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LOC-VII Build Thread

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jmuck78

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The LOC-VII arrived yesterday, and since I'm already doing a build thread on the LOC 7.5'' Hawk, I thought I would do a build thread on the LOC-VII as well to show how the modular motor system (MMS) works. Here are the unboxing photos as well as the first test fit. {Jason @ LOC also promised me that if I order another 7.5 inch kit, he would throw in a free garage upgrade so I have more build space.}

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The kit consists of:
  • 30 inch booster tube, pre-slotted with three fin slots
  • 15 inch booster extension
  • 22 inch payload tube
  • 2 15 inch couplers
  • Nose Cone with bulkhead
  • 78 inch parachute, nylon shock cord, and recovery hardware
  • Five inch diameter motor tube with two Loc-n-ring centering rings (one pre-drilled for the MMS attach points) and one centering ring with holes pre-drilled for the two included U-bolts
  • Modular Motor Adapter for the 54mm motor
  • Three 3/8 inch fins
  • 1515 Rail Buttons

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Initial dry assembly looks good, although I will have to sand the centering rings a bit to get them to fit over the motor mount tubes.

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Some considerations during the build:
- The kit is not quite ready for dual deployment since it's missing a drogue and electronics bay. That's not a big deal as I happen to have two LOC Ebays sitting around from other rockets.
- The nose cone is also unused space, so I am considering either modifying the nose cone to use the RNWS, or reusing the nose cone from the LOC 7.5'' Hawk. I also have a third cone modified to use Chris Atteberry's head end altimeter bay, so I have some design trades / options to consider.
- I am fiberglassing the LOC Hawk, and I may do the same thing here if it works out well on that rocket.

OpenRocket File for the stock configuration:
 

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jmuck78

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Motor Mount Assembly

I decided to assemble the fin can completely before I insert it in to the airframe, which seems simpler for these fins, so the motor mount assembly here includes the fin attachments.

The motor mount consists of a 5.5 inch (!) motor mount tube and three centering rings. The aft centering ring has four fin loc attach points for the three fins as well as four pre-drilled holes for the 1/4-20 t-nuts that receive the mounting hardware for the MMAS. The forward centering ring has four holes for the included U-bolts.

I installed the U-Bolts and the T-nuts into their respecting centering rings. I also removed the glassine paper from the 5.5'' motor mount tube, and while I was doing this, removed the glassine layer from the 54 mm motor mount tube for the included optional 54 mm motor adapter. I then position all three rings on the 5.5 inch tube and tacked the forward most ring in place with CA. I placed the middle ring on the tube roughly in the middle of the tube and then placed the aft centering ring on the tube about 1/8'' above the aft end of the tube. Using the fins as a guide, I installed all three fins and adjusted the middle centering ring so that there was a good, tight fit between the two rings and three fins. I then tacked the remaining two rings in place with CA. I mixed some RocketPoxy and put the first fillet on the forward centering ring and let it sit over night, making sure to get some epoxy on the U-Bolt attachment nuts and also around the t-nuts in the aft ring.

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I liked the look of the simple, rounded edges on the LOC VII fins instead of the full beveling, but this was simply my personal preference. I used the sander to round the edges and set the fins aside for the next step.

Once cured, I mixed another batch of RocketPoxy and added fillets to the remaining centering rings and buttered and mated the fins, using the remaining epoxy to add fillets to the fin roots and anywhere else the assembly needed fillets.

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I did a test fit with a 7.5 inch body tube to make sure the fin can would slide in and the fins would clear the outside of the body tube.

I also began epoxying the centering rings on the 54 mm motor adapter.

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I'm still considering whether to glass the body tubes.
 
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jmuck78

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Booster Assembly

I decided against fiberglassing the airframe. I had already cut on the aft end of the fin slots to fit check the fin can / motor mount assembly, and didn't have a way to hold the shape of the tube at the aft end while I applied the glass and epoxy. I would have also needed to epoxy the slotted tube, first coupler, and second main body tube before applying the glass, which would have required me to place the coupler exactly at the right height in the body tube before I installed the motor mount, else I would risk having inserted the coupler too far, which would force the motor mount tube further aft than is desired.

