7.5" LOC DOORKNOB Level 2 Build Thread

[pathtouch]

Figured its about time to start the build thread on this pig. After a 10-year hiatus from high powered rocketry I figured it was time to dust off the fleet and go for a Level 2 this year. Since I'm east coast, most waivers are fairly low and I figured I'd go with the low and slow approach. I might as go with something short and stubby, and well you can't get much fatter than 7.5" tubes. I wanted something with enough room to do a conventional AV bay and dual deploy so the Goblin/Warlock/Mega-Magg's didn't excite me. DOORKNOB it is!

Options for waivers around here are a drive south a couple hours and stay under 3,000 feet, or head north several hours and get up to 12-17,000 feet. Oooooor I can wait until winter when lakes start to freeze over and I have options to go to 17,500 two hours away or 9,700 nearby. Flying on ice sounds relentless on body tubes and fins, so I plan on glassing the tubes and doing tip to tip on the fins. Not so much that it's needed to handle the motors I plan on flying but for durability. I've also never experimented with composites so this will be a good learning platform.

Speaking of motors, I have an order in with Rob and Gloria for a CTI 38mm 6-grain skidmark J381, simming to 1,100 feet. Once the mild J is out of the way I plan on flying 54mm 6-grain K's simming to 4,000 feet.

It wasn't long after my order a neat looking package arrived. True master's of their craft, Jay and Dave fit over five feet of rocket in a rather compact box.




Everything was perfectly protected and it wasn't long before it was mocked up for funsies.


Parts spread for the standard kit.


Toddler for scale. Originally got an airframe for a different kit on accident but the LOC guys got the right one sent out pronto and some additional goodies I decided to add on to make this Dual Deploy.


Here's a video going much further into the details of what comes with the kit if you're interested. I ordered some extras to make an AV bay and I'll explain more as I go.

 

[pathtouch]

Playing some catch up here as I have a few videos already posted. This kit uses the MMAS system, which is pretty sweet. Basically the motor mount is a 5.5" tube, and that can accept all sorts of adapters. On hand I have a single 38mm, single 54mm, and then a crazy (7) 38mm cluster that keeps eyeing me from across the room.

I wanted to assemble the fin can completely outside of the airframe so I could do decent internal fillets. Then I'll slide this up into the booster section and do external fillets and tip to tip for more ice proofing.

Also I have some decade old unopened TotalBoat epoxy resin and slow hardener that I'm using. Xyla Foxlin, a famous YouTuber, recently built her L2 rocket with the same stuff so I have enough faith in it to charge forward.... and I'm cheap.

There's a page of written directions with the MMAS system but the gist of my approach:

• Peel glassine layer off mother tube
• Sand laser char off all wood components and get all rings sized as needed
• Press in t-nuts in aft rings (mine had some long prongs that I trimmed with a pair of pliers before heading to the vise)
• Mark fin locations
• Attach u-bolts to forward ring NOT FORGETTING TO PUT Y-HARNESS ON
• Use fins to mark where centering rings go distance wise along the mother tube axis
• Epoxy all rings on mother tube, all rings on 54mm adapter, and all but the front ring on the 38mm adapter
• Add some 1" hard maple blocking to the hidden mother tube centering rings incase I need attachment points for rail guides later. Will most likely use the lowest one, and another near the center of gravity once I have things fully assembled.
• Use 54mm adapter to set the internal thrust ring in the mother tube and epoxy that in
• Once internal thrust ring is cured, use it to set location of forward ring on the 38mm adapter
• Round over fins at router table. Decided to go with a round over versus knife edge because I could care less about altitude and want it to be able to land on ice without chipping constantly.
• Tack fins on and square up, using a ratchet strap as a clamp because it works
• Two heaping teaspoons of West Systems 406 colloidal silica mixed into every pump of TotalBoat epoxy. Mixing brands but whatever, nothing's caught on fire yet. This was a great non-sag combo that let me make some beefy internal fillets.

