A little 54mm airframe rocket for fun

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Issus

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So i've been planning to build a little rocket that will be pretty cheap to launch and not go too high. Originally, I was going to do this properly with milled VK airfoils on the fins and tip-tip carbon, however some plans for that launch were canned. Now the idea is something I can pretty much build in a weekend from scratch without costing very much money yet still be strong enough to take some medium sized motors and to be durable enough to not need to worry about it too much. Basically, just a rocket I can have fun with. I went with a 54mm blue tube and plastic nose cone so I don't have to spend a week doing layups, sanding and epoxy to make the body tube/NC and am using a 38mm motor mount to take CTI or Aerotech motors to keep my options open for reloads.

It will take up to a J, however stability is getting down to 1.30, and breaking mach with 6mm square fins, and this fin design isn't likely to work too well. A I366 will be my biggest load for this, which is going to get up to 0.98M anyway, so I probably wont bother with it.

AV Bay is detailed here:
https://www.rocketryforum.com/showthread.php?42018-Rethinking-my-AV-Bay-designs

Tower Launcher is here:
https://www.rocketryforum.com/showthread.php?41954-High-power-launch-tower-design-review

This is how the rocket looks with subsonic fins:
RockerRender.Final Color Output.Final Color Output.jpg

These fins will be identical in shape, but without the airfoil milled I can mill 3 square fins in under 2 minutes for all of, vs 20 mins of setup/CAM and then 14mins per side (about 32mins per fin including setup).

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Some of these photos are a bit crappy, I'm using my Fiancee's point and shoot rather than switching lenses on my DSLR in the dust of the workshop (or running inside to dust of the camera and then switch lenses).

All of my parts in one place, ready to start building. All of the milled parts are milled to within about 0.05mm of the actual dimensions on the rocket, so when test fitting it was actually hard to pull it apart. I'd say I could almost have flown this friction fitted with a G and not using any glue. The RSO wouldn't like that very much though!
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I cheated and glassed the motor mount tube with two wraps of glass earlier in the week. Peel ply leaves a nice surface for secondary bonding of the centring rings. I also filled the spirals in the BT and sanded them this morning before taking the photo.

First up, is putting the forward centring ring and thrust ring on the motor tube. I find by using a second ring with an ID the same as the ID of the motor tube I can put less effort into a fillet on the top side of the mount. There is a lot more surface area for contact with 12mm of plywood. Seeing as this is a quick and dirty rocket, it works fine for me.
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I'm using some BSI 30m slow cure epoxy as it cures pretty quickly and this isn't a high load area. It's also nice and thick.

Likewise, the front ring of the AV bay is also glued together. The nuts and eyebolt have also been thread locked with loctite.
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While that is curing I cut the slots in the bodytube using the dremel as I don't have a fixture to put 54mm tubes in the mill. The slots, while not totally straight ;) do appear to be more than adequate for the job.
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Now that the forward CR for the fins to sit up against is cured enough to work on (a quick trip to the epoxy oven speeds things up) I can get to fitting the fins to the motor mount.

Another test fit to make sure my TTW tabs are the correct depth on the MMT, and ensure I haven't made the slots on the fin guide too small. It's a very very tight fit, being that the wood ended up being 6.3mm and these were milled for exactly 6mm. Nothing a bit of pushing and shoving can't solve.
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I applied a liberal amount of JB weld to the root of the fin, slid it through the slot until it hit the forward ring and then pushed it down hard onto the body tube. The JB weld will fillet itself well enough with the amount I have applied. I can't find my box of epoxy fillers since I cleaned up the workshop (i'm sure you know how it is... you know where everything is until you clean up!) - hence my preference for thick epoxies today.
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The elastic bands are holding the whole thing nice and tight while it cures.

To help set the JB weld some time this month, I put a hair dryer blowing through the tube from the nosecone end, and a 38mm motor case in the MMT to help transfer the heat coming through into the MMT/JB weld.
 
After the JB weld is cured, the fin can is removed and the aft centring ring is installed with some west systems epoxy. Not shown, it's also lightly filleted to the fin and MMT.
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Next up, is putting a big ring of West systems epoxy inside the body tube where the forward rings will sit. The ring at the front of the fins is mostly ornamental and for the fins to attach to/stop at during assembly - so it's not going to be glued in. It's too much of a hassle to get glue on it when I'm doing a fast build anyway.

As with any rocket which is slotted all the way to the end of the airframe, I find the tube has difficulties sitting against the aft ring when everything is installed. So once the rear CR has epoxy on and everything is in place, I use the fin alignment board as a clamp. The fin jig is an extremely tight fit on normal tube, so pressed over a CR that is if anything a tiny bit too big makes everything fit very nicely.
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As west systems is rather runny and I can't find my fillers, I have to leave it here or else ruin my "fillets" by having them all run onto one side of the rocket as I cure the epoxy at higher temperatures.
 
I've spent about 15 mins each night working on the rocket.

The final things left to do from my last post were:
* Fillets
* Airframe drilling
* Paint (who needs primer!)

So, first up is fillets. I found my box of epoxy fillers, so i'm just using some west systems with west's colloidal silica/cotton for filler until it's a thick paste. I'm mixing batches of 6g of epoxy + filler, and being left with about 50-60% unused.

So, masking off the body tube as usual, because this is messy :)
IMG_0931.jpg

No pictures of it in progress, I don't want epoxy on the camera from my gloves.