So, this step became substantially simpler without the fiberglass step. I added some epoxy inside the slotted body tube at the three locations I the centering rings would be placed and also added epoxy for the first coupler. Immediately after inserting the motor mount tube into the first body tube, I would insert the coupler so that it rests on the top centering ring of the motor mount tube.

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Once the tube was slathered with epoxy, I slid the motor mount assembly in through the back of the tube until the fin tabs met the top of the fin slots. After inserting the motor tube, I set the new booster section on the floor and slid the first coupler in from the top.

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I then epoxied in the 15 inch upper tube for the booster, after first confirming (this time) that I could reach the U-Bolts to install the Y-harness later. I did test fit a quick link just to make sure I could install it as well since the clearance between the U-Bolts and the coupler is tight.

The final step here was to add some epoxy around the aft most centering ring and clamp the air frame to the aft centering ring.

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Normally I would have installed the rail button t-nut before I installed the motor mount tube, but the set of 1515 rail buttons and t-nuts I had allocated for this project had an issue where the screws weren't the correct thread for the t-nuts (or had already been cross threaded and didn't work). Either way, I will install the aft rail button later by drilling in to the middle centering ring and epoxying the screw in there.

I also did a quick check with the second coupler to see how much I could insert it in to the booster section. The second coupler only inserted about 5.5 inches instead of the 7.5 that would mark the mid-point of the coupler. The instructions did note that the second coupler would not be able to go in to the midpoint, so I considered that a successful fit check. I also inserted my electronics bay to verify and it works also, but only if I adjust the switch band (which hasn't been attached yet) so that the switch band is not in the middle of the electronics bay coupler. Again, this may be desirable since the new LOC electronics bays only take up half the length of a 15'' coupler, which means attaching the switch band exactly in the middle of the electronics bay would conflict with the interior e-bay bulkhead.

**I have been carrying those orange handled spring clamps for years, but have never used them (I have others I like better). I'm glad I kept them as they were perfect for holding the airframe to the aft centering ring while the epoxy cured.
 
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jmuck78

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Fin Fillets and Rail Buttons

I added the external fin fillets, which were fairly straight forward.
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For the rail buttons, I used two different methods, one Aft, one Forward. I aligned the line for the rail buttons with the fin on the opposite side using a carpenter's square, then used an angle iron to extend the marks all the way up the booster.

For the Aft rail button, I marked the aft and forward sides of the aft most centering ring, since I will be using that centering ring to secure the rail button. I drilled a small pilot hole into the CR and then enlarged it. The drill bit is slightly smaller than the #10 wood screw used to install the 1515 rail button. For the forward rail button. I drilled a through hole and then used a #8 machine screw with a washer and nut on the interior side of the booster. The forward rail button is located a few inches aft of where the bottom of the electronics bay will sit. The rail button fastener goes through both the airframe tube and the coupler tube, so there should be substantial strength to the joint. I want to leave the rail buttons removable before I apply paint, so I will not epoxy either rail button fastener in place until after the rocket is finished.

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jmuck78

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Fiberglassing Tubes

I elected not to fiberglass the LOC VII.
 
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jmuck78

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Motor Mount Adapter Installation

UPDATE 1/27/19: Based on discussion later in this thread, I opted to use the Aeropack 54 mm flanged retainer instead of the LOC retainer to provide a mechanical connection between the motor mount tube and the aft ring that transfers the launch load to the airframe. That ring is a single failure point, and merits some mechanical redundancy despite the epoxy calculations below. The details of the 54 mm flanged retainer are described in post #73.

I had assembled the 54 mm motor mount adapter in a previous post, so this post simply covers the bolting of the Motor Mount Adapter to the aft centering ring using the included four 1/4-20 fasteners.

