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alternatives to centering rings?

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jimbo_slim

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designing a HPR that is as modular as possible for R&D purposes. as such, I'd rather not have to glue in any centering rings in case I need to fiddle around with/in the lower body tube, but am struggling to find alternative motor constraint methods. any resources out there or tried and true alternatives?
 

KennB

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If you give the folks here some ideas of the parameters you want to vary in your research they'll be better able to make good suggestions.
 

MikeyDSlagle

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Use rings that are thick enough to where you can mechanically attach them to the rocket via screws. You can actually build a glue free rocket. Bolt/screw/rivet everything together.
 

BABAR

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If you give the folks here some ideas of the parameters you want to vary in your research they'll be better able to make good suggestions.
I am with Kenn on this one. What kind of “fiddling” do you need to do?

You can build a motor mount with rings or with centering “strips”. In either case, you can make it slide in and out pretty easily, and use screws or cotter pins or pegs or some other device/removable mounting that allows you to fully remove it for “fiddling”. If you use side strips (think of them as fins that extend radially from the mount to the inner side wall of the outer body tube) you need to think your deployment force. If motor deploy, you will need some sort of forward “ring” to seal the mount to the body tube, otherwise your ejection force just shots out the back between the strips. This only needs to be strong enough to resist the ejection charge, but doesn’t have to be as strong as a structural engine mount ring.
 

JohnCoker

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The original ARLISS rockets used a modular design with all parts attached using mechanical fasteners. Centering rings and bulkheads were glued into short pieces of coupler for example.
jcrocket.com/arliss2000.shtml
 

Sooner Boomer

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What I built for MPR would work for (slightly) higher power stuff. The body tube is 54mm phenolic. 3 surface-mounted fins. The motor mounts are in long "couplers" that slide into the body and are retained by a forward thrust ring and three machine screws equally spaced around the body about half-way down the motor mounts. There are nuts epoxied inside each motor mount adapter (coupler). I've made motor mount adapters with a single 24mm engine, three 24mm engines, and a single 29mm engine. Sorry, no pics right now. I'd posted some on the old forum, if the pictures still exist under my account, I could re-post them.
 

Dave A

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When I built my last large rocket I just bonded a few coupling centering rings sized for the largest motor it would fly, no motor tubes. You can insert ribs only to sandwich the fins in place.
 
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jqavins

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To Jimbo:
These are all fine ideas that boil down to "use centering rings but incorporate them into a removable motor mount assembly." And that probably scratches the itch you initially described, but it doesn't answer the question you originally asked.

So, if you want to have an alternative to centering rings, how about struts? Take straight pieces of plywood or aluminum angle. To one end, attach small plates, curved to fit the outside of the motor mount tube. Glue these to the tube. To the other end attach plates curved to fit the inside of the airframe tube. Glue (or bolt) these into the airframe. Using three or four of these around, repeated in two planes, you've got your motor mount in place with no rings. And if you take the bolt option for attachment to the airframe then it's removable, like the suggestions above, so you've got the best of both.

Keep in mind that if you do this, your booster section is open to the air, so all of your recovery laundry must be above a solid bulkhead, and motor ejection is not an option (but altimeters would love it).

(Hmmm, now I'm thinking of designing a whole booster section out of nothing but struts, like an Erector Set rocket.)
 

Sooner Boomer

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mmount.jpg
What I built for MPR would work for (slightly) higher power stuff. The body tube is 54mm phenolic. 3 surface-mounted fins. The motor mounts are in long "couplers" that slide into the body and are retained by a forward thrust ring and three machine screws equally spaced around the body about half-way down the motor mounts. There are nuts epoxied inside each motor mount adapter (coupler). I've made motor mount adapters with a single 24mm engine, three 24mm engines, and a single 29mm engine. Sorry, no pics right now. I'd posted some on the old forum, if the pictures still exist under my account, I could re-post them.
Found a picture of the three motor mounts!
 

Chad

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What I built for MPR would work for (slightly) higher power stuff. The body tube is 54mm phenolic. 3 surface-mounted fins. The motor mounts are in long "couplers" that slide into the body and are retained by a forward thrust ring and three machine screws equally spaced around the body about half-way down the motor mounts. There are nuts epoxied inside each motor mount adapter (coupler). I've made motor mount adapters with a single 24mm engine, three 24mm engines, and a single 29mm engine. Sorry, no pics right now. I'd posted some on the old forum, if the pictures still exist under my account, I could re-post them.
What is the wear and tear like on the airframe where the machine screws connect the couplers? I've thought about combining this technique with rear ejection to make like a unibody airframe. Imagine a nose cone, airframe, and fins as one piece and it all slips over an ebay, recovery, and motor, securing at the bottom.
 

Sooner Boomer

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What is the wear and tear like on the airframe where the machine screws connect the couplers? I've thought about combining this technique with rear ejection to make like a unibody airframe. Imagine a nose cone, airframe, and fins as one piece and it all slips over an ebay, recovery, and motor, securing at the bottom.
Almost none. The body is 54mm phenolic, and there is a thrust ring just above the motor mounts. The screws just hold the mounts in place, but don't take much stress.
 

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