A rocket for a smurf

[Funkworks]

Step 40 –

Shock cords and smurf-sized clothes pins making themselves useful for test fitting the boards. Smurf rightfully resting because sanding all of them was a smurf of a job. The very slight curvature of an extra board's inner surface can be seen by his head.

 

[Funkworks]

Step 41 –

And here on the backside is the gap that's been expected from the beginning. It turns out to be about ½” wide. With 24-fold symmetry, at least there are many options to choose from as to which ones to narrow.

 

[Funkworks]

If I were to narrow all of the boards, I’d have to trim 0.5 mm on all 24 of them. I just can’t too that with my current tools, so I’ll only trim the ones around the 3 fins. Six boards or so. I doubt any normal human being will complain about that.

Next up: to display the smurf with the rocket, it needs a door step.

 

[Funkworks]

Step 42a –

Here’s the idea for the door step:

- ¼” thick balsa x 1” wide x 9/4” long (3 x 3/4” board widths) for the step itself.
- ¼” thick balsa x 1/2” wide x 9/4” long (3 x 3/4” board widths) for a brace underneath, sanded to fit the inner tube and allow a good gluing or velcro surface.

 

[Funkworks]

Steps 42b and 42c

Sanding the brace until its surface matches the inner tube.

 

[Funkworks]

Step 42d and 42e -
Gluing the door step. I made sure to leave one board-width hanging to match the comic, but this is terrible for drag, so the doorstep might end up being removable. Actually, the whole rocket might end up having 2 configurations: one for display and one for flight.


Testing the fit on a rocket arc (balanced).

 

[Funkworks]

Step 43 –

Gluing together the door. To cut these balsa boards, all you really need is to put your weight on a chisel.

 

[mo2872]

This is modeling at its best, IMO. Well done. (And I’m not afraid to admit I used to watch this cartoon while waiting to go to the bus stop in my younger years). Great job!

 

[Funkworks]

I probably watched it for a year or two, missed the Astrosmurf episode (no YouTube back then, so when you missed a show, it was gone!), and then quickly felt too old for it. Going back now for a while is quite enjoyable.

Just found out yesterday I could adjust picture size in posts. How about that.

 

[Funkworks]

Step 43 -

Built most of the door and cut out the entrance in the tube.

Sanded and dry fitted the motor mount.

Thought out the payload sections and bought plywood to separate the recovery equipment from the occupant(s?).

Now Astro says he wants to take advantage of the thick fins to try out a mechanism to have them pop-off at apogee and come down tethered to the main tube. Understandably, some are having strong doubts about this. But he says that if it doesn’t turn out well, the gimmicks can easily removed and the fins can instead be glued on permanently like with every other rocket out there.

 

[mbeels]

Now Astro says he wants to take advantage of the thick fins to try out a mechanism to have them pop-off at apogee and come down tethered to the main tube.

Interesting! Sounds like a fun spectacle!

 

[Funkworks]

Actually, instead of “pop-off”, it’s probably more accurate to say “fin release, drag separation and tethering”. On a note pad sketch anyway, it works great!

 

[jqavins]

His "If it doesn't work that we can glue them on later" approach sounds reasonable, mostly. If it fails on first flight, I only hope that no smurfs die needlessly. Is he planning some sort of ground test? (And just what is he trying to accomplish by this mechanism?)

 

[Funkworks]

His "If it doesn't work that we can glue them on later" approach sounds reasonable, mostly. If it fails on first flight, I only hope that no smurfs die needlessly. Is he planning some sort of ground test? (And just what is he trying to accomplish by this mechanism?)

Well, rockets this heavy, using 29 mm or 38 mm motors, usually have plywood fins because it's much stronger than balsa. Granted, 1-inch balsa might be thick enough to take a hit, but we don't really "know" that. So to reduce the risk of breaking a fin, they come off at apogee, and contribute to drag on descent. Basically, if glued to the rocket, a direct hit might break a fin. However, on its own weight alone, a fin certainly cannot break.

Since the mechanism is based on Apogee's "strap-on" boosters, I'm quite sure they won't be coming apart on lift-off, so the system doesn't seem to be losing any points on safety here either.

Be assured that no smurf will be harmed in this adventure, the parachute's coming out no matter what. Ground test? Maybe... Ok. Sure! Why not.

 

[Funkworks]

Step 45 –

To make the fin release system, in the previous step, I epoxied short sections of ½” ID transparent tubing to the upper part of the fin roots (2 cm from top). Now this step is for the … let’s call them “fin retainers” (really just 3 sticks):

• Selected a 3/8” dowel to fit in the ½” ID transparent tubing, because a nose cone ejection should easily slide them out of the tubing
• Selected and purchased eye screws to insert at the end of a 3/8” dowel
• Cut a 30 cm long dowel section (3x)
• Drilled 1/16” holes at one end of the dowel and epoxied in an eye screw (3x)
• Skinned the opposite end of the dowel to make it somewhat pointy and sanded it with 60 grit (3x). This will make it easier to insert the stick in the tubing from way up top.

