"Positive Motor Retention"

[Cougar93]

Okay, what gives with the terminology?

As you guys know by now, I've been strictly an LPR to this point in my life. However, as I get into MPR, I could use some help with terminology. I know retention means a device to keep the motor from flying out during ejection. However, I've seen several of you use the term "positive" in front of "retention." Is that just a redundant term to imply "keeping the engine in" vs. the implied opposite, "negative retention" which I can only surmise means letting the engine fly out?

While I'm on the subject, I could use some advice on the use of epoxies, what brand, and tips for application.

Thanks in advance!

 

[tazzdevl1]

Positive retention is by means of a clip, screw, bolt, nut, retainer, etc. Friction fitting the motor in is not a form of positive motor retention.

 

[Cougar93]

Thanks. Examples? Pictures? Who offers the kits. Aeropack?

 

[Dark Matter]

Originally posted by Cougar93
Thanks. Examples? Pictures? Who offers the kits. Aeropack?

There aern't any kits, although some rocket kits come with retention. Aerotech for example. Mostly it is do it yourself. Here are some examples:

https://www.info-central.org/construction_retention.shtml

Also do a search for motor retention, click the button for searching titles only.

Dark

 

[rstaff3]

Aeropack sells top-end retainers; Giant Leap sells several styles and shapes of retainers, from the top-end 'SlimLine', screw down models, to a simple 'Kaplow klip' style that come with 2 clips, threaded inserts, and the screws; Rowes (UK) sells a line of retainers. PML makes a retainer. There are others.

Aluminum mirror hangers will work, as will simple washers. Just depends on how much you want to spend, how much planning you do during the build, and what type motors you will use them with (AT, vs 'Kosdon-style' reloads, or SU). Ask more questions if need be.

 

[Cougar93]

You guys are awesome. I was asking because someone told me I would need positive motor retention for the Binder Designs Thug.

Is that because MPR kits cannot hold a motor in with friction retention? My second question is, if you're using single use motors, why is positive motor retention important?

Thanks again!

 

[rstaff3]

You will get different stories of friction fitting reliability. There are always some people who always use it on their expensive M powered hardware and have never had a problem. Then there's the rest of use that would rather pay a few bucks for insurance. BTW, tape works good for keeping a SU motor from going up through the rocket, keeping it in at ejection is where there is dissention.

Why plan for it on MPR: 1) because you will probably move to reloads if you keep flying; 2) When you pay for them you want to keep them; 3) so the motor doesn't eject before the nose cone; 4) because all the cool people do it

If you plan right, the same mounting hardware can work for SU and any style reload. On my rockets, I generally use various clips that are screwed into threaded inserts or T-nuts. I have a pile of clips in the range box that will work with most anything on any rocket. I have one Aeropack and one Slimline. They are nice if you want to spend the money. You can economize by buying only the motor tube section and relocating the cap from rocket to rocket.

Oh you can also mount chep screws through the rear CT and uing clips or washers.

If you use Loki motors, remember the retention has to mate with the retaining ring, not the rim of the casing.

 

[MarkM]

Dick provided a detailed and excellent answer. I just want to add one thing concerning reloads vs SU motors.

You'll find SU motors are significantly more expensive than reloads/flight. True, the initial investment in the casing and closures is a bit expensive, especially when getting into the really large 54mm and above motors, but over the length of your flying activities you'll save a lot of $$$ by going with reloads. Besides, once you get beyond G motors, there is a very limited selection of motors so you'll most likely go with reloads. Therefore, you'll want to eventually learn how to assemble reloads and fly them. So, you'll want to design positive motor retention so you don't lose those expensive casings.

Plus, without positive motor retention a good portion of the energy delivered by the ejection charge firing will be disapated as the motor is expelled rearward, and not enough will be left to effectively expel your recovery system. That's bad news. With retention, the motor is kept in place and the full energy of the ejecton charge will go to expelling your recovery system.

HTH

PS: I use Aeropack retainers on all my HPR rockets, though there are cheaper methods that a lot of flyers use that are just as effective if done right.

 

[Daedalus]

I use Rowes retainers because once you get over 24mm the whole retainer is interchanged between models so you only need one in each size (unless you want a flat retainer and a cone retainer for different looks, oh, and different colours too). Each new model just requires captive nuts building in to the rear centering ring - they can also be retrofitted to existing models with the supplied threaded inserts.

https://www.rowesretainers.co.uk/

Just a happy customer.

 

[geof]

Originally posted by Cougar93
You guys are awesome. I was asking because someone told me I would need positive motor retention for the Binder Designs Thug.

Thanks again!

I was that person but I'm being misquoted slightly. You probably *want* postive motor retention because the 29/40-120 casing costs almost as much as the Thug itself! (Note: that's also a tribute to Binder's affordable pricing.) It is not required. Binder gives explicit instructions about friction fitting and recommends that approach, but I prefer the extra security. I want that casing back!