Completed Motor Mount Adapter Assembly for a 54mm motor:
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Motor Mount Adapter inserted into the booster section:

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I did have one concern / observation. Every other rocket, particularly high power rockets, have two or three centering rings that share the launch loads from the motor. With the MMAS, the load is transferred from the motor thrust ring to the motor mount adapter to the rocket via one centering ring only. The other centering rings on the adapter are there purely to keep the motor and thrust axis aligned with the CG of the rocket.

The aft centering ring is attached to the motor mount tube with a single epoxy fillet that is about 1/4 inch high. For a 54 mm motor mount tube, that equates to about 1.7 in^2 of epoxy surface area reacting the force of the motor (area = pi*d*h_fillet). For a 54mm motor, I used the ~285lbs max thrust for an L1000 motor as an example. With that thrust spread out over the 1.7 sq inches of bond area, I end up with a shear load of 171 psi on the epoxy joint. I could not find the shear strength of Rocketpoxy, but I did look at John Corker's epoxy tests here: http://jcrocket.com/adhesives.shtml

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John's lap shear tests for Bob Smith epoxy (which I assume here is inferior to the shear strength of Rocketpoxy) shows a minimum shear strength of about 950 lbs over 2.25 sq inches, or a capability of 422 psi before the plywood failed. John used 3/8 inch plywood, I believe, for his tests, so they aren't exactly comparable, however, he did note that the plywood failed before the epoxy joints in all of the cases.

Using the same approach with larger motor diameters increases the bond area, so the same thrust in a larger diameter motor means less stress on the bond joint. An N2000 motor (98mm motor) for example has about 700 lbs of max thrust, which translates to 232 psi of shear load on that one joint.

So, I think I have convinced myself that the MMAs should be safe under most 54mm motors, but its better to use 75 mm or 98 mm motors for higher max thrust motors just to be safe. The Modular Adapter is still a single point failure, but probably a low risk one.
 
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jmuck78

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Nose Cone

The nose cone for the Hawk arrived in the stock configuration with the plastic aft end in place. The Hawk doesn't necessarily require nose weight since its a long rocket, so the stock configuration comes with a simple bulkhead and eye bolt for harness attachment. I ordered another RNWS setup from LOC so I could adjust the nose weight and also optionally use the Head-End Altimeter bay that I am designing for the Hawk. The Head-End Altimeter bay can be used for a separate tracker or to initiate deployment of the main chute if I fly the LOC VII without the payload bay and avionics bay (i.e. the stubby version).

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I hammered the four t-nuts in to the forward side of the nose cone bulkplate so that the aft end of the t nut is flush with the aft end of the bulkhead and epoxied them in place (epoxy not shown here).

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I epoxied the 75 mm bulkhead with the two holes into the motor mount tube.

I used the mitre saw to cut off the end of aft end of the nose cone and sanded the inside the 100 grit paper to rough up the portion at the shoulder and a portion near the nose.

I then rinsed the inside with alcohol and acetone. When dry, I epoxied the fixed portion of the bulkplate in place. After the bulkhead epoxy had cured, I inserted the 75 mm motor mount tube (which in this case was much too long because I had ordered a longer version so I could use the excess for something else) and marked the length so that the motor mount tube would be flush with the aft end of the ring when the motor mount tube is attached to the nose at the forward end. I cut the motor mount tube, poured some epoxy into the nose, and then slid the tube in as far as it would go.

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jmuck78

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Head End Altimeter Bay

The Nose Cone was modified to use the LOC Reusable Nose Weight System in a previous post; this post replaces the spacer between the nose weight cartridge and the nose cone bulkhead with an altimeter/tracker bay.

The "spacer" cartridge used in the LOC RNWS is a standard 3 inch diameter coupler, which is the same coupler used in the LOC 3'' avionics bays, so I decided to start with the stock 3'' electronics bay and modify it to fit in the RNWS spacer cartridge slot. It's important that the nose weight cartridge have the anchor / attachment point recessed into the coupler so that the anchor doesn't protrude outside of the standard coupler volume.

