Motor retention

[MattieShoes]

Hello, my name is Matt.

I used to build Estes kits as a kid and just got back into the hobby, and now I'm enjoying the mid power rockets. The Estes kits accomplished motor retention via clips mostly, and my first mid-powered rocket (AT Barracuda) also used a clip, but the one I just got (LOC Onyx) has nothing like that. I'm looking for something that works without costing as much as the fancy retention kits, preferably something I can build with parts from the hardware store... Tape feels unsatisfactory to me in the realm of G motors.

I've searched a fair amount on the net for information, but most everything seems like they're talking to somebody who has already gone through this before, drawing on shared experience. Could anybody direct me to some basic, explicit reading material on retention methods? Are there any gotchas I should know before I screw up my centering ring?

If it matters, I haven't glued anything yet -- I wanted to figure out the retention issue beforehand.

 

[ScrapDaddy]

Tape acctually works GREAT I have heard you can use it for up to a G80T although I see why you don't want to, losing a reload casing is rough..... But it acctually is VERY strong and the problem isn't the power with the motor it is the power of the ejection charge, for this reason I belive tape will work fine


Notes: DONT use an engine block; there are over 100 types of 29mm motors you would be limiting your choises

AT kits have an hook because they are designed to fit with a AT motor very snugly, as far as I know AT is the only one who uses them in MPR

 

[Mike Di Venti]

People have been known to tape "m" motors in place.
I used tape for a long time until I got tired of cleaning the goo off my motor hardware(38mm)
You can use a t-nut and a screw w/ a washer or kaplow clip to hold your motor in.
add a little epoxy or thick ca to the underside of the t-nut wheel inserted into the pilot hole. be careful not to get the glue into the threads.(you'd have to chase it then w/ a tap)

That's probably the cheapest way of doing it.
The easiest and more costly would be to use an aeropac retainer.

 

[mikeyd]

Here is what I did for my 29mm powered rockets.
https://members.cox.net/1radsaved/motoret/pvcret.htm

 

[Chrisn]

Tape acctually works GREAT I have heard you can use it for up to a G80T although I see why you don't want to, losing a reload casing is rough..... But it acctually is VERY strong and the problem isn't the power with the motor it is the power of the ejection charge, for this reason I belive tape will work fine


Notes: DONT use an engine block; there are over 100 types of 29mm motors you would be limiting your choises

Say that to the many people who thought their tape was sufficient at launch only to find they are missing a reload case when they get home. Why do you come to the conclusion it will be fine? Not to sound harsh but please fly some rockets like this first.

Can you show me the over 100 different length 29mm motors available commercially?

 

[MarkII]

I usually employ some variation of a screw-on clip system. Drill a pair of holes 180° apart in the rear centering ring that are just large enough to allow the posts of a pair of blind nuts (T nuts) to pass through. Don't use nuts that are so large that the holes significantly weaken the ring. I have used some that were as small as 2-56. Epoxy the blind nuts into the holes so that the posts face rearward. If the rear centering ring is thin, reinforce it. Then use a pair of machine screws to attach small brass strips (Kaplow Clips) or else screen door tabs (if they fit) that will overlap the outer edge of the motor casing when it is inserted into the mount.

Any type of retention system, such as this type or the familiar motor clips, that mechanically fastens the motor to the mount is called positive motor retention. The opposite of PMR is friction-fitting (not negative motor retention, which would be an oxymoron). A type of motor retention that is often used in competition models but rarely used in mid-power or larger rockets involves wrapping tape around the aft end of the motor tube and the motor together where it exits the tube, in essence taping the motor directly to the tube. This is also a form of PMR. Friction-fitting was the very first method of model rocket motor retention; the very first motors were wrapped with rubber bands before being inserted into their mounts in order to prevent them from being ejected. The main drawbacks to friction-fitting are that it is not always reliable and that the stresses involved in forcing a tightly-fitting motor into a motor tube and pulling it back out afterward can damage the motor mount or the rocket.

