Rear Eject Spool Design

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lakeroadster

When in doubt... build hell-for-stout!
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I'm a huge fan of rear eject rocket designs. These are rockets that have a spool that holds the motor and the parachute in place until the motor ejection charge fires off, which pushes the spool out the back of the rocket.

Is there any scenario that you could envision that could cause the spool to fall out of the rocket during coast phase? The spools are friction fit into the rocket.

My Columbine rocket flew and recovered great. But I'm just thinking through this and trying to stave off any possible issues when I launch my P-40

Columbine

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P-40

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I have wondered about this. Have yet to build my first rear-eject but I have had one waiting on the drawing board for quite a while now.

The ejectable spool is not subject to drag separation forces, in fact it would be the opposite (airframe wants to slow down, spool wants to keep going, so it would tend to push in even harder). So I would think that as long as it doesn't fall out when you hold it by the nose, then it's not going to fall out in the air. I wonder if there's any more to it than that?
 
I have wondered about this. Have yet to build my first rear-eject but I have had one waiting on the drawing board for quite a while now.

The ejectable spool is not subject to drag separation forces, in fact it would be the opposite (airframe wants to slow down, spool wants to keep going, so it would tend to push in even harder). So I would think that as long as it doesn't fall out when you hold it by the nose, then it's not going to fall out in the air. I wonder if there's any more to it than that?

You would think since drag forces act on the fuselage, but not the spool, that the fuselage slowing down would actually mean the spool could even be loose, as the fuselage is decelerating faster than the spool.
 
You would think since drag forces act on the fuselage, but not the spool, that the fuselage slowing down would actually mean the spool could even be loose, as the fuselage is decelerating faster than the spool.
Yes, exactly... but there's probably no compelling benefit to making it *loose*. Just a light amount of friction to hold it in against normal gravity would seem to be sufficient, and that amount of friction should not impede ejection. Again, going by intuition here, not experience.

On the other hand there's what @Cape Byron just wrote above. :)
 
Thanks for this thread, I was about to ask someone about making "Bullet Bobby" into a rear eject as he'd look great coming down nose-first!
That would be a good project... it already has a spool style motor mount. I'd add a full-length coupler in the body to ensure fuselage rigidity... and add whatever nose weight Open Rocket says it needs.

Then paint it like Fat Man. Drop the F-Bomb.

https://www.rocketryforum.com/threads/launch-lab-rocketry-bullet-bobby.165277/#post-2116024
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That would be a good project... it already has a spool style motor mount. I'd add a full-length coupler in the body to ensure fuselage rigidity... and add whatever nose weight Open Rocket says it needs.

https://www.rocketryforum.com/threads/launch-lab-rocketry-bullet-bobby.165277/#post-2116024
View attachment 544227
Well, that's my thoughts as well, and also possibly using a 24 mm motor mount - you can always adapt down for small fields. Steve (mfgr) said it was doable, but he didn't say if any mods besides the motor mount tube/centering rings was needed.

Now back to your thread!
 
I guess every rocket is essentially a double spool with fins, a nose cone, tube, and recovery.
 
I should do a rear eject sometime. Maybe I'll put that on my goals for 2023. After looking hard at a mental free-body diagram, I don't see any forces that push the spool out. So at least the theory matches up with experimental test data (Thanks @Cape Byron!). I would probably do as @neil_w says and make it have just enough friction so the pod doesn't fall out on the way to the pad. That's really just so nothing embarrassing happens during RSO inspection and/or you don't have to do a re-pack at the pad. You could just carry it nose down to the pad, but I like to have rockets be at least idiot-tolerant if not idiot-proof.
 
I should do a rear eject sometime. Maybe I'll put that on my goals for 2023. After looking hard at a mental free-body diagram, I don't see any forces that push the spool out. So at least the theory matches up with experimental test data (Thanks @Cape Byron!). I would probably do as @neil_w says and make it have just enough friction so the pod doesn't fall out on the way to the pad. That's really just so nothing embarrassing happens during RSO inspection and/or you don't have to do a re-pack at the pad. You could just carry it nose down to the pad, but I like to have rockets be at least idiot-tolerant if not idiot-proof.

It can be a way to reduce zippering.
 
Poor man’s “Remove Before Flight” tags.

Although I prefer to prep rockets right before launch, sometimes I have a bunch of rockets to fly and a small launch window (or it is really hot or cold or mildly windy, making field packing a PITB.)

I used to use regular masking tape after packing to hold nose cones on as sometimes during transport they come off and undo all my careful comfortable cozy rocket room prepacking. Then at at NSL I had an embarrassing ballistic recovery……. You guessed it, forgot to take tape OFF.

Now I use this stuff, pink masking tape.

https://smile.amazon.com/STIKK-Pain...668535469&sprefix=pink+masking,aps,154&sr=8-3
I leave a “tail” about 4 inches folded back on itself (so tail isn’t sticky) that SO FAR catches my attention as I put the model on the rod to “remove before flight.”

Should work to keep the pod in place from packing to launch rod for rear eject just as well.

Bullet Bobby looks like a rocket begging for rear eject. Going back down memory lane, Estes PortaPot Shot was also a model where rear eject would have been both more practical and perfectly theme oriented.
 
Oddroc and spool flyers are immune to embarrassment. Anyone with the guts to walk up to the RSO table with such abominations is already a lost cause in the eyes of the 3-4FNC, HiPo hipsters...OMG is that a 1/2" rod in your hand and an eight motor clip whip around your neck? Five one Five 0, better call the Po-Po! :)
 
This is my version of the spool. After a bit of experimenting with rear ejection I found that a piston design worked best for me. It ejects very cleanly and never snags. The important thing is to be sure that the piston is sanded down so it runs smoothly. I doubled the wall thickness of the piston and sanded it down quite a bit. It also helps to clean it out every few launches and lubricate it waith talcum powder.

I made this version around 2010 and have lost count of the number of successful launches its done.

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