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The all Important Motor Retention Question....

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moocrew

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Alright so I've always kind of wondered....Bulkheads are good for keeping the rocket out of the nose cone...but what about ejection charges?.....I'm not real big on out side contraptions for keeping the motor in there. I've heard of people making a "friction fit" by wrapping masking tape around the motor then inserting it..does this work?....what about other retention systems..I've done all kinds of research on this over the last few months, but I can't seem to find an explanation that actaully makes sense. If you could point me in the right direction or provide any kind of info or help I'd be incredibly grateful.
Thanks!
-Matt
 

Mister Rogers

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Hi Moocrew,
If I understand your question, the bulkhead does a lot more than just keep the "rocket out of the nose cone". That's not the job of a bulkhead at all really. Keeping the motor from moving forward inside the rocket is the purpose of the engine block, a ring or "donut" glued immediately above the engine which prevents the engine from moving but allows the ejection gases to pass through the hole.
A bulkhead is usually a solid ring or circle of thicker material to block the ejection gases. This can protect vital things on the other side of the bulkhead from the gases or, it can just be to reduce the internal volume the ejection charge has to pressurize. For example, if your rocket was 2" wide and 6ft tall, it might be hard to pressurize all six feet of 2 inch tubing. Putting a bulkhead at the 3 foot point means you only have to pressurize half the volume. Of course that means your rocket should seperate just below that, and below that is the parachute and wadding, then the engine retainer block, then the engine.
At the other end, to keep the motor from kicking out the rear when the ejection charge goes off, something has to "hold" it in the rocket. Friction fit works fine, especially for smaller motors, and is cheap and easy. It does have the drawback of needing to be just right though, too loose and it pops out anyway, too tight and it is a bear to get out afterwards. If you design the rocket so that the motor sticks out a ways, say 1/4 to 1/2 inch, it is relatively simple to grasp it with pliers and twist.
The most popular way to retain a single smaller moter is the classic engine hook. This premade flat spring metal piece allows the rocket engine to slip in easily but is retained positively. After the flight, you just bend it back out of the way and voila! Of course is springs back into place immediately for your next launch.
Other methods such as threaded rods with washers and nuts work better for large engines or multiple engines. The idea there is that the threaded rod is permanently inside the rocket. The engines fit with ease until you screw on the washer/nut and then the motors are "trapped". The washer/nut is sized to only cover the outside lip of the engine/engines, and not obstruct the nozzle in any way. After the flight, unscrew the nut and remove the old motors.
There are other ways as well. I recently built a rocket with a rubber band atached to an "L" shaped metal bracket to retain the motor. I needed to do that because my clearance was too limited to use a traditional hook. This allows the engine to move rearwards about a 1/4 to 1/3 inch, but still be held in, while it can be cocked sideways to get the motor in/out after recovery. I hope all this helps and is not too basic for what you were asking for.
If not, maybe someone else will find it usefull.:rolleyes:
 

rabidsheeep

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masking tape works beautifully...

what i do is put alittle on the inside, making sure that its a VERY tight fit, but not too tight so it cant come out

then i leave about a 1/4th inch peice of the motor out of the tube, where i can wrap the tape onto the outside

i can upload pics to make it easier, if you launch at a club just ask somone about masking tape retention
 

moocrew

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No, of course not. Nothing is ever too basic...heck the basics are what everything else is built on so you explained a few techniques I was unaware of even after almost a year of building.
It was a typo thou...i tend to get BulkHead and Engine Block Easily confused when typing. I wasn't paying attention to what I was writing. :rolleyes:
I've actaully built a few clusters..but have used hooks on all of them....I'm starting to tire of them. I think a friction fit is prolly the easiest as you said. I have seen the "screw and nut" fixture before..but that a little too large for my usual 13mm rocs.
Thanks for your input and help!
-Matt
 

lalligood

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Originally posted by rabidsheeep
masking tape works beautifully...

what i do is put alittle on the inside, making sure that its a VERY tight fit, but not too tight so it cant come out

then i leave about a 1/4th inch peice of the motor out of the tube, where i can wrap the tape onto the outside

i can upload pics to make it easier, if you launch at a club just ask somone about masking tape retention
^^^^^^
An excellent explanation of friction fitting with masking tape. It has become my preferred method of motor retention. It is also the lightest & cheapest solution out there...

I have tried (& like) the Aero Pack retainers, but you pay for them. If I ever do a LARGE project (say Level 3), I would consider using them again but I didn't know any better back when I was building my PML Phobos, which I flew for my Level 1 cert.

Kaplow Klips (do a search here & on Google) work very well & can be made cheaply (less than $2 per rocket).

