L3 Project - Modular 98MM

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ether

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Getting closer to starting my build and wanted to share the project. I've been away from HPR for around ~30 years, got back into the hobby last year as a way to live vicariously through my two little boys. Achieved L1 and L2 certifications last year at Rocstock 50 and decided to start planning my L3 certification.

This first post will provide background and general approach, will post additional parts as the project develops, this may take some time as I'm trying to avoid re-work by re-confirming the design as I gather the components, have about 95% of items, but I'm keen on double checking everything and iterating as needed as I get the last few items. Also this Virus thing has thrown in a spanner into life, so build updates may come sporadically.

Been in communication with RocStock TAP members on this project and following the prescribed process to a T.

Beyond achieving L3 certification, want to scratch build a design that (1) looks good, (2) carries a HD camera, (3) highly re-usable - easy to re-launch the same day multiple times, (4) fly on a wide range of motors (really, really wide range), (5) robust recovery system and (6) incorporate know-how of the people who have pushed the envelope and succeeded and more importantly failed, but have been gracious to share their experiences on forums such as this one.

The rocket will be 85 inches long, with a conical (4:1) nose cone, 3 fins, tail cone, modular fin can (for L3 attempt will fly it with a 75MM motor mount).

Inspiration/goals for the design comes in all forms:

(1) From my prior time in rocketry - loved the feeling of launching, recovering and relaunching the same rocket over and over again. Especially when changing up the motors, thrilling to see how they would perform.
(2) Love long duration motors, want to build a rocket that can stay under the Lucerne waiver and use an L395 motor - 12 second duration.
(3) On-board rocket videos are awesome, especially looking at the horizon.
(4) Clean sheet design, but leverage what works and others have tested - don't re-invent the wheel, but approach things fresh.

From this, I made the following choices:
  • 4:1 conical nose cone - (1) not looking for the most aero design, would have gone with a 5:1 or 6:1 VK. In fact a Conical shape helps in trying to stay under the lucerne waiver. It looks cool, and my kids can easily draw the rocket using crayons.
  • HD carbon fiber airframe, sourced from public missiles (slotted cut to length) - reduce weight where possible, but overbuild the rocket to handle the largest 98MM motor there is (O)
  • Airframe will be in two pieces, and not have the traditional location of the break and a different nose cone attachment than typical.
    • The nose cone (Madcow fiberglass) has a shoulder that typically has a coupler glued in place, then that coupler woudl slide into the main airframe. I'm inverting this, the coupler will be glued into the top of the airframe, with it exposed past the top of the airframe and the nose cone sliding onto the coupler. Couple of reasons for this, (1) Coupler acts a stop for the 98MM aeropak motor retainer, (2) the coupler double ups the main airframe above the aeropak retainer, creating a very strong airframe to take on the biggest motors, (3) allows use of the inside of the nose cone base as space for recovery gear, (4) noticed others pushing the envelop had failures of the airframe where there is a break in the airframe, typically a bit of distance from the top of the nose cone (usually the AV/Payload bay meeting the rest of the airframe), hence wanted to move the break in the airframe as far forward as possible.
    • The rocket has another break, but this is found at the bottom, it will have a "fin can" made out of the same HD carbon fiber, but it will only be as tall as needed to attach the fins. This fin can will not be glued to the airframe, instead it will have a coupler glued in place, and that coupler will slide into the bottom of the airframe. Using an aeropak 98MM retainer for the motor, this allows (1) to use the motor to sandwhich the fin can between the end cap and the main airframe, (2) take away the compressive forces off most of the airframe and have the motor only be compressing a small section of the airframe (which is double walled) via the aeropak retainer (3) allow to change the fin can for different motors / fin designs as needed, thereby leveraging all the components and while in essence having a different rocket. Already have the components to switch it to a 98MM min diameter configuration, for example.
  • 3 Fins made out of aluminum, sourced from Binder Design (awesome craftsmanship). Doing research was tempted to use carbon fiber or fiberglass fins, however if the rocket is ever pushed to the max, fin flutter could have been a concern, gave some thought to using 4 or more fins to reduce the risk of fin flutter by having "shorter" spans. However, decided to go with aluminum since I rather over build this component and not worry about potential cracks one could not see/notice developing, again going to inspiration of item (1) above. Fins will extend beyond the bottom of the airframe to the tailcone. The shape is inspired by the wing tips of F22 fighter, and did not want them to extend past the bottom of the rocket to reduce fin damage on landing. They will be attached in a traditional through the wall method, will create epoxy dams to affix them to multiple surfaces not just the motor mount tube.
  • 3D printed tailcone. Reason is it looks cool and aerodynamic, I'm sure it helps a bit with base drag, but it is more an artistic feature, did not spend time trying to come up with the most aero shape, it is conical in design, for visual symmetry with the nose cone. The tailcone is not a load bearing part and will not be touching the motor case in any way (don't want it to melt), will be using a fiberglass motor mount tube and even the motor end cap will not directly touch the tailcone.
  • Will be using dual egg timer protons for deployment activation. will use centrifugal tube charge in a PVC charge tube holder to separate the rocket at apogee. 24" drouge chute (rocketman). At lower altitude main chute (rocketman) will deploy by a tender descender moving the connection of the drouge chute from the main airframe (it will disconnect it) to the top of the deployment bag (rocketman). The main chute will be held inside the main airframe until the drouge pulls it out. Grateful to all that posted videos of their chute deployments, really insightful to see what worked and what did not and the near misses of tangles that magically un-tangled before the main. This led me to using a deployment bag and designing it to stay inside the main airframe.
  • AV bay will be a slide in sled from the top of the rocket that will be followed by the recovery gear. The AV sled will be locked in place to the top of the aeropack retainer, then the recovery gear will be place inside the main airframe and bottom of the nosecone. The AV sled will have the two eggtimer protons, each with its own battery and screw switch accessible via vent holes, it will also have a HD camera recording the flight, the camera will look out horizontally via window epoxied in place. The window will be flush with the curvature of the airframe, using a watch lens cover that perfectly matches the curvature of the 98MM airframe, eliminating any potential drag from a camera setup while giving clear views.
  • The nose cone will house a Eggtimer GPS transmitter. I'm splitting the nose cone into two functional areas to maximize the use of space, the top half will house the GPS TX and battery and the lower part will have a portion of the recovery gear.

