You're Gonna Like The Turbo: A spinning 2-stage

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boatgeek

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This design appeared first in the Half Baked Designs thread. The concept is a booster with canted fins that spins up during boost, then throws a finless sustainer on to the heavens. 24mm to 18mm with a small (~1") gap stage, likely D12-0 to A8 for the first flight. Despite a great deal of input in the half-baked designs thread that a built up balsa system would be lighter than 3D printed, I liked the ease of 3D printing and I'm willing to accept the loss of performance. The sustainer nose will also be moderately heavily weighted to keep the whole stack stable.

Open questions:
Will the spin be enough to keep the sustainer stable? If not, I'll add canted fins.
Will the booster tumble enough (probably) and have the right drag to weight to tumble recover at a safe speed?

First flights will be at a private launch with heads up on a soft grass field. If it all works then I'll go more aggressive.

I'll add some pics in a day or two (hopefully not boatgeek time!), but you can see a model of the turbo booster in the linked thread above.
 

jqavins

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1) "Will the spin be enough to keep the sustainer stable?" I have a finless, spin stabilized model I've build and never yet simultaneously had the opportunity and guts to try out. It uses a two engine cluster in a twisted motor mount assembly to make the spin and will be launched from a tube. And I keep going back and forth on the likelihood of success. On the one hand, spinning provides gyro stabilization, which resists turning, but it doesn't provide any restoring force when some turning occurs anyway, so it shouldn't work. On the other hand, this is exactly how bullets work, and they fly straight. We'll both just have to try it.

2) "Will the booster tumble enough (probably) and have the right drag to weight to tumble recover at a safe speed?" I bet it will come down as a spinning saucer, which will make for a plenty soft landing and be really cool too. And if not, based on the picture you posted in the Half Baked Ideas" thread, I'm comfortably confident it will tumble at a safe speed. (But then, with that super heavy 3D printed stuff it might weigh a couple of tons, so one never knows. ;) )
 

boatgeek

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A review of the principles of bullet spin stability might be worthwhile: https://bisonballistics.com/calculators/stability

They say I'm not stable (and who am I to argue? :D), but this projectile is also well outside their assumptions. It's far longer, turns more slowly, and also is weighted toward the front.

If on was to place a small conical tail on the aft end of the spinning sustainer

The sustainer is minimum diameter, so I don't have any tailcone to play with unless I glued something to the back of the motor (not doing that).
 

jqavins

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The sustainer is minimum diameter, so I don't have any tailcone to play with unless I glued something to the back of the motor (not doing that).
But it is a nifty idea for a single stage min diameter. Is that original, or am I just ignorant?
 
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BABAR

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1.Does sustainer have a recovery system?
I ask because while it is relatively easy to build a symmetrical cylinder and nose cone, it may be tough to load a chute or streamer that is radially symmetrical from a mass perspective to the long axis of the rocket spin, so it may wobble. Think a potters wheel with the lump of clay off center. Spin it fast enough and something tears loose. The faster it spins the more likely even a slight eccentricity can throw it out of wack. A long coupler run the length of the tube will give you extra mass symmetrically on the outer diameter that will HELP to inertially maintain spin, at a cost of extra net mass and less room for recovery gear.

Does it have a launch lug?
I am guessing no. If so, it has the problem above as a radially asymmetric mass option AND. a lot of drag to rotation. Two small lugs will be better than one large lug for both of the above purposes, although will cost you more parasitic drag (I think that is the right term) on boost, likely negligible for your purposes.

you probably have already seen this


remove the xx’s to get it link, otherwise I can’t get link to work

xxhttps://www.flickr.com/photos/oflittleinterest/9108286136/in/photostream/


 

boatgeek

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1.Does sustainer have a recovery system?
I ask because while it is relatively easy to build a symmetrical cylinder and nose cone, it may be tough to load a chute or streamer that is radially symmetrical from a mass perspective to the long axis of the rocket spin, so it may wobble. Think a potters wheel with the lump of clay off center. Spin it fast enough and something tears loose. The faster it spins the more likely even a slight eccentricity can throw it out of wack. A long coupler run the length of the tube will give you extra mass symmetrically on the outer diameter that will HELP to inertially maintain spin, at a cost of extra net mass and less room for recovery gear.

Does it have a launch lug?
I am guessing no. If so, it has the problem above as a radially asymmetric mass option AND. a lot of drag to rotation. Two small lugs will be better than one large lug for both of the above purposes, although will cost you more parasitic drag (I think that is the right term) on boost, likely negligible for your purposes.

you probably have already seen this


remove the xx’s to get it link, otherwise I can’t get link to work

xxhttps://www.flickr.com/photos/oflittleinterest/9108286136/in/photostream/




The sustainer will have streamer recovery. I'm thinking this is the best shot at radially symmetric weights since I can pack it into a relatively homogeneous cylinder so that it fills the inside of the tube. The motor block and shock cord are already installed, so I don't think I'll be able to add a full length coupler at this moment.

The booster will have a launch lug, but not the sustainer. If necessary, I'll put a second lug on the booster opposite the first, but I think that will be less of an issue because the booster has so much more inertia.

One more challenge is making sure that the sustainer sits as perfectly straight in the booster as possible. I'm not 100% sure how to manage that, but it will probably involve some fiddling. This will definitely be an "earn your paint" scenario. I expect to need to make a bunch of tweaks as we go down the road and see how it flies.
 

jqavins

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The sustainer will have streamer recovery. I'm thinking this is the best shot at radially symmetric weights since I can pack it into a relatively homogeneous cylinder so that it fills the inside of the tube.
You could use break-apart recovery, like the Whacky Wiggler.
The motor block and shock cord are already installed...
Oops, maybe it's too late.
One more challenge is making sure that the sustainer sits as perfectly straight in the booster as possible. I'm not 100% sure how to manage that, but it will probably involve some fiddling.
The obvious answer is usually a long coupler, but with the sustainer being minimum diameter that's off the table. You could use a sleeve on the booster that the sustainer fits into. That can go as far up the sustainer body as you want, since there are no fins in the way.
 

shockie

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They say I'm not stable (and who am I to argue? :D), but this projectile is also well outside their assumptions. It's far longer, turns more slowly, and also is weighted toward the front.