The 3'' electronics bay kit comes with a stiffy coupler that sits inside the nominal 3'' coupler. Two quarter inch bulkheads then sit inside the main coupler and use the stiffy coupler as a backstop. This set up is close to what I need. The anchors that attach to the payload bay on the aft side need to be completely recessed so as to not interfere with the large bulk plate on the nose cone, so I need to reduce the stiffy coupler by about 1.5 inches. I also reduced the electronics sled so it would fit in the new, smaller ebay volume.

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The RNWS uses the anchor on the weight cartridge as a "pull" to be able to remove the nose weight. In order to be able to retrieve the weight cartridge, I need to be able to attach it to the electronics bay so that when I pull the ebay out, the weight cartridge pulls out too. The ebay will thus have a recessed eye bolt on the aft side and a protruding eye bolt on the forward side. The protruding eye bolt on the forward side will sit inside the recessed portion of the weight cartridge and also be tied to the anchor on the weight cartridge. The asymmetry gives me some flexibility to fly only the ebay (by reversing its orientation) and using a spacer cartridge.

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The rest of the assembly is similar to the standard RNWS nose cone assembly, except in this case, the anchor point is much further aft in the nose cone.

The ebay has enough room in it for an Eggtimer TRS with the antenna placed beside the flight computer and connected via coax cable.
 

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jmuck78

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Electronics Bay


I wanted to make the electronics bay modular enough that I could reuse it in several different rockets, particularly the LOC VII and the LOC Hawk, but also in the extended Mega Magg I built a long time ago that was built with motor deployment as the recovery method. This electronics bay ended up being a bit heavier than I had expected, primarily due to the replacement of two of the three 450 mAh LiPo batteries with an 1000 mAh LiPo and a 1300 mAh LiPo. After having to stretch (with a ladder) to arm the electronics on my Wildman Extreme, I also wanted to use the wireless arming capability of the Eggtimer Quantum. The other design consideration was that the LOC VII is not fiberglassed, while the LOC Hawk is. So, in order to make this e-bay work for both, I had to either live with the slight difference in tube diameter due to the fiberglass, or make two different ebay containers and line up the switch holes perfectly in both. Wireless arming solved that problem since I no longer need to line up the switch holes in the vent band. I can use the same bulkheads and sled, I will just use a different coupler+switch band, which is easy to swap between flights - not that I expect to be able to fly both the LOC VII and the LOC Hawk on the same weekend since I only have so much space in the car.

Given those constraints, I elected to use the following:
- Eggtimer Quantum as the primary
- Eggtimer Classic + Eggtimer WiFi switch as the back up
- Eggfinder TX (which I don't have yet) as the optional on-sled tracker.
- Three 2S LiPo batteries - one 450 mAh, one 1000 mAh, and one 1300 mAh.

I have assigned the 450 mAh battery to the eggfinder TX (~ 6 hrs of runtime), the 1300 mAh battery to the WiFi Switch + Eggtimer Classic (should last about 12 hours for just the switch), and the 1000 mAh battery to the Quantum (should last "all day" according to the users guide).

I had also paid the price recently on another rocket for accidentally leaving the LiPo batteries connected to an Eggtimer TRS, and have two dead LiPo batteries as a result. I decided I should install two manual switches to allow me to connect the batteries and still have an easier method of disconnected them without having to disconnect the connectors. I won't be able to access the screw switches from outside the rocket, unless I decide I need to drill the holes to do so, but being able to flip the switches makes bench top testing substantially easier.

Since I'm going to (eventually) integrate an Eggfinder TX onto the sled, I added an RP-SMA cable to the main bulkhead that will allow me to mount a 900 MHz antenna on the outside of the electronics bay, which should help minimize interference from the two all-three rods inside the electronics bay.

