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Aeropack min dia retainer - why???
#1
I am thinking of building a minimum diameter rocket and was considering using the Aeropack minimum diameter retainer.

I also took a look at Tim's build instructions for his Wildchild 38.  He uses a coupler to secure the recover harness, but suggests tape and friction fit for the actual motor.

My first thought was concern about the motor casing kicking out.

But then I started thinking -  if I use the retainer it basically prohibits you from using motor eject and you need to use electronics.

So if the motor has no ejection charge, it can't kick out - right? 
I can install a bulkhead to keep the electronic's charge from pushing onto the motor (and use this bulkhead to secure the harness)

The motor's thrustring on the base of the tube will keep the motor from going forward.

So except for the fact it provides you a an eyebolt for the attachment of the harness, what benefit does the retainer really provide?

Or am I missing something??

Thanks
Les
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#2
It gives piece of mind that your casing won't fall out.

My suggestion to you however, is to attach an eyebolt to your motor instead of a bulkhead. That way, if your motor does kick out for whatever reason, it's attached to something.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
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#3
Poster answered most of his own questions. 

I agree.  The aeropack retainer provides an attachment point and keeps the motor from falling out from the ejection force. A bulkhead will accomplish the same thing.

Since it sounds like he is asking about 38 mm, most 38mm motors don't offer a threaded option for attaching an eyebolt directly to the case that I'm aware of.
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#4
(11-08-2015, 08:24 AM)MarkH Wrote: Poster answered most of his own questions. 

I agree.  The aeropack retainer provides an attachment point and keeps the motor from falling out from the ejection force. A bulkhead will accomplish the same thing.

Since it sounds like he is asking about 38 mm, most 38mm motors don't offer a threaded option for attaching an eyebolt directly to the case that I'm aware of.

If he's using a CTI motor, aeropack makes an adapter that accepts a 5/16 (I think) threaded rod/eyebolt. The intent of the adapter is so CTI motors can be used with their MD retainer. If he's using AT, then he should just use a threaded forward closure if he has one, it uses the same threading as the aeropack MD retainer.

Here's the CTI adapter installed in a motor:
[Image: attachment.php?aid=689]

And here's the motor with an eyebolt installed:
[Image: attachment.php?aid=690]

On a side note, I use this attachment method of the shock cord on all of my rockets that use dual deploy. I feel it's a much more reliable way of attaching a shock cord and it makes it 100% serviceable for replacement.

But for my 38mm MD, I use friction fit since the av-bay sits right above the motor. I just make sure to use enough tape that I'm confident the case won't slide out at any point during the flight.


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NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
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#5
Cool. Good info. I've never flown a 38mm CTI so I'm not that familiar with it.
I   think rousetech offered a tapped closure that is hard to come by now for the Aerotech 38mm hardware.
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#6
Yes - I am planning for a 38mm CTI using the adapter if I use the retainer.

Note that Aeropack does make the statement

     Adapters are for retaining motors to Aero Pack bulkhead retainers only and not to be used for attaching recovery hardware.


I was more questioning if I have another means to secure the harness plus there is a bulkhead to isolate the motor from the rest of the rocket (so the deploy charge does not hit the motor) then to me there should be no forces trying to eject the casing. 

The only thing that I just thought of was the kick of the ejection charge and/or the shock of the rocket coming to the end of the cord could dislodge the casing.  Remembering some RAVEN plots there are some serious G forces at work when the ejection occurs....
This may actually be the answer to my question
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#7
(11-08-2015, 12:42 PM)les Wrote: Yes - I am planning for a 38mm CTI using the adapter if I use the retainer.

Note that Aeropack does make the statement

     Adapters are for retaining motors to Aero Pack bulkhead retainers only and not to be used for attaching recovery hardware.


I was more questioning if I have another means to secure the harness plus there is a bulkhead to isolate the motor from the rest of the rocket (so the deploy charge does not hit the motor) then to me there should be no forces trying to eject the casing. 

The only thing that I just thought of was the kick of the ejection charge and/or the shock of the rocket coming to the end of the cord could dislodge the casing.  Remembering some RAVEN plots there are some serious G forces at work when the ejection occurs....
This may actually be the answer to my question

Even though aeropack doesn't recommend it, I do it anyway... If your not comfortable with it, then don't. But if there's already a bulkhead in place (or planning to put one in) then make sure there's a bolt there to hold the motor in if your not comfotable with friction fit.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
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#8
Just a reminder to all that those curved, unclosed eyebolts can pull apart pretty easily if the harness gets tangled. Better to use forged or welded eyebolts.

http://www.grainger.com/product/GRAINGER...36;smthumb$

http://www.grainger.com/product/RONSTAN-...36;smthumb$
John S.
NAR #96911
TRA #15253
MDRA
Level 1, 2014-Mar-15 -- Aerotech Sumo, H133BS
Level 2, 2014-Jun-21 -- Giant Leap Vertical Assault, J240RL
Level 3, 2016-03-12 -- MAC Performance Radial Flyer, M1101WH, 13,028 feet
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#9
(11-09-2015, 11:50 AM)Bat-mite Wrote: Just a reminder to all that those curved, unclosed eyebolts can pull apart pretty easily if the harness gets tangled.  Better to use forged or welded eyebolts.

http://www.grainger.com/product/GRAINGER...36;smthumb$

http://www.grainger.com/product/RONSTAN-...36;smthumb$

Yes the forged and welded eyebolt a are a much better idea, although I've never pulled one apart. I'll still pick some up at some point.

NAR# 98194
Level 1: CTI I-216, 3,043'
Level 2: CTI K-740, 5,999'

Personal altitude record: 12,400' CTI L395
2014 total impulse: 9,018.2 Ns (76% M)
2015 total impulse: 7,171.7 Ns (40% M)
2016 total impulse: 18,664.2 Ns (91% N)
2017 total impulse: 8,281.1 Ns (80% M)
Reply
#10
The only real reason to use one of the MD retainers is they look cool before you glue them in. Friction fit works fine for motor eject so it would stand to reason that it works super awesome with a bulkhead set up and no motor eject.
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