Blue Variable - 38mm K627 Altitude Record Attempt - My Failure

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Alright, huge life transitions are happening right now. Some for better, some for worse. I just have more time on my hands now. Hopefully more money starting December. C'est la vie.

Chapter 2: Construction​

2.1 - Sanding the airframe tube​

I chose a roll wrapped tube for two reasons.
1. The OD of the mcmaster tube is smaller than any commercial rocketry airframe tube.
2. Filament winding is not optimal for the forces an airframe experiences. Fibers are good in tension, resin is good in compression. Filament winding is great in pressure vessels like scuba tanks, paintball tanks, motor cases. Roll wrapped tubes are used in structural applications which require resistance to bending and crushing forces. This is much more optimized for rocketry.

The roll wrapped tube comes with 1 glaring issue though. To fit the loki case, a LOT of sanding needed to happen. This ended up being the most tedious task of the whole build.
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Here is the beautiful tube from McMaster.
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Here is the sanding setup. Some things to note:
I purchased a flap sander off mcmaster and had to make some modifications to speed up the process.
1. I added larger 80 grit pieces with hot glue (I bought a 150 grit or something similar)
2. I purchased a threaded rod so that it would be able to reach all the way down the tube.
3. I marked the threaded rod so that I could sand where it was needed based off how far the motor case could go in.
4. CRITICAL: During sanding I would feel less torque and see little to no carbon dust come out of the ends. This means little to no sanding was happening. To increase sanding, I would add pieces of shop towel in between the sanding flaps. This increases the diameter of the tool and applies more pressure during sanding. This must be done or you will spend days with little to no progress. I figured this out after Day 3 of sanding.
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Above is the general setup. However this was prior to really focusing on adding paper towels to increase pressure. As you can see, little to no carbon dust is coming out the end during sanding. There should be much more than this if you are sanding efficiently.
The next two photos are of the flap sander after the tube was finished.
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After sanding, a significant amount of the wall thickness was removed. I didn't measure (my mistake) but the hard work actually benefited me well. It reduced the amount of my total weight (more altitude) while also slightly increasing stability. This definitely reduced strength by a lot. However, less than 0.25 inches of airframe was not supported by either the motor or the nosecone! Due to this along with it being roll wrapped, I was not worried about airframe failure!

Regarding cutting the tubes to length, I used a chop saw that I setup with a machinist square and a blade without teeth. I think there is a better way to do this but I have not figured that out yet. If anyone has a better way please suggest it!

With this finished it was time to prep the fin edges and bond them to the airframe!
 

Chapter 2.2 - Beveling the leading and trailing edges of the fins​

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The fins are 1/16th inch which means that they were too thin to be beveled by my machinist. I decided to bevel them by hand with sandpaper that I taped onto a board. This was done in the way that one would sharpen a knife on whetstone.
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As you can see in the back there are various grits of sandpaper going all the way up to 3,000 grit. Wet sanding to keep dust down.
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Look at that shine.

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Regarding technique, it really is just like whetstone sanding a knife. Take your time and practice, practice, practice. This helps with altitude no doubt but I also wonder if this helps in preventing delamination? I think the smooth and polished surface would prevent air from delaminating the layers. I know aerodynamic heating is worse on thin sharp edges but it didn't seem to be an issue during flight. Let me know what you all think!

Up next is bonding the fins to the airframe!
 

Chapter 2.3 Fin attachment​

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I always use the method described in https://www.rocketryforum.com/threa...our-government-doesnt-want-you-to-know.58389/
I find that this method produces bonds that are better than anything I did prior. Sometimes I am unsure if I hit 'water break' but I generally just do my best. I use Hysol E120Hp for the root and fillets. In the future I may explore using Hysol 9394 for fillets since it has a much thicker viscosity compared to 120HP. I only do 1 fin at a time. This allows me to really focus on getting a solid bond. I will put wrap tape around the fin and airframe tube so that there is a force holding the root and airframe tube tightly against eachother. I then put this into a 'oven'. Video below (SOUND WARNING):

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The hairdryer at max setting will heat up the inside to about 180-190F while the low setting will heat up to about 150F. As always have a fire extinguisher around and dont leave it unattended! When E120HP is cured it is white whereas when it isnt cured it is clear. Using the oven increases curing time significantly. After about 30 minutes, I am ready to move onto the next fin!

Fillets are done one at a time also. This prevents sagging that 120HP is prone to. Once all fins are done with fillets, I do one last long cure in the oven. This usually lasts for a few hours.

Things I would do differently:
1. Cut out a custom fin jig out of MJF. The mach1 guide really was not ideal for this build. It didn't quite fit the airframe tube (too loose) and was quite flimsy. It really took a lot of time for me to get each fin perfectly straight.
2. Use a thicker epoxy like hysol 9394 for the fillets. This is up in the air and I will be doing testing to see how it performs and ease of use compared to 120HP.
 
Alright, huge life transitions are happening right now. Some for better, some for worse. I just have more time on my hands now. Hopefully more money starting December. C'est la vie.


The roll wrapped tube comes with 1 glaring issue though. To fit the loki case, a LOT of sanding needed to happen. This ended up being the most tedious task of the whole build.
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My cats are jealous of the tree in the background. Their tree needs a little TLC.

Very nice construction.
 
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