Instead of custom-made brass strips or screen door clips, people also use loops of coat hanger wire that are also attached to the rear ring via the machine screws. I have also used electric conduit washers if I can find any that are the right size. I drill pairs of holes in them on opposite sides so that I can affix them behind the aft end of the motor. Whatever type of clip, loop or ring that you use, make sure that it is shaped so that when it is installed, no part of it extends into the jet plume exiting the motor's nozzle. That would negatively affect the stability of the rocket (by causing vectored thrust) as well as being a violation of the Safety Code (modifying a motor).

Some people (including me, at one time) have reversed the process by fixing studs or sections of all-thread through the centering ring, projecting rearward. The retaining clips are then simply bolted onto the studs with nuts. But having a hard stud or bolt projecting out of the rear end of the rocket can present a safety hazard, so I always construct my retention systems now so that the clips are fastened by machine screws or bolts that thread into the centering ring. Most of the time it is no harder to construct the system that way, and on the few occasions when it actually is, the improved safety margin is worth the little bit of extra hassle.

MarkII

 

[Scott Evil]

It's good that you don't have the aft centering ring on yet.

Here's a couple shots of how my son has retention set up on his Onyx.

These parts are all aluminum and were purchased at an Ace True Value for just a few bucks ($5 or 6)

EDIT-There are actually just two clips. I placed an extra T-Nut on the CR just to show you the part but the photo didn't turn out real well.

 

[MattieShoes]

My barracuda is 100% stock, from the motor clip to the paint job I think it'll only be flying aerotech reloads, but that's fine. It's a good rocket for that.


The Onyx pushed an unrelated order over the "free shipping" limit, so I effectively got it for less than an Estes kit. In all likelyhood, it'll probably only use aerotech reloads too, but figuring out a flexible, cheap retention method I like on a seems like a good idea before I get around to building a 38mm beastie and trying to qualify L1 or something. Then again, if I build a 38mm rocket, I probably wouldn't wince at paying for a nice looking retention kit.

What is a Kaplow clip? I've seen references to it all over, but nothing that really explains what they are/how to make them.

That's a good point, that the retention is to hold the motor against the ejection charge, not the thrust portion... I guess I really have no idea how strong an ejection charge is, since I'm always hundreds or thousands of feet away when it happens.

It's good to know tape will work, but somewhere in the back of my head, there's a video of a rocket and a relatively expensive motor casing parting ways half a mile above the earth, and then a relatively expensive casing coming in ballistic, straight through somebody's windshield... Even if it's purely psychological, I think I'll be installing some form of more significant retention.

 

[MattieShoes]

Doh you guys are too fast for me! My questions were answered before I finished asking them! Thanks, I think I'll be going to the hardware store sometime this week... Thanks for all the help, I really appreciate it.

 

[Initiator001]

My barracuda is 100% stock, from the motor clip to the paint job I think it'll only be flying aerotech reloads, but that's fine. It's a good rocket for that.

Yah! Another Barracuda fan.

(Okay, I am the designer of the kit. )

 

[MattieShoes]

Yah! Another Barracuda fan.

(Okay, I am the designer of the kit. )

Haha no kidding!? My dad sucked me back into the hobby when he got an Initiator, so naturally I had to get something just a little bit bigger. :neener: Now he's talking about getting a 6 foot rocket and doing his L1 cert. I'm egging him on, telling him to get the 38mm special from Aerotech...
...so I can "borrow" it.

 

[cjl]

Say that to the many people who thought their tape was sufficient at launch only to find they are missing a reload case when they get home. Why do you come to the conclusion it will be fine? Not to sound harsh but please fly some rockets like this first.

Can you show me the over 100 different length 29mm motors available commercially?

If they're missing a reload casing, they did it wrong. I've successfully taped many motors, including an I600 in a minimum diameter rocket and a K motor, with a 100% success rate. Tape works great, but it's a bit of a pain to do correctly.

 

[The EGE]

Being the crazy person I am, most of my MPR and small HPR rockets don't have room for most commercial systems or Kaplow clips, and Aeropac retainers are nice but too expensive for me.

For anything using SU motors, I just tape the motor in. It works well, but you have to do it *right*. Doing it wrong is a very easy way to lose the reload case, hence why I onl do it for SU motors.

For most of my mid-power fleet, I epoxy a 3/16" threaded rod on a spacer to the MMT. Slide the motor in, slide on a washer, screw on the nut. Can take plenty of kick.