Notes:

  • I mounted the three electronics components to the plywood sled using #4 nylon standoffs and #4 nylon screws.
  • The batteries were attached to the sled with 3 large zip ties (each) and mounted down the center of the sled.
  • I also attached the WiFi passwords so that I don't lose them (unless I lose the rocket, of course).
  • For the switches, I used to missile works screw switches and just glued them to the sled with CA. The switches are connecterized so I can by-pass them altogether if I decide I don't need them.
  • For charging, I plan to bring the sled to the charger and just leave the batteries attached unless it becomes necessary to replace one of the batteries.
  • Final build weight for the electronics bay (excluding only the Eggfinder TX) is about 2100 grams.

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jmuck78

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Recovery System:

For the booster section, the recovery system consists of a Recon Recovery drogue, a 45 foot long 7/16 inch tubular kevlar harness from OBH, and a 24 inch square nomex blanket.

The main section recovery consists of a Recon Recovery 60 inch chute, 35 foot long 7/16 inch tubular kevlar harness, and a 24 inch square nomex blanket.
 
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jmuck78

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Finish and Paint

Since I decided against fiberglassing the airframe, I have some spirals to fill. I decided to apply a thin coat of finishing epoxy to the fins and the airframe to fill in the spirals and other imperfections. There was almost no imperfections in the plywood fins, so no bondo filler was required. I used West Systems finishing epoxy for the finishing coat.

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After the epoxy cured, I sanded most of the epoxy off and applied a first coat of rustoleum filler primer (red).

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After some more sanding and few coats of grey primer, I finally applied the finishing coat of white paint:

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Applied the decals from stickershock:

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jmuck78

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Deployment Tests

Initial Deployment tests were successful. Using the black powder calculator from insane rocketry (https://www.insanerocketry.com/blackpowder.html) I calculated 2.3 grams of black powder for both the booster (drogue) and payload (main) sections. The deployments were initiated using the eggtimer Quantums' built-in test function over wifi. I managed to initiate the deployment test and record using the same iphone, but I had no idea when the count down from 10 hit zero, so the deployment was a bit of a surprise.

Main chute deployment test:

Drogue chute deployment test:
 
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jmuck78

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Launch Report

It launched! I brought the LOC VII to NSL and planned to launch it on a K1000. Unfortunately, yours truly didn’t read the instructions for the motor until i was preparing it, and the k1000 requires grain bonding, so I went to plan B, and launched it on an L1000. The flight was perfectly straight up to roughly 3700 ft. I had used 3 #2 shear pins to hold the nose cone to the payload tube, which apparently wasn’t enough for a big heavy nose cone, and the main deployed at apogee as a result. There was little wind and the rocket landed very near the pad it launched from, so i didn’t have a terrible walk. I will say that i underestimated how much effort it takes to walk that rocket out to the pad, wow.

The booster managed to stick the landing in full view of the crowd, which was enthusiastically received.

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ThirstyBarbarian

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I’m really looking forward to this build thread! From the placeholders, it looks like you are planning a very thorough build, covering almost every aspect of HPR.

This rocket looks extremely versatile and could be flown in lots of different configurations. MMAS gives you countless motor combos. You could fly it in long or short configurations. Very cool!

I love working with the big 7.5” tubes and large components.
 

DRAGON64

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The build id looking pretty good so far, that thing is huge!
 

jmuck78

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Updated post #2 with the motor mount and fin sub assembly. Checked fit with the body tube and the fin alignment looks spot on.
 

Pyropetepete

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Whoop been waiting for someone to do this. Super high level of detail thank you.

Pushing to get this as wedding gift and go for my L2 with it.
 

jmuck78

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Whoop been waiting for someone to do this. Super high level of detail thank you.

Pushing to get this as wedding gift and go for my L2 with it.
This rocket might be a tad large for an L2 flight, most of the J motors are underpowered according to the sims. You might be able to lighten it up a bit, or get a really big J.
 

Padseven

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This rocket might be a tad large for an L2 flight, most of the J motors are underpowered according to the sims. You might be able to lighten it up a bit, or get a really big J.
I flew this rocket on a J 520 last summer. The flight was the definition of low and slow. She pitched about 10 degrees off the rail but ran true from there. Crowd pleasing flight but on the lower limit for thrust ratio.
 
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