These 3 sticks will eventually be positioned along the inside surface of the rocket tube, and keep the fins against the rocket. At ejection, they'll be pulled out from the top by the nose cone, releasing the fins in the wind.


As an added bonus, these sticks will slightly shift the CG towards the front, because:

• the 3 eye screws are by the nose cone (and will be connected to more hardware)
• they come down to a point
• they don't go all the way down

 

[neil_w]

Hey that's nifty.

 

[Funkworks]

Step 46 – Motor Mount, Staining, Fin Leashes and Door Hinges

So the main things here are that:

• The motor mount is dry-fitted with all the parts at their approximate locations. The wooden parts are all painted with the complete Minwax system: pre-stain, stain and clear coat. This matches the ladder and is apparently a "golden oak" color which fits perfectly with the characters' universe.

• The middle MM ring will be the “floor” of the payload area, and the upper ring will be the “ceiling”. The “smurf area”, and the “parachute area” will be separated by 2 wooden “walls”, not shown. The "ceiling" over the parachute area will be removed so that the parachute can follow the ejecting nose cone.

• I cut the top ring myself with hobby plywood from Michael’s, and a brand new 1-5/8” hole saw, because the MM kit I got only came with 2 ring.

• The 1 white nylon cord is simply a 20” loose loop around the motor mount, and each of the 3 smaller hemp cord "leashes", loosely links an eye screw on a fin, to the white cord. The hemp cords leashes are thin because they’ll have to pass through a slot at ejection, and they can easily be replaced if I find out hemp is too weak.

• The 3 eye screws on the payload “floor” will become door hinges.

The next steps will be to cut out body tube slots for the fin leashes and attachments, and to decide how much of the payload area to allocate for the smurfs, and how much for the recovery equipment.

 

[Nytrunner]

Will the hemp cords be exposed any ejection gas/particles?

 

[Funkworks]

Will the hemp cords be exposed any ejection gas/particles?

All these cords live between the 2 lower motor mount rings, which will be epoxied all around so they should be safe during ascent.

At ejection, I would assume that little of the hot gas ejecting with the nose ever reaches the rocket fins and if any does, it's probably cool by then.

Not quite sure what's the best cord to use here, but I'm not very fussy about it either. More than anything, it has to be thin or with a small diameter.

 

[Funkworks]

Just thought the cord setup needed a better picture and description.

Basically, once the motor mount will be epoxied in place, the white nylon cord will form a loose loop between the MM rings, but it won't be easily accessible and be hard to replace. The 3 hemp cord “fin leashes” however, will allow a user to pull out enough of the white cord to attach a replacement leash. I chose hemp because it was thin, cheap and smurfy-looking. (about $0.25 worth used here).

 

[Funkworks]

Step 47 – Fin Slots

Cutting out the fin slots took much longer than I first thought it would. This is what worked:

• finding the arc distance between 2 fin slots center-to-center so I could mark it on a cord: Arc length = [(5.38 inch) x π x (2.54 cm/inch)]/3 = 14.3 cm,
• wrapping the cord around a motor mount so I could mark a motor mount face where the fin slots should align,
• inserting the motor mount at a tube edge, so I could mark the tube by the 3 motor mount marks,

And for each slot:

• extending the tube marks along the tube with the aluminum angle,
• measuring the fin tab length (including fin leash eye screw), and drilling 2 small holes at the end points of where each slot should be,
• using an X-Acto blade to join the 2 holes and make,
• drilling increasingly bigger holes, swiping from one to the other with the spinning bit each time, and sanding the inner slot with a 120 grit sponge, until a fin tab could just easily slide in and out.

Astro enjoyed the process, but Clumsy, still not too sure if this is a good idea, stayed out of it and decided to build a few crates instead.

 

[SCooke123]

I'm really enjoying this build. It looks like you are having a lot of fun too!
Steve

 

[Ted Cochran]

One short cut to measuring for fin slots that I've found helpful is to wrap a piece of paper around the tube, trimming it to fit. Then remove it and fold it into thirds (or quarters, as the case may be.) Now you have an instant fin-slot marking template.

