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  #1  
Old 09-15-2007, 02:06 AM
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Tau Zero Tau Zero is offline
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Exclamation 29mm Motors

Okay, you mid-power guys who use 29mm motors (especially in Semroc Large Scale [SLS] models)...

1) Exactly how long are those motors?

2) Do you friction-fit them, or do you usually have some sort of engine block/thrust ring?


I'm trying to "standardize" the minimum number of parts for flyers who might use either 24 or 29mm motors in Semroc's SLS tubing.


Thanks in advance, and cheers,
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  #2  
Old 09-15-2007, 06:41 AM
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Jay,

The length depends upon the motor. You'll just have to decide what motor you plan on using. Me, I design for the longest (I think that's like 120) and then use spacers, something similar to Estes designing 24mm for E motors and providing a spacer for D.
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  #3  
Old 09-15-2007, 08:19 AM
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The standard Aerotech reload case is 124mm, and the single use vary all over the place. While they USUALLY cap around 124, some of the G's get into 170-200mm length.

Ellis Mountain tend to be longer, but also burn longer, and Road Runners are 112 for the F and 140 for the G.

If using the reload, the aft ring acts as a thrust ring, so all you need to worry about is retention. You could go with a clip, but that limits your flexibility. If you have room on the centering rings, use blind nuts. Otherwise, friction fit AND a layer of tape across the aft ring and body tube is a good idea. The reload case is expensive and you want redundancy.

Some single use have thrust rings molded into the case, but most don't. Instead, just use masking tape to build a thrust ring. Friction fit is also the standard for retention.

If you absolutely have to go with a planned/fixed length, I'd opt for the AT reload length of 124, which will accomodate about 75% of the 29mm low power motors out there.
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Old 09-15-2007, 09:41 AM
snaquin snaquin is offline
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Semroc instructions recommend a friction fit with both 29mm motors and 24mm motors in the SLS kits. Unless you decide to permanently mount the 24mm adapter into the 29mm tube of one of the SLS kits, the 24mm motor is retained inside the adapter with a hook but technically the adapter is still friction fitted inside the 29mm motor tube. You can always make a 1/4" wrap of masking tape around the end of the adapter to achieve the same goal, same as you would a motor to keep the adapter from moving up when fitted inside the 29mm main mount. Essentially treating the adapter same as you would a motor in the 29mm motor mount tube.

I never use thrust rings in these types of models. Masking tape is a simple and effective way to build a thrust ring for single use motor flights without a need to plan for motor lengths & placement of internal thrust rings.

I have some SLS kits already prepped using this method in the "Tripoli Louisiana Saturday September 15th Launch Announcement" thread. You can see from the wide variety of my motor selections why I use this method. I can pop any one of these motors out right now, and plug it into any other rocket in that photo without any problems, even the longer Ellis Mountain G35's.

http://forums.rocketshoppe.com/show...756&postcount=2

.
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  #5  
Old 09-15-2007, 12:28 PM
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the masking tape idea works well, tho I don't care for it myself

I like to put a motor block in for the longest motor I plan to use , I have an assortment of spacers in my range box to accomidate whatever I use. just pop one in and Im done.

I'm a little more picky than alot of folks,I like to have a clean looking rocket without the motor sticking way out or a bunch of masking tape . probably because I spend "too much" time trying to make them look good.

I have used hooks for retention but I've found it's important to secure the hook with some wire or a tywrap, with the heavier ejection charges, there's a tendancy to squirt past a non-secured hook.
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Old 09-18-2007, 09:16 PM
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Quote:
Originally Posted by CenturiGuy
Okay, you mid-power guys who use 29mm motors (especially in Semroc Large Scale [SLS] models)...

1) Exactly how long are those motors?
2) Do you friction-fit them, or do you usually have some sort of engine block/thrust ring?


Jay.

Good questions. With 24mm and 29mm motors, there are too many lengths to keep up with. I've simply adopted the HPR approach of using the aft thrust ring and no forward motor block. That way, amost any length (up to some maximum determined by MMT or airframe length) can fit in the rocket without adapters (spacers).

There are several types of thrust rings. The aft closures of the reload cases are bigger around than the motors and thus act as the thrust ring. Aerotech has a glue-on ring with some of their single use motors IIRC. RoadRunner SU motors have one molded into the case. Some folks use a hose clamp. I use the tried-and-true three wraps of 1/4" wide masking tape.

At first, I was skeptical about using tape, but as I gained experience, it became clear that it's well up to the task. If you think about it this way, you'll realize how capable it is. If the rocket weighs three pounds and accelerates at 3g, then the motor must apply 9 pounds of thrust plus an additional amount to overcome drag. Something between 12 and 25 pounds is a good guess. Now, apply three or four wraps of masking tape to the end of the motor being careful to press it in place. Maybe scrape your thumbnail along the tape edge to help it get a good bite. Now, wrap your hand around the motor and try to slide the ring off. You can easily apply many times the aforementioned amount of force without budging the tape. If you can't move it here, it's not going anywhere during the flight.

As for retention, I fly a lot with Dave Schaefer. He's the NAR L3 certification committee chair, and he advocates using friction fit AND positive motor retention. I've started doing that, too. That way, if there is some sort of anomaly, you've got some backup. For example, if the motor fits a bit loose in the tube, and there's some play in the retainer, then at ejection, the motor can begin moving aft and have a head of steam built up when it impacts the retainer. I lost a 24-40 case once that might have been saved if I'd put a little tape on it to make it hard to slide. Plus, some retention hooks can be bent out of the way. Friction fitting diverts the force away from bending the hooks to sliding the hard-to-slide motor thus protecting the hooks.

Here's a pic of a motor with 1/4" masking tape thrust ring. It's brown from the swirling exhaust, but never budged.


Here's a pic of a typical retention setup. That's a 29mm RoadRunner and a 29-38mm adapter. The retainer wire is galvanized 16 or 18 gauge wire with #8 screws. The screws mate to T-nuts mounted thru the aft centering ring.


Here's a different style retainer made from a thin brass plate. This one is not for this rocket, but you get the idea.


I use other stuff - hooks - with reloadable cases, but for single use, there's not much for the hooks to grab, so you need retainers which encompass the motor as the wire and plates do.

HTH.

Doug
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