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Motor mount adapters and high thrust

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Threemorewishes

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Hello everyone,
Does anyone have any experience with high thrust motors being used in motor mount adapters? Say a 54mm very high, initial impulse K stuffed into a 98mm mount using an adapter. Are they a good idea? Are some designs better than others?
Thanks for any input you may have.
Greg
 

bobkrech

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Greg

You could be more specific, however a properly designed and installed motor adapter for a high thrust motor, should work fine in a larger motor tube.

Bob
 

THarrison

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Make the aft ring on the adapter a thrust plate where the ID of the ring matches the OD of the motor case. This way there is no possible way the motor can fly through the rocket. Tie together with allthread.

Todd Harrison
 

Bill P

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Dave at Red Arrow Hobbies makes a great 98mm to 54mm adapter, it'll handle all you throw at it.

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troj

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Dave at Red Arrow Hobbies makes a great 98mm to 54mm adapter, it'll handle all you throw at it.
For a really hard-hitting motor, I don't like that design -- it puts all the load as shear load on the rear ring of the adapter.

Instead, at the rear of the adapter, place a centering ring where the ID of the ring matches the OD of the case, and the OD of the ring matches, or better yet, exceeds, the OD of the motor mount in the airframe.

That way, that ring transfers thrust directly from the motor to the aft end of the rocket, and not to components of the adapter. The rest of the adapter just becomes motor alignment.

-Kevin
 

Gabe Osborn

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Kevin,

I may be picturing your description incorrectly, but the rear ring on that motor adapter does exceed the OD on the 98mm case. It fits nicely in an Aero Pack retainer. I dont know how much of a difference it makes in transferring the motors thrust but, if requested, Dave will drill a couple holes through the rear ring and 98mm tube and run at least a 1/4" dowel rod through them, then sand them smooth. They seem to hold together pretty well.
 

troj

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I may be picturing your description incorrectly, but the rear ring on that motor adapter does exceed the OD on the 98mm case.
The piece that exceeds it is the outer tube sleeve, as opposed to a piece of plywood.

So, what you have in the motor transferring thrust to the adapter body, then the adapter body transferring it to the motor mount.

Basically, the thrust ring on the motor transfers its thrust (mostly) to the motor mount tube in the adapter. That tube, via shear load, transfers it to the rings. Those rings, via shear, transfer it to the 98mm outer tube of the adapter. That tube transfers it via shear to the thrust ring (a sleeve of tube) on the adapter.

With construction like that, it's tricky to get a full bonding surface on the tube-to-tube joint.

Instead, I prefer when the piece that takes the load from the motor's thrust ring and transfers it to the rear of the rocket is one solid piece of plywood.

if requested, Dave will drill a couple holes through the rear ring and 98mm tube and run at least a 1/4" dowel rod through them, then sand them smooth.
That puts the dowel pins in a position to help bear the load, and isn't a bad idea. But, by redesigning to use a single piece of plywood, that's no longer necessary, and assembly becomes much easier.

I've seen an adapter built similarly to the one you show fail, resulting in a J570 shooting up through a rocket. We examined the adapter afterwards, and there was a complete glue joint that had failed; the rest of the adapter was built with the same batch (including mix) of epoxy, and it appeared to be rock-solid.

Needless to say, this changed how I look at adapters....

-Kevin
 

Bill P

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I see your point.

If you look at the pictures, you will see that Dave does put the dowels in. There are four. This wasn't a special request feature.
 

Threemorewishes

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The reason I ask is I want to design my next rocket for both 98mm L3 motors but be able to fly it on the far more economical 54mm hardware I already own.

The distance between the 54mm support elements and the end of the 98mm mount under high thrust is what causes the concern. Having never used an adapter for any reason I have no experience with the possible failure modes.

Everyones input has helped but I still need to get that warm fuzzy feeling before proceeding.

Thanks again.

Greg

Greg

You could be more specific, however a properly designed and installed motor adapter for a high thrust motor, should work fine in a larger motor tube.

Bob
 

terryg

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I would highly recommend Aeropack adapters and retainers. They are made from aluminum and you can nest the adapters. My current build is a 3 inch rocket with a 54mm mount. With the adapters I can fly any combination from a 54 mm K to a 29 mm H. They all fit together to retain the motor and adapters with one screw on cap and give a flush almost continuous appearance at aft end. You can share the parts among multiple rockets since only one part is permanently attached to the rocket. No muss, no fuss.
 

troj

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I would highly recommend Aeropack adapters and retainers.
The Aeropack adapters and retainers are indeed top-notch.

-Kevin
 

butalane

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Make the aft ring on the adapter a thrust plate where the ID of the ring matches the OD of the motor case. This way there is no possible way the motor can fly through the rocket. Tie together with allthread.

Todd Harrison
The thrust plate design is bulletproof. If you are really worried have the thrust plate made out of aluminum.

I have a 98mm-75mm adapter that has survived 6 Ms and an N that is made around a 1/8" aluminum thrust plate. 2 of these were 3000 plus Newtons average impulse.
 

