LOC-VII Build Thread

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jmuck78 said:
Motor Mount Adapter Installation

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:
View attachment 371955

View attachment 371956

Motor Mount Adapter inserted into the booster section:

View attachment 371957 View attachment 371958

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: https://jcrocket.com/adhesives.shtml

View attachment 371975

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.
Click to expand...

I am reluctant to mention this...but i did see this exact failure at a launch in Ohio. It was on this rocket with an L1000. The motor tube (with motor) blew clean through the rocket at peak thrust. Based on what you said...as I understand it....this glue joint is absolutely critical to get right. I wonder if there is any way to transfer the thrust load to more points. Just an FYI. Not trying to stir it up....but I could not believe it when I saw that particular concern you had....having actually seen that failure mode in action.
Peace
Andrew
 
jmuck78 said:
As I work on this rocket, I often put the nose cone in the incomplete booster section just to keep the nose cone out of the way, and it occurred to me that a short version of the LOC-VII would be fun too. The rocket looks stable and launchable without the payload bay, so I modeled it in OR and it looks good as a simple motor eject rocket:

View attachment 371915
Click to expand...

Hmm . . . The "LOC-ETTE VII" . . . Looks good, nice modification !

Dave F.
 
jmuck78 said:
Motor Mount Adapter Installation

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:
View attachment 371955

View attachment 371956

Motor Mount Adapter inserted into the booster section:

View attachment 371957 View attachment 371958

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: https://jcrocket.com/adhesives.shtml

View attachment 371975

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.
Click to expand...

I would not do the motor mount adapter that way. I would incorporate either a metal thrust washer with an O.D. larger than the hole in the aftmost centering ring and an I.D. the same size as the motor mount tube I.D. or simply use an Aeropack retainer which has an internal shoulder. I would not rely on the shear strength of the epoxy in your MMA.
Use of either a thrust washer or metal retainer with an internal shoulder positively prevents the motor mount adapter motor mount tube from passing through that aft centering ring.
 
Steve Shannon said:
I would not do the motor mount adapter that way. I would incorporate either a metal thrust washer with an O.D. larger than the hole in the aftmost centering ring and an I.D. the same size as the motor mount tube I.D. or simply use an Aeropack retainer which has an internal shoulder. I would not rely on the shear strength of the epoxy in your MMA.
Use of either a thrust washer or metal retainer with an internal shoulder positively prevents the motor mount adapter motor mount tube from passing through that aft centering ring.
Click to expand...

A metal thrust washer is an interesting Idea. If I understand the suggestion correctly, the thrust washer would sit between the motor thrust ring and the motor mount tube, right? The OD of the thrust washer would have to be larger than the hole in the aftmost centering ring - do you mean the aft most ring on the adapter, or the aftmost centering ring on the larger motor mount tube that receives the motor mount adapter?

I have a flanged 54 mm Aeropack adapter I had considered using here, too. I just need to make sure the holes for the flanged retainer don't interfere with the epoxy bond on the opposite side of the retainer. With the flanged Aeropack retainer, I also have to worry about the fasteners taring out through the plywood centering ring. Alternatively, I could use a bonded Aeropack retainer, but I would have to remove the 5.5 inch diameter thrust plate that is bonded on already.
 
BSNW said:
I am reluctant to mention this...but i did see this exact failure at a launch in Ohio. It was on this rocket with an L1000. The motor tube (with motor) blew clean through the rocket at peak thrust. Based on what you said...as I understand it....this glue joint is absolutely critical to get right. I wonder if there is any way to transfer the thrust load to more points. Just an FYI. Not trying to stir it up....but I could not believe it when I saw that particular concern you had....having actually seen that failure mode in action.
Peace
Andrew
Click to expand...

Interesting - I was wondering if there had been failures. The only way to make sure the joint is solid now is to proof test it, which would be interesting to see. There may be a temperature factor here too that a hot motor is able to transfer enough heat to the bond to weaken it at peak thrust, so even a proof test wouldn't be totally definitive.
 
Here's what the aft end looks like with the 54 mm Aeropack flanged retainer in place (but not attached). The holes on the aluminum flange are maybe 1/8'' from the inner edge of the plywood ring.

