A rocket for a smurf

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Funkworks

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Having rediscovered this lately, after decades of it being off my mind, I decided it had to exist. I don't know if it will ever fly, but here goes.

Step 1- Designing the thing on Openrocket, roughly scaled to the figure: Done.
Step 2 - Sourcing the required parts and tools: Done.
Step 3 - Marking the balsa and cutting it. The shorter pieces are to extend the fins, because the planks I found were only 4 inches wide, but the fins need to be a bit longer:

Cut Fins.jpeg
 

les

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uhm - which is the root edge? If they are standing in the planned orientation the grain is the wrong way.
And what thickness is that wood?
 

BABAR

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An open rocket pic would be welcome. Looks like it is going to be a smurf size field flyer! (Don’t get me wrong, I like flight where the entire course of the flight is easily in sight and I only have to climb one or fewer fences for recovery)
 

Funkworks

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will it be pedal-power-propeller driven?! :D
It will be set up for 38-mm first, easily adaptable for a 29-mm, and I'll probably find a RC aircraft propeller for display.

uhm - which is the root edge? If they are standing in the planned orientation the grain is the wrong way.
And what thickness is that wood?
The leading "face" is the curved one. The grain is vertical for the main part and horizontal for the tips. The fins will be detachable (and replaceable), so I'm not worried about grain orientation for now. Also, cartoon shots show a vertical "crack" in the wood so ... authenticity?.

That balsa wood is 1 inch thick so there might a few world records right there: thickest square fins on a model rocket, highest drag, etc. But I'm not about to argue with a smurf.
 
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Funkworks

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An open rocket pic would be welcome. Looks like it is going to be a smurf size field flyer! (Don’t get me wrong, I like flight where the entire course of the flight is easily in sight and I only have to climb one or fewer fences for recovery)
Here's an OpenRocket shot. Nothing is set in stone and I'm not sure how accurate drag is with 1 inch fins but at least I got the CG ahead of the CP, and there's still plenty of room left in the nose for extra mass.
Screen Shot 2020-06-14 at 6.45.58 PM.png

A low altitude for this experiment is perfectly fine!
 
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Funkworks

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Step 4 - Glued the fin tips to the fins. I actually like the huge gaps, but I’ll fill them up. The grains for the bulk of the fins match the cartoon drawings. The grains for the fin tips are roughly along the leading edge, as suggested by model rocket theory.
02 Fin tips.jpeg

Step 5 - Ordered most of the main rocket parts (tube, motor mount, nose cone balsa).

Step 6 - Brainstormed ways for a landing not to destroy the fins. Most likely, the ejection charge will release them from the fuselage. Either using shear pins or simple mechanics like with Apogee’s strap-on boosters.

Step 7 - Sawed off the rounded tips from tongue depressors I found at some dollar-type store. These are purely for aesthetics. Three of them will be glued to fin roots, and the rest to the main 5.4” tube. Sanding them produced saw dust I can use with wood glue to fill in gaps. Apparently, sawing and sanding was quite exhausting.
03 Surface planks.jpg
 
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neil_w

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Here's an OpenRocket shot. Nothing is set in stone and I'm not sure how accurate drag is with 1 inch fins but at least I got the CG ahead of the CP, and there's still plenty of room left in the nose for extra mass.
That rocket will have a huge amount of base drag, especially with those 1" fins. It's halfway towards being a saucer. :) You might not need much more stability margin than that...
 

les

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You will also benefit from base drag associated with large diameter rockets. I know there is a way with Rocksim, not certain with OR, to sim this base drag (basically, a zero mass cone mounted on the rear of the rocket)
 

neil_w

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You will also benefit from base drag associated with large diameter rockets. I know there is a way with Rocksim, not certain with OR, to sim this base drag (basically, a zero mass cone mounted on the rear of the rocket)
Same thing works. However, it's only good for calculating CP, you need to remove it to do your actual flight sims. Also, not sure how to include the effect of the thick fins.

I think I'm gonna propose that OR include a way to automatically factor in base drag for CP calculations. Seems like it should be straightforward (famous last words).
 

BABAR

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Fin grain direction is wrong, although with planks this size won't matter. You have plenty of room for rear ejection if you want,
 

dr wogz

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i'm curious as to the whether the flight is as per the book or per NAR / TRA / what we want to see! :D :D

(For those that don't know the story, it's a fun read!)
 

Nytrunner

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2-3" between CG and CP is what I'd aim for if I was building it..

The grains for the fin tips are roughly along the leading edge, as suggested by model rocket theory.
Grain along leading edge is a guide because normally the leading edge isn't parallel to the airframe. Think of the grain lines as weaker material between many long thin pieces of wood. When the fin flexes/flaps inflight or bends on landing, airframe parallel grain means the outer segments of the fin have no direct connection to the tube other than the weaker grain substance, and they often fail along those lines. With most normal fins, the leading edge intersects the airframe, so each "segment" of wood in the piece is directly glued to the airframe at the end.