So many solutions... Just depends on your level of comfort & budget!

HTH,
 

powderburner

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If your rocket is designed with a short portion of the BT projecting aft of the fins, and with the motor case sticking out beyond the BT, you can wrap some masking tape around the outside of the motor and overlap it onto the BT. Two or three wraps are enough to hold the motor case in place.

Friction fit is cheap and fast but is more "art" than science. Chances are that for every "good" fit you will have one (or more) that end up too tight and one (or more) that end up too loose. And if you are a little late in recovering your rocket and the masking tape adhesive has time to soften and re-set, then the motor might be permanently stuck in the MMT. Until you develop the right judgement for getting the fit just right, you can have lots of problems. I recommend clips or some other form of retention.
 

11bravo

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See if I got this correct, the SlimLine Retainers from Giant Leap acts for both "keeping the rocket out of the nose cone" and keep it in the body at ejection.
Correct?

Greg
 

rstaff3

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Only the latter, as they were designed for AT style casings that have a thrust ring built into the aft closure. As far as I know they will not work with Kosdon-style motors either. The newer versions may have corrected this.
 

11bravo

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I was under the impression that on AeroTech motors the little ring at the bottom (lower left in the linked photo) ((I guess it's the rear closure)) was held captive in the SlimLines.
Of course, the pic is a 29mm, maybe the 38s work differently.

http://store1.yimg.com/I/ehobbies-com_1816_1328486612

Greg
 

rstaff3

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Sorry if I confused the issue. The AT aft closure itself keeps the motor from going up through the rocket. The Slimline of course doesn't affect this. The Slimline ring (snap or screw in type) goes in behind the motor to keep it from coming out on ejection.

Oh, yes, all the AT/Dr Rocket cases all have the aft closure that is larger than the motor diamater to act as a thrust ring.
 

lalligood

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Originally posted by rstaff3
Sorry if I confused the issue. The AT aft closure itself keeps the motor from going up through the rocket. The Slimline of course doesn't affect this. The Slimline ring (snap or screw in type) goes in behind the motor to keep it from coming out on ejection.

Oh, yes, all the AT/Dr Rocket cases all have the aft closure that is larger than the motor diamater to act as a thrust ring.
But all SINGLE USE motors require that you add a thrust ring--usually a few wraps of masking tape is easiest & plenty sufficient--to provide the same feature. I've heard of people epoxying slices of motor tube or plastic zip ties to make a thrust ring for single use motors too.

HTH,
 

Daedalus

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For larger motors (24mm and up) Rowes Retainers are hard to beat. For 24 mm a threaded adaptor is glued to the mmt and the motor retained by a screw on cap or cone (expensive but nice). In the larger sizes the retainers are held on by captive nuts and screws so you only need one retainer for each size you fly as they swap between models (unless you are sad like me and have to have both flat and cone retainers and colour co-ordinated to the model). The cone retainers are VERY nice.

http://www.rowesretainers.co.uk/

Just a satisfied customer (currrently owner of 5 retainers)
 

rstaff3

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Originally posted by lalligood
But all SINGLE USE motors require that you add a thrust ring--usually a few wraps of masking tape is easiest & plenty sufficient--to provide the same feature. I've heard of people epoxying slices of motor tube or plastic zip ties to make a thrust ring for single use motors too.

HTH,
Good clarification point lalligood. I have done the epoxying nylon ty-wraps on thing. Never had a problem, but switched back to tape. For upwards retention, tape has been 100% for me also. For downwards retention, the success rate is good, but not perfect. Still use tape on 24mm and below.

BTW I see Giant Leap has a cool new bell-shaped retainer.
 

Brian Barney

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since we are on the subject of motor retention... I am wondering if anybody has had a good idea of how to retain motors in min diameter rockets other than friction fitting... I had a friend lose a K1100 casing because of friction fitting letting loose, still haven't found the casing. I have been throwing around abunch of ideas in my head, none of which i really like other than a plugged foreward closure with a forged eyebolt for the aerotech/drrocket casings... but that doesn't work for cesaroni's, single use, or hyperteks... I guess for single use engines it doesn't matter too much cause if you lose one of those no big deal (unless it lands on your head) Anybody have any good ideas? I dont like the slimline retainers for min diameter cause then its not min diameter anymore and adds even more evil base drag... Just thinking out loud here :)

-Brian Barney
 

rstaff3

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The only other things that come to mind involve something on the outside of the airframe. There was somebody selling a thin bracket that connected to the outside of the airframe. I forget the manufacturer. Then, I have made retainers from very thin brass tubing and piano wire. The tubing is mostly mounted/buried in the fin fillet and there is a piece of wire that is bent so that in the 'closed' position, it overhangs the motor on one end and the leading edge of the fin on the other.
 

moocrew

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Seems like this is turning into a very informational thread...
minimum diameter was going to be my next question. I think that the wire deal closed over the butt end is probably the best idea....I may have to try this one....I'm real big on aesthitics though so I may have to try some way to disguise this feature.

