Quentin's Scratch-Built L3 Build Thread

[qtrull]

I just recently received approval to begin construction on my Tripoli L3 project, so I figured this would be a great place to post the documentation for the project. The rocket is a 4" diameter fiberglass build with a 75mm motor mount and dual sep dual deploy. It will be flying on an Aerotech M1350. Tentative launch date is January 27th at the monthly TRAPHX launch.


Some details about the build:
70" TopFlight main parachute
24" TopFlight drogue parachute
18" Nomex blankets for both chutes
3/8 Kevlar Shock cord
3/16 G10 Fins
All Wildman fiberglass parts
Proline 4500 Epoxy for holding everything together

PerfectFlight Stratologger CF primary computer
Altus Metrum EasyMini backup computer
Featherweight GPS primary tracker
Eggfinder Mini backup tracker
All computers will be powered by 2S lipos since that's what is most readily available to me.



Progress so far:
All of my parts are ready to be assembled. To cut my fins I printed out paper templates and glued them onto the G10 as cutting guides and cut the fins out by hand with a band saw. I was originally planning to use a waterjet that is available at my school but unfortunately, it's down for repairs until the end of January.

Once the fins were cut to size, I added chamfers to the leading and trailing edges with a belt sander. I then test fit the fins and they fit in the slots perfectly!

After finishing the fins, I cut both of my body tubes to size and test fit all of the parts together with tape to get an idea of what the final product might look like.

Now that I'm all done with my finals, I should be able to start the bulk of the build in the next few days!

 

[Handeman]

Should be a great L3 rocket. What altitude, 10 - 15K?

 

[qtrull]

Should be a great L3 rocket. What altitude, 10 - 15K?

OpenRocket says around 12k. I have yet to run it in RAS aero but I plan to.

 

[qtrull]

Today was pretty productive. I started off by marking out where my centering rings would go, and adding the tri symmetry lines from my fin slots so I have a good idea of where everything will end up during the assembly. Once everything was marked out, I cut some shock cord to length for my Y harness and started sanding all the bonding surfaces as well as slots in the forward centering ring to pass the Y harness through.

Next, I wiped everything down with acetone to get rid of any unwanted dust or oils and started to tape off where the Y harness would mount to the motor tube. I also used CA glue to tack the centering rings in place, so they won't slide around. Around this time, I also taped off my avionics bay coupler so I could use any leftover epoxy to bond my switch band to the tube.

Before applying epoxy, I went over the areas the kevlar would be sitting one more time with some sandpaper and cleaned it again with acetone. Afterwards, I laid down some epoxy and pressed the cord into it.

After a few minutes, I went over the cord with epoxy to seal it in. After I completed the first strap, I used the remaining epoxy in the batch to bond the switch band to my av bay coupler. I meant to add tri symmetry lines to this coupler as well using the fin slots, but I forgot. I suppose that will have to happen later.

I repeated the same steps for the second kevlar line and removed all the tape. Leftover epoxy from the second batch was used to attach the motor retainer to the motor tube. I'll add fillets to the centering rings when I bond the motor tube to the body tube later down the line.

During the rest of this process, I also had my fin jig that I designed last night printing. Early in the morning I stopped the print early to double check the fit. It fit great, so I let the full jig print for the rest of the day.

 

[thzero]

How are you doing the internal filets for the fins if you've already bonded on the motor retainer?

 

[qtrull]

How are you doing the internal filets for the fins if you've already bonded on the motor retainer?

I'll be drilling holes near the fin slots and injecting the internal fillets with a medicine syringe after everything is tacked down. In my experience, trying to do it other ways gets really messy since there isn't much room between the motor tube and the airframe.

 

[qtrull]

I was able to get a lot more work done today. First, I market out areas to drill holes near my fin slots for injecting internal fillets. After the holes were marked, I drilled them out. These holes will be filled in with epoxy later during the application of the external fillets.