Pounder of Bissel Brother's Substance IPA for scale. My biggest rocket before this was a LOC Nuke Pro Maxx... 54mm body tube. Yea this is going to take a while to get used to.



Respective YouTube videos of this fin can assembly!

 

[wonderboy]

Nice choice and nice job so far! I also used this kit for my L2. I flew it on an Aerotech J350W to about 800 feet... a bit lower than anticipated, but successful and perfect for a small field.

From a scale perspective, the factory dimensions are a bit short for the diameter. If you want a slightly more scale-like appearance (perhaps this can work with your upper airframe mods when you add the av-bay) stretch the forward end by about 5" (5.2" to be precise).

 

[StreuB1]

I love this rocket. I am a huge fiberglass fan but my old faithful's are all LOC cardboard kits. This one is going to be added to my fleet, without a doubt.

 

[Tim51]

Really nice vids. Interesting kit - following with interest.

 

[pathtouch]

While I wait to find the time to edit the fiberglassing video I figured I'd post some more useful info I've assembled.

Like any good project, start with a paper study to see what's been done in the past. These gave me some inspiration and some of the discussion about motors, altitude, and nose weight were useful.

Example Build Threads
Doorknobs:
https://www.rocketryforum.com/threads/loc-doorknob-arrived.42276/https://www.rocketryforum.com/threads/loc-door-knob-level-2-project-build.20085/
Bruisers & Bruiser EXP's:
https://www.rocketryforum.com/threads/loc-bruiser-build-thread.34645/https://www.rocketryforum.com/threads/help-with-loc-bruiser-exp.168002/https://www.rocketryforum.com/threads/loc-bruiser-exp-build-thread.32531/

Here's my table of stock component weights from the factory as of this kit version. If you didn't glass the airframe you could probably achieve the advertised 13lb weight easily. I'm actually projecting to be near there without motors or electronics with 2 layers of 6oz fiberglass. Filler and paints will probably tip me over the mark though.



The Simulation of Short Wide Rockets using RockSim Version 8: An Additional Base Drag Consideration for Rockets With Less Than a 10:1 Length to Diameter Ratio.
https://www.apogeerockets.com/education/downloads/Newsletter154.pdf
I've also created a crude model of the rocket with current component weights. The model includes the new AV bay, glassed tubes, and a conical transition behind the airframe to simulate the extra base drag rockets with length to diameter ratios below 10:1 tend to exhibit. This is based on the the linked above. It seems the jury is out on whether you also use the sim with this transition to check altitude and stability or just stability. We'll see this winter after the first launch.

 

[pathtouch]

Well I stole 2" from the booster stage and created a switchband from that. Then I used some Total Boat Epoxy and 6oz cloth to add two wraps of fiberglass to all of the airframe. Process went something like this:

• Peel glassine off LOC tubes
• Cut cloth for two wraps around tubes
• Weigh cloth and mix enough epoxy for a 1.3:1 by weight ratio of epoxy to cloth
• Masking tape inside of fin slots
• Wet out cardboard tube
• Apply cloth to tube wetting out with as you go
• Finish with some porous teflon from https://www.aircraftspruce.com/
• Trim when fiberglass is in its "leather" stage
• Strip porous teflon the next day

The porous teflon is neat, it actually seams to sweat out excess epoxy. I am going to make sure to wash parts well after using it before I sand, I feel like skipping that step could embed some undesirable particles in the surface of the airframes during sanding that would affect bonding of subsequent layers, like when I add the tip to tip.

So far this added about 19.5 additional ounces to the flight weight. I'm coming in at about 1.25:1 epoxy to cloth by weight. I know 1:1 is a better goal but is that realistic with peeled cardboard tubes? That surface is thirsty.

Hard at work in the rocket lab. You can spy a scale model on the bench of the scale model in this thread. Rocket-ception.


Cool to see the porous teflon sweating out excess epoxy.


Looking rocket-y


Typical surface finish, some minor filling will be needed but not much!


Follow along on the build series!

 

[Adam3836]

Things a beast !!! Very

 

[jnmiller]

Good choice on the LOC Doorknob for your L2 !

