Using motor to place Aeropack MD retainer

Help Support The Rocketry Forum:

jkovac

Well-Known Member
TRF Supporter
My exact question is hard to put in the title. I've seen a lot of threads on the MD retainer in general but nothing about this: The instructions that come with the Aeropack 54mm minimum diameter retainer say you should put the motor together with everything except the propellant -including the spacer, liner and O-rings -before using it as a guide to place the MD retainer inside the body tube.

I'm concerned about possibly damaging the O-rings when doing this. Beyond that, I don't recall seeing a gap between the forward and aft closures on fully built Aerotech motors (we're talking an LMS K250 in this instance) and the motor tube - I believe both closures screw the whole way in. So it would be exactly the same length whether the O-rings were in there or not if that's true. But I'm not looking at an assembled motor now, so maybe I'm wrong.

So the question is - does one really need to put the motor together with the 0-rings, spacer and liner to place the retainer correctly, or will just the motor tube with forward and aft closures attached give me the same length? Any input appreciated.

tfish

Well-Known Member
2-12. Finish tightening the aft closure by hand until it is seated
against the case. NOTE: There will be some resistance
to threading in the closure during the last 1/32" to 1/16"
of travel. It is normal if a slight gap remains between the
closure and the case after tightening and the grain
rattles slightly inside the liner

Tony

ttabbal

Well-Known Member
For a threaded case like Aerotech, I don't see any reason to worry about it. I would thread the closures on and place the retainer. For a snap-ring case, you would need something inside to keep the forward closure from moving backward into the motor case while assembling. If I didn't have a load available, I'd probably just cut a piece of PVC or even body tube the right length.

NateLowrie

Well-Known Member
My exact question is hard to put in the title. I've seen a lot of threads on the MD retainer in general but nothing about this: The instructions that come with the Aeropack 54mm minimum diameter retainer say you should put the motor together with everything except the propellant -including the spacer, liner and O-rings -before using it as a guide to place the MD retainer inside the body tube.

I'm concerned about possibly damaging the O-rings when doing this. Beyond that, I don't recall seeing a gap between the forward and aft closures on fully built Aerotech motors (we're talking an LMS K250 in this instance) and the motor tube - I believe both closures screw the whole way in. So it would be exactly the same length whether the O-rings were in there or not if that's true. But I'm not looking at an assembled motor now, so maybe I'm wrong.

So the question is - does one really need to put the motor together with the 0-rings, spacer and liner to place the retainer correctly, or will just the motor tube with forward and aft closures attached give me the same length? Any input appreciated.
Why not just build the motor according to instructions and use that? When the retainer is glued in place just take the motor out and take the forward or rear closure off until you're ready to fly. If it's a few months from now seal the whole assembly in an anti-static bag to protect against moisture.

watermelonman

Well-Known Member
I never glue Aeropack MD retainers, and use three 6-32 rivets when they are strictly for retention.

BDB

Absent Minded Professor
I never glue Aeropack MD retainers, and use three 6-32 rivets when they are strictly for retention.
Can you explain just a little more? I assume you insert the MD retainer and then drill through the airframe and the retainer before inserting the rivet. Correct? Any special requirements for the rivet? Thanks.

rc dude

Well-Known Member
Can you explain just a little more? I assume you insert the MD retainer and then drill through the airframe and the retainer before inserting the rivet. Correct? Any special requirements for the rivet? Thanks.
I did the same idea but used 10-32 machine screws with a nut inside the retainer. That way it is removable.

NateLowrie

Well-Known Member
I never glue Aeropack MD retainers, and use three 6-32 rivets when they are strictly for retention.

I like this idea. Is that mainly because you can install them in a different spot if you use a different sized casing?

cherokeej

Well-Known Member
That's it. It let's you change your mind.

If you set up thinking the longest motor you'll ever run will be a 1706, if you glued it in, you're SOL when your buddy offers you his 2560 load.

Use screws, and you can move it.

watermelonman

Well-Known Member
Can you explain just a little more? I assume you insert the MD retainer and then drill through the airframe and the retainer before inserting the rivet. Correct? Any special requirements for the rivet? Thanks.
Exactly! Like shear pins I drill through the two together, then remove the inner piece, tap the outer, and drill the inner again to the next size up. I like the rivets Missile Works sells but any should do.