Once all the holes were drilled, I prepped the bonding surfaces for internal fillets on the fins, motor tube, and I also did some initial sanding on the body tube. I will be re-prepping those surfaces before I do my external fillets. While I was there, I also sanded some area around the centering rings so I could make some fillets on those before sliding the assembly into the airframe. Once I sanded the surfaces I cleaned them with acetone.

After all the surfaces were prepped, I moved on to tacking the fins down and filleting the centering rings. I added some extra epoxy to the forward-most centering ring in hopes that it might flow into the inner wall of the airframe and add some additional strength. Once all of this epoxy was added, I inserted the motor tube assembly to the airframe, added some epoxy near the aft centering ring, and inserted all the fins. Then I set the whole thing up to dry with the fin jig I printed previously.

Once that epoxy cured, it was time to start the internal fillets. I started by mixing a batch of 501 West System epoxy and adding some chopped carbon fiber to it. Once it was all mixed together, I poured it into a syringe and prepared to inject it to the root bond of my fins through the holes I drilled out previously.

Next, I injected the epoxy into the holes and moved the rocket back and forth to get it to flow out along the entire root of the fin.

I'll repeat this process for the other 2 sets of fillets tomorrow and hopefully I'll be ready for external fillets in no time!

 

[OzHybrid]

Can you post the OR sim file. 10 Kg empty weight sounds a bit heavy. But from your build so far you could probably weigh it....

 

[qtrull]

Can you post the OR sim file. 10 Kg empty weight sounds a bit heavy. But from your build so far you could probably weigh it....

Here's the openrocket. The empty weight is 4.9kg. The first photo is with the motor installed.

 

[OzHybrid]

Here's the openrocket. The empty weight is 4.9kg. The first photo is with the motor installed.

Parts list in post #1 says 3/16" G10 fins. Sim says 3mm should be 4.76mm(3/16") or have they been changed?

 

[qtrull]

Parts list in post #1 says 3/16" G10 fins. Sim says 3mm should be 4.76mm(3/16") or have they been changed?

You're correct. The version I uploaded is not up to date. The current version has the correct fin thickness of 3/16". Once the rocket is built up, I'll override the mass and CG. I'm intentionally attaching the forwardmost bulkhead to the nosecone last so I can add nose weight if the actual values stray from the simulation too much for some reason.

 

[qtrull]

Yesterday was very productive. I started the day by injecting the last set of internal fillets. I wasn't able to get a super good photo from the inside, but all the fillets flowed nicely, and the carbon fiber is distributed relatively evenly.

Once these fillets were cured, I moved on to the external fillets. I started by mixing up a batch of proline and letting it set up for 15-20 minutes before it was applied. While I was waiting, I prepped the bonding surfaces with sandpaper and cleaned it with acetone.

After the surfaces were prepped, I taped off the areas where the epoxy would sit and form the fillets. For shaping the fillets, I used a set of teardrop shaped tools I 3D printed to pull the radius down the length of the fins. When slicing the tools to be printed, I ensured the seam was at the point of the drop so the round edge was smooth.

Once the fillets were shaped, I removed the tape and left them to cure. I tried my best to fill in the low spots leftover from the injection holes, but I will still likely need to fill them in later and sand it smooth before I paint the fin section.

I repeated the same process for the other 2 sets of external fillets, each spaced 3-4 hours apart. During the last set, I used the leftover epoxy to bond the section of coupler tube that would form the shoulder of the nosecone. I followed the same steps of roughing it up, cleaning with acetone, and applying epoxy before letting it cure overnight with the last set of fillets.

Now that the bulk of the epoxy work is done, I'm waiting for some parts to arrive so I can assemble the avionics bay and attach all the parachutes. Then it will be ready for paint!

 

[qtrull]

Home for the holidays. This means it's time to do some sanding! I started the prep process by knocking off some of the high spots around the fillets with 120 grit sandpaper. I spent some extra time shaping the area by the leading edge of the fin.

After I knocked down the high spots, I decided to fill in the areas where the epoxy flowed into the injection holes. I started by bond prepping the inside of the holes with a Dremel and cleaning with acetone. I then used JB Weld putty to fill in the low areas. Once it was cured, I used 120 grit sandpaper to make the indents smooth with the rest of the surface. I also used extra putty to fill in an area on my payload section where some fibers had broken off when cutting the tube to size. I also sanded this area smooth. It should look pretty seamless when it's painted.