I have not yet tried commercial retention systems; they seem to run about $20 each in the US. For about $2 I can get enough t-nuts, bolts, brackets from Home Depot to do 4-6 rockets.

Geof

 

[Cougar93]

Thanks again to everyone. I love this site!!!!

Geof, you mentioned:

I have not yet tried commercial retention systems; they seem to run about $20 each in the US. For about $2 I can get enough t-nuts, bolts, brackets from Home Depot to do 4-6 rockets.

Can you walk me through the process of building with your methodology?

For what it's worth, I'm not against a $20 outlay on the Aeropack equipment. Then again, if it can be done just as well more cheaply, I'm certainly not against that either.

 

[fyrechaser]

Cougar,

Here's pic that may help you visualize the set up. The clip in the picture is a screen door clip I got at Ace Hardware. A reload casing is pictured but you can easily use this same set up with SU motors. A few wraps of masking tape around the aft end of the motor will make a *thrust ring* that will keep your SU motor from moving forward, the clip will keep it from ejecting.

 

[fyrechaser]

Here's one using an AT F20 SU.

 

[geof]

Well, what I did on my Thug was a retrofit, so I wouldn't advise doing quite what I did there!

What I am recommending is essentially like the first 3 photos on this website:https://www.info-central.org/construction_retention.shtml

T-nuts (see top photo) have a barrel threaded on the inside and smooth on the outside. At one end of the barrel is a larger, circular platform with spikes. (The spikes are facing down in the top photo. In this photo, the nose of the rocket would be up. The bottom of the motor tube extends strangely far beyond the bottom of the CR, so I'm guessing that the t-nuts in this photo were actually used for some other purpose). The circle platform and spikes go on the inaccessible side of the CR (i.e. the nose cone side). The barrel extends down towards the rear of the rocket through a hole you drill in the CR. (See photo 2: the barrels are the smaller metalic things. I don't know why there are two other screws with washers in that photo.)

[Edit: In photo 1, the t-nuts are only partly in. Hammer them all the way down for the spikes to grab. The bolts will go *up*, from below the CR, into the barrelsof the t-nuts.]

Two t-nuts seems to be a popular number for 29mm motors.

Installing the t-nuts must be done before the CR is glued in the body. They can just be hammered in, but I filled mine with wax then added a bit of epoxy to the contact point. Whatever you do, don't gum up the threads with epoxy!

Now, after the CR is glued in place, you will need bolts matching the size/threads of your t-nuts. These can be screwed into the barrels of the t-nuts. See photo 3.

In photo 3, they used washers and "screen door panel clips". I used the smallest shelf L-brackets at Home Depot, cutting off one side of the L to get a linear bracket. These brackets also happened to be just the right width to fit through the notches in the 29/40-120 casing nozzle end.

Now there is one more complication. Your RMS motor casing may extend beyond the surface of the rear CR by about .25 inch even if the motor mount tube is flush with the CR. This is not really seen/present in photo 3. Therefore, a flat bracket will not really work when bolted in, because it would need to slant downwards. The "screen door panel clips" in photo 3 appear to have a small set of bends in them to accomodate this. For my straight brackets, you could simple use a nut or a stack of small washers to build up .25 inch. But plan for this extension when you select the length of your bolts.

[Ignore this last part, hopefully: For my Thug, I didn't plan for positive motor retention until the CR/MMT was glued in. Therefore, I had no access to the back of the CR. Thus, I attached the t-nuts to pieces of .25 plywood approx 1 inch square, then expoxied these assemblies to the back surface of the CR. (This also took care of the offset issue mentioned above.) For the Thug, all this business is still hidden since the CR is recessed slightly in the body tube. This thread (https://www.rocketryforum.com/showthread.php?s=&threadid=22910) describes my dilemma and some suggestions that are potentially superior to the approach I took. I was/am fairly confident that this wood-to-wood bond with generous epoxy "fillets" will stand up well. ]

My original message said to "consider positive motor retention" during the build process, so that you accomplish it without having to do a retrofit like I did.

BTW, a few people have commented on your transition to MPR. Having recently done so myself, I would personally recommend making the 29mm Thug, rather than the 38mm. It's plenty of rocket, you have easy access to motors, and I think you'll be fully satisfied on a 29mm G64 motor. No need to go bigger yet.

Hope that helps!

Geof

 

[Cougar93]

Thanks. Well, it appears I have a lot of learning to do. The good news is... I'm totally ready to learn. Glad you guys are here to teach.



PS: fyrechaser, I'm originally from Odessa! Are you a native Odessan, or did you move there later in life?

 

[fyrechaser]

Cougar,

I got here about 18 years ago. Moved here from the Dallas area, but I'm originally from the Houston area. Was born in Pasadena....small world!

Lets see, that makes me how old? Sheesh.....I'd rather not think about it.

 

[Cougar93]

Funny, I lived there until I was 21, then moved to the University of Houston to finish up my college degree. Been here since. I still go home though. Was just there over Thanksgiving. Maybe I can hook up with you guys for a launch next time I'm home. Where do you launch?

