're not all the way in back, they're not nearly as effective, so I was hoping at least hoping to save some weight.
Realistically, I could make them 3/32" (which would definitely be plenty strong IMHO) and all it'll cost me is a bit more nose weight, so I probably shouldn't worry about it.
I've used a replaceable bamboo skewer mounted between the motor mount and the BT that sticks out 4 or 5" to protect fragile fins at landing.
Just a thought...
Looking at your fin shape.
I am assuming that the fins are supposed to be invisible. and also assuming that you are not shooting for altitude, so DRAG efficiency isn't a big deal, but CP efficiency IS a big deal.
I am using nomenclature from this post for fin terms
Note sure how you are picking the root chord length, but figure you have a fixed length in mind.
That being the case, would a square or rectangular fin shape (take out the delta in front) allow you to use a shorter semi-span for the same CP benefit?
POSSIBLE advantages of such
1. The smaller the semispan, the lower the flutter and also less stress/probability of breakage if/when it DOES bump into something.
2. Less weight (probably negligible.)
References online say 450-550 ksi, which matches up with the 3 GPa along the grain in the chart above. That goes down to 0.1-0.2 GPa for stiffness across the grain. As already mentioned, natural materials are pretty variable, and this seems especially true of balsa from other users I've seen here. I don't have a lot of experience, but I see people talking about buying stiff balsa or soft balsa.
I'm getting 2.3 GPa from the linked website for unfilled polycarbonate, so nominally less than balsa along the grain but much more than balsa across the grain. One interesting aspect of this is that with the grain aligned along the leading edge, spanwise flexing of the fin is a combo of along the grain and across. That makes an engineering analysis get really unpleasant really quickly, as it is with most non-isotropic composites. Best to stick with the push-on-it-with-your-thumb test!
Constraints are as follows:
1) Fin cannot be attached to the ferrule.
2) I thought at least 1.5" of attachment (just in front of the ferrule) seemed like a reasonable minimum
3) No more than 1.5" hanging over the ferrule. This is a completely arbitrary decision. In the current design it's 1.125" overhang.
Preference is to be more durable to the extent possible.
In OR, squaring off the front, given the same fin root, brings the CP *forward*.
Given fixed fin root, biggest CP benefits come from additional sweep, although at the possible cost of some durability, and also maybe the fins become a bit more visible.
This shape gains me .05 calibers of stability, by a combination of slight improvements in weight and CP.
I don't know if this would look better or worse or be less durable or what.
I'm not sure if I should worried about appearance at all, because in real life the Lexan fins seem to disappear pretty well regardless of their shape. Originally I had thought that the swept-back fins would be more conspicuous than the design I posted previously. Dunno.
To help them disappear, not that I've done this, polish the edges after cutting. Probably round the edges and then polish. And by "polish" I mean until it's all the way back to clear.
I think "jeweler's polish" is used for that.
Like the edges of eyeglass lenses.
You pay extra to have them clear(polished) instead of cloudy(unpolished).
Personally, not worth it to me to pay the extra bucks.
I'm not that vein, er vane, er narcissistic to do that.
Less talking, more doing. Build it lol! You got a motor mount, right?
Oh, is that what it's for?
Wow, Okay, I hadn’t figured on the fins being THAT close to the CG.
Does making the rocket a little bit LONGER (move the nose further forward) throw the “scale-pencil-look” off to much?
Also, any chance of recessing the motor to bring CG forward? IIRC
Krushnic allows you up to about 1 tube diameter before he starts stealing your thrust.
Of course, then your Eraser takes the landing impact, unless you put some dowels out the back.
Or if you really want crazy, try the Estes Scout trick. Motor shifted back at deployment, I think maybe had a long engine hook? So you recess the motor less than one tube diameter, at deployment ejection force pushes motor back say one inch, should still have a seal and enough force to deploy the laundry. Tricky, though, if the hook fails and the engine ejects........
Just trying to come up with something other than adding more fins that will give you a bit more stability. It would be something of an irony if this thing ended up SkyWriting.
You could put something that leaves a smoke trail on the tip of the nose cone. Sky writing either way.
Seriously though, looks really cool so far Neil! That eraser tip looks perfect. What is your plan for attaching the fins?
The real one will look better.
The fins will be fit into TTW pockets at three of the vertices just above the ferrule. I think they'll hold quite solidly. This will all be covered in a build thread, eventually; I think this rocket is interesting enough to warrant one.
Construction of the body is still a ways off, though.
Longer is the best answer I think. I added a few inches to the front, and gained some margin and was still able to reduce the semispan a bit. Small amounts of nose weight should be sufficient to make it all work if it becomes too tail-heavy during construction.
It'll still be within the aspect ratio of a standard pencil.
The motor mount will be very standard here.
I just remembered the 1/200 Estes Sat. V has clear plastic fins. So I eyeballed the thickness.
Definitely less than 1/16". Perhaps 1mm?
Flies OK on a C6, really good on a D16.
So good I had to replace it.
What a clever idea
Great idea, only concern is to round the rear ends or put rubber caps on them.
Safety issue if it comes down in the spectator area. Although if you landed in grass with pointy tips you would get extra points for sticking the landing!
Catching high power rockets is against the safety code (also a Darwin Award nomination.). Not so for low power, and I have seen kids and adults try to catch descending low power rockets. I saw one kid get cut from the plastic fin can, I believe it was an AstroBeam.
How big are the fins for the plasma dart? Although you said I should be able to figure that out, I can't. Or do you have an ork file you could put up in a post?
Give the man what he wants Neil!
I'm not trying to be demanding here, unlike what you make it sound like
I think it's meant to be in jest :-D
Nytrunner kids everybody, and this time he's mostly kidding Neil. And Neil is well known to give back as good as he gets. Don't sweat it.
I cannot deny the will of the people! Attached are two versions of the ORK for the price of none. I experimented with a *lot* of variations on this one.
Needless to say (and yet I'm gonna say it anyway), if you build it, feel free to make any changes you like, nothing sacred here. Oh, and a build thread would be much appreciated (but I don't need to have my name in the thread title).
Above discussion for some reason made me think of this:
Yeah, sorry there chap. You got caught in the crossfire of long-running thread banter. Jqav was right, that was directed at Neil.
I'm a fan of his original plasma dart design, and if he isn't going to build it, I'll enthusiastically encourage him to give someone else what they need to work on it!
Separate names with a comma.