Plan on multiple flights before going for the L3 certification on a M1101. Want to test out all the systems and airframe robustness, will likely have its maiden voyage on a K motor then step up to a L motor (unless it needs more shake down flights on K motors) before going for an M launch.

My pie in the sky goal, is to take it to Balls and push it to max, be great to fly the same rocket to 2-3k feet and then go gangbusters without really changing anything on main part of the rocket.

See through picture
Roketto_see_through.JPG

75MM fincan version - L3 cert design
Roketto.JPG

98MM min diameter version - fin can swapped out for a full aluminum fin can (post L3 if it works)
Roketto 98MM.JPG
 

Theory

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sounds like a VERY cool project. ...and i too dig the conical nose cones

be sure to run the entire build by your TAPs before ordering anything. honestly, i am not sure where they will come down on the aluminium fins. might be a non-issue, but then again, you dont want to be the proud owner of fins you cant use.
 

ether

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Absolutely, everything being run through them, including the fins
 

Tim51

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Agree with Theory this is a very cool looking design...coincidentally bears a little resemblance to the very first Black Brant, before the more famous
BBII and III shapes evolved:
20200713_160104.jpg

Very best of luck with the build and L3 cert. Will be following with interest.
 

ether

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28823015611_7920a4d6b9_b.jpg

The initial mockup of the corporal missile was also an inspiration. Conical nose cone and tailcone.
 

Glasspack

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Looks very nice...……..Really like the tail Cone version.
 

OverTheTop

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Nice looking project.

Not sure how you are going to do your NC, but I have 3D printed them in ABS. Successfully flown to M2.14 for my 3" version. Info here if you are interested:
Feel free to steal any ideas. You might need to switch to a fiberglass NC if you put an O in it ;).

Good luck. Enjoy the L3 journey :).
 
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ether

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Going with a fiberglass nose cone from madcow, it has an aluminum tip

Using an eggtimer gps transmitter in it, so had to avoid carbon fiber

My tailcone looks very similar to your 3d printed transition, I also printed it in ABS.

I'm thinking about trying Kapton tape as an ablative for the nose cone if I get the chance to push it, simmed some motors pushing it ~ 2.5 mach. Think Kapton tape should handle the brief period of heat at that speed.

Got the idea from the Australian forum, someone was going to try doing a metal wrap of a conical nose cone, he used Kapton tape while securing the metal. Got me thinking Kapton tape itself should work, it should handle the heat and I can mechanically secure it with the aluminum tip, besides it's adhesive.
 

ether

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kicked off the assembly process, had the parts for a while, clearing out the assembly techniques cobwebs

Epoxied (rocketpoxy) the nose cone attachment bolt to the tip of the nose cone.