Now your fins have their grain parallel to the airframe, but they're also 1" thick which makes their moment of inertia incredibly larger (decreasing bending stress greatly). You're
 

BABAR

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Step 4 - Glued the fin tips to the fins. I actually like the huge gaps, but I’ll fill them up. The grains for the bulk of the fins match the cartoon drawings. The grains for the fin tips are roughly along the leading edge, as suggested by model rocket theory.
View attachment 420551
Step 5 - Ordered most of the main rocket parts (tube, motor mount, nose cone balsa).

Step 6 - Brainstormed ways for a landing not to destroy the fins. Most likely, the ejection charge will release them from the fuselage. Either using shear pins or simple mechanics like with Apogee’s strap-on boosters.

Step 7 - Sawed off the rounded tips from tongue depressors I found at some dollar-type store. These are purely for aesthetics. Three of them will be glued to fin roots, and the rest to the main 5.4” tube. Sanding them produced saw dust I can use with wood glue to fill in gaps. Apparently, sawing and sanding was quite exhausting.
View attachment 420552
Good to see he has full protective gear on. Although he may already have blue toe syndrome
 

Funkworks

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That rocket will have a huge amount of base drag, especially with those 1" fins. It's halfway towards being a saucer. :) You might not need much more stability margin than that...
The very existence of saucers is what convinced me that this model wasn't completely ridiculous!

Fin grain direction is wrong, although with planks this size won't matter. You have plenty of room for rear ejection if you want,
I just haven't actually built a rear-ejection device yet, so I'd have to look this up in detail. Maybe I will.

i'm curious as to the whether the flight is as per the book or per NAR / TRA / what we want to see! :D :D

(For those that don't know the story, it's a fun read!)
Me too! I watched this particular episode for the first time only a few months ago while planning. It's all part of the drama!

2-3" between CG and CP is what I'd aim for if I was building it ... the outer segments of the fin have no direct connection to the tube other than the weaker grain substance, and they often fail along those lines. With most normal fins, the leading edge intersects the airframe, so each "segment" of wood in the piece is directly glued to the airframe at the end.

Now your fins have their grain parallel to the airframe, but they're also 1" thick which makes their moment of inertia incredibly larger (decreasing bending stress greatly). You're
Great advice and explanation, much appreciated!

Good to see he has full protective gear on...
That's what I thought too, but he tells me it's for COVID protection.
 
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Funkworks

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Step 8 -
Found smaller 0.75” tongue depressors, so I sawed and sanded a bunch of them as well. Once I get the tube, I’ll have the option between 1”- or 0.75”- wide boards, or a combination. I also have even smaller ones to fill in gaps because in practice, I wouldn’t count on a single width being an exact factor of the circumference. So having a few different widths will give me options when I try wrapping the tube.
04 Boards 17x1.jpeg

05 Boards 23x0point75.jpeg

Grade 5 math told me a 5.38” diameter tube has a 16.9” circumference, so possibilities are:
  • Using 1”-wide boards only. About 17 are needed to go around. This width conveniently matches the fin thickness.
  • Using 0.75”-wide boards only. About 22 are needed to go around. This seems to better match the comic.
  • Using a combination of boards widths. I’ll have more tubing than required so I’ll get to test a few patterns.
I’ll probably paint all of these boards before glueing them on. That way, I only glue on quality ones. Also, no need to seal because it's not secret smurfs use actual wood.

Step 9 -
Received these 3”-wide balsa boards! This is the raw material for the nose cone. I just have to saw them in equal lengths and glue them into a 6”x6”x6” cube.
06 Nose cone boards.jpeg

Step 10 -
Securing a lathe and owner-operator-instructor, to sculpt the upcoming 6”x6”x6” cube into a nose cone. - Done.

This shot is one of the best references I have for proportions:
07 Best Comic Shot.jpg

A fin tip is about the size of a smurf’s head, and you can count 11 boards on a half-circumference. The rocket diameter was either going to be 4” or 5.38” but the latter seemed more interesting. I made life easy by choosing a fin height equal to the rocket diameter. Lengths and nose dimensions followed. Incidentally, there's a huge vertical crack in a fin, which is why I say having the grains in that direction is technically "authentic".
 