For minimum-diameter designs that use reloads, I gotta get creative; usually I epoxy ends of a piece of piano wire to two adjacient fin roots, then loop it around the aft end of the casing and secure with tape. Free, and very strong.

 

[hardinlw]

The TARC team I'm mentoring made retainers out of mirror mounting clips from Home Depot. The ones used were steel and were cut with a Dremel tool and bent to form a "Kaplow Klip" type of retainer. I think the bolts and blind nuts are 6-32 since that's a pretty common size for my junk box and most of their design decisions are based on what I have laying around that they can grab. With 29mm, they only used one clip which has proven to be adequate even with the rather energetic ejection charges of the F50 engines.

 

[MattieShoes]

Thanks for the tips guys I went to the hardware store on my lunch break and picked up some mirror clips and parts that should make up to 4 retainers. Even though one would be sufficient, I think I'll use two. Something to distribute the force, and the symmetry appeals to me on some irrational level. I'll share when I've got it together

 

[dave carver]

Most of my rockets I slot the bottom of the motortube with a razor saw, 4 slots usually. Wrap masking tape around the nozzle end of the motor. Just enough to slightly expand the slots a bit then I use a 3/8th inch wide radiator hose screw clamp and screw it down. Had an 18-20 case kick out never to be seen again so I came up with the screw clamp thing and have not lost a case since. I used this method to hold a Kosdon M2240 in place on my 3rd level rocket.

 

[MattieShoes]

I ended up going with what I assume are standard kaplow clips. A couple T-nuts, a couple screws, and some clips that were originally for screens....

I posted a picture thread in the mid-power forum, here

 

[Evo666]

Hi, Does that method of motor retention allow you to use estes motors too? I've been searching through these threads so I can do both Estes and aerotech motors.

 

[MattieShoes]

Hi, Does that method of motor retention allow you to use estes motors too? I've been searching through these threads so I can do both Estes and aerotech motors.

That's a 29mm diameter motor mount -- Estes engines are 13mm, 18mm, and 24mm in diameter so none would fit without an adapter. I could in theory make an adapter with a thrust flange and an engine hook, then use Estes motors, but they aren't powerful enough for an Onyx with the possible exception of the D12. Jack is going to be flying on G's for the forseeable future...

I was under the impression that the Aerotech 18mm and 24mm motors will fit into the standard estes clips... Am I mistaken? I've never bought an AT motor less than G so...

 

[Evo666]

Thanks for answering my question. I found my answer on Apogee's website and their video helps a ton.

 

[ben_ullman]

If they're missing a reload casing, they did it wrong. I've successfully taped many motors, including an I600 in a minimum diameter rocket and a K motor, with a 100% success rate. Tape works great, but it's a bit of a pain to do correctly.

very true. There was someone at BALLS a few years back that used like 2 rolls of tape to tape a MD N motor into his rocket. One roll made a thrust ring, and the other criss crossed the case.

Never saw a picture but it sounds pretty wild.

Ben

 

[UPscaler]

That's a 29mm diameter motor mount -- Estes engines are 13mm, 18mm, and 24mm in diameter so none would fit without an adapter. I could in theory make an adapter with a thrust flange and an engine hook, then use Estes motors, but they aren't powerful enough for an Onyx with the possible exception of the D12. Jack is going to be flying on G's for the forseeable future...

I was under the impression that the Aerotech 18mm and 24mm motors will fit into the standard estes clips... Am I mistaken? I've never bought an AT motor less than G so...

Yes Aerotech cases will fit in standard Estes mounts. However, if you get the 24mm by 95mm motors or case you have to have the Estes E sized mount to fit them.

 

[Thermo]

Hi all,

I started my first "big" rocket build the other day, and after epoxying the engine mount in, I realized I'd painted myself into the proverbial corner. I did not let enough of the engine mount extend from the rear plate to allow tape retention to work. The 1/4" that I did let it extend out is entirely taken up by the epoxy fillet...so any tape I use for retention would not have that much surface area to grip.