 

[pathtouch]

Small update since it's been a while. I managed get the fin can epoxied into the booster and add external fillets.

Started by cutting out the fin tabs all the way to the end of the booster section so I could slide the fin can up into it. I used a Dremel but found I got the best results by just using the cutoff wheel to get through the fiberglass, then a razor knife to cut through the cardboard.


Then epoxied the fin can in, using some clamps to keep everything tight to the centering rings.


Once that was in it was business as usual spending four separate nights installing external fillets.



And finally I filled in the stock bulkhead as I'll be going from a single eyebolt to a u-bolt with two new holes. I debated adding a layer of 6oz cloth and epoxy to both of these but figured that's just unnecessary weight and the ply should be good enough for this ~13lb rocket.


That's it for now. I'll be getting the tip-to-tip done on the fins this week I hope. The goal is to launch in February once we get some good ice on the lakes.

Associated youtube video:

 

[wonderboy]

Nice work! Just FYI: you don't have to completely open the slots at the end of the body tube to the full fin width. You can just slit them. When you slide the fins in, the gap opens up enough to let the fin through. This has the advantage that once everything is slid up all the way, the gap closes and captures the bottom of the fins and holds everything in place. No worries with how you did it, just FYI for next time.

I'm looking forward to seeing your continued progress on this one.

 

[Five]

Nice thread. Detailed, step by step instructions with pictures and video. I love it

 

[user 35280]

Excellent rocket

 

[rfjustin]

Great fiberglass work, well done!

 

[Theory]

very nice!

 

[pathtouch]

Nice work! Just FYI: you don't have to completely open the slots at the end of the body tube to the full fin width. You can just slit them. When you slide the fins in, the gap opens up enough to let the fin through. This has the advantage that once everything is slid up all the way, the gap closes and captures the bottom of the fins and holds everything in place. No worries with how you did it, just FYI for next time.

I'm looking forward to seeing your continued progress on this one.

Good to know, I am definitely going to try that on the next one.

 

[kswing]

Nice!

 

[pathtouch]

Howdy Folks! Got a tip to tip layer of 6oz cloth on the fins last week. I wasn't worried about fin flutter or flight stress, just wanted some durability for hard landings on ice. So I opted for a single layer of 6oz. That on top of 3/8" plywood should hopefully handle rough landings. All said and done it added 8oz to the rear of the rocket. Process was pretty typical:

• Trace a template out of the fins with some craft paper
• Cut out 6oz cloth with template
• Sand surfaces with 220 grit on the orbital sander
• Wipe surfaces down twice using lint free cloth and denatured alcohol
• Mix up some epoxy
• Working in halves, apply epoxy to airframe, then apply one half the cloth and wet out, making sure to keep it tight to fillet at fin root
• once one half was wet out, fold other half of the cloth over and repeat (the video makes more sense than the description)
• Apply porous teflon peel ply and wet that out
• Wait until the next morning and trim excess while epoxy is in "leather" stage
• Rip off peel ply and enjoy

Like the other posts I continued the video saga!

 

[wonderboy]

Lookin' good!

 

[pathtouch]

Things are pretty much ready for my launch this weekend as long as the ice holds up on Lake Winnepesaukee.

I’ll make a follow up thread with photos and text, but for now here’s a video of the AV bay construction, recovery setup, and deployment charge testing.

 

[Tim51]

Another great video. Are you leaving it all over white or doing the classic Doorknob flourescent roll pattern?

 

[Bat-mite]

Enjoying your thread.

 

[pathtouch]

Another great video. Are you leaving it all over white or doing the classic Doorknob flourescent roll pattern?

Sneak peak from paint session tonight. I have chrome vinyl tape to add once this cures 48 hours. Might not have time to clear coat it, but I imagine getting dragged across a frozen lake bed is gonna patina this up no matter what.

I’m a little behind on the videos and this thread but hoping to launch Saturday as long as our lake doesn’t thaw.