I typically size for a 6XL motor and adapt down with threaded rod, but I still like for it to be removable for cleaning and maintenance.

rcktnut

Well-Known Member
For a threaded case like Aerotech, I don't see any reason to worry about it. I would thread the closures on and place the retainer.

+1 Absolutely no reason to assemble the motor parts. Screw closures on tight to case, done.

ColumbiaNX01

Red blooded white American male
DONT USE A MD AEROPACK!!!! It is to expensive and fancy for what it does. Waste of money. Make your own. Cheaper and easier. For example what I did. My MD rocket was 3 inch. Take a piece of 3 inch coupler 1 inch in length. Take a 3 inch avebay lid sold my Madcow, the ones that have a step on it. Then take another piece of 3 inch coupler 1 inch in length. Epoxy the avebay lid on the coupler piece. Drill a hole in the center of the lid for the thread rod. If you need a place for shock cord attachment install a I-bolt or U-bolt on the lid. Slide assembly into booster and epoxy in place. Once dried slide the other coupler piece into the booster. This creates a sandwich. That bad boy is not coming out. All the space you really need is room for shock cord and space for the avebay to slide into. All the other space is for motor. Find out the maximum length motor that you would ever use. For example Aerotech RMS has the threaded forward closure. Thread rod goes in that and then goes through the hole you drilled in the ave bay lid. Use Stainless Steel I bolt or U-bold, never have to worry about corrosion. Use Kevlar cord for shock cord, never have to worry about it getting burned.

Enough said.

MD 75mm aeropack is 39 bucks plus all their fancy extenders for different lengths of motors. So easily over 50 bucks.

A 75mm avbay lid from madcow is 10 bucks, (2) 1 inch 75mm pieces of coupler less then 10 bucks. Threaded rod 5-10 bucks. All and all less then 30 bucks!!!!

I am betting Fiberglass epoxies better to fiberglass rather then fiberglass to aluminum.

Last edited by a moderator:

dixontj93060

Well-Known Member
DONT USE A MD AEROPACK!!!! It is to expensive and fancy for what it does. Waste of money. Make your own. Cheaper and easier. For example what I did. My MD rocket was 3 inch. Take a piece of 3 inch coupler 1 inch in length. Take a 3 inch avebay lid sold my Madcow, the ones that have a step on it. Then take another piece of 3 inch coupler 1 inch in length. Epoxy the avebay lid on the coupler piece. Drill a hole in the center of the lid for the thread rod. If you need a place for shock cord attachment install a I-bolt or U-bolt on the lid. Slide assembly into booster and epoxy in place. Once dried slide the other coupler piece into the booster. This creates a sandwich. That bad boy is not coming out. All the space you really need is room for shock cord and space for the avebay to slide into. All the other space is for motor. Find out the maximum length motor that you would ever use. For example Aerotech RMS has the threaded forward closure. Thread rod goes in that and then goes through the hole you drilled in the ave bay lid. Use Stainless Steel I bolt or U-bold, never have to worry about corrosion. Use Kevlar cord for shock cord, never have to worry about it getting burned.

Enough said.

MD 75mm aeropack is 39 bucks plus all their fancy extenders for different lengths of motors. So easily over 50 bucks.

A 75mm avbay lid from madcow is 10 bucks, (2) 1 inch 75mm pieces of coupler less then 10 bucks. Threaded rod 5-10 bucks. All and all less then 30 bucks!!!!

I am betting Fiberglass epoxies better to fiberglass rather then fiberglass to aluminum.
Too much talk of $'s! Sometimes it's just a nice feeling that you can afford to bury a shiny, sexy hunk of anodized aluminum deep inside your airframe that no one will every see--just because you can! ColumbiaNX01 Red blooded white American male Too much talk of$'s!

Sometimes it's just a nice feeling that you can afford to bury a shiny, sexy hunk of anodized aluminum deep inside your airframe that no one will every see--just because you can!
That funny lol.

jkovac

Well-Known Member
TRF Supporter
2-12. Finish tightening the aft closure by hand until it is seated
against the case. NOTE: There will be some resistance
to threading in the closure during the last 1/32" to 1/16"
of travel. It is normal if a slight gap remains between the
closure and the case after tightening and the grain
rattles slightly inside the liner

Tony
I should have caught that. Thanks.

jkovac

Well-Known Member
TRF Supporter
Why not just build the motor according to instructions and use that? When the retainer is glued in place just take the motor out and take the forward or rear closure off until you're ready to fly. If it's a few months from now seal the whole assembly in an anti-static bag to protect against moisture.
I was reluctant to open before being ready to fly it, but your suggestion of putting it in an anti-static bag afterwards makes sense. Thanks.

