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DrewW

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I've been messing around with this design for the past several weeks and finally did the work to build it...since Arizona has been afflicted by winter this week, I'm thinking it wont get past the primer stage for a few more days. But for now, it looks good in OR, or at least I think so.

First look at the "Mob Missile"

Mob Missile v2.JPG
 

DrewW

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It strikes me that I didn’t bother to explain this at all. My bad. Was playing around with some ideas in my head related to stiffening a tube like structure for higher loads when I realized under the tension of flight string (200# Kevlar in this case) could be used to create a floating ring tail; that is, rigid under thrust but a spaghetti noodle otherwise. Ended up using the case from a lovely bottle of single malt for an extra stiff ring.

While I’ve been messing with it both in OR and I happened to get curious about a behavior I saw where a small bird will chase off a larger bird protecting territory and nests. This is called mobbing, the “mob missile” came out of that idea of the smaller object chasing the larger object and lent to the color scheme and hints at feather-like fins on the ring.

I have one built just as modeled I’m planning to check fly this weekend, before dressing up, but if anyone has any suggestions for a version 2 have at it.
 

BABAR

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Hmmmmm. I figured the lines were carbon fiber rods. So alignment was fixed

I like it with firm fixation.

just hanging on....not so sure.

@Daddyisabar did a Tapeworm with drag tentacles




things that MIGHT get this to work.

very long rod.

set up on rod so that the lines are already taught and ring is definitely oriented from the get go.

possible a lug on the ring to keep it straight on the rod, but you then need a stand-off on the body tube, and probably a matching fake Stand-off on the opposite side to keep it balanced. Kinda messes with the “no fins” on the tube look.
 

BABAR

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In engineering, you can’t push on a rope. In medicine, we do it all the time with catheters, but they are stiff and they are constrained inside tubular structures (usually vessels or gastrointestinal or urinary or biliary segments) so we “cheat.” Even then, it’s challenging, often as much or more art than science.
 

DrewW

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Hmmmmm. I figured the lines were carbon fiber rods. So alignment was fixed

I like it with firm fixation.

just hanging on....not so sure.

@Daddyisabar did a Tapeworm with drag tentacles




things that MIGHT get this to work.

very long rod.

set up on rod so that the lines are already taught and ring is definitely oriented from the get go.

possible a lug on the ring to keep it straight on the rod, but you then need a stand-off on the body tube, and probably a matching fake Stand-off on the opposite side to keep it balanced. Kinda messes with the “no fins” on the tube look.
After stringing the uppers which route through a brass ferrules which are flared toward the empty space. I marked my lines at a consistent 8.5” then after assembling the ring and anchoring in the ferrules, I tied off....and discovered it was a lot harder to set alignment on the ring than I anticipated. I lost about half an inch to additional knots balancing out the ring. End result, I’m off center by less than half an 18mm motor case at 8”, so about 2.5 degrees...good enough for first effort. Great idea on the jig though, if i build another I’ll definitely be taking the approach whether making it rigid or not.
 

DrewW

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In engineering, you can’t push on a rope. In medicine, we do it all the time with catheters, but they are stiff and they are constrained inside tubular structures (usually vessels or gastrointestinal or urinary or biliary segments) so we “cheat.” Even then, it’s challenging, often as much or more art than science.
Anyone who cheats the “rules” in physics and engineering with enough understanding to know why it’s totally fine is okay in my book.
 
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jqavins

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Hmmmmm. I figured the lines were carbon fiber rods. So alignment was fixed

I like it with firm fixation.

just hanging on....not so sure.
My thoughts almost exactly. The only difference is that with firm fixation I really like it. Hanging on strings I don't think the fin section can transfer any restoring torque to the drive section. I'll be tickled pink if I'm wrong.

Using CF rods I might even try it myself. But I say that about a lot of cool ideas that I never wind up doing.
 

David Schwantz

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Carbon rods will still flex. The only way my mindsim can see it is with cross pieces between all the rods. A lattice work, if you will.
 

neil_w

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The drag of the tail ring thing should keep the string taut under thrust, and at least somewhat taut during coast. My mindsim says it'll work, to say nothing of the similarity with working oddrocs by @Daddyisabar.