 

[fyrechaser]

We fly model rockets in a field/park near my house. We haven't found a site close by to fly high power yet, so we usually make the trip up to Wayside to fly high power.

Let me know the next time you get up this way. We'll fly some model rockets, then retire to the rocket room for interesting disussion and brewski.

 

[Cougar93]

Sounds like fun... deal!

 

[DeWain]

I'm also new to midpower. I have a new Semroc SLS Laser-X upscale and an abandoned and unclaimed LOC Graduator that was in our club's lost and found.

Here's my question. Since the Graduator is already built *without any nuts or threaded inserts in the centering ring*, would I be able to effectively epoxy in a T-nut from the bottom? The Graduator was rather poorly built (no wonder no one claimed it), but I plan on refinishing it and giving it some decent decals.

Anyone have any tips on the best approaches to retaining a motor on the Laser-X 29mm motor mount?

Thanks

 

[f16fan12]

Hi, I just have a question for you folks. I had built my PML Io and I didn't put any motor retention in during construction. After, I put a threaded insert into the CR, and then a bolt went through that and there are a couple nuts and a washer holding the case in. I had bought two sizes of threaded inserts, and I used the smaller one. I still have the bigger one...

Now, I'm going to start work on my Binder Excel, and I want to put in motor retention now. I bought some #8-32 x 1/4" t-nuts, cause they were out of the #6-32s at home depot. I also bought some 1 inch bolts to go in those t-nuts, and two lengths of metal. I was recommended 1/4" by 0.025" brass strip and when that seemed a little thin, I got the 1/2" by 0.025" brass strip too. Now, I hold this stuff in my hand and I can easily bend the metal. Is this seriously going to retain my 38mm /240, 360, or 480 cases? It looks like as soon as the ejection charge blows, my whole case, closures and all, will come out.

Any suggestions?

 

[m85476585]

Does your IO have a 29mm or 38mm motor mount? My Callisto is 38mm and motor retention was a big pain. The gap between the motor tube and the BT is so small that I had to file the sides of #4-40 nuts to get them to fit.

If you are concerned about the strength of your motor retention, you can always add some masking tape and friction-fit engines. I always make the fit as tight as possible just-in-case (maybe a little less tight if I have good retention). Some of my 29mm casings are so tight in the 29-38 adapter that I have to push with a wood rod from the top to get them out. But I haven't lost one yet! (of course I have only done 3 flights (A single-use F, a G64, and an H128 for my L1.))

 

[cjl]

I personally like Aeropack retainers. A little expensive, but it WILL not come out (and the retainer is cheaper than a new casing).

 

[m85476585]

But T-nuts and masking tape are cheaper than a retainer.

 

[cjl]

Yes, but I've had one blow past t-nuts and masking tape before - that was more expensive than a new retainer (almost - I found the case, a Rouse 38/480 by pure luck about half an hour later)

 

[f16fan12]

Fine, I'll probably tape it. But do you have any thoughts on how malleable this metal is, and whether that would even stand a chance at retaining the motor? I was gonna go for Kaplow Clips, but now that doesn't seem like a great idea. I'll probably just use these big threaded inserts...

Oh, and my Io is 29mm. The threaded inserts I had barely fit into the centering ring. I like I had 1 cm of space, and I made a 1 cm hole.

 

[MarkM]

Originally posted by cjl
I personally like Aeropack retainers. A little expensive, but it WILL not come out (and the retainer is cheaper than a new casing).

Me, too.

True, they are expensive and the T-nut/metal clips work fine for most people. I just find the Aeropack easier not only to install on the rocket, but it takes all but 10 seconds to secure the motor in and you're guaranteed that it won't slip out or blow by the clips. Plus, it's just plain cool

If you go with the retainers just be sure you leave enough of the motor tube extended beyond the aft centering ring in order to secure the retainer on.

 

[ben]

I use clips on my rockets you can get them at hardware stores use some T-nuts and for less than 50cents a piece you have motor retainers I spray the clips with heat resistant paint just in case. I would go with Aeropacks but I only use those on bigger MPR and HPR rockets that i care if I loose

thanx, ben

 

[f16fan12]

Ben, could you be a little more descriptive about what you mean by "clips?" If you mean panel clips, can you make it so they're not flush with the CR? Cause for the Excel, there's about an inch of space between the CR and where the aft closure sticks out of the motor mount tube. I think I'll probably end up using threaded inserts, unless I can understand how I'm supposed to use this very-bendable metal to retain a H or I motor.

Thanks

 

[loopy]

I picked up some brass strips for motor retention as well, and also thought it seemed a bit flimsy. However, it only seems flimsy when you play with the whole strip. I grabbed the strip where I thought the bend would be to hold the motor so it only spanned what I thought would be over the motor, and it didn't flex at all. However, since it is a bit flexible in it's longer state, I'm using three of them on my Norad Pro Maxx. I figure three of them should hold it since the force required to bend three brass strips should be much greater than the force required to eject the recovery system of the Norad...