38mm Motor Mount length?

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Charles_McG

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Is there a recommended length for motor mounts - L:D, or otherwise spec'd?

I'm working on a 1/5.2 scale Sandhawk and I need to cut the motor mount tubing - but nothing about the design jumps out to suggest a length.

Is 5:1 (7.5") reasonable? Is there a rule of thumb for this kind of thing?
 

jmasterj

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What motors do you want to put in it? I don't know what size body tube you're dealing with, but making the MM longer makes the volume smaller for ejection charge purposes. As long as there's still room for your laundry I don't think you can go too long.
 

Jmhepworth

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The motor mount does not need to be as long as the motor. You can have an 8-10” motor mount and still use the long 1080 case. As pointed out, the only limitation on the size of the motor is whether the recovery stuff will fit.
 

Zeus-cat

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I don't think the ratio of the motor mount to the rocket is important. You need the motor mount to hold the motor securely and be centered properly.
 

Charles_McG

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Which motors? Likely 29mm in an adapter- so the small end of the range would be just longer than the adapter.

But if I fly it as a Dualhawk (Sandhawk-Tomahawk) then it would probably get a 2 grain 38mm motor.

Most of my builds have had something that made the choice of length easier - one tube length or fin tab length just suggested how to go. This one is more open ended.
 

boatgeek

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Which motors? Likely 29mm in an adapter- so the small end of the range would be just longer than the adapter.

But if I fly it as a Dualhawk (Sandhawk-Tomahawk) then it would probably get a 2 grain 38mm motor.

Most of my builds have had something that made the choice of length easier - one tube length or fin tab length just suggested how to go. This one is more open ended.
For that motor (and honestly, probably most of the 38mm line), you'd be fine with 6"-8". I think one could make a very strong engineering case that any MMT forward of the fin tabs has minimal benefit for holding the motor straight.
 

afadeev

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I don't think the ratio of the motor mount to the rocket is important. You need the motor mount to hold the motor securely and be centered properly.
Exactly THAT.
The only OTHER consideration is the role your motor mount tube (MMT) may play in securing the centering rings (CRs) forward and aft of the fins. Especially if you plan to inject internal fin fillets.

On larger rockets that warrant internal fin fillets, it is helpful to use CRs to create "dams" blocking the flow of injected epoxy on both sides of the fin tabs. Thus, you may choose to size your MMT to be of the length that accommodates that function.

Lastly, on larger diameter airframes (5+"), you may want to build an extra long MMT to minimize the volume that will need to be pressured by the ejection charge. In that case, your forward CR location may be chosen much further inside the airframe just so that the standard 1-2gram ejection charge will be sufficient, and then the MMT length will be extended to reach that FWD CR.

a
 
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Rocketjunkie

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Long enough to hold the motor straight. As a practical matter, long enough to put a centering ring at the front of the fin tabs.
 
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