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  #1  
Old 01-08-2009, 05:07 PM
dogcaller dogcaller is offline
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Default Who ever thought "friction fit" was a good idea?

OK, what am I missing? On a couple of occassions (including just recently, unfortunately) I have purchased kits with the "FF" engine mount style. What a pain! I'm always worried about getting a "good enough" fit, but still being able to get it back out w/out destroying the rocket. What's the problem with a simple wire hook? Plus adding the tape is a pain. Many rockets don't list what method they use.

Am I missing something? Just showing my own inexperience? What is the possible advantage, and, more importantly, how can I turn a FF kit into something easier?

Thanks!
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  #2  
Old 01-08-2009, 05:15 PM
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cas2047 cas2047 is offline
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I'm with you. I don't lke the idea. I've never purchased a rocket that uses the FF method.

Different strokes for different folks I guess.
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Old 01-08-2009, 05:41 PM
Peartree Peartree is offline
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Quote:
Originally Posted by dogcaller

Am I missing something? Just showing my own inexperience? What is the possible advantage, and, more importantly, how can I turn a FF kit into something easier?

Thanks!


I have one two rockets that use friction fit. One is for competition to save weight and the other was something of a speed build in which I deliberately skipped that step to save time. I have had miserable luck with friction fit although there are others here who have techniques that seem to work for them (my success rate is well below dismal).

Once you've built a few LPR kits with motor hooks for retention it is generally very easy to adapt a FF kit to use an engine hook (purchased or homemade from a windshield wiper spring).
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  #4  
Old 01-08-2009, 05:52 PM
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gpoehlein gpoehlein is offline
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Well, there are a couple things at play here.

First, if the rocket is a minimum diameter model, you either use friction fit to hold the motor in or you have to use some method to hold the hook in place. This usually happens right in the middle of the fins, so you then have to make allowances in the fin root (take a look at the Hi-Flyer fin for an example. Whatever you use (in the case of the Hi-Flyer, it is a mylar ring) will also add drag to the model.

Second, a lot of older designs used friction fit - I'm assuming that the hooks were not available at the dawn of rocketry. So, to maintain the look of the original, you gotta go back to friction fit.

Third, in a multi-stage model, the hook can get in the way of the staging mechanism (ie two motors taped together with cellulose tape).

You do have a few options, however. If you are building an 18mm minimum diameter design, you could always change the mount out for a 13mm mount with a hook. I've done this with the Hi-flyer and it works fine. Remember, the Estes Goony Birds were BT-60 models (think of the Baby Bertha, but a bit shorter) and they flew on 13mm motors. Anything that flies on an A8-3 will also fly on an A10-3T or A3-4T.

You might also try what I do - tape the motor in place at the bottom on the outside. Use a good quality tape - I like the Tamiya masking tape for this. If the fin goes all the way to the back edge of the BT, you can use three (or four if a 4 fin model) short pieces of tape in between the fins. Just make sure the tape is stuck down very well to both the rocket and the motor.

I was also informed (I think on TRF) that when using tape to friction fit the motor, wrap the tape around the FRONT end of the motor, not the middle. The motor can swell from the pressures during flight and putting the tape in the middle might make it a lot more difficult to get the motor out. You want it tight enough so the "path of least resistance" is the nose cone and recovery system popping out, not the motor popping out.

Hope this helps.

Greg
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Old 01-08-2009, 06:15 PM
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Bazookadale Bazookadale is offline
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Quote:
Originally Posted by dogcaller
Who ever thought "friction fit" was a good idea?


To answer your original question it was Orville Carlisle in the 1950s - the motor hook system wasn't invented until the early '60s
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  #6  
Old 01-08-2009, 06:37 PM
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Mark II Mark II is offline
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Quote:
Originally Posted by Bazookadale
To answer your original question it was Orville Carlisle in the 1950s - the motor hook system wasn't invented until the early '60s

They also used rubber bands wrapped around the motor for friction-fitting in those days! Hey, it's worth a try!

Greg makes a good point. The upper end of the motor is often slightly smaller in diameter than the nozzle end - the case bulges slightly when the black powder is pressed in. The logical place to put the tape is near the upper end. I agree though - I do not care for friction-fitting, and I avoid it whenever possible when I build my rockets. The only rockets that I have that I tend to use FF for are my Micromaxx models and in my saucers and odd-rocks. The insertion and extraction of the MMX motors is less traumatic than it is with motors in other sizes. In saucers with open-ended motor mounts, I don't need to make the fit very tight, because the motor won't be kicked out in such a design.

In some designs where the difference in the motor diameter and the outside airframe is very small - too small to effectively use a springy motor hook - I have made rigid motor hooks out of music wire that I twist to lock and unlock the motor. The trick is to give the wire a little extra bend at the top of the hook so that it can be twisted 90 degrees without coming loose and pulling out.

Was it Vern Estes who invented the spring-steel motor hook?

Mark \\.
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  #7  
Old 01-08-2009, 07:30 PM
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dwmzmm dwmzmm is offline
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My vintage Centuri 1/100 Saturn - V's cluster engine mount (believe it or not) uses friction fit for flight. I've already flown her twice over the past 1 1/2 years without problems.....
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Old 01-08-2009, 07:33 PM
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Intruder Intruder is offline
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Quote:
Originally Posted by dogcaller
OK, what am I missing? On a couple of occassions (including just recently, unfortunately) I have purchased kits with the "FF" engine mount style. What a pain! I'm always worried about getting a "good enough" fit, but still being able to get it back out w/out destroying the rocket. What's the problem with a simple wire hook? Plus adding the tape is a pain. Many rockets don't list what method they use.

Am I missing something? Just showing my own inexperience? What is the possible advantage, and, more importantly, how can I turn a FF kit into something easier?

Thanks!


Here's how I do it. First, take a piece of tape that can wrap almost all the way around the motor but not overlap itself. Tape doesn't stick anywhere near as well to the back of itself than it does to the motor. If it overlaps, the tape could slip and bunch up causing the motor to fit super tight.

After you put the first strip on. Slip it in the mount. If it gives some resistance, it's a good fit. If it goes in really easy, pull it out because it needs more tape. Take a second strip of tape the same size as the first. When you apply this strip of tape, do it a little above or below the first strip so that some of the second strip is in direct contact with the motor and the other part is laid over the first strip. This gives the extra diameter while reducing the chance that the tape will bunch up. Repeat this as much as necessary.
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  #9  
Old 01-08-2009, 08:07 PM
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cas2047 cas2047 is offline
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Quote:
Originally Posted by dwmzmm
My vintage Centuri 1/100 Saturn - V's cluster engine mount (believe it or not) uses friction fit for flight. I've already flown her twice over the past 1 1/2 years without problems.....


You are a brave man Dave!

I'd be sweating hitting the button sending that beauty skyward. In the back of my mind I'd be afraid that at least one of the engines might just want to pop out.
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  #10  
Old 01-08-2009, 08:12 PM
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hcmbanjo hcmbanjo is offline
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If I'm building a kit with a 3/4" diameter body and no (outside) engine hook:
I stay away from the thinner Estes BT-20 tubes. If you try to pull a friction fit engine out of a BT-20 you run the risk of the tube crimping .
I've always had better luck with the old Centuri style ST-7 tubing in minimum diameter models. The tube wall is thicker and stronger.

Hans "Chris" Michielssen
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