24mm Cluster Mount

[BobCox]

I am currently building a BT-70 rocket with a 3x24mm cluster mount.

Constraints:

• I do not trust friction fit. For motor retention, I want to leave a threaded rod sticking out the back between the engine tubes and hold the motors in place with nuts and possibly washers.
• I want the option to be able to fly with C11, D12, or E9 motors.
• I am trying to hold the weight down, at least on the C motors.

My question: How long to make the engine tubes?

Option A: Make the engine tube 3.75 inches long, install a 1/4" engine block in each tube, and leave 0.5 inches of threaded rod sticking out. Install three 1-inch spacers in the tubes when using the shorter C11 or D12 engines
Pros:
+ Moves weight farther forward when using E9 engines.
+ Less threaded rod weight.
Cons:
- Spacers adds extra weight when using the lower-powered engines.

Option B: Make the engine tubes 2.75 inches long, install a 1/4" engine block in each tube, and leave 1.5 inches of threaded rod sticking out.
Pros:
+ No spacer tubes required for shorter engines.
Cons: The extra length of metal rod hanging out the back:
- may get bent on landing
- may impale something or someone during landing
- may get gunked up by the engine exhaust.

Option C: Other.

Comments? Suggestions?

 

[wwattles]

Option C-1:
Use only 1.5" of threaded rod, leaving 0.5" hanging out the back (epoxy the rest in place between the 3 MMT's). When prepping the engines, wrap 10-12 layers of 1/4-wide tape around the nozzle end of the motors, forming thrust ring, just like what you find on a reloadable. Use a washer held in place by a hex nut on the threaded rod for motor retention.
With this configuration, you can make your MMT's be nice and short, if you want, and you aren't using friction fit.

WW

 

[Mad Rocketeer]

A couple more possibilities.

C.) What wwattles said. (He beat me to it by mere seconds.)

D.) (Or should this be option C-2?) Epoxy a nut between the motor mounts, rather than a rod, (if there is space in this tube configuration, which I'd have to check to know for sure). Use the shorter MMTs. Do the tape at the nozzle end of the motor, like wwattles describes, (which isn't the same as friction fitting, of course). This gives you no extra rod poking out on any configuration. (Even A. and C. leave a little bit of rod poking out.) This way, instead of a threaded rod in the rocket and a nut screwed onto it at prep time, you have a nut in the rocket, and a machine screw or a bolt screwed into it at prep time.

In both cases, include no glued-in thrust ring in the MMTs.

There are probably a lot of other ways to handle the same thing. So far, I'd lean to either (C.) or (D.), whichever fits your fancy and the available space.

 

[brianc]

Epoxy a NUT in the center of the MMTs, then use a bolt/washer to retain the
motors. PRO- no extra rod to bend or impale on recovery; won't get gummed
up with exhaust. CON- might be too tight a fit for a decently sized nut (but I've
sucessfully retained 29mm motors with 4-40x3/4 - BP should be OK too).

If your rocket is long enough to handle pulling the CG back a bit, I'd
build the MMTs with a C11/D12 thrust ring, and hang the E9's out the back
when you use'em.

Then again, you could use tape to build up a thrust ring on each motor
(at the nozzle end- like a reload) and skip the forward thrust ring entirely.
CON- would add to the rocket prep time.

EDIT:

Mad Rocketeer wrote
C.) What wwattles said. (He beat me to it by mere seconds.)

And it looks like they both beat me by a minute or so...

 

[Mad Rocketeer]

Originally posted by brianc
Epoxy a NUT in the center of the MMTs, then use a bolt/washer to retain the
motors. PRO- no extra rod to bend or impale on recovery; won't get gummed
up with exhaust. CON- might be too tight a fit for a decently sized nut (but I've
sucessfully retained 29mm motors with 4-40x3/4 - BP should be OK too).

[CLIP]

EDIT:
And it looks like they both beat me by a minute or so...

Yep. And my (C.) was wwattles's (C-1) suggestion, while your suggestion was my (D.) (or C-2). Great minds thinking alike, man!

One more thing to keep in mind if you epoxy in a nut, you'll want to put some Vaseline in the threads first. Be sure to keep it only in the threads though, as you want the epoxy to stick elsewhere on the nut. This keeps the epoxy from cloging the threads. Tough to fix if it happens but easy to prevent. The grease lubricates the threads too and helps to prevent corrosion from exhaust gasses, so it's not a bad idea to apply a bit to your machine screw or bolt occasionally and thread it into the bolt.

 

[jflis]

Here's a thought that has come to me from time to time and it involves this form of motor retention so this may be the best place to suggest it for discussion...