SpaceManMat

Space Nut
Here's my 38mm MD retainer

Not sure why you would weaken the air frame with rivets and also suffer a drag penalty. I also don't know why you would build a MD rocket only to place a large piece of metal inside it.

jkovac

Well-Known Member
TRF Supporter
DONT USE A MD AEROPACK!!!! It is to expensive and fancy for what it does. Waste of money. Make your own. Cheaper and easier. For example what I did. My MD rocket was 3 inch. Take a piece of 3 inch coupler 1 inch in length. Take a 3 inch avebay lid sold my Madcow, the ones that have a step on it. Then take another piece of 3 inch coupler 1 inch in length. Epoxy the avebay lid on the coupler piece. Drill a hole in the center of the lid for the thread rod. If you need a place for shock cord attachment install a I-bolt or U-bolt on the lid. Slide assembly into booster and epoxy in place. Once dried slide the other coupler piece into the booster. This creates a sandwich. That bad boy is not coming out. All the space you really need is room for shock cord and space for the avebay to slide into. All the other space is for motor. Find out the maximum length motor that you would ever use. For example Aerotech RMS has the threaded forward closure. Thread rod goes in that and then goes through the hole you drilled in the ave bay lid. Use Stainless Steel I bolt or U-bold, never have to worry about corrosion. Use Kevlar cord for shock cord, never have to worry about it getting burned.

Enough said.

MD 75mm aeropack is 39 bucks plus all their fancy extenders for different lengths of motors. So easily over 50 bucks.

A 75mm avbay lid from madcow is 10 bucks, (2) 1 inch 75mm pieces of coupler less then 10 bucks. Threaded rod 5-10 bucks. All and all less then 30 bucks!!!!

I am betting Fiberglass epoxies better to fiberglass rather then fiberglass to aluminum.
Already $26.50 poorer on this build, but I will try something like what you've suggested next time. BDB Absent Minded Professor Here's my 38mm MD retainer Not sure why you would weaken the air frame with rivets and also suffer a drag penalty. I also don't know why you would build a MD rocket only to place a large piece of metal inside it. I would love to do that, but you friction fit, right? I just found out that my HPR field requires positive motor retention. (Although I I may try to argue that tying the motor case to the recovery harness is the ultimate motor retainer.) watheyak Barnstormer I would love to do that, but you friction fit, right? I just found out that my HPR field requires positive motor retention. (Although I I may try to argue that tying the motor case to the recovery harness is the ultimate motor retainer.) I do the same thing. I'm pretty sure that's a carbon fiber bulkhead near the top that gets permanently glued in place. Cheaper, lighter, simpler. watermelonman Well-Known Member DONT USE A MD AEROPACK!!!! It is to expensive and fancy for what it does. Waste of money. Make your own. Cheaper and easier. For example what I did. My MD rocket was 3 inch. Take a piece of 3 inch coupler 1 inch in length. Take a 3 inch avebay lid sold my Madcow, the ones that have a step on it. Then take another piece of 3 inch coupler 1 inch in length. Epoxy the avebay lid on the coupler piece. Drill a hole in the center of the lid for the thread rod. If you need a place for shock cord attachment install a I-bolt or U-bolt on the lid. Slide assembly into booster and epoxy in place. Once dried slide the other coupler piece into the booster. This creates a sandwich. That bad boy is not coming out. All the space you really need is room for shock cord and space for the avebay to slide into. All the other space is for motor. Find out the maximum length motor that you would ever use. For example Aerotech RMS has the threaded forward closure. Thread rod goes in that and then goes through the hole you drilled in the ave bay lid. Use Stainless Steel I bolt or U-bold, never have to worry about corrosion. Use Kevlar cord for shock cord, never have to worry about it getting burned. Enough said. MD 75mm aeropack is 39 bucks plus all their fancy extenders for different lengths of motors. So easily over 50 bucks. A 75mm avbay lid from madcow is 10 bucks, (2) 1 inch 75mm pieces of coupler less then 10 bucks. Threaded rod 5-10 bucks. All and all less then 30 bucks!!!! I am betting Fiberglass epoxies better to fiberglass rather then fiberglass to aluminum. dixontj93060 said: Too much talk of$'s!