 

[Tim51]

Sneak peak from paint session tonight. I have chrome vinyl tape to add once this cures 48 hours. Might not have time to clear coat it, but I imagine getting dragged across a frozen lake bed is gonna patina this up no matter what.

I’m a little behind on the videos and this thread but hoping to launch Saturday as long as our lake doesn’t thaw.

View attachment 503939

That looks superb.

 

[TheTank]

Lookin good. Wish I could make it up to the lake this weekend to see it.

 

[pathtouch]

So to recap building out my AV Bay.

• Glued stiffy coupler in midway of 7.5" coupler that came with the kit. In hindsight I wish I set this back an inch or two to give more room to pack the main parachute. Oh well I fixed this in the next step.
• I cut a 2" switch band off the booster section and then glued this on the couplers offset to gain me some extra room for the main recovery gear.
• Drilled appropriately sized holes for vents. I went with the old Adept22 research that the area should equal a 1/4" diameter for every 100 cubic inches of AV bay volume.
• Oddly enough I got the best finish on the holes going fast on the drill speed, using no pilot hole, and taping the surfaces off before I drilled to prevent tear out. I CA glued the interior of the cardboard tubes.
• I ended up going with (4) 7/32" diameter holes, and then just used the same bit to make vents for the booster and payload bay as well.
• I drilled the bulkheads for 1/4" u-bolts, installed them with decent backing plates and loc-tite
• I cut the sled out of 1/4" ply, but smartly sized it to also work within a 4" fiberglass AV bay that'll be part of my next build, so the sled is modular
• Assembled my Eggtimer Quantum, a black friday purchase
• I created a little vacuum chamber to test the new altimeter, and my old Adept 22, firing christmas lights instead of charges. That little 10mL syringe in an 8oz mason jar simulates a 700 foot flight.
• A little bit of wiring and then added 1/2" copper charge wells. They were originally sized for 2g charges but since I'm using Hodgdon Triple 7 smokeless powder I need 3.0 and 3.5 gram backups. I extended them with 1" and 1.25" coper pipe stubs. These worked great as long as I put 8 layers of masking tape over them. I've read Chris from Eggtimer uses a very similar approach.
• Attached recovery gear, I have 30" nomex shields on each chute. 24" Recon Drogue has 30 feet of 3/8" kevlar from One Bad Hawk with a 3rd loop sewn in. Main is the 78" nylon that came with the kit on the 25" nylon webbing supplied by LOC. I put the main on a 1287lb rated swivel, and it looks like the same one that came with the recon drogue. Both attached with 827lb rated quicklinks to the harnesses. To minimize quicklinks I attached directly to AV bay u-bolts and girth hitched to the nose cone. I Since kevlar and nylon are rated for like 5000lb and 2000lb respectively the limiting factor besides the chutes is the quicklinks so I'm good to almost 100 g's on recovery gear.
• I attached AV bay to payload with (4) #6-32 countersunk machine screws to nuts epoxied inside.
• For now I'm use (2) #2 nylon shear screws at each separation point. This is about 62lbs of resistance. I did some fuzzy math (difference in booster and remaining airframe flight ready is like 3lbs, max 20g burnout acceleration). This seems good for the drag separation I expect at burnout and can handle 30gs at the nosecone connection for a harsh drogue deployment.
• Masking taped all harnesses exposed to deployment charges.

 

[mIcahel]

Great looking project. How are you arming the electronics?
Hope the weather cooperates for your cert flight. Good luck!

 

[pathtouch]

Great looking project. How are you arming the electronics?
Hope the weather cooperates for your cert flight. Good luck!

Thanks! The quantum will be armed wireless with the built in wifi switch. The club I'm launching with is okay with that approach. The Adept22 will just use two leads twisted and taped through a vent hole in the AV bay. I'll change it up to a double pull pin eventually.

Funnily enough I almost need some more bad weather. It's been above freezing during the day and that has some people spooked about the ice it seems. I'm brining an auger with me to check along the way.

 

[Bat-mite]

Best wishes. It will look nice going up.

 

[boomtube-mk2]

I'm not judging.

I'm just pointing out the obvious.