Loctited t-nut to the mounting screw, to help anchor it into the epoxy, made sure the epoxy formed a plug in the nose cone. The nose cone tip area has a cylinder hole about 1/2 inch deep then the conical shape takes over. Filled the epoxy below this cylinder, making sure to create a plug that was bigger than the cylinder hole. The nosecone from MadCow comes with a washer and screw, one could mount the tip by just tighten it down, but would need to be re-center anytime the tip is removed.

Using a motor mount ring in the nosecone to function as the attachment point for the base plate (will make the bolts captive) that has the nose cone eye-bolt and holds the GPS TX and Battery. Drilled four holes for M4 screws and epoxied the nuts on the top side of the motor mount ring, next step will be to epoxy the ring in place inside the nose cone.

here are some pictures, so glad the nosecone is translucent, easier to get comfortable with clearances. Next on the nosecone todo list is to trim the L bracket holding the GPS/Battery sled, solder in the switch and secure all the components in place (loctite and epoxy as needed). Then will probably move on to assembling the 75MM motor mount fin can.

feels great to actually start assembling the components, been a openrocket design file and a bunch of parts in boxes for too long

20200716_231013.jpg20200716_231030.jpg
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ether

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nose cone completed, epoxied in the retaining ring and finished installing all the components for the eggfinder GPS TX. Tested it out works perfectly with LCD receiver. Only missing piece is a Velcro strap to keep the wires and battery balance port in place for flight

Waiting to receive the AV bay sleds - laser cut plywood. Will move on to the fin can assembly, mostly sanding/trimming items to fit.
20200719_005940.jpg20200718_230621.jpg20200718_225253.jpg20200718_234410.jpg
 

ether

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Getting the fin can ready

75MM fiberglass tube - need to trim length about 1/4 above the top motor mount centering ring

20200719_235548.jpg

Test fit of the fins, will epoxy the abs tailcone and centering ring, then CA in balsa epoxy dams. Fin tabs will be sanded to aid adhesion, will sand all areas of the motor tube, inside of fin can airframe, motor mount centering ring and tailcone that will have epoxy when the epoxy dam is filled in

20200720_000037.jpg

Test fit of complete fin can - need to trim fin slots
20200720_000355.jpg
 

ether

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All pieces together

Another benefit of having the fin can detachable, it is so much easier to move the rocket around and transport it, in a regular car or even check it in, the longest part is about 52 inches long.

20200720_003558_1.jpg
 

ether

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midway in assembling the fin can, cut down motor mount tube, epoxied in the top centering ring and the tailcone (epoxied inside area not just the top - leaving about 3/8 of the motor tube below the bottom of tailcone exposed - will use epoxy to fill in to taper tailcone all the way to the end and keep the ABS material away from the motor end cap).

Switching from balsa dams to using 1/2 of jumbo paper straws to make the dam walls, wider epoxy dam where it matters, once those are in place will epoxy in motor mount tube then after the fins one at a time

Lots of prep effort to get it to fit exactly right and promote the best adhesion

20200721_210608.jpg
 
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ether

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Internal motor mount assembly done, epoxying into the fincan airframe at the moment, all areas prep (sanded and cleaned) for fin attachment. Next step is prepping fins and fin alignment guides (get in a week or so), then can move to the final step on the fincan and start on the main airframe and recovery/camera sled.

Stayed with the balsa epoxy dams, the space between the MMT and the fincan airframe is not large enough to benefit from using semi-circular dams.

20200723_220549.jpg
 

ether

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Plywood pieces arrived, have 100% of the components to finish the build

Alignment guides and recovery/camera sled
20200724_114107.jpg

It all fits 100% perfectly, was bracing for lots of sanding, but the tolerances calc'd were perfect. Great to have be in 3D, all the design work was in 2D

Will confirm all the electronic components fit as expected without obstructing each other and enough room for wiring. Will modify slightly to allow for pull pin per the new rules covering eggtimer proton to disconnect the deployment side.

