Min Diameter Motor Retention

[ac2fv]

I'm gather the materials etc for my first MD build, the Loc 1.63. I saw this product on the Apogee website which looked simple for a first timer to use for motor retention. I'm sure there are pros and cons, but for a first time build, is this a good way to go? I'm not a fan of friction fit.... thanks!

https://www.apogeerockets.com/Build...eter_Retainers/38mm_Minimum_Diameter_Retainer

 

[kjhambrick]

IMO, if you can stand the weight, those AeroPac retainers along with a set of extensions look really good ...

OTOH, I am old and set in my ways and masking tape is a lot lighter

Kidding !

I am studying all the Min Diameter threads I can find to decide the same thing for a pending 38mm min diameter rocket followed by a 29mm version.

-- kjh

p.s. The only thing I've seen that might be better is @Adrian A's home-made gadget made from two pipe-testing plugs ...

This is one Link: Violent Agreement: Two Stage 38mm to 33,290 feet but I know I've seen a picture of his gadget here on TRF

 

[pugachu]

I'm gather the materials etc for my first MD build, the Loc 1.63. I saw this product on the Apogee website which looked simple for a first timer to use for motor retention. I'm sure there are pros and cons, but for a first time build, is this a good way to go? I'm not a fan of friction fit.... thanks!

https://www.apogeerockets.com/Build...eter_Retainers/38mm_Minimum_Diameter_Retainer

That would work, but you are permanently limited to one motor casing length. This is because its epoxied inside of the rocket. Also, it looks like you need tapped forward closures (plugged, no motor eject). And i don't think it would work with casing spacers.

You may be able to use different lenght of threaded rods for different casings.

 

[David Schwantz]

I have several of the Aeropacks, they work great. But my favorite is aluminum tape on the rear of the motor around the rear of the airframe.

 

[ac2fv]

That would work, but you are permanently limited to one motor casing length. This is because its epoxied inside of the rocket. Also, it looks like you need tapped forward closures (plugged, no motor eject). And i don't think it would work with casing spacers.

You may be able to use different lenght of threaded rods for different casings.

Yes, Apogee sells threaded extension rods at various lengths for different casings...

 

[ac2fv]

I have several of the Aeropacks, they work great. But my favorite is aluminum tape on the rear of the motor around the rear of the airframe.

I've been reading where that is a method. On the Loc 1.63 the fins are flush with the aft end of the airframe. I guess I could move the fins forward 1/4-3/8" so I could use that method without the performance being affected. Thanks!

 

[cwbullet]

If you do use the threaded rod options, I recommend aluminum because it is relative ly cheap and lighter.

 

[SolarYellow]

I can't figure out what that AeroPack will do that a plywood bulkhead, maybe with a chunk of coupler adjacent to it, won't do for a lot less money and weight.

 

[Neutronium95]

I'd lean towards anchoring the shock cord to the forward closure and then just using a good tight friction fit to retaib the motor.

 

[SolarYellow]

I'd lean towards anchoring the shock cord to the forward closure and then just using a good tight friction fit to retaib the motor.

All the threaded forward closures I can recall reading about have explicit manufacturer warnings that they are for holding the motor into the rocket only and are not to be used as a recovery anchor.

 

[tsmith1315]

Similar product, sans grooves, sold as a recovery anchor:

https://giantleaprocketry.com/collections/fins-1/products/shockcord-hardpoint-38mm

edit: quite a bit cheaper, too

 

[tsmith1315]

And i don't think it would work with casing spacers.

There is a tapped floating closure for 38mm:
https://www.csrocketry.com/rocket-m...20-hardware/38mm-floating-closure-tapped.html

Or this cool 38mm retaining ring that adds a thread at the end of the case, but must be used along with a spacer:
https://www.csrocketry.com/rocket-m...ch-38mm-forward-closure-ring-with-anchor.html

 

[SolarYellow]

Similar product, sans grooves, sold as a recovery anchor:

https://giantleaprocketry.com/collections/fins-1/products/shockcord-hardpoint-38mm

Absolutely. The AeroTech unit includes the eye bolt, so it can be used as an anchor. The warnings are to not use the threads in the forward closures as a recovery anchor point.

 

[Neutronium95]

All the threaded forward closures I can recall reading about have explicit manufacturer warnings that they are for holding the motor into the rocket only and are not to be used as a recovery anchor.

I've only ever seen that warning on the adapter for using Cesaroni 38 and 54mm motors with the Aeropack system.

Aerotech 38mm motors have a 5/16" thread tapped straight into the aluminum. That's more than good enouth.

 

[SolarYellow]

I've only ever seen that warning on the adapter for using Cesaroni 38 and 54mm motors with the Aeropack system.

Aerotech 38mm motors have a 5/16" thread tapped straight into the aluminum. That's more than good enouth.

You're right. I have looked at AT, but the cases I actually have are CTI, so I've been a lot more serious about the details of those parts.

https://aerotech-rocketry.com/products/product_fb123c37-83ac-350c-f81e-887bc7a2845e

The anchor retaining ring has a central 5/16-18 thread for a forged eyebolt or similar recovery attachment and is vented to allow passage of ejection charge gases.

Sez it right there in the description.

With a little searching, I also found this post:
https://www.rocketryforum.com/threa...ns-for-cti-54mm-hardware.146139/#post-1788394

 

[David Schwantz]

Ok, first of all, the Aeropack retainer not only does the job, but looks way cooler than ply bulkhead and sloppy glue job. And as far as the fins at the rear of the rocket, just cut a slot in the aluminum tape for the fins, I have done it quite often. If you burnish the tape down there is almost no ridge to disturb airflow. The only thing on a cardboard rocket, make sure you have clearcoat on the backend, or paint, cause the tape WILL peel the layers of cardboard off.

 

[SolarYellow]

The only thing on a cardboard rocket, make sure you have clearcoat on the backend, or paint, cause the tape WILL peel the layers of cardboard off.

Not a terrible idea to reinforce the cardboard in that area prior to primer & paint by soaking with thin CA or thinned epoxy. I'd probably do that after fillets, in any case, to avoid interfering with that glue's soak into the cardboard. You can prep epoxy to get adhesion of epoxy on top, but it's not as good as a soak into a porous surface. And I'm not aware of any glue that sticks decently to cured CA.

 

[kjhambrick]

Not a terrible idea to reinforce the cardboard in that area prior to primer & paint by soaking with thin CA or thinned epoxy. I'd probably do that after fillets, in any case, to avoid interfering with that glue's soak into the cardboard. You can prep epoxy to get adhesion of epoxy on top, but it's not as good as a soak into a porous surface. And I'm not aware of any glue that sticks decently to cured CA.

Yes.

I learned the hard way to coat the tail with thinned epoxy after I pulled off the paint and I started to flay my LOC tube after the using plain-old masking tape motor retention method.

I was able to clean up the the peeling skin with a circular cut and post-paint the naked tail with Epoxy when I got home but that was the only flight that day for that particular rocket.

Anyhow, yes do prep the tail of the rocket if you plan to use tape retention.

-- kjh

 

[Sooner Boomer]

For motor retention, I use aluminum standoffs (a round or hex rod, threaded on the inside), machine screws, and washers. Right now, they go between the motor tube and body tube. For my minimum diameter build (54mm), I plan on embedding them at the aft end of the fin root. No reason why this would not work in 38mm. You can also use smaller washers, or grind them to the shape you want.