Sometimes it's just a nice feeling that you can afford to bury a shiny, sexy hunk of anodized aluminum deep inside your airframe that no one will every see--just because you can!
No doubt. Most of my rockets could have been made lighter or less expensively, but I am not hunting for records and convenience is also of value.

SpaceManMat said:
Not sure why you would weaken the air frame with rivets and also suffer a drag penalty. I also don't know why you would build a MD rocket only to place a large piece of metal inside it.
We pushed generic fiberglass tubing over 100k at mach3 with 4x 8-32 rivets in the one piece retainer thrust plate; I understand being concerned about strength or drag but I think we have evidence that the impact is small.

SpaceManMat

Space Nut
I do the same thing. I'm pretty sure that's a carbon fiber bulkhead near the top that gets permanently glued in place. Cheaper, lighter, simpler.
Correct.

SpaceManMat

Space Nut
We pushed generic fiberglass tubing over 100k at mach3 with 4x 8-32 rivets in the one piece retainer thrust plate; I understand being concerned about strength or drag but I think we have evidence that the impact is small.
Probably not as big a deal for the big ones, but for me if I'm going to go all out then I'll avoid it.

cherokeej

Well-Known Member
DONT USE A MD AEROPACK!!!! It is to expensive and fancy for what it does. Waste of money. Make your own. Cheaper and easier. For example what I did. My MD rocket was 3 inch. Take a piece of 3 inch coupler 1 inch in length. Take a 3 inch avebay lid sold my Madcow, the ones that have a step on it. Then take another piece of 3 inch coupler 1 inch in length. Epoxy the avebay lid on the coupler piece. Drill a hole in the center of the lid for the thread rod. If you need a place for shock cord attachment install a I-bolt or U-bolt on the lid. Slide assembly into booster and epoxy in place. Once dried slide the other coupler piece into the booster. This creates a sandwich. That bad boy is not coming out. All the space you really need is room for shock cord and space for the avebay to slide into. All the other space is for motor. Find out the maximum length motor that you would ever use. For example Aerotech RMS has the threaded forward closure. Thread rod goes in that and then goes through the hole you drilled in the ave bay lid. Use Stainless Steel I bolt or U-bold, never have to worry about corrosion. Use Kevlar cord for shock cord, never have to worry about it getting burned.

Enough said.

MD 75mm aeropack is 39 bucks plus all their fancy extenders for different lengths of motors. So easily over 50 bucks.

A 75mm avbay lid from madcow is 10 bucks, (2) 1 inch 75mm pieces of coupler less then 10 bucks. Threaded rod 5-10 bucks. All and all less then 30 bucks!!!!

I am betting Fiberglass epoxies better to fiberglass rather then fiberglass to aluminum.
If what you seek is pure economy, make one on a machine lathe. Cost in materials is less than 2 bucks. An inch of aluminum tube, some 1/4" aluminum plate, and an hour in front of a machine.

Used to be that you could take a drawing to the local high school and get just about anything done in exchange for some materials donated to their shop. Alas, it seems those days are gone.

Last edited:

fyrwrxz

latest photo
Here's my 38mm MD retainer

Not sure why you would weaken the air frame with rivets and also suffer a drag penalty. I also don't know why you would build a MD rocket only to place a large piece of metal inside it.
Okay- I'm really dense. Explain to me slowly how this sexy assembly goes together. I really like this but can't understand what goes on inside the case to secure the eyebolt. Modified delay/charge well? CTI off-the shelf-hardware I'm not aware of? Duh....

blackbrandt

That Darn College Student
The retainer screws into the plastic threading of the charge well.