Will swap out one screw switch intended for the camera and replace with 2 micro switches to disconnect the deployment side and a push on/off switch for the camera

long development journey to this point.
20200724_115429.jpg20200724_115440.jpg20200724_115500.jpg
 

Bat-mite

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Awesome. I like your color scheme (see left rocket in my avatar).
 

ether

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nice, like the style

used the same checkered pattern of the original LOC laserloc 3.1 and the simplicity of early HPR of making different aircraft sections contrasting colors, like the Mongrel at LDRS-1

should be easy to paint, will get the checkered pattern as decal, need it to be as simple as possible, painting is not a strength
Advertisement.jpgMongrel.LDRS1.jpg
 

BrendanH69

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Stayed with the balsa epoxy dams, the space between the MMT and the fincan airframe is not large enough to benefit from using semi-circular dams.
I like your idea of fin dams for internal fillets in tight clearances. Might use that in a future build.
 

ether

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I'm also drilling holes into the fin tabs to add an additional method securing the fins

Taking notes of minor changes I would make on the build, since the fin can is removable, i can easily built another one with stronger methods if needed. One of initial mock-ups was a 6 fin version, with very short fins, may do that later on just for fun.

been working on the recovery/camera sled, minor adjustments to add the additional switches, its a very tight package and been thinking through the assembly process to make sure it all comes together in the right order. Although almost everything are tried and tested components, putting them together into a scratch built design takes time and lots of little tools to assemble.

The last part of the build, cutting the camera window into the CF main air-frame gives me the most apprehension, everything else just takes patience and not that much skill.
 

ether

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progress on the recovery/camera sled, everything fits.

Here is the 4k camera, it also has an FPV camera but wont be using that at the moment, but have placed the FPV controller on the sled just in case, the TX is super small and can be attached on the side, but likely won't ever need it, not really interested in using it and probably only work for less than a second off the launch pad.
 

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ether

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Minor tweaking of how fastners will be oriented, to allow for disassembly, but everything fits.

Next step is gluing the sled together and soldering the wires.


20200726_223048.jpg
20200726_223248.jpg
 

ether

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Sled glued together, wiring is the next step.

The aeropak min dia retainer is perfect, the chutes are directly connected to the motor case. It is the keystone of the rocket design.

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ether

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Appreciate the compliment

1st fin being epoxied into place, one fin per day

Tested out the Eggtimer Protons and the camera, all working 100% as expected, need zipties to secure the wires, else 100% done with that part.

Waiting for a tungsten dremel wheel to work on the main airframe assembly, likely next week. less and less parts in the boxes, spending time to think through all the assembly steps/plans and measuring twice to try avoid assembly errors

20200728_223446.jpg
 

ether

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Fincan complete

Looks good together and apart, next the main airframe, need to drill vent holes, camera window and epoxy in motor retainer and coupler


20200729_190548.jpg

20200729_190634.jpg
 

ether

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epoxy on the motor retainer and air-frame curing tonight. nerve racking drilling the vent/switch holes and getting them to line up with the switches/sled, all worked out, lots of effort measuring and using guides to get them right. All that is left is cutting the air-frame for the curved glass for the camera, then cutting the coupler to allow the lens to look out and serve as the platform for the glass to be epoxied into place.

the window is made from mineral crystal used in watches, it measures 40MMx30MM and has a radius of 50, it matches perfectly the curvature of a 4 inch tube. difficult to figure out exactly which one would work from the specs on websites, ended up buying a package with the full spectrum of radiuses, glad I did, as i had calculated a radius of 45 would fit and while close the one with radius of 50 fits perfectly. It should allow the camera to view outside without a drag penalty of an open hole or an obtrusion.

Assembly has gone rather smoothly, biggest effort has been on sanding items to fit, some easier than others, and masking parts to avoid epoxy getting into things and protecting components while test fitting/adjusting components.

aiming to complete assembly by next week, will not paint it till rocket has survived multiple test flights
 

ether

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cut the camera window on the main air frame, window will be epoxied to the coupler. need to cut holes in the coupler for the lenses, making sure the camera gets un-obstructed views

20200802_185410.jpg

20200802_185333.jpg
 

OverTheTop

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the window is made from mineral crystal used in watches, it measures 40MMx30MM and has a radius of 50, it matches perfectly the curvature of a 4 inch tube. difficult to figure out exactly which one would work from the specs on websites, ended up buying a package with the full spectrum of radiuses, glad I did, as i had calculated a radius of 45 would fit and while close the one with radius of 50 fits perfectly. It should allow the camera to view outside without a drag penalty of an open hole or an obtrusion.
What a great idea!
 

ether

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Camera window and coupler installed

20200803_004445.jpg

L3 Build is complete!

Rocket is ready for motor and deployment charges

Will finish up documenting the build and create checklists

Hope to find a launch for a shakedown flight soon
 

OverTheTop

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Looks great. I assume you have a breather hole in that section of airframe with the window? You wouldn't want the internal pressure to blow the window out on ascent. If you have made it captive from behind you can ignore my question :).
 

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