IMG_7509.jpgIMG_7508.jpg
 
BSNW said:
I am reluctant to mention this...but i did see this exact failure at a launch in Ohio. It was on this rocket with an L1000. The motor tube (with motor) blew clean through the rocket at peak thrust. Based on what you said...as I understand it....this glue joint is absolutely critical to get right. I wonder if there is any way to transfer the thrust load to more points. Just an FYI. Not trying to stir it up....but I could not believe it when I saw that particular concern you had....having actually seen that failure mode in action.
Peace
Andrew
Click to expand...

Andrew, I'd also be interested to know how comparable their build was to this one. Not that I expect you to know all of these answers, but it would be good to know if they used an Aeropack retainer, or just the stock LOC retention system, what epoxy they used, etc, and if the epoxy failed or the plywood ring failed. If the plywood failed, the solution here would be different than if the epoxy failed.
 
jmuck78 said:
A metal thrust washer is an interesting Idea. If I understand the suggestion correctly, the thrust washer would sit between the motor thrust ring and the motor mount tube, right? The OD of the thrust washer would have to be larger than the hole in the aftmost centering ring - do you mean the aft most ring on the adapter, or the aftmost centering ring on the larger motor mount tube that receives the motor mount adapter?

I have a flanged 54 mm Aeropack adapter I had considered using here, too. I just need to make sure the holes for the flanged retainer don't interfere with the epoxy bond on the opposite side of the retainer. With the flanged Aeropack retainer, I also have to worry about the fasteners taring out through the plywood centering ring. Alternatively, I could use a bonded Aeropack retainer, but I would have to remove the 5.5 inch diameter thrust plate that is bonded on already.
Click to expand...

Larger than the aftmost ring (its purpose is as a thrustplate, but it’s still just a big centering ring) on the adapter. The fasteners for the Aeropack withstand very little force and none of the thrust. I would just wrap tape around a motor case to make it a slightly snug fit in the Aeropack retainer and MMT, put the retainer in place, and drill the fastener holes. Don’t worry about drilling through epoxy. Once that retainer is coupled to your thrustplate the epoxy is no longer as critical.
Generally speaking, don’t rely solely on glue joints if you have the ability to create a mechanical interface. Obviously there are some applications, especially with minimum diameter fin attachments, where epoxy becomes structural, but as a rule the epoxy should just hold things in place, rather than being a structural component.
 
I think in general, you should never rely on the sheer strength of adhesives in any rocket design. Ideally, the motor tube is just for aligning the motor in the rocket, not bearing the force of the thrust and transferring it to the airframe. Forces should be transferred by compression from the motor thrust ring, to a motor retainer or thrust plate, to a centering ring, to the fin tabs, to the airframe. If you dry fit the rocket together and weakly held everything in place with something like masking tape, and then you pushed hard against the motor, would the rocket hold together and move forward, or would the joints fail due to sheer? If anything would slide against another part due to sheer, it’s not the best design.
 
jmuck78 said:
Here's what the aft end looks like with the 54 mm Aeropack flanged retainer in place (but not attached). The holes on the aluminum flange are maybe 1/8'' from the inner edge of the plywood ring.

View attachment 372008View attachment 372009
Click to expand...

I’ve never seen one of these retainers up close. Does it have an internal flange that the motor thrust ring rests against? I assume so, and the motor thrust ring is not pushing directly against the inner edge of the plywood ring or against the aft end of the cardboard motor tube. As long as there is an internal flange, and the external flange that we can see, I think this is fine. Thrust is transferred directly from the thrust ring to the retainer to the plywood centering ring, and there are no sheer forces on adhesive joints to fail. For this to fail, the internal flange would have to fail, or the plywood wood have to fail with the entire retainer pushing through it.

The problem with the z-clip retention is that the thrust ring either pushes against the cardboard tube, in which case if the glue joint fails, the motor shoots through the rocket. Or it pushes against a very narrow ring of plywood around the centering ring hole, in which case if the tiny lip of plywood fails, the motor shoots through the rocket.
 
ThirstyBarbarian said:
I’ve never seen one of these retainers up close. Does it have an internal flange that the motor thrust ring rests against? I assume so, and the motor thrust ring is not pushing directly against the inner edge of the plywood ring or against the aft end of the cardboard motor tube. As long as there is an internal flange, and the external flange that we can see, I think this is fine. Thrust is transferred directly from the thrust ring to the retainer to the plywood centering ring, and there are no sheer forces on adhesive joints to fail. For this to fail, the internal flange would have to fail, or the plywood wood have to fail with the entire retainer pushing through it.