That said, I don't wanna be the RSO when someone brings that to the table. :)
 

jqavins

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Flexing carbon rods takes a nontrivial force, so even if they wind up bent out of alignment for a quasi-static moment, they're still transferring torque. Are they stiff enough? Well, they're certainly way ahead of string, but there's only one way to really find out. (OK, yeah, it's calculable. But I'm not going to do it. I have only one way.)

More* of a truss structure would probably be needed if the size is increased. Come to think about it, Drew, what is the size you've designed there? It would need a truss if it gets much bigger than the version in my head, which has a BT-50 or -55 body tube. If I were to do it on 3 inch scale I'd expect to need more truss members, and probably before that (BT-80?). But it might not even then if the rod diameter grows in proportion; stiffness goes up with the 3rd (or is it 4th) power of thickness. And there mustn't be too many members or it might risk interfering with air flow through the ring.

* Strictly speaking, it's already a truss, just a really sparse one.
 

DrewW

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My hypothesis under test here is that the ring has enough drag/mass to induce adequate tension in the Kevlar strands. The strands themselves are in a flared ring truss configuration which should be very rigid with sufficient tension and provide three points of contact at each body element ideally constraining both the body and ring. When appropriately constrained (ideal not required) restoring forces should pass both ways along the truss.

while I’m greater than 80% confident this will work under thrust. I have concerns in coast to apogee. But that shouldn’t be dangerous to anything but the rocket.

I was going to fly this on my own before taking to a club launch but I need to build a launch controller that can manage the cluster before doing that.
 

DrewW

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Flexing carbon rods takes a nontrivial force, so even if they wind up bent out of alignment for a quasi-static moment, they're still transferring torque. Are they stiff enough? Well, they're certainly way ahead of string, but there's only one way to really find out. (OK, yeah, it's calculable. But I'm not going to do it. I have only one way.)

More* of a truss structure would probably be needed if the size is increased. Come to think about it, Drew, what is the size you've designed there? It would need a truss if it gets much bigger than the version in my head, which has a BT-50 or -55 body tube. If I were to do it on 3 inch scale I'd expect to need more truss members, and probably before that (BT-80?). But it might not even then if the rod diameter grows in proportion; stiffness goes up with the 3rd (or is it 4th) power of thickness. And there mustn't be too many members or it might risk interfering with air flow through the ring.

* Strictly speaking, it's already a truss, just a really sparse one.
I can’t recall at the moment, but I believe I used a BT-55 for the main body. And I used a 3.75”heavy walled ring that used to be a scotch bottle sleeve.

In this case, the truss is analogous to those used for telescopes which are very very stable. Key difference is under compression the rods need to be thicker to minimize bending; this version is in tension and Kevlar as I recall doesn’t have much stretch.
 

Daddyisabar

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Like C3PO said in the the movie "This is such a drag!" Any draggy bits in back, flexible or not, will help. The more flex, surface area, mass, the wilder the ride! More power needed. FEEL THE BURN! If it goes squirrely then hopefully it has some altitude for sky writing. Punch it up there, trust in quickly applied thrust. Tell them you are using Gas Dynamic Stabilization and that your are a rocket scientist! A mindsim is a terrible thing to waste. :)
 

David Schwantz

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What are all the little round things that look like train wheels?

Like the rocket, by the way :)
 

David Schwantz

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Damn, and I thought I had drank too much coffee :)

I'll go and look for the thread. Dave.
 

K'Tesh

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This could be really interesting...
 

OverTheTop

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Nice design. I think the fluid dynamics might bite. Fast flow equals low pressure, so the trailing ring may get sucked towards the exhaust.

Looking forward to the flight :).
 

K'Tesh

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Nice design. I think the fluid dynamics might bite. Fast flow equals low pressure, so the trailing ring may get sucked towards the exhaust.

Looking forward to the flight :).
Perhaps a boat tail might help
 

jqavins

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I feel rather hypocritical urging @Senior Space Cadet not to launch his dirt dive recovery rocket, even on a deserted soccer field miles from anyone, while saying this looks like a cool experiment and eagerly waiting for the results. I assume you've got a 'chute or streamer in there, but that's not the point. It