When the epoxy is still a little green I trim off the excess to get the tip and tail nice and flush. This makes sanding in the leading/trailing edges easier later.
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Unsanded, I have a nice smooth fillet with just a little lip from the masking tape.
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After doing all 6 fillets and heat curing, I sanded away the masking tape edge and filed in the leading/trailing fillet edge.
 
After the fillets were done, I drilled the vent holes and static hole in the airframe and cleaned them up. Nobody seems terribly interested in this thread so I didn't bother to take any photos of that phase.

I'm using some cheap spray which came up looking a lot better than I expected. As I've been working on this at night, I'm using clicky cans because the compressor is too loud. It's only a small rocket so clicky cans are easier anyway and give less waste. For small rockets, half of my build table converts to a painting area with the use of a 72c drop sheet cut in half. This keeps the paint from making a silvery layer on everything.

I'm leaving the nose cone unpainted and the AV bay body tube ring is going to be yellow for a bit of colour. I was hoping to mask and paint a pattern on the fins with the yellow paint, however the shiny on the paint sticks to the masking tape so thats a no go. I also tried to clear coat a test peice of wood with this paint on it, however all the shiny went away when I did that.

This rocket has no primer on it, just a single layer of paint that I mostly sanded away. This leaves me with some spirals still showing, and grain in the fins, but as this is meant to be a quick and dirty build I forced myself not to strip all the paint off and spend the next few nights doing it properly. I'm curious to see how it goes/wears like it is.
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After doing 2 layers of paint, I unfortunately realised I had forgotten to sand one of the fillets :(
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Fully assembled, it just fits between the table and the roof :)
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I just need to add electronics and parachutes and it's ready to fly for next weekend.
 

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Thank you for sharing your build. For a quick-and-dirty rocket, this is some nice CNC work. I wonder if you can talk about the mill you have, and the type pf plywood you use for fins and CRs.

Ari.
 
The milling is easy, it's the CAD that takes the time ;)

I have a DIY CNC mill (3rd generation now). It's all stepper motor driven ball screws running a 2.2KW watercooled spindle. I feed it 1200x600mm sheets of wood (a standard size here) if i want to do a large amount of 2D work, with 3mm MDF spoilboard underneath so i can cut all the way through without ruining the finish of my vac bed. For smaller parts, i'll just vacuum them down and mill. These fins are milled with a 2mm endmill in about 35-40 seconds each - it's not pushing the machine to it's limit but it certainly keeps thinks quiet. VFD spindles are worth the investment over sticking a cheap router in there for both accuracy and your hearing. I don't wear earmuffs when milling unless using large endmills or chipbreakers - or i have the vac + compressor going. With a 1mm end mill, the fans on the machine are twice as loud at least (every stepper motor has two heatsinks and fans the VFD is pretty darn loud and there are 3 fans on the watercooling - 1 of which is used unless pushing the spindle hard.)

The mill really isnt anything fancy, but it does everything I ask of it within very reasonable tolerances. I like doing my own thing, so building a mill was good fun and it gives me plenty of pleasure being able to design a part in solidworks and then watch it appear in real life. I have a 3d printer as well for the same reason ;)

The plywood is all liteply. Its $6-12/sheet vs $120 for the same thing in aircraft quality spruce or birch ply. Normally I'd mill home made glass, carbon, or kevlar (or mix of all 3) plates to make centring rings but I don't foresee this rocket taking a lot of load.
 
I feed it 1200x600mm sheets of wood (a standard size here)

Very cool! I notice all your dimensions are metric, rather an exception on this forum. I wonder where you are geographically.

Ari.
 
Haha, yeah well the USA is a little behind the times in this regard, the rest of the world uses metric ;) We only use imperial for telling stories where you're exaggerating - "The fish was 5 foot long!".

Is it even possible to do engineering in imperial??
 
Haha, yeah well the USA is a little behind the times in this regard, the rest of the world uses metric ;) We only use imperial for telling stories where you're exaggerating - "The fish was 5 foot long!".

Is it even possible to do engineering in imperial??

We pushed for SI years ago, but the public did not accept it.
 
Unit discussions come up regularly here, as in my-units-are-better-than-your-units. I'm pretty much staying out of them now :=) So the guessing game continues... You do use English units in vernacular but not for actual measurements, and you write excellent English... Britain? Ireland? Australia?

Ari.
 
You appear to have missed Canada, Scotland, New Zealand, Singapore, South Africa and Bahrain just off the top of my head.

It's very frustrating dealing with the mix of SI and Imperial.. the TRA L2 test for example is all in imperial (and all the safety questions are about ATF/FAA/US legislation... sigh). What temperature does NOX gas off at? Who knows, it's in farenheit :p
 
In Canada, many people use English units in everyday speech. Scotland is part of Britain (or so this University of Edinburgh alum thinks). There are indeed many other options. How you describe your units preference suggests growing up metric, but it's possible that you're an expat living in a country whose first language is other than English. Packaging in your photos is all in English, and I recognize many of the brands, which suggests North America.

I'm content to live with your location as a mystery :=)

Ari.
 
I posted this in the launch tower thread, however thought it was relevant here too.

DSC_6423.jpg

Rocket test fitted in the tower. I'm hoping it's 1.5m of total rail length is going to be enough for it's 980mm-to-stable-flight simulation on the smallest motor i'll fly.
 
Well, it was too windy over both days we had to launch - so I wont be able to fly this until next year :(
 
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