 

[Funkworks]

Step 48 – Filler Putty to turn each of the 3 Fin Release Tubes from an O-shape to a U-shape

It could be debated for ages whether the 3 fin release tubes should have been U-shaped instead of O-shaped, but this new recruit wouldn’t have it. To make sure they'll do their job and readily slide out of the upper (yet-to-be-made) slots in mid-air, he rolled up some FixIt epoxy clay, stuffed it in the 6 grooves, and capped it with very thin, 1/32-inch, slippery balsa sheets. After cutting off the excess putty with a small blade, the O tubes became clean U’s, at least on the outside where it matters. This was a somewhat delicate job.

Meanwhile, Clumsy is keeping busy building crates somewhere off-camera, but no one really knows why.

 

[Funkworks]

Step 49 – Cutting the 3 Fin Release Slots (Squares)

Spread a little CA on the 24-hour cured FixIt, and also on the thin balsa sheets covering it, making very solid U-tubes (HA! U-tube!). Once the CA was cured, took measurements and went after the “Fin Release Slots", which are 3 squares in the body tube, each about 2 cm x 2 cm, for the U-tubes to fit in.

Pencil wasn't sharp enough to make thin lines, so this was a lot of X-acto work, and a lot of patience, slowly increasing the square size until the U-tubes fit in well, and the fins could easily fall out, making sure none of the square hole became wider than a fin itself (which is 2.5 cm wide). That would have been ugly.

In the first figure:
The left-most fin shows a finished “U-Tube”, with the FixIt at the base, balsa shell, and CA all over.
The middle fin is fitted in without a “Fin Release Stick”. It should fall out if I lifted the tube up.
The right fin is fitted with a “Fin Release Stick” through its U-tube. An extra “Fin Release Stick” is leaning on the outside just for show.


In the second figure, an empty square hole is shown on the left and a fully assembled fin on the right, with the square hole, the U-tube, and the stick.

A little more tuning, sanding, cutting, or whatever, will be needed to make sure the fins fall out as they should. And then I’ll add a little CA to prevent the cut cardboard from fraying.

 

[Funkworks]

Step 50 – Test Fits for the Motor Mount, Fin Mounts, Recovery Gear, and Cockpit.

Drilled 3 holes in the forward ring to allow the fin release sticks to reach the fins. Having earned its seat on the rocket stand, the motor mount is being tested in combination with the fin mounts. Considering that a door has to be opened towards the inside, that leaves about ¾ of the pie for the payload.


Step 51 – Payload Test Fit and Roof

The left half of the pie will be dedicated to the shock cord and parachute. A NOMEX cloth or maybe a baffle will sit inside the motor mount, and the right half of the pie will be the cockpit. Two rectangular walls, not shown, and a roof, the half donut proudly displayed below, will separate the cockpit from the recovery gear and protect the crew from ejection gases.

Ah yes. In case anyone is wondering, the smaller holes, for the 3 sticks, they were drilled with a Forstner bit, because someone told me they made cleaner holes, and someone else lent me a kit for a while.

 

[Funkworks]

Step 52 – Preparing Motor Mount Parts for Permanent Assembly

Slow day at the Smurf Rocket plant. Assembled a 38mm-to-29mm adapter to make sure it was actually a removable one and it would fit the motor mount no matter how I glued it together. Still thinking about what retainer to use. Not sure an aluminum one is worth it.

Thinking about having the aft MMT ring 1"inside the body tube, and leaving 1/2" of motor tube hanging out the ring.

Basically, I prepared the MMT parts for assembly. Final checks before permanent bondings.

 

[mbeels]

Glad to see that he's getting some help finally, that's a big job for just one Smurf. I'm curious what Clumsy is going to do with those crates. Well, with a nickname like that, better work on the crates, not the rocket!

 

[Funkworks]

... with a nickname like that, better work on the crates, not the rocket!

Few people my age dabble with the smurfs, but because of this project, I have become one of them. At least while this lasts. And by dabbling, I learned yesterday that a new 3D-animated TV series will be released in 2021 (Nickelodeon). And your post freaks me out because ... um ... (promo shots) :

 

[Funkworks]

Step 53 – Motor Mount Assembly and … Smurfberries?

So that’s pretty sweet. I understand they asked Clumsy to build crates, because Sassette wanted to go out and pick some smurfberries for the crew.

Hmm… Still a bit strange because they usually eat at break time, not on the job. Oh well.

Upper motor mount ring / cabin floor was glued on. Measurements to position ring just below door/hatch and doorstep assembly done twice to make sure this was right.

Now Sassette isn’t just a gritty and handy smurfling, she’s also got an eye for aesthetics, and she’s the one who insisted these two 1/32”-thin balsa boards get stained. I note they match the cabin floor. Maybe that’s a hint as to why, I’m not sure.

The stain finish isn’t the same brand as the pre-coat and stain itself. That’s Clumsy’s fault (but it shouldn’t be a problem).