So, having already epoxied all together, with no access to the back side of the rear plate (thru which I could have put a T-nut had I thought this through properly), I instead tried this:
Drill out two shallow holes (one each side of the motor) in the rear plate, about half the way through the 1/4" plywood, and seat into each hole a nut, and epoxy all around. I also drilled smaller through-holes to allow the screws to pass all the way in. The screws will of course hold the clips that hold the motor in. The epoxy is all around the nuts, but did not go into the threaded holes. (I can get the screws to go all the way in and out smoothly.)

Is this "quasi-surface mount" going to be strong enough for motor retention you think?

(The kit is a LOC Iris, can use 38 mm motors)

Thanks!

 

[chadrog]

Hi all,

I started my first "big" rocket build the other day, and after epoxying the engine mount in, I realized I'd painted myself into the proverbial corner. I did not let enough of the engine mount extend from the rear plate to allow tape retention to work. The 1/4" that I did let it extend out is entirely taken up by the epoxy fillet...so any tape I use for retention would not have that much surface area to grip.

So, having already epoxied all together, with no access to the back side of the rear plate (thru which I could have put a T-nut had I thought this through properly), I instead tried this:
Drill out two shallow holes (one each side of the motor) in the rear plate, about half the way through the 1/4" plywood, and seat into each hole a nut, and epoxy all around. I also drilled smaller through-holes to allow the screws to pass all the way in. The screws will of course hold the clips that hold the motor in. The epoxy is all around the nuts, but did not go into the threaded holes. (I can get the screws to go all the way in and out smoothly.)

Is this "quasi-surface mount" going to be strong enough for motor retention you think?

(The kit is a LOC Iris, can use 38 mm motors)

Thanks!

I personally would not trust those nuts, I suspect they could be pulled off rather easily. I use and recommend threaded brass inserts that screw through the centering ring for this situation. I can post a pic of one in the morning if you'd like. If you do use the nuts, I'd highly recommend a nice tight friction fit on your case as well.

 

[Thermo]

I personally would not trust those nuts, I suspect they could be pulled off rather easily. I use and recommend threaded brass inserts that screw through the centering ring for this situation. I can post a pic of one in the morning if you'd like. If you do use the nuts, I'd highly recommend a nice tight friction fit on your case as well.

Chadrog, thank you. I posted a reply to this but it seems to have vanished...oh well, all I was saying was that I'd use your suggestion for a tight friction fit...and also use tape AND my screws/clips to be extra super duper sure the motor stays in, the parachute deploys, etc. Don't want to disgrace myself during the L1 attempt.

 

[dave carver]

Just the other day I saw a kit for guitars that has an insert that you drill a certain sized hole and the insert threads itself into the hole like a screw. Then you use a screw threaded into the insert. Seems to me a good wood working shop would have these as well as someone like Fastenall.

I just wanted to add that the guitar kit was made for Fender Guitars that have had the neck mounting screw holes stripped out for some reason. As the tension on a guitar neck is measured in the thousands of pounds I'm pretty sure it would hold up to a rocket motor

 

[Rex R]

those would be the type Chad mentioned . ya gotta get the holes just the right size else you end up splintering your centering ring.

 

[o1d_dude]

Threaded inserts can be bought at most big box stores. Look in the fastener drawers for these. They might be categorized as "furniture hardware".

My LHS carries them as well. If all else fails, check Amazon.

6-32 is pretty commonly used size for mid power and small high power birds. Or you could go with 8-24 if you don't mind the small weight penalty.

 

[qquake2k]

Here's what I do for retention. I epoxy 6-32 tee nuts in the aft centering ring. See posts 12 and 27:

https://www.rocketryforum.com/showthread.php?t=33493

Then I make clips from brass or aluminum. Lately I've been using 1/8" x 1/2" aluminum exclusively. See posts 116-120:

https://www.rocketryforum.com/showthread.php?t=30829&page=4

I use 6-32 machine screws with external tooth lockwashers to attach the clips to the rocket. Haven't had one fail yet, knock on wood.

 

[skip_dye]

Here is another inexpensive method of motor retention:

https://www.drsuesrocketworks.com/CrayPuppy1.html

Also, I don' remember where I saw it, but somebody did some type of motor retention is a twist off cap from a sports drink bottle.

Personally, I use masking tape on all my 29mm MMT rockets.

I use slimeline retainers on 38mm and above.

Alan