Handeman

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For a really hard-hitting motor, I don't like that design -- it puts all the load as shear load on the rear ring of the adapter.

Instead, at the rear of the adapter, place a centering ring where the ID of the ring matches the OD of the case, and the OD of the ring matches, or better yet, exceeds, the OD of the motor mount in the airframe.

That way, that ring transfers thrust directly from the motor to the aft end of the rocket, and not to components of the adapter. The rest of the adapter just becomes motor alignment.

-Kevin
First and foremost, I completely agree that having the rear ring of the adapter being the load bearing/transfer piece in an adapter is the right/best design.

I just don't know that if doing that adds a significant amount of dependability to the design since there are already shear load points in place. Unless the rocket is a minimum dia., the aft CR in the rocket has shear loading between it and the MMT and the BT. Even with the rear ring of the adapter extending over the rocket MMT, that ring still has a shear joint between it and the adapter MMT.

Reducing the number of fail points is important, I just don't think that I would recommend avoiding/modifying or not using the commercial adapters when I don't see that the proper assembly of those adapters doesn't significantly change/increase the risk of failure. It is just another design/risk/cost trade off each rocketeer has to make.
 

troj

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I just don't know that if doing that adds a significant amount of dependability to the design since there are already shear load points in place. Unless the rocket is a minimum dia., the aft CR in the rocket has shear loading between it and the MMT and the BT. Even with the rear ring of the adapter extending over the rocket MMT, that ring still has a shear joint between it and the adapter MMT.
To be honest, I'm not real fond of construction, especially on bigger rockets, that relies on a shear load between the motor mount and centering rings.

I've got a couple rockets that have rear rings that have rabbets on them -- the ID of the ring is the ID of the motor tube, and the OD of the ring is the OD of the airframe. There's a rabbet on the ID and the OD that lets the ring fit partially up into the airframe, and the motor mount to slip partially into the ring. But the ring takes the motor thrust directly, and transfers it to both the motor mount and the airframe, directly.

On our really big (several hundred pound) projects, we build so that the motor thrust is directly transferred to the airframe and supporting structure via similar methods. Nothing in shear, as we just don't trust it, especially on P & Q motors.

-Kevin
 

butalane

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First and foremost, I completely agree that having the rear ring of the adapter being the load bearing/transfer piece in an adapter is the right/best design.

I just don't know that if doing that adds a significant amount of dependability to the design since there are already shear load points in place. Unless the rocket is a minimum dia., the aft CR in the rocket has shear loading between it and the MMT and the BT. Even with the rear ring of the adapter extending over the rocket MMT, that ring still has a shear joint between it and the adapter MMT.
If the rear ring (thrust ring) of the adapter is so created that it is the load bearing and transfer peice of the design then it eliminates shear forces on glue joints in the adapter entirely. This is an example of a simple method of adding dependability without that much extra modification and no weight addition. For this ring to fail the thrust ring would need to fly through the thrust ring material. Lots of force for that to happen.

As an example the previous thrust plate I mentioned is not even glued or otherwise attatched to the rest of my adapter. It floats between the motor and the rocket and is pinched during the flight. The only other thing holding it on is the motor retention. My adapter simply sits in the motor tube around the motor. If there were any shear stresses here there were obviously be failure.

I agree with troj on the "rabbets" around the aft centering ring. Awesome design.
 

troj

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As an example the previous thrust plate I mentioned is not even glued or otherwise attatched to the rest of my adapter. It floats between the motor and the rocket and is pinched during the flight. The only other thing holding it on is the motor retention. My adapter simply sits in the motor tube around the motor. If there were any shear stresses here there were obviously be failure.
Never really thought about a floating plate -- quick and simple to do, too!

-Kevin
 

patelldp

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The thrust plate design is bulletproof. If you are really worried have the thrust plate made out of aluminum.

I have a 98mm-75mm adapter that has survived 6 Ms and an N that is made around a 1/8" aluminum thrust plate. 2 of these were 3000 plus Newtons average impulse.
Didn't I make that?
 

bobkrech

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Before you go and make a lead sled, do some simple calculations.
  1. The highest impulse K motor that I could find has a peak thrust of ~500 pounds or 2250 N.
  2. The circumference of a 54 mm motor tube has an OD ~2.25" and a circumference of ~ 7"
  3. The shear area for a 1/4" ring is 1 3/4 sq.in. so the shear load is < 300 PSI.
  4. What kind of epoxy and tubes are you using that can not withstand a 300 PSI shear load?
Assumptions:
  1. You are using a good 1/4" aircraft grade baltic birch for the rings.
  2. You are using a good high temperature epoxy such as JB Weld for the MT-Ring shear joint.
  3. Plenty strong even when hot.
Roll Your Own Motor Adapter Design Rules:
  1. These simple rules don't even rely on shear strength. The motor tube simple acts a thermal insulation and a torque rod to prevent off-axis torque from assymetric thrust.
  2. Use (3) 1/4" aircraft grade baltic birch for the rings: two rings fit inside 98 mm MT; third ring has o.d. = to 98 mm motor thrust ring.
  3. Top ring is flush mounted to top of 54 mm motor tube.
  4. Lower rings are doubled with larger ring flush mounted to aft end of 54 mm motor tube.
  5. 54 mm motor thrust ring attached to larger birch ring by 54 mm motor positive retention method which carries thrust load.
  6. 98 mm thrust ring adapter uses existing 98 mm motor positive retention method which transfers thrust load to lower rocket ring.
Bob
 