Banzai88

Lvl 1,Wallet....Destroyed
TRF Supporter
Here's my 38mm MD retainer

Not sure why you would weaken the air frame with rivets and also suffer a drag penalty. I also don't know why you would build a MD rocket only to place a large piece of metal inside it.
I would love to do that, but you friction fit, right? I just found out that my HPR field requires positive motor retention. (Although I I may try to argue that tying the motor case to the recovery harness is the ultimate motor retainer.)
While one could friction fit this, to make it positive retention, glue the bulkhead into place. The red adapter threads into the stub bolt of the eyebolt. Use one with a longer shank. BOOM, positive retention! :wink:

Okay- I'm really dense. Explain to me slowly how this sexy assembly goes together. I really like this but can't understand what goes on inside the case to secure the eyebolt. Modified delay/charge well? CTI off-the shelf-hardware I'm not aware of? Duh....
As presented, to be friction fit, just screw the eyebolt through the bulkhead to the red adapter. To make it positive retention, you can mount the eyebolt permanently attached to the black bulkhead with another thin nut on the back, then the black bulkhead is permanently glued into the tube the way any other bulkhead would be done. Recovery harness is attached to the eyebolt, and deployment is handled by the flight computer. The red retainer between the motor and the eyebolt stub is the adapter (remove black powder ejection charge and thread the adapter into the plastic charge well). Simple and elegant, nicely done!

Last edited:

OverTheTop

Well-Known Member
TRF Supporter
I machine my own retainers in the Aeropack style. To align them in the airframe, if I don't have the casing I am targeting on-hand, I use an aluminium tube with a thread machined into one end that matches the motor thread and is coaxial with the rod. The mount is fitted to the rod, glue is applied to the inside of the airframe and then the mount is slid into place, keeping the rod centralised in the airframe (at the aft end) to maintain correct alignment at the mount end inside. Before using the rod I mark the length of insertion on the rod so I just push it in until it is level with the rear of the rocket.

I like positioning the mount to be able to take the longest case available for that diameter. Shorter motors or other brands are accommodated using a short spacer rod that screws onto the end of the motor to make up the extra length.

The other thing I like to do is to make sure that when the motor is screwed in it bottoms out on the mount end, leaving a slight gap at the rear near the thrust ring. That means that the motor pushes only on the mount, and the airframe behind the mount is in tension, rather than compression. Composites always work better in tension, so that is what I try to get working for me!

mpitfield

Moderator
Staff member
Global Mod
Size and weight wise the Aeropack MD retainer is a pretty efficient retainer, so I am personally not that concerned about the weight it may add. If I was going for an altitude "best" or record attempt then sure I get it but beyond that it's a moot point from that perspective.

I have installed this retainer using the removable method with 3 holes drilled 120 degrees apart, small machine screws and self clinching nuts epoxied inside the retainer, twice now. I used it on my 54mm CF MD, launched 8 times now, including one free fall from 10,000 and no damage, and just installed it last week on my 38mm FW/FG MD and I have one more 38mm CF MD that I will be installing it in, over the next month or so.

What I have attempted to do is set the rocket up for the biggest motor, then I used adapters and threaded rods to make up for shorter cases. It adds some weight however the extra weight helps to counteract the loss of mass difference between the larger case, which the rocket was initially set up for and the smaller cases, so it has worked out well.

I also had Teddy from one bad hawk sew harnesses directly into the eye hooks so I can do away with a quick-link, which has worked out very well.

In regards to removable vs. permanently fixed, I like the idea of being able to remove the retainer for maintenance. This has proven useful in the ease of cleaning up post flights but also in one case where I had an overly energetic apogee deployment that bent the eye-hook so it was a quick swap out, which would have been doable with a fixed retainer but was much easier with a removable.

As others mentioned when you are laying this out beside the airframe to measure where to mount it make sure you don't screw the retainer all the way in. I screw it half way in when figuring out where to measure it, that way when it is installed it does not bottom out in the retainer but allows you to snug the thrust ring up against the base of the airframe to ensure you are transferring the load.

One other bit of advice. I had a main apogee deployment at 18,000', which resulted in a 3.5 mile drift and when I found the rocket the motor was about a half turn from falling out of the rocket. Ever since I had that experience I used some blue thread locker when assembling the motor and let it sit for a couple of hours to set up a bit before launch. It is easy to remove with a bit more force and provides me with a bit more security. Since then I have not had a repeat. This is also another reason for a removable setup as it allows you to clean things up on the retainer threads.

Good luck with your build, and lots of opinions for you to choose from.