WOuld it be possible to take a bolt (probably about 4-40 (though 6-32 may also fit)) and treat the threads with a good lubricant (something that will get into the threads but not *fill* the threads (perhaps WD-40?)), then epoxy this bolt into the small gap between the three motor mounts. When fully set, would the lubricant allow you to *unscrew* the bolt from the epoxy?

If you can, then you will have effectivly "tapped" the epoxy. Then you have a threaded insert without the bulk of an actual nut taking up all that extra space.

just a thought...

 

[wwattles]

You probably could, if you coated the bolt with a spray-on teflon release agent. Regular lubes might not work as well, as epoxy has a nasty way of getting past those.

WW

 

[BobCox]

I figured I could count on this group to be awash in good ideas. Thanks for all the suggestions so far.

Jerry Irvine sent me a PM and asked me to post this for him, since he is currently banned from TRF :

NEVER use thrust rings!!

Okay, fine. Message received -- No thrust rings.

I would like to use a cluster configuration similar to the US Rockets Interchangable Motor Mount (TM). (See attached photo). The wooden ring in the front will act like an engine block. With this arrangement I will not need to install thrust rings in each tube and I won't need to make thrust rings out of masking tape.

I had already thought about embedding the nut in the epoxy instead of the rod, but wasn't sure how I would get it aligned properly. The vaseline trick sounds like a good idea. I think I'll do some experimenting tonight ...

 

[Micromeister]

Heres a 4 D motor retainer system, this works just as well with 3 D's using a 10/24 riv-nut. You may have to file away a little of the flange but these things work wonderfully.

WW:
the easiest way to embed these threaded inserts is to install the machine screw in the insert then use masking tape to seal the end of the insert and machine screw, make sure to seal the bottom as well. set in 30 minute epoxy, devcon 2-ton is my perfered for this operation. After the epoxy as set for several hours simply back the screw out, TA DAAaaa! instant retainer. I ALWAYS use AR-2050 rings or 1/4" sections of spent D-motor casings as Thrust rings epoxied in place with this type motor retention system.

Take care, be very vey careful listening to Mr. Irvine, the man is a NUT!
Hope this helps

 

[Fore Check]

Did you post the option of having your engine tubes be 3.75" long, have the threaded rod for retention set so that the motors would stick 1/4" outside the engine tubes when in place, and using 1" spacers on the forward end of the engines when using C11's and D's (no spacer needed when using E's) ?

 

[BobCox]

Originally posted by Fore Check
Did you post the option of having your engine tubes be 3.75" long, have the threaded rod for retention set so that the motors would stick 1/4" outside the engine tubes when in place, and using 1" spacers on the forward end of the engines when using C11's and D's (no spacer needed when using E's) ?

Yes, that was Option A in my original post. With the 1/4-inch engine blocks in front, an E engine would extend 1/4 inch out the back. By having the threaded rod stick out 1/2 inch, there would be room for 1/4 inch of nuts and washers for retention.

 

[Fore Check]

It wood hulp if i wuznt ulliterit and kuld reed............

 

[BobCox]

Originally posted by Micromeister
Heres a 4 D motor retainer system, this works just as well with 3 D's using a 10/24 riv-nut. You may have to file away a little of the flange but these things work wonderfully.

Forgive my ignorance, but what is a riv-nut?

the easiest way to embed these threaded inserts is to install the machine screw in the insert then use masking tape to seal the end of the insert and machine screw, make sure to seal the bottom as well. set in 30 minute epoxy, devcon 2-ton is my perfered for this operation. After the epoxy as set for several hours simply back the screw out, TA DAAaaa! instant retainer.

That looks like a great system for a 4x24mm mount. I'm not sure it would fit in a 3x24mm configuration. According to my calculations, the gap in the center of a 3x24mm pattern is 0.151 inches. A 6-32 bolt is 0.1372 inches in diameter to the outside of the threads, so it would just fit inside a 3x24mm.

The 0.25-inch face-to-face distance of a 6-32 nut is too big to fit into a 3x24mm opening. It would fit nicely in a 4x24mm where the gap is 0.404 inches.

Take care, be very vey careful listening to Mr. Irvine, the man is a NUT!

Please let's not start a Jerry-bashing thread here.

 

[Micromeister]

Bob:
this system does indeed fit 3 motor configurations I just don't have a photo of it, as stated instead of a 1/4-20 riv-nut (threaded insert) a 10/24 or even 8-32 can be substituded without fear of pullout, I've used as small as 4/40" inserts in 3 motor clusters.
The system allows ANY length motor tube to be used , my 60" OT has tubes with inserts for the Old Estes E15 BP motors but flys mostly now with D12s and the insert blocks.