It was around this time that I noticed how bad the fillets sagged while they were curing. This meant the bond area onto the airframe was a lot less than intended. I did some research and decided that it would probably be best to fix the issue with more epoxy. I sanded down each of the fillets so that most of the low spots were gone. I then bond prepped the fillets with 120 grit sandpaper and acetone. I decided that my best course of action would be to lay a new set of fillets on top of the old ones. I taped everything up and prepared to start fresh.



This time I increased the size of my fillet tool by 0.1 inches. It wasn't much, but it should ensure that I had adequate coverage and a nice concave surface when I was done. This time for the fillets I used regular JB Weld. I've done a few projects with it in the past and it seems to hold its shape very well. I laid the rocket horizontally to see how the first set of new fillets would turn out. They looked great! I decided to do all 3 sets of fillets at the same time and let the rocket dry vertically since the JB weld is viscous enough that flowing shouldn't be an issue.



All 3 sets turned out great. There was complete coverage on all but 1 fillet. There are also a couple low spots near the trailing edge on a couple of the fins that I'll fix when I sand and shape the fin section again.

 

[qtrull]

It's been a while and it's time for some more updates. After the new external fillets were done, I filled in the gaps toward the trailing edges of the fins with epoxy putty and sanded the whole surface smooth. I'm really happy with how they turned out.

Before painting I also worked on some avionics. I 3D printed my sled. The build volume on the printer I was using was a little too small, so I had to use a shorter sled. The print also failed around an hour before it was completed so I just decided to print the part that didn't finish separately. I attached all both computers and trackers along with batteries and screw switches and then wired everything up. I added an extra screw switch and spot for an extra battery in case I ended up with enough time to add a camera (I probably won't before the cert flight).

Once I got back to school, I created the wire pass throughs for the ejection charges. I decided to use lever terminals instead of screw terminals since I've had way better luck getting them to hold onto the thin ignitor leads. After the pass throughs were created, I wired up the ejection leads.

 

[qtrull]

Once I finished the avionics, I attached all the parachutes and checked the CG and mass of the vehicle. The mass ended up being around 6kg and the CG was further forward than I anticipated. My stability caliber with the updated CG is around 2.7 instead of 2.4. Now that I knew the rocket would be stable without nose weight, I attached the bulkhead to the nosecone.

To do this, I smeared a lot of proline around the inner edge of the coupler and all over the eye bolt nut and then twisted the bulkhead into place. I ended up deciding to make the bulkhead flush with the bottom of the coupler. In hindsight, I somewhat regret not recessing it a little so I could add an extra fillet to the other side for added strength. A fillet was created on the inside edge of the bulkhead as the excess epoxy flowed down onto the inner surface of the bulkhead.
View attachment 625269View attachment 625270View attachment 625271

Now that everything structural was done, I could finally paint. I started by modeling the paint job in CAD. It's nothing I'm super proud of, but any paint is better than no paint. I decided on an inverse color scheme to my L2. I also decided to name the vehicle "Potentia"

View attachment 625275

Given the wintery conditions where I was at, painting wasn't a super smooth process. I had some pretty bad wrinkling. I suspect the layers weren't drying as fast as I'm used to so when I put more layers on top, they wrinkled. I also decided to use a paper stencil. I've had varying levels of success with it. This time was not super successful. Overall, after about 5 days of struggling with it, I ended up with a product I was okay with. Once I added the clear coat though, the surface finish was destroyed from wrinkles in a few spots. I'll probably fix those areas before the flight, but for now it's okay.

View attachment 625279

I drilled vent holes and screw switch holes after painting. I haven't decided on what size of rail button I'll use just yet, but otherwise this build is done! I can't wait to see how the flight goes.

 

[kjhambrick]

@qtrull --

Heads up !

None of the pictures show in your most recent post ...

-- kjh