-matt
 

lalligood

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While this wouldn't exactly be the most visually aesthetic solution, you could make small slits in 2 or 3 places around the aft end of the MMT tube & use a metal hose clamp (tightens with a screwdriver). I have seen guys use this technique on 38mm and larger MMT tubes. (Probably wouldn't be worth it on anything smaller than 38mm--too bulky & heavy for 24mm & 29mm.)

Good thread discussion indeed!
 

rstaff3

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I cut the slits on a 38mm Art Applewhite saucer...didn't like the technique...never used a clamp.
 

rabidsheeep

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Heres a few examples of retentions ive used:

For aerotech kits i like to do add alittle tape around the hook for *alittle* extra protection (it just adds alittle peace of mind)

This is the aerotech hook. On the right is how I wrap alittle tape around (making sure no tape gets near the nozzle.



For single use motors, I like to make alittle rim, almost like the rim of the aft closure on casings, that prevents the motor from shooting up into the rocket (I only do this when i dont put in an engine block.)

I also add alittle tape in the middle to make it fit just a tiny bit more snug.



Heres an example of the overhang left out of the bottom of the rocket. On the right is how I wrap the tape so that it touches both the overhanging MMT and the engine to hold it in place so it doesn't shoot out the back when the ejection charge fires.



When i do have an engine block installed, just nothing to hold the motor from ejecting out the back, I do something like this. Adding the little bit of tape on the inside again for a snug fit, then some on the ouside holding the engine to the MMT.



Another I've used uses the hooks on screen doors (go ahead, look. I couldn't figure out what anyone was talking about untill i did) By securing them with screws to the bottom CR of a rocket. The screws i secured with apoxe, making sure that none got on the inside.



The last Ive used is the washer one for 3 motor clusters, where in the middle I apoxed one of those wall fastiners (sp) and then screwed in a screw with a washer to hold the engines.



Heres a picture of the little fastiner (sp) thingy



Hope that helps
 

daveyfire

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Originally posted by Brian Barney
I am wondering if anybody has had a good idea of how to retain motors in min diameter rockets other than friction fitting... I had a friend lose a K1100 casing because of friction fitting letting loose, still haven't found the casing.
Ouch. Was that David Roy's Shock? Awesome flight in August, BTW :)

I go to West Marine and get some of their 1/8" KevCord. It's 1/8" Tubular Kevlar in a nylon sheath. You could also do this with Kevlar from Pratt Hobbies, or anywhere else. I tie the cord onto the forward closure of the motor and friction fit it in. If the tape doesn't hold, the Kevlar cord will. I use Kevlar because the ejection charge is right next to the cord. If you are using altimeter ejection, I would imagine any small round cord that you could snug around the closure would be fine. Loki motors have a screw point for a forged eyebolt... attach your shock cord there too!
 

brianc

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sandman

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Now this may sound goofy but...how about glue? Like Elmer's or yellow glue.

It should hold well during launch and ejection but wouldn't "stick" that well to the aluminum case so you could "beak" it loose after the flight.

Just rambling...move along...we don't need to see their papers ...these are not the droids you're looking for...
 

moocrew

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The nut and washer system is usualy used on rocs with wood CR's?...could it be modified to use it on a smaller roc...say a clustered 1/2A?
Does it only need to be glued to the bottom CR...or both top and bottom?....I'm just trying to clarify some things.
 

rstaff3

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I have attached the 'Kaplow clip' style retainers (including procured ones, home made ones, and steel mirror brackets which are now what I mainly use) using T-nuts, threaded inserts, and bolts inserted in the rear CR. All these only go into/thru the rear ring. I personally have done this with only wood and thick fiberboard rings.

When I use a bolt, I insert it from the inside of the ring and usually add a washer inside, although if the bolt has a large head I have skipped the washer. I usually thread a nut onto the bolt from the outside and hold it in place with a dab of epoxy. This method allows me to use the clip method or a washer.

You could downscale the bolt method but you should get a sense of the amount of weight you are adding to the rear of the rocket. I've used a central bolt on a 3 x 24 mm cluster but nothing smaller. Here I cut off the head and epoxied it in the gap between the tubes.

....hmmm....I do have these realy tiny botl/nut/washer sets....
 
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