Scratch-Built HPR MMT

[Elapid]

is this overengineered, underengineered, or just right?
i think i went a bit heavy, but maybe i'm underestimating the forces involved. all joints are 30-minute epoxy, the plywood is 5/32" aircraft plywood. I made 4 centering rings from the plywood and epoxied them together to form a double thickness. the rings at the top of the mount are attatched such that the lower ring encircles the tube and the topmost ring butts against the end of the tube.

the tube is a standard Staples brand mailing tube.

the assembly fits a standard BT-80 body tube, though i would assume that if i want to make something to fly on a 38mm motor, it had better be glassed!

once i get some feedback, i'm planning to make a couple more of these for my "Project".


i'm working on an 8" coupling ring right now...
woo wHooo!

 

[Elapid]

an AT 38/480 motor
here's the thrust ring i fab'd
it is generously epoxied in place

 

[Neil]

Have you tried to fit a motor in that thing yet? staples tubes are VERY tight with motors... Maybe its different with the humidity wherever you are, but they are no good for MMTs up here...

Nice looking motor mount.The 5/32 plywood might be a little overkill, but hey (for a 2" tube, with a 38MM motor mount, I would proabably use 1/8"

 

[edwardw]

For my centering rings on my L1/L2 bird the top and center ring are 1/4 5 ply. The bottom centering ring where the motor is is 3/8 7 ply. I usually go with three so that when I do TTW fin mounts the MMT will be center while I do the internal fillets. Then I just add the back centering ring and ka-bam - a MMT in a BT.

Edward

 

[Elapid]

Originally posted by Neil
Have you tried to fit a motor in that thing yet? staples tubes are VERY tight with motors... Maybe its different with the humidity wherever you are, but they are no good for MMTs up here...

lol!
no...i haven't tried to fit a motor in it...i don't have one.
i did measure it and it seemed close. i asked in another thread about it but no replies to that... so i figured wtf...

i guess i'll have to pick up a motor casing so i can see if this was mostly wasted time. i did figure out a couple things i'd do differently next time.

i guess if it is too tight, i can bore it out a little with a dowel wrapped with sandpaper?

 

[Elapid]

Originally posted by edwardw
For my centering rings on my L1/L2 bird the top and center ring are 1/4 5 ply. The bottom centering ring where the motor is is 3/8 7 ply. I usually go with three so that when I do TTW fin mounts the MMT will be center while I do the internal fillets. Then I just add the back centering ring and ka-bam - a MMT in a BT.

Edward

no offense intended but, methinks yer mmt is backward...
there's almost no force (relatively speaking) on the mount at the rear of the rocket, all the force is at the front of the motor and forward...until the ejection charge goes off.

i am planning to use this in a modular cluster arrangement.
the fins will attatch to the main MMT which will have at least 3 (maybe 5 if they fit) BT-80 sized mounts to accept this style of mount. recovery will be via electronics so the native MMT will have a solid bulkhead which these will butt up against.

 

[Neil]

I wouldent make any more untill you know your not wasting time, thats for sure... Its enough to fit a motor in, if you REALLY shove... Its toatally impossible to do if you dont want to crimp your rocket in the process... And the tube gets a lot weaker if you take out the inner layer of tube from that size mailing tube... It aint purty... I would reccomend going to The Hobby Connection and get yourself some nice LOC tubing... I made the same mistake you did, only on a much larger scale... I bought 20 of those things for the MMTs on my kits.... Much to my dismay, IT DIDNT FIT! So now I have a big order of LOC tubing on its way to me....


At least they are not expensive! Besides, I can always made tube-fins out of em, or BB gun targets if they prove REALLY useless....

 

[Elapid]

well...
*shuffles feet*
i guess i'll go buy the right thing so i don't waste anymore time...

thanks!

 

[Neil]

I think it definetly will be worth it... If you arrive at the range, load up a J570, get all exited, then notice the motor wont fit in your motor mount....

Youd proabably wish you spend a few extra bucks...

 

[edwardw]

But, at the back end is where I need the motor retention. I use a PML retainer and want that to hold. If I put that in 1/4 ply about 1/2 of the wood insert would be sticking through. With 3/8 ply only about 1/16 of an inch sticks through - and I can get a better hold.

Edward

 

[Elapid]

that makes sense!

i'm thinking of 4x 4-40 T-Nuts epoxied to the forward side of the rear ring and the bottom of a Spam can for a retainer if i can figure out how to dispose of the contents safely...

 

[Ryan S.]

you dont need 4, one will do it, I would say 2 max. I just use tape. The reason I say dont put so many is it is going to take a long time to take out and put in your motor

 

[edwardw]

I used to do T-nuts and such..but now I'm addicted to slimline retainers. They are so easy to use and just look great. Contack Midpwrguy and he will hook you up with one

Edward

 

[Ryan S.]

never used one of his retainers but midpwrguy is the man so yeah I would talk to him if I were you

 

[MarkABrown]

Originally posted by Elapid
...there's almost no force (relatively speaking) on the mount at the rear of the rocket, all the force is at the front of the motor and forward...until the ejection charge goes off.

Actually, in a properly designed MMT, the thrust is transferred along the MMT and Aft centering ring to the airframe through the adhesive bond of the MMT to the centering rings. You sometimes see others using all thread in their HPR MMTs to assist in this transfer of forces to additional centering rings and thus additional CR to airframe adhesive bonds. Bottom line, the more force you can transfer to the most number of bond points, the stronger the MMT is.