Rivit-nuts are sold by many manufacturers under different names. The brand we buy are Riv-nuts, however look up McMaster-Carrs on-line catalog current page 3070 Rivit nuts. are shown in sizes 6/32 to 1/2-13 in both 5056 aluminum and steel. Alum. product numbers #94020A311 to A364. I've used the standard flanged types from McMaster on 3 to 7 motor clusters in 13, 18 and 24 mm motor tubes. As mentioned on 3 motor clusters, a round file IS needed to cope out some of the flange around the motor tubes but the shafts fits nice and snug between the motor tubes. You ask for a good bp cluster motor retainer system, I've shown you a great one. it's up to you to make use of the info Here a close up of the retainer system and a 1/4-20 aluminum riv-nut siting on the table 3rd item from the left. Note how much longer the machine screw is then the insert. That machine screw is screwed all the way into the insert before applying the masking tape form tip of the MS to the bottom edge of the insert, the assemble is then set in 30 minute epoxy poured between the motor tubes allowing just enought to come within about 1/8' of the top of the insert and machine screw. Hope this helps.

 

[BobCox]

That looks like a great system, Micro. I believe you that you got it to work, but I'm still trying to figure out how you made it fit. Forgive me while I think out loud and go through the calculations for a 3x24mm mount:

Case 1: Permanent Mount in a BT-70 tube
Motor Tubes: Estes BT-50 0.976" OD
Airframe Tube: Estes BT-70 2.180" ID

If the motor tubes are pressed against each other, the space in the middle is 0.151" diameter and there is a 0.0385" gap around the outside. This was the assumption I was using for my previous post, and explains why I didn't think it would fit.

If the motor tubes are spread apart so that they touch the inside of the airframe, the space in the middle is 0.228" diameter.

According to the McMaster-Carr catalog, the 6-32 rivet nut has a collar diameter of 0.230". That is bigger than the calculated gap, but probably within the measurement tolerances, so that's why it works. Cool!

Case 2: Removable Mount assembled in a tube coupler
Motor Tubes: Estes BT-50 0.976" OD
Tube Coupler: Estes JT-70A 2.090" OD

In this configuration, the motor tubes must be pressed tightly together. The available space in the center is only 0.151", so a 6-32 bolt or threaded rod will fit, but not a rivet nut.


I should have mentioned earlier that the removable mount is what I am trying to build.

 

[Micromeister]

OK a removable mount Does make it a bit more difficult but you'll still be able to use the 6/32 or 8/32 rivit nut insert. You Will need a dremel tool with a 1/4" sanding drum or a millend cutter, well a hand file will also work but it takes more time
Please note the aluminum body on these Rivit nuts is very thick, to allow compression with a tool we will not be using. We can file away all but a tiny fraction of the insert body and flange without distrubing the threads. I actually make a triangle out of the round insert the fits snuggly but sits between the already glued together 3 stack of motor tubes. You only need the shaft of the insert and the machine screw threaded all the way in to make this work.
Hope this helps.

 

[BobCox]

I hadn't thought of filing the barrel down too, but yeah, that sounds like a good idea. The rougher surface would even give the epoxy more to grab onto.

While I was at Home Depot, I stumbled across an aluminum binding post. I don't have the exact measurements with me, but it looks a lot like McMaster-Carr part # 93121A325. This looks a lot like your rivet nuts except the end is sealed. Home Depot had barrel lengths from 1/4" up to 1" long. Being aluminum, they are pretty light weight.

 

[BobCox]

In case anybody is interested, here is what I finally settled on.

The front is a US Rockets CB-22-3 Bulkhead with 3 factory holes. The bulkhead comes with an indentation at the center so it was easy to accuratey drill a small hole through the center for the threaded rod.

Each engine tube is 2.5 inches of thick-walled BT-50. Around the back is a 1-inch long TC-70 tube coupler and 0.5 inch band of BT-70 tubing (painted red) which is the thrust ring for the whole assembly. 5-Minute epoxy holds everything except the threaded rod together.

Down the center is 4.5 inches of 6-32 threaded rod with a T-nut on each end. When loaded with E9 engines, the threaded rod is fully extended to the rear. With C11 or D12 engines, the excess rod is recessed into the airframe so it is not a landing hazard.

The entire assembly weighs 0.93 oz.

If I wanted to shave a little more weight, I could use a 3.5-inch piece of threaded rod when using C11 and D12 engines. The savings would be about 0.05 oz.


Here is a picture of it loaded with three E9 engines.

 

[BobCox]

Another view with 3 E9 engines.

 

[BobCox]

Loaded with three D12 engines.

 

[BobCox]

Another view of 3x D12.