The sustainer is minimum diameter, so I don't have any tailcone to play with unless I glued something to the back of the motor (not doing that).
You could do a 13mm bullet with a bt 5 to 20 shroud to the 24 mm booster .

There's an old article about that shrouded rocket body . I'll see if I can find it.
 

boatgeek

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BABAR

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For alignment, make the 3D printed booster forward inside diameter 0.5 mm larger than the sustainer outside diameter. The full sustainer tail nests inside the booster.

The may be a case where no gap staging may help. If motors are tape and friction fit, the booster and sustainer will rotate together.

If not, and the coupling is rigid to lateral bending movement (essential) but loose to longitudinal movement (also essential as you WANT them to separaye) it MAY also be loose to rotation if you gap it.

Thus your booster may be spinning like mad with minimal rotation on sustainer.
 

mooffle

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The booster will have a launch lug, but not the sustainer. If necessary, I'll put a second lug on the booster opposite the first, but I think that will be less of an issue because the booster has so much more inertia.

Looking forward to seeing this plan in action. I'm curious about this point though, when you say opposite are you going to use two launch rods on either side of the tube? That would keep it from spinning until off the pad but then you have to build a whole new pad.
 

jqavins

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I'm struggling with the launch rod in a few ways. With a single rod, the rocket would not only spin, but that would force it to orbit the rod, so the rod would be whipping around in a counter circle; the rocket would experience both rod whip and it's own tangential momentum when it reaches the end. There are plenty of spinning designs that don't seem to suffer from this. Is this one's spin enough greater to cause a problem?

With two rods, the spin would be suppressed, but the rods would flex and, again, nasty rod whip. and a lot of friction where the launch lugs are pressed against the rods. Also, you need it to spin up before exiting the rod for the spin stability to be working; I guess I should have led with that one.

Tube launch might work, but there might not be sufficient length of engagement between the booster ring and the tube wall to keep it straight.

The ideal would be a single rod up the center, but the engines are in the way, booster and sustainer both.

Motorized GSE to spin it up before ignition, and then the rocket going up with no launch guide at all might work. The igniter circuit would have to go through slip rings, but that's doable. You'd need some sort of hold down/release mechanism to keep it from lifting like a helicopter as it spins up; synchronizing the release and ignition would be a major challenge. I think I've gone into whacky territory.
 

neil_w

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Spin rockets of various types are launched all the time off single rods without a problem. Why do we think there would be a special problem here?
 

mooffle

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Also, you need it to spin up before exiting the rod for the spin stability to be working; I guess I should have led with that one.
I think this is not a concern here. The rocket is stable off the pad without spinning right? The spin is only needed at separation to keep the sustainer stable
 

jqavins

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At first I was going to write "No, Boat Geek's project here is all about the spin", but then I realized that the pinwheel that makes it spin is also a ringtail. So you're right. I retract the comment.
 

boatgeek

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I've done a couple of other spinning rockets that flew fine off the pad as long as their fins were large enough to make them conventionally stable. I didn't notice any objectionable amount of rod whip--I'd guess that inertia prevents too much spinning while it's still on the rod.
 

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With your ring fin, you could consider a tube launch, something along the lines of a mortar or bazooka. Not sure how much a piece of PVC pipe with an internal diameter slightly larger than the external diameter of your ring would be.
 

boatgeek

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So I've already glued in a motor block into the sustainer, as well as the shock cord mount. That's all pretty standard stuff.

I wanted to keep the sustainer body fairly short so that it's less likely to have weirdness around not being exactly centered. That means I need quite a bit of no-good performance-robbing nose weight to balance it out. Here's how that went down. I cut some scrap copper house wire into small pieces that I could stuff into the nose cone. Then I made a small shock cord hard point out of a paper clip since I didn't trust the plastic loop under the extra weight. After that, I injected epoxy into the hole with a small syringe. There were a couple of flaws in this plan:

My syringe was a hair too big, so I couldn't direct the epoxy through the hole and then down into the nose cone. I kind of came in through the side of the plastic loop. That made it really hard to get the epoxy down where I needed it to be, even though it was laminating epoxy. It also didn't vent well, so the epoxy going in was fighting the air coming out. I hand-drilled a small hole in the back of the nose cone midway through the process. That helped, but it wasn't ideal. So here's the finished nose cone, propped up in a handy spent E motor:

IMG_3215.JPG

And here's a picture of the full stack. As the epoxy was curing, the launch lug drifted under surface tension to be right up next to a fin. I may glue on another one on the opposite side if it seems to make a difference in the flights. I also used a bit of epoxy to reinforce the bottom of the sustainer tube.

IMG_3217.JPG

God willing, the creek don't rise, my package arrives from BMS, and I can get the last bits and bobs done, it might fly on Sunday.
 

boatgeek

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How thick did you make that ring and fins? :shocked:

Too thick and heavy*. That's something that I always do when 3-D printing stuff. I'm trying to back off, but it's hard to break bad habits. I deliberately made the fins thicker so that they'd have a rounded leading edge that was less likely to stall, but it was probably still too much.

* 4mm ring, 6mm max fin thickness. It's got pretty light infill, but the fin can is still too heavy. Bulletproof, but too heavy.
 
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