Motor shift rocket glider

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Jeff Lassahn

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I'm thinking about a glider design where the GC shifts by having the motor move back, like in an Astron Scout:
Screen Shot 2021-05-14 at 5.32.03 PM.png
Screen Shot 2021-05-14 at 5.32.52 PM.png

glider1.png

It seems like it ought to work, but I've never seen anyone actually do this before, which makes me nervous because it seems pretty simple and obvious.
Is this actually a really bad idea?
 

shockie

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I'm thinking about a glider design where the GC shifts by having the motor move back, like in an Astron Scout:
View attachment 464253View attachment 464252
View attachment 464254
It seems like it ought to work, but I've never seen anyone actually do this before, which makes me nervous because it seems pretty simple and obvious.
Is this actually a really bad idea?
RocketGliders have been using motor CG shift since at least the late 60's.....in model rocketry. SO no, it's not a bad idea. It's just a really old idea. Nice design.
 

Rktman

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I'm thinking about a glider design where the GC shifts by having the motor move back, like in an Astron Scout:
View attachment 464253View attachment 464252
View attachment 464254
It seems like it ought to work, but I've never seen anyone actually do this before, which makes me nervous because it seems pretty simple and obvious.
Is this actually a really bad idea?
It's actually a great (and workable) idea. Mine uses a burn string but I can't get it to release reliably (except once; about 10 feet above the ground. Glided fine for all of 9 seconds). 😬 So it's been a work in progress for 5 years. (I've since shortened the boom and replaced the basewood with carbon fiber).
Anyway, following to see how yours progresses. Interested to see if you find a better method to shift the motor/weight.

Twin Boom.JPG
 

Jeff Lassahn

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Cool!
How much distance does your motor move?
I'm thinking about something really simple, the ejection charge forces the engine backwards, and it gets caught by a long retaining hook. Basically identical to the way the old Scouts do CG shift for tumble recovery.

My two concerns are that the engine won't move properly or stay in the proper position during glide, or that the GC shift isn't big enough. I've only got about an inch (2.4cm) of CG change between boost and glide.
 

Rktman

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Cool!
How much distance does your motor move?
I'm thinking about something really simple, the ejection charge forces the engine backwards, and it gets caught by a long retaining hook. Basically identical to the way the old Scouts do CG shift for tumble recovery.

My two concerns are that the engine won't move properly or stay in the proper position during glide, or that the GC shift isn't big enough. I've only got about an inch (2.4cm) of CG change between boost and glide.
Slides about 2", but the front end is kinda heavy because I reused an old nosecone that had epoxy and BBs loaded on the end of the shoulder, and there's a dense balsa grain running through the leading edge of both wings, so I'm not surprised your motor doesn't need to move that far back at all.

I like your idea of just using the ejection charge to kick it backward. I was going to do a larger version using this idea:

Gemini slide pod concept.jpg

But maybe I'm overthinking and over-engineering it when a much simpler solution to keep the motor in place once it slides backward is to just glue on a locking catch made from a piece of bent music wire:

Bent wire catch system.jpg
 
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Rktman

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Oh, and there's no need to use a swept wing. It requires the CG to be further back then with a non-swept wing. The way you have your concept set up is fine.
I only used swept wings because I didn't want to mess with figuring out how to attach the twin booms on a wing with dihedral.
 

mooffle

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Cool!
How much distance does your motor move?
I'm thinking about something really simple, the ejection charge forces the engine backwards, and it gets caught by a long retaining hook. Basically identical to the way the old Scouts do CG shift for tumble recovery.

My two concerns are that the engine won't move properly or stay in the proper position during glide, or that the GC shift isn't big enough. I've only got about an inch (2.4cm) of CG change between boost and glide.
I have a scratch tumbler with shifting cg, there is near zero issue of having motors slide around on you after ejection. I would also be concerned about having enough cg shift, though it probably doesn't take much.
I say make it!
 

Jeff Lassahn

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Well, here it is. A quick toss in the front yard with an expended engine suggests it will glide, so we'll see how it does on a real launch. It built as light as I can make it, so it's pretty fragile and I figure at least a 50:50 chance of significant damage. But the more weight and strength I add the less the CG moves on engine shift. If this boosts straight but dies on landing I might build another one heavier.

DSC_0209.JPGDSC_0208.JPGDSC_0207.JPGDSC_0206.JPGDSC_0205.JPG
 

Rktman

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Well, here it is. A quick toss in the front yard with an expended engine suggests it will glide, so we'll see how it does on a real launch. It built as light as I can make it, so it's pretty fragile and I figure at least a 50:50 chance of significant damage. But the more weight and strength I add the less the CG moves on engine shift. If this boosts straight but dies on landing I might build another one heavier.

View attachment 465081View attachment 465082View attachment 465083View attachment 465084View attachment 465085
Looks cool. But no dihedral in the wings for lateral roll stability?
 

BABAR

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Well, here it is. A quick toss in the front yard with an expended engine suggests it will glide, so we'll see how it does on a real launch. It built as light as I can make it, so it's pretty fragile and I figure at least a 50:50 chance of significant damage. But the more weight and strength I add the less the CG moves on engine shift. If this boosts straight but dies on landing I might build another one heavier.

View attachment 465081View attachment 465082View attachment 465083View attachment 465084View attachment 465085
Love the twin boom with the box tail. Great and relatively light way to avoid singed tail feathers and provide great boost stability!
 
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