O Motor retention - Need ideas

Building a 7.5" diameter rocket that will have 6" motor mount. The O motor is a 152mm, 3 grain. I'm not sure how to best provide for motor retention.

Perhaps as simple as insert motor, insert a centering ring over motor case down to thrust ring, screws through body tube into centering ring. But I would prefer something along the lines of an aeropack retainer, but there doesn't seem to be anything such as that in the market place.

Your ideas would be appreciated. If you know someone who can fabricate and is reasonable and reliable... I'm interested.

Thanks

I have no idea if they do - I'm just speculating, but have you checked if Aero Pack would be willing to do a custom retainer? Probably be pretty pricey though if they agree to.

Dennis - How about a bulkhead plate forward of the motor and a threaded stud? Just screw the motor in.

The Canepa book might have some more ideas.

Dennis,,
I just called Bob M. of Aeropack just to inquire...
I left a message..
He'll call me back,, he always does...

Teddy

H_Rocket said:

Dennis - How about a bulkhead plate forward of the motor and a threaded stud? Just screw the motor in.

Click to expand...

I am using this concept on may of my future large rocket builds. Threaded rod in forward closure through bulked in airframe then tightened down with a nut. Then attach eyenut to this rod for recovery attachment points.

If you want to go conventional Nat Kinsey (Upscale CNC) can cut a retaining ring out of G10 than can be attached into your after mmt ring.

Used fender washers [2doubled up] & 1/4in lag bolts to hold in O & 54's.




Have also used large cable clips with short 1/4 lags.....when flying just the O.




I know other O and P fliers who do same.

I did look on the aeropack site. They have an image of a 150mm retainer for the CTI 150, but it requires a min. 8+" body tube. If sure a custom job, wold be very expensive.

Fender washers and cable clips.. good ideas. Clips sound interesting.

Got the voicemail and email from Teddy. Everyone is correct:

We make a retainer for the CTI Pro150 motor, but they are $340. each due to only making 6 at a time. We are currently sold out of those (so maybe we should make more next run). Typically sold to research firms, NASA, and a few HPR flyers.

Making a custom retainer is possible but it too would be very expensive, as one-offs usually are.

The cable clamps shown by Jim are a good way to go, but they are not heat treated and can bend so just get a large heavy duty size and use several (4 or 6) and they will be fine. Even with 6 they do not take very long to install.

Regards,

Bob
Aero Pack

Also a good idea. Somewhat concerned about getting it out

Thanks Bob for responding

Hey Dennis. How about having 6 1/4-20 threaded inserts pressed into to inside of you aft ring , slide your motor in , take another ring , sandwich the rear external snap ring, and bolt it downright .

3stoogesrocketry said:

Hey Dennis. How about having 6 1/4-20 threaded inserts pressed into to inside of you aft ring , slide your motor in , take another ring , sandwich the rear external snap ring, and bolt it downright .

Click to expand...

+1.....
Me like.......

Teddy

Thank you all for responding. Great replies, great ideas.

Next question. The rocket is 7.5" hawk mountain to the 6" motor mount. Thinking of attaching upper section to the motor's forward closure via 1/2" welded stainless eye bolt screwed into top of closure. Expecting the rocket to weigh approx. 125 - 135 lbs. Would you trust single 1/2" welded eye bolts? Double them or 2x 3/8" ubolts.. etc. Ideas....

dmlappert said:

Thank you all for responding. Great replies, great ideas.

Next question. The rocket is 7.5" hawk mountain to the 6" motor mount. Thinking of attaching upper section to the motor's forward closure via 1/2" welded stainless eye bolt screwed into top of closure. Expecting the rocket to weigh approx. 125 - 135 lbs. Would you trust single 1/2" welded eye bolts? Double them or 2x 3/8" ubolts.. etc. Ideas....

Click to expand...

I retain my 6" O and P cases from the top end. My forward closure is tapped for 2 3/8" eyebolts. I usually get one big quicklink and link the eyebolts together and attach my harness so they can't unscrew themselves under any circumstances.

The forward closure is already tapped for a single 1/2" eye bolt. The welded eye bolt has a WLL of 1,800lbs. Thinking that's sufficient, just hate single point of connection.

To add an addition connection point I would need to add a bulkplate forward of motor. Could do two 3/8" in bulkplate. Center cut bulk plate and have the motor attachment as third point.

I'd think a 1/2" welded stainless eye would do it...
That comes from my gut,,, not experience...
If you're willing to build retention into the rocket then I'd suggest U bolt's for sure...
The whole point is to spread the load,, a single U bolt spreads the load twice what a single eye bolt does...
If you have the room and will do it a single U bolt on either side of the airframe with a "Y" harness attachment you just won't beat for spreading load.....

Teddy

Room is an issue. So may have to do double 1/2" eye bolts

Or two 3/8" per hang point.

I'd like to thank all those who have read, and responded. After much consideration.. I will be retaining the motor with a custom aft retainer. All hang points will be with dual 3/8" U-Bolts. Thanks again. Plan is to fly the beast in the spring at Red Glare.... Thanks again.

So the O project is progressing well. There is 4x 1/4" threaded rod from aft centering ring to forward bulk plate. Then for additional strength and motor retention there is a 1/2" threaded rod screwed into the forward motor closure, up through the forward bulk plate, in the center between 2x 3/8" 3800' stainless ubolts. If this puppy fails, then there's bigger problems. The 1/2" threaded rod serves as both motor retention and strengthening of the forward bulk plate.