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Short motor mount!

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DexterLB

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I got my Zafod out of a closet... And I discovered that there isn't a motor block ring. So I decided to fly it. I measured the motor mount... It's exactly 70mm. There's not enough space to fit a motor block for an US motor, like the ones I've ordered.

I've built the motor mount for the 50mm long Bulgarian motors. So now I can't figure out how to mount the motor inside. The centering rings are stuck with epoxy, so there's no way of removing the motor mount. Is there any hope for this rocket?
 

adrian

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The motor is not supposed to be completely inside the mount tube. If you take a 70mm long tube and glue a 5mm thick blocking ring into the front end, a 70mm long motor will then stick out of the back end by 5mm. This is good - it gives you something to grab hold of when you want to remove the expended motor.

However far your 50mm motor sticks out of the back of the mount, a 70mm motor will stick out by 20mm more. This is not necessarily a problem, so long as there is enough of the motor inside the tube to be secure. Also, with the motor further back, the CG will be slightly further back as well. Check that the rocket is still stable, either by software or by swing test, and add a little nose weight to put the CG back where it should be if the rocket is unstable.
 

jflis

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I got my Zafod out of a closet... And I discovered that there isn't a motor block ring. So I decided to fly it. I measured the motor mount... It's exactly 70mm. There's not enough space to fit a motor block for an US motor, like the ones I've ordered.

I've built the motor mount for the 50mm long Bulgarian motors. So now I can't figure out how to mount the motor inside. The centering rings are stuck with epoxy, so there's no way of removing the motor mount. Is there any hope for this rocket?
If I understand what the problem is...

Place several wraps of 1/4" masking tape around the nozzle end of the motor casing. This will function as a motor block, well more of a "reverse motor block"... The motor will still extend past the motor tube, but that will be inside the model where you really don't care...

jim
 

DexterLB

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The motor is not supposed to be completely inside the mount tube. If you take a 70mm long tube and glue a 5mm thick blocking ring into the front end, a 70mm long motor will then stick out of the back end by 5mm. This is good - it gives you something to grab hold of when you want to remove the expended motor.

However far your 50mm motor sticks out of the back of the mount, a 70mm motor will stick out by 20mm more. This is not necessarily a problem, so long as there is enough of the motor inside the tube to be secure. Also, with the motor further back, the CG will be slightly further back as well. Check that the rocket is still stable, either by software or by swing test, and add a little nose weight to put the CG back where it should be if the rocket is unstable.
I'll do that. Thanks for replying.

If I understand what the problem is...

Place several wraps of 1/4" masking tape around the nozzle end of the motor casing. This will function as a motor block, well more of a "reverse motor block"... The motor will still extend past the motor tube, but that will be inside the model where you really don't care...

jim
That's also a solution, but I think in this case the first one is better. :)
 

rocket9005

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Don't sell the masking tape on the outside on the motor short. I've used in on a Loc Mini-Mag with a G125 motor and it worked ! This way you are not limiting yourself to one size for type of motor.
 

jflis

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I'll do that. Thanks for replying.



That's also a solution, but I think in this case the first one is better. :)
The one advantage to using the tape-on-the-end-of-the-motor method is that you don't have to worry about stability anywhere near as much. fwiw :)
 

DexterLB

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The one advantage to using the tape-on-the-end-of-the-motor method is that you don't have to worry about stability anywhere near as much. fwiw :)
Yep, I noted that. But this rocket is waaaaaaaaaaaaaaaay stable! It has enormous fins.

Later I will post the VCP .cp file and an image. (When I find the file...)
 
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adrian

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Just to clarify, what is the exact problem? Presumably one of:

1. You cut the 70mm motor mount to be shorter in order to take 50mm motors, and now it's too short for 70mm motors;
2. You glued a motor blocking ring into the motor mount so that 50mm motors wouldn't disappear inside, and now 70mm motors stick too far out of the back.

Jim's solution works well for 1. Mine is for 2.
 

Pem Tech

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Don't sell the masking tape on the outside on the motor short. I've used in on a Loc Mini-Mag with a G125 motor and it worked ! This way you are not limiting yourself to one size for type of motor.

That is correct...
I use tape thrust rings and tape "friction" retention on about everything I fly. Only one motor casing that has popped free, that was a 29-40/120, which was shortly recovered. This was caused by the arch-enemy of tape, Evil Humidity.
*dramatic music*
:y:

This has been the only incidence of tape retention failure I have experienced, for motors up to and including AT H180.
Never really understood why so many people insist on stopping up a perfectly good motor tube with a thrust ring. It limits the max length of the motors you can use in that bird and offers no significant advantages over a tape ring.

And your motor does not have to be completely encased in the motor tube. Sometimes you just gotta' let it all hang out...
 

DexterLB

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Just to clarify, what is the exact problem? Presumably one of:

1. You cut the 70mm motor mount to be shorter in order to take 50mm motors, and now it's too short for 70mm motors;
2. You glued a motor blocking ring into the motor mount so that 50mm motors wouldn't disappear inside, and now 70mm motors stick too far out of the back.

Jim's solution works well for 1. Mine is for 2.
Neither!!! :D
I had started the rocket, almost finished it. The motor mount is 70mm long - that's NOT enough for a 70mm motor, because it will stick 5mm out of the motor mount if I put a 5mm block-ring. I had planned to put a motor block at 50mm, and put a 50mm motor, but then they stopped production.

So, now if I put a 5mm block-ring at 65mm, the hook won't be able to hold the motor from kicking out at deploy, because it's too short. :(

So I'll use some tape round the motor to make a tight fit.

You see, the problem is far more insegnificant :D
I just posted that thread to see if there's another solution except tape. Sorry for not being clear in my first post.
 
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Bazookadale

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Neither!!! :D
n.

So, now if I put a 5mm block-ring at 65mm, the hook won't be able to hold the motor from kicking out at deploy, because it's too short. :(
Is the hook 70mm ( maybe 70.5)? If so mount the front end of the hook in front of the block, not behind it!
 
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