RMS Motor Block Required?

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doug_kosty

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I was looking at an Aerotech 24/40 casing placed in an appropriate sized motor mount tube. It looks to me as though no motor block should be required since the aft closure is larger in diameter than the motor mount tube. Is this reasonable, or would this place too much stress on the threads of the aft closure?

I am aware of the method of making a motor block by wrapping masking tape around the end of the casing, but I am having a hard time seeing the motor distorting the motor mount tube to any large extent.

Just looking for ways to make things neater and more flexible at the same time!
 

WillMarchant

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That's correct, the aft lip on the motor casing will prevent it from flying up and through your rocket.

I expect that Layne will pop in with a plug for the "Llamas against engine blocks" campaign which seems to have lost some of its momentum in the forum of late... :happydeer:
 

The EGE

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The threads are plenty strong, even on the tiny 24/40 case, to use as a thrust ring. Never put engine blocks on any motor mounts larger than 18mm , otherwise someone will come out with a larger motor that you then can't use :D

Just make sure that the motor mount tube is strong enough to handle the force; this shouldn't be a big deal unless you have a ridiculously thin motor mount tube and a high-thrust motor.

Also, make sure to include positive motor retention - either a mechanical device like a thread rod with a nut and washer, or a threaded retainer like Aero-pac. Even wrapping masking tape around the end of the motor mount and the aft closure is sufficient for 18mm-small 29mm cases.

Take good care of the case and don't lose it; it's a lot of fun.
 

ScrapDaddy

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Acctually the Apogee D3 (OOP) will be back soon for 18mm and it is 7 mm longer than standard so dont put and engine block in period! :neener:
 

doug_kosty

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Also, make sure to include positive motor retention - either a mechanical device like a thread rod with a nut and washer, or a threaded retainer like Aero-pac. Even wrapping masking tape around the end of the motor mount and the aft closure is sufficient for 18mm-small 29mm cases.

Take good care of the case and don't lose it; it's a lot of fun.
Learned this lesson already - first composite launch the casing ejected and nearly hit a spectator. I was so excited that I didn't notice that - fortunately the casing missed, and a friend retrieved it from the sand.
 

SCE to AUX

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Does anyone sell a 24mm version of the Aeropac or HAMR type retainer? Smallest I have seen is 29mm.

Some Estes rockets use a plastic bayonet locking retainer setup. These DO NOT work with RMS casings unless you modify the retainer cap. Because of the thrust ring, the cap needs to have the ID bored out about .070". Easy to do if you have a lathe, but kind of a tricky job without one. Fortunately, the caps will still work with BP motors after modification.

I've had poor luck with the standard Estes metal engine hook when using the 24/40 RMS case. The ejection charge on reloads seems a bit more potent than the Estes BP motors, and I have had the motor kick out at ejection a couple times. I will say that the brightly colored Rouse-Tech cases are easier to find on a grass field than the black Aerotechs...:eek:

A small nylon wire tie (4" length), secured around the MMT and engine hook just above the thrust ring beefs the hook up enough to prevent it deflecting out of the way under the ejection force.
 

ScrapDaddy

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I've had poor luck with the standard Estes metal engine hook when using the 24/40 RMS case. The ejection charge on reloads seems a bit more potent than the Estes BP motors, and I have had the motor kick out at ejection a couple times. I will say that the brightly colored Rouse-Tech cases are easier to find on a grass field than the black Aerotechs...:eek:
You do realize rouse tech MAKES areotechs hardware
 

SCE to AUX

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You do realize rouse tech MAKES areotechs hardware
Yes, I do.

You do realize that the Aerotech-branded ones are anodized black, while the Rouse-Tech branded ones are gold or blue, right?

The Rouse-Tech closures get a heavier (clear) anodize than the cases, and they seem to hold up a bit better after repeated use.
 
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The EGE

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Acctually the Apogee D3 (OOP) will be back soon for 18mm and it is 7 mm longer than standard so dont put and engine block in period! :neener:
Where are you getting this information from? I've heard nothing about the return of the D3... :confused:
 

Handeman

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Does anyone sell a 24mm version of the Aeropac or HAMR type retainer? Smallest I have seen is 29mm.

Some Estes rockets use a plastic bayonet locking retainer setup. These DO NOT work with RMS casings unless you modify the retainer cap. Because of the thrust ring, the cap needs to have the ID bored out about .070". Easy to do if you have a lathe, but kind of a tricky job without one. Fortunately, the caps will still work with BP motors after modification.

I've had poor luck with the standard Estes metal engine hook when using the 24/40 RMS case. The ejection charge on reloads seems a bit more potent than the Estes BP motors, and I have had the motor kick out at ejection a couple times. I will say that the brightly colored Rouse-Tech cases are easier to find on a grass field than the black Aerotechs...:eek:

A small nylon wire tie (4" length), secured around the MMT and engine hook just above the thrust ring beefs the hook up enough to prevent it deflecting out of the way under the ejection force.

What he says. If you have a question about the wire tie, here's a pic of the retension using a piece of copper wire. Same thing.

 

SCE to AUX

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I usually install the wire tie ABOVE the thrust ring, about where the yellow/black banding is showing on your casing. I wouldn't trust a nylon tie not to melt that close to the exhaust.

The copper wire trick looks just as good, though...
 

Crazyrocket

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Instead of a wire tie, you can also just use a few wraps of masking tape to accomplish the same thing. That's all I've ever used and never had a problem. Use whatever method is easiest for you.
 

Pem Tech

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That's correct, the aft lip on the motor casing will prevent it from flying up and through your rocket.

I expect that Layne will pop in with a plug for the "Llamas against engine blocks" campaign which seems to have lost some of its momentum in the forum of late... :happydeer:

Well, since you mentioned it....

LAME (Llamas Against Motor-blocked Engines) is formally protesting the use of motor blocks in all model rockets. They are unsightly (even if you can't see them), restrict the over all length of motors you can use, are a bugger to remove once you realize your mistake, cause hair loss and gas, and are just all around naughty on general priciples.
:bangbang:
(Death to Motor Blocks)
 

Pem Tech

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Is there a petition I can sign somewhere;)
We'll send you one...
Sealed with Llama spit.

But seriously....
If your nerve can stand it, don't use a motor block, you will regret it.
RMS cases and some single use motors (AT, CTI, and Road Runner) have built-in thrust rings at the aft of the motor. For those that don't, use a few wraps of masking tape for a thrust ring and a wrap or two to hold it in place during ejection. Personally I have been using that method for years and have yet to lose a casing. The added flexibility is well worth the expenditure of masking tape.
 
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