troj

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Before you go and make a lead sled, do some simple calculations.
  1. The highest impulse K motor that I could find has a peak thrust of ~500 pounds or 2250 N.
  2. The circumference of a 54 mm motor tube has an OD ~2.25" and a circumference of ~ 7"
  3. The shear area for a 1/4" ring is 1 3/4 sq.in. so the shear load is < 300 PSI.
  4. What kind of epoxy and tubes are you using that can not withstand a 300 PSI shear load?z


  1. Up until the launch where I saw it happen, I likely would've agreed with you, Bob.

    However, having seen a J motor fly up through a rocket, because a joint failed in an adapter, has made me prefer a plate that prevents it.

    It hardly qualifies as a "lead sled" -- it's a difference in the size of the rear ring, and nothing more.

    -Kevin
 

patelldp

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Up until the launch where I saw it happen, I likely would've agreed with you, Bob.

However, having seen a J motor fly up through a rocket, because a joint failed in an adapter, has made me prefer a plate that prevents it.

It hardly qualifies as a "lead sled" -- it's a difference in the size of the rear ring, and nothing more.

-Kevin
The LOC type adapters with the 1" or so of motor mount tube glued to the couplers can work if care is taken. My 54-38mm has worked for years, but the thing has more epoxy than cardboard at that joint.

Every adapter that we make now has an aluminum backplate, exactly the same as the one that I made for Ryan that he described before. The O.D. of the ring is larger than the host tube (so 3.75" OD for a 76mm to 54mm adapter) and knurled. The center hole is stepped, a through hole to accept the motor casing with very little play, and then a countersunk diameter to accept a motor tube. We then J.B. Weld it to the tube, and put 2 centering rings made of plywood on the tube. The aluminum ring could float if we wanted it to, but the J.B. Weld is nice because we never have to worry about losing the aluminum ring.

My newest project (in the works for a long time) is a 3" M.D. rocket with all aluminum plates for bulkheads. The 54mm adapter is as described earlier.
 

m85476585

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Dave at Red Arrow Hobbies makes a great 98mm to 54mm adapter, it'll handle all you throw at it.
I had an adapter like that fail when the rocket hit the ground and the motor, which stuck out the back a bit, took most of the impact. The ring that keeps the adapter from going into the tube slid right off. Sure, that wasn't caused by the motor, but it was a small rocket under chute, so the forces involved were not huge.
 

DKrause

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The LOC type adapters with the 1" or so of motor mount tube glued to the couplers can work if care is taken. My 54-38mm has worked for years, but the thing has more epoxy than cardboard at that joint.

Every adapter that we make now has an aluminum backplate, exactly the same as the one that I made for Ryan that he described before. The O.D. of the ring is larger than the host tube (so 3.75" OD for a 76mm to 54mm adapter) and knurled. The center hole is stepped, a through hole to accept the motor casing with very little play, and then a countersunk diameter to accept a motor tube. We then J.B. Weld it to the tube, and put 2 centering rings made of plywood on the tube. The aluminum ring could float if we wanted it to, but the J.B. Weld is nice because we never have to worry about losing the aluminum ring.

My newest project (in the works for a long time) is a 3" M.D. rocket with all aluminum plates for bulkheads. The 54mm adapter is as described earlier.
This is what happened to my LOC adapter, with an I600 in it..

http://www.youtube.com/watch?v=mqtNhcODfCk

The motor went right through the rocket.

Was funny though...

=Don=
 

Bill P

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This last weekend I did see a thin glued thrust ring on an adapter fail. The motor flew marvelously.
 

Sgt Blowfly

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at the rear of the adapter, place a centering ring where the ID of the ring matches the OD of the case, and the OD of the ring matches, or better yet, exceeds, the OD of the motor mount in the airframe.
Does anyone have a pic they might share of their adapter built like this? Putting together a LOC MMA-4 (38/54) and not too keen on using the thin thrust ring supplied.
 
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Bill P

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Does anyone have a pic they might share of their adapter built like this? Putting together a LOC MMA-4 (38/54) and not too keen on using the thin thrust ring supplied.
I've been using mine for about 2-1/2 years, but it came pre-assembled,
it has well over 20 flights on it. The trust ring is the thickness of a motor mount tube and 3/4" long.

As mentioned in some above posts, you can peg the thrust ring with some pieces of dowel. Just make sure you epoxy it very well.
 
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