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Easy, Cheap 29mm Motor Retainer

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DynaSoar

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Here's how I made a positve motor retention system for my 29mm mach buster. It's also very suitable as a replacement for Aerotech's huge stiff engine hook.

1 Estes BT55 tube coupler
3 Estes 2.75" engine hooks (with the "Z" shaped bends in the bottom)
Thin CyA

A BT55 couple sides very nicely over a 29mm heavy wall body/motor tube with just enough slack. If you don't have a coupler, an inch or so of BT55, sliced down the side with a sliver removed so the result doesn't overlap itself when wrapped around the main tube is just fine (Probably better in fact, because it'd be lighter and thinner).

Mark the main tube 1/4" from the aft end. This will be where the aft end of the coupler goes.

Bend the top bend of the engine hooks backwards until they snap off. Flatten the end with pliers or a hammer.

Slide the coupler into place over the main tube.

Slide an engine hook between the coupler and tube, and push it up so that 1/4" is below the aft end of the tube (1/2" below the coupler). Mark the hook where it comes out of the forward end of the coupler. Do this for all three hooks.

Take the coupler off the main tube. Bend the hooks over 180 degrees at the mark, so they'll lay against the main tube, and bend OVER the coupler. Go ahead and pinch the bend down flat.

Slide a hook into place on the coupler (stick it through the coupler and pull the new bent over tab over the outside). Mark where it goes with a pen and remove it. Put a drop of thin CyA between the marks and pull the hook back into place. Repeat for the other hooks, 120 degrees apart. Once dry, the retainer is done.

Slide the retainer onto the main tube to check the fit. If too tight, take off, sand the main tube, and replace. Mark the main tube at the forward end of the retainer. Remove, paint the main tube with CyA, and replace. Paint CyA along the fore and aft joints of the coupler and main tube, all the way around. You're done. The diameter difference is so slight that an even fillet would be difficult, and unnecessary. I tried, with Fix-It epoxy clay. Wasn't worth the effort.

If you want to use epoxy for the main tube cementing (due to the reasonable worry that the CyA may grab before you get the coupler to the right spot), use only enough to paint a film on the cementing region, so there's not a glob pushed up when you slide the retainer on.

If you have the Estes engine hooks with the "Z" style bottom end, be aware that the bend that holds the motor and the bottom most bend in the hook are not the same; there's a 1/8" difference. Just make sure you're measuring things based on the part of the hook that'll hold the motor. Also, you can bend the very end of the finger tab down towards the aft, or even cut off the end with diagonal cutters, so it's more streamlined.

Weight: betwen 2 and 3 grams including all the CyA. Maybe 4 to 5 grams if you use epoxy.

Cost: If you bought it all from say, Balsa machining, $2.00 total.
50 cents for the coupler. If you don't have any surplus Estes 2.75" engine hooks laying around to use for free, I'm betting you could find someone who did. Same with a scrap of BT55 if you don't have a coupler.

Size considerations: This takes a bit more tube length, but no more diameter, than a Slimline retainer. It fits very well over an AT 29mm motor tube. It also fits very nicely between said AT motor tube and a PML "1.1" (29mm motor tube size) urethane tail cone. My money say this will handle any ejection charge the AT monster hook will handle, and is a lot easier to use.

Note: some places (like BMS) sell 2.75" engine hooks that are thinner than the Estes style with the "Z" shaped bottom. Their bottom bend is just a 90 degree angle bend. Given the tiny weight savings, I prefer the Estes hooks for this, because the "Z" bend makes it much easier to push an engine in, and because the greater amount of hook under the aft end of the motor makes it less likely that the engine could slip out. The "Z" bend would act as a spring and push back on the motor as it pushed down; the 90 degree bend hooks would only catch it. And be careful handling the hooks after you break/cut them: the broken end is sharp.

Also, this requires you to hold back three hooks simultaneously to get the motor out. It takes practice, or help. It shouldn't be easy, or it wouldn't work, but it's still easier than that stiff AT hook. And if you replace the AT hook with this, be aware your only engine block for thrust will be the thrust ring inside the motor tube. You might want to give it an extra epoxy fillet.

I tried to take a picture to post, but the cheap disposable camera won't focus well close enough to see the details. If I can find someone with a decent camera, I'll get some pics to post.
 

DynaSoar

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Originally posted by DynaSoar

I tried to take a picture to post, but the cheap disposable camera won't focus well close enough to see the details. If I can find someone with a decent camera, I'll get some pics to post.
Well, it wasn't as bad as I thought. At least you can get the idea.

And here's the 29mm minimum diameter mach buster built on it.
 

Bill

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Originally posted by DynaSoar
Here's how I made a positve motor retention system for my 29mm mach buster. It's also very suitable as a replacement for Aerotech's huge stiff engine hook.
Great idea!

I never cared for the Aerotech hook and like your concept better. This is also a good way to use up those old 3.5" long "Estes E15" hooks which have been ganging up in the parts box; using them with your technique is easier than converting them into 2.75" long conventional hooks.

If I can find a short length of tube just large enough to slide over the aft thrust ring of the motor, that could be used to spread the motor hooks apart as great motor insertion/removal tool. People who glue a motor block in the MMT and use only single use motors can just use a short length of MMT for this.

The Aerotech motor block can be dispensed with if flying only with RMS motors or with single use motors to which an aft thrust ring is created by wrapping with 1/4" wide masking tape found in the sewing department of the local "rocket store".


Bill
 

DynaSoar

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Update on the design.

For more positive retention, especially when using a 24mm motor and adapter (you want to retain both, I assume), cut 1/4" from a spent 18mm D21 and slide that over the hooks up to the first bend. It'll hold the ends of the hooks almost to the nozzle, but not quite. No old D21s? I guess a slice of a paper casing 18mm would work, but you'd probably need to remove a few wraps from the inside diameter, and then strengthen it with a touch of CyA. No way the hooks can spring enough to let the engine get loose.

The bird is ready for its first MPR test with an F21 (hence the adapter). Not likely to bust Mach. I've got plans for that.
 

saunassa

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Dybasoar
amazing to look at your mach buster, i have been playing around with rocksim to also make one. i remembered how loc had some fins like that so when i was done my rocksim file looks amazingly like yours.
post a pic after you launch so we can see the results
 

DynaSoar

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Originally posted by saunassa
Dybasoar
amazing to look at your mach buster, i have been playing around with rocksim to also make one. i remembered how loc had some fins like that so when i was done my rocksim file looks amazingly like yours.
post a pic after you launch so we can see the results
I'm testing it on an F21 Saturday. I'll *try* to get pics. I only have a disposable and those don't do well. Hopefully someone at METRA will get better pics.

I wish I'd used thicker fins so I could have given them a double wedge shape.

I may actually downscale this to a 24mm model. I've got a G110 coming. With a carbon tube it might hit Mach, but it'll definitely go over a mile.
 

DynaSoar

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Originally posted by DynaSoar
I'm testing it on an F21 Saturday. I'll *try* to get pics. I only have a disposable and those don't do well. Hopefully someone at METRA will get better pics.

Very nice flight on an F21-4. Unfortunately this was the second bird of mine today to have its Rockethead mylar chute just come right off. Cheap, non-locking snap swivels apparently.

It lawndarted, even with the nose off. Being built for speed, it lawndarted big time. Straight down into the dirt six inches.

But, I built it strong because it's a Mach buster. I pulled it out and knocked the dirt out with a launch rod, and it's ready to fly again, no repairs needed. I can't swear that the heavy appliance epoxy paint had something to do with it, but I suspect it did.
 
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