The problem with the z-clip retention is that the thrust ring either pushes against the cardboard tube, in which case if the glue joint fails, the motor shoots through the rocket. Or it pushes against a very narrow ring of plywood around the centering ring hole, in which case if the tiny lip of plywood fails, the motor shoots through the rocket.
Click to expand...

Yes, the thrust ring of the motor presses against a shoulder inside the retainer.
You’re exactly right about the thrust ring pressing only against the cardboard tube. If enough of the tube protruded aft of the rear thrust plate a good reinforced fillet would probably be sufficient. After all, for years people have built up thrust rings using tape.
 
I took the advice and used the 54 mm flanged retainer. I completely agree that relying on a single epoxy joint is risky and considered a single point of failure, particularly given the focus we place on the joints for the other centering rings. The only other comment here is that in all of the test data I've seen so far, the plywood failed before the epoxy did. However, all of those tests were stressing the plywood in tension, or the plywood glue in shear. None of those tests demonstrated a failure mode where the plywood failed in shear where the direction was perpendicular to the plywood layers.

Here's the installed motor retainer, which should transfer the load to the plywood centering ring instead of the glue joint. You can see the threaded inserts protruding from the epoxy on the back side of the plywood ring, and the threaded inserts don't seem to have disturbed the epoxy fillet on the motor tube, just on the plywood ring.

IMG_7523.jpg IMG_7522.jpg IMG_7521.jpg
 
I have a 75 mm motor adapter I need to build now as well, which means I will need to order a 75 mm flanged retainer for that adapter, too.
 
jmuck78 said:
I have a 75 mm motor adapter I need to build now as well, which means I will need to order a 75 mm flanged retainer for that adapter, too.
Click to expand...

I was thinking of having my 75mm cover the whole base and push against the body tube of the main airframe.
 
I have started the Nose Cone modifications to accommodate the LOC Reusable Nose Weight System as well as a custom Head End Altimeter Bay (see post #7). The RNWS is an optional upgrade for the LOC VII since the VII doesn't require much (if any) nose weight.
 
I built the 75 mm motor adapter today, or at least most of it - I do not have the 75 mm flanged adapter. I am planning to build this one the same way I build the 54 mm adapter, using the 75 mm flanged motor retainer to add strength to the joint. Yes, I realize the 54 mm adapter is a bit redundant now since I do have an Aeropack 54 mm to 75 mm adapter, but the 54 mm adapter came with the kit, and I had the flanged 54 mm retainer laying around.
 
I have ordered the Y-harness from OBH (thanks for your help, Ted!). Should be here in a few days (not that I'm waiting on it, lots of other stuff to do). I have also ordered the 3'' payload bay for the head-end altimeter/tracking bay from LOC. The 3'' payload bay will replace the empty spacer coupler in the RNWS and, when in use, will provide the nose anchor as well as the tracker bay (and the deployment electronics bay when I fly the stubby version of the LOC VII).
 
I added the electronics bay to the nose cone RNWS (see post #8) so that I can use both the RNWS and a nose cone tracker / deployment if I want to.
 
Yep. Just finished mine - best xmas present ever!! Your build thread is very comprehensive and helpful. I launch at Three Oaks in SW Michigan. Might not be painted and ready for March 9th, but April 26 is looking good. I posted earlier about motor selection with data for delays. Anybody have suggestions for my first flight?
 
JP Shaw said:
Yep. Just finished mine - best xmas present ever!! Your build thread is very comprehensive and helpful. I launch at Three Oaks in SW Michigan. Might not be painted and ready for March 9th, but April 26 is looking good. I posted earlier about motor selection with data for delays. Anybody have suggestions for my first flight?
Click to expand...

Thank you! If you are interested in openrocket, I posted the sim file at the beginning of this thread that includes launch results using different motors. I am going to be using electronic deployment, but openrocket does calculate the optimum ejection delay for any given motor.
 
After more sanding, more filling, and several coats of grey primer, I applied the finish coats of white (see post #11). I'm going to go with the stock paint scheme and the stock decals, with the exception of the fins, which I'm going to paint metallic blue.

The stickershock decals arrived this week, so I'm pretty close to a deployment test. My to do list:
- wire up the electronics bay
- cut the switch band OR permanently install the ebay coupler into the payload bay
- install the 75 mm aeropack retainer to the 75 mm motor adapter
- drill for shear pins and rivets
- install the recovery hardware
- deployment testing
 

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