Originally posted by Neil
The 5/32 plywood might be a little overkill, but hey (for a 2" tube, with a 38MM motor mount, I would proabably use 1/8

1/8" might be a little light for some 38mm motors. I use 1/4" as a minimum thickness for HPR motors. I'll use 1/8" for 24mm Mid-Pwr motors.

 

[loopy]

Originally posted by edwardw
But, at the back end is where I need the motor retention. I use a PML retainer and want that to hold. If I put that in 1/4 ply about 1/2 of the wood insert would be sticking through. With 3/8 ply only about 1/16 of an inch sticks through - and I can get a better hold.

Edward

Not to mention that if you're using reloads, the thrust ring on the aft closure of the RMS casing transfers all of the force of the thrust of the motor to the MMT, and the aft CR is the first bond it encounters. I think having the stronger aft CR makes alot of sense in that case. It really depends on if you're using the RMS casing with the thrust ring at the aft end, or if you're using an internally mounted thrust ring.

Loopy

 

[Neil]

Well, your not going to put a J570 in a 2" diameter 2.25' long rocket, are you

Thats what I though.

Yes, I would use 1/4 on pretty much anything bigger... Though I think the LOC Forte has 1/8" plywood CRs, and that has a 38MM motor mount...

 

[llickteig1]

Elapid said:
no offense intended but, methinks yer mmt is backward...
there's almost no force (relatively speaking) on the mount at the rear of the rocket, all the force is at the front of the motor and forward...until the ejection charge goes off.

Perhaps your misconception in this area is because you're thinking too much in terms of model rocket designs that have an engine block at the forward end of the motor tube. Forget about that if you are. Most HPR RMS motors have an integrated thrust ring at their AFT end (typically the larger diameter of the AFT closure) which prevents the motor from going forward and which transfers the energy to the rocket and no engine block is needed or desired. Due to this ring, the forces of the motor are transferred to the rocket through the MMT from the AFT end. This also enables different lengths of motors to be used in a single rocket.

The exceptions to the above statements are the line of Aerotech kits. Aerotech designs-in an engine block (and motor clip) into the motor mouhts of their kits for use with the 29mm single use and 29/40-120 RMS. For other HPR models and motors, engine blocks (and motor clips of this type) are generally not used.

To ease the transition to HPR RMS designs, I would suggest acquiring or at least looking at and handling an RMS motor before scratchbuilding a rocket that uses one.

If I have a misconception of your misconception, I'll slither back into my hole.

With regard to your MMT, the strength looks a little overbuilt, but not terribly so. You might be just as well or a little better off with single centering-rings in 3 places rather than double thickness rings at 2 places.

HTH, --Lance.

 

[Elapid]

that happens occasionally!
hehehhe!


first off...
i measured the id of the tube and it measures a perfectly incompatible diameter of 37mm

secondly...
having never built an HPR i didn't know about the lack of a forward thrust ring. it all makes sense now.

lastly...
i used 4 - pieces of 5/32" birch plywood to create 2 - 5/16" centering rings.

so, i think it's stout enough if the motor would fit...i think i'll take the dremel to the tube to reclaim my centering rings.

 

[Elapid]

i'm handling...

i have an AT Cheetah that takes a reloadable motor system and that's what i made my decisions based on. a third centering ring pair was to be added once design parameters were finalized.

i like to wing it.
like my "project"
you can see the payload bay in the background of the attatched photo.
i don't know what the rocket will be, but i can build components while i decide.

anyway, the cheetah uses an engine retainer clip as well as a thrust ring. i can see now that this has more to do with the capability to use SU motors in addition to the RMS motors and that that isn't an issue with the larger diameter motors.

thanks for the illumination!

 

[aksarben10]

Aerotech kits do have a thrust ring in the top of the motor mounts and their SU motors and motor mounts in their kits are designed around the 29mm 40/120 motor. However, this is just about the only mid-high power rocket kits that you will find this a stock option.

Most people do not like to be limited in the motor choices which is what the thrust ring in effect does as you can only go up to, I believe, a G80 in the Aerotech kits with the SU motors. Many of the Aerotech kits are capable of flying on H and I motors but only if you modify them by taking out the thrust ring.

Also, do not trust that motor clip on the Aerotech kits as they do not always hold and can kick the motor out the back, expensive habit.

Scott

 

[Elapid]

thanks!
it is difficult to find that info on the web...
maybe i should finish that book i bought...

specific design parameters and alternative sourcing of materials are topics that are not well covered from what i have found so far. most of my previous experience was with the estes/centuri models in the '70s, though i had an F100 powered egg crate from enerjet that i got to watch fly once. and i had an E-powered enerjet model also but it lawn-darted in a massive grassy field and ignited the grass...fortunately my mom had brought a fire extinguisher and made us take it out toward the rocket as it was screaming toward earth.

i put out the fire within seconds of it igniting!

thanks, mom!


I just ordered a kit with a native 54mm MMT so i'll see how it's done right.

as far as that engine clip...
i found out on its maiden flight that it is unreliable and will bring some masking tape with me next time. it ejected the motor and half of the 24mm adapter...fortunately it also ejected the parachute!

i only flew it once that day because i was worried about a second faillure and less luck!

 

[llickteig1]

aksarben10/Scott,

Thanks for the explanation about Aerotech's motor mount/engine block design. I liked it so well, I went back and added a bit about it to my previous post in the interest of completeness.

This is valuable information for the HPR neophytes that we 'old-timers' sometimes take for granted as common knowledge. It is a change in the design paradigm with which model rocket flyers are not familiar.

TRFers are the bomb!

--Lance.