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Funkworks

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Step 11 -
Cutting and sanding the balsa boards and test fitting the parts into a pre-nose cone. A small coping saw was again sufficient. Balsa is so easy to work with. Everything glides like butter. But since it’s expensive, and since pine is cheap, I might do a pine version next, glue it together and get it in a lathe, just so I can do some drop tests. If a glued pine nose cone doesn’t fall part, neither will a balsa one.
08 Nose Cone Test Fit.jpeg
 

Funkworks

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Step 12 –
Priming the boards. These tongue depressors are so light the spray can flip them. Contrary to common wisdom, I primed with a darker red, and will probably go with a brighter one at the end. The thinking here is that I don’t want this to look perfect, but an imperfect bright red will look better on this dark red than on white.
09 Boards (primed red).jpeg

Step 13 –
Starting the nose cone assembly. Using Gorilla wood glue. I only have one set of clamps this size, so it's going to be one pair at a time.
10 Nose Cone Glueing.jpeg

Step 14 –
Watching the part of the cartoon where the propeller spins, so I can count how many blades it has and what diameter it is. Animated, it definitely has 4 blades and it’s a little smaller than the tube, so about 4.5 inches will do. Now I know what to shop for.
11 Propeller.png
 

Funkworks

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Step 15 -
Assembling the nose cone layer by layer, with Gorilla wood glue and "paperweights".
12 Nose Cone Assembly.jpg
 
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Funkworks

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Step 16 -
Waiting for the pre-nose cone to dry up as well as possible. I spread the glue evenly with my index at each one of about 15 interfaces, just so I had the peace of mind that I had done the best I could. This will be ready for a lathe shortly and at the price balsa is going for, I won't have many chances at this.
13 Pre-nose cone.jpg

Having found an extra board locally, I added a few layers just to protect the middle part during transport and the carving process. My simple mind is somewhat fascinated by this assembly since it could very well be the lowest-density part this size I've ever seen up close. (Yeah. It doesn't take much but that's ok. 😁)
 

neil_w

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My brain really wants to see one board in the middle pulled out, Jenga-style, but I suppose the glue would make that difficult.

In the meantime: extremely unfair to show a stack of books like that without showing us the spines, so we could all critique your book collection. ;)

Very entertaining so far!
 

mbeels

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I really like the smurf working on the rocket, it's too bad that he has to do the entire thing by himself, and I can only image what a heck of a time he must have had moving those books, and operating that clamp.

In the meantime: extremely unfair to show a stack of books like that without showing us the spines, so we could all critique your book collection.
I had the exact same thought! The choice of books matters here! That should be a de-facto rule of the scratch building forum.

This is a fun build, enjoying it greatly!
 

Funkworks

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My brain really wants to see one board in the middle pulled out, Jenga-style, but I suppose the glue would make that difficult.

In the meantime: extremely unfair to show a stack of books like that without showing us the spines, so we could all critique your book collection. ;)
I really like the smurf working on the rocket, it's too bad that he has to do the entire thing by himself, and I can only image what a heck of a time he must have had moving those books, and operating that clamp.

I had the exact same thought! The choice of books matters here! That should be a de-facto rule of the scratch building forum.
Had the same thoughts about Jenga, but yeah, glue.

If I had my rocket books with me, I definitely would have used those and showed them off. I think I have 6 that would have been thick enough to matter here, including Rocket Propulsion Elements by Sutton. But I don't, so just think of these as physics and math bricks with titles so scary most people would run away in mental agony and never come back.
 

mbeels

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But I don't, so just think of these as physics and math bricks with titles so scary most people would run away in mental agony.
Based on that description, I'll guess that the blue hard cover second from the bottom could be Jackson, "Classical Electromagnetism".
 

SDramstad

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I was thinking Readers Digest Condensed Books but I could be wrong......;););)
 

SDramstad

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Step 4 - Glued the fin tips to the fins. I actually like the huge gaps, but I’ll fill them up. The grains for the bulk of the fins match the cartoon drawings. The grains for the fin tips are roughly along the leading edge, as suggested by model rocket theory.
View attachment 420551
Step 5 - Ordered most of the main rocket parts (tube, motor mount, nose cone balsa).

Step 6 - Brainstormed ways for a landing not to destroy the fins. Most likely, the ejection charge will release them from the fuselage. Either using shear pins or simple mechanics like with Apogee’s strap-on boosters.

Step 7 - Sawed off the rounded tips from tongue depressors I found at some dollar-type store. These are purely for aesthetics. Three of them will be glued to fin roots, and the rest to the main 5.4” tube. Sanding them produced saw dust I can use with wood glue to fill in gaps. Apparently, sawing and sanding was quite exhausting.
View attachment 420552
Probably way too late but what if you drilled a 1/8th or 3/16th hole from fin tip to root and then glued in a wooden dowel. Kind of like adding rebar.
 

jqavins

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  • Using 0.75”-wide boards only. About 22 are needed to go around. This seems to better match the comic
Using 0.75" boards, I calculate that you'll need 22.53 of them. So, if you use 23, you can sand just a little off the width of each one; that would be 0.015" off of each.

Or, use 24 of them, an even number that makes attaching the fins a lot easier. Then you'd have to remove 0.046" from each one. That's the way I'd go.
Priming the boards. These tongue depressors are so light the spray can flip them. Contrary to common wisdom, I primed with a darker red, and will probably go with a brighter one at the end. The thinking here is that I don’t want this to look perfect, but an imperfect bright red will look better on this dark red than on white.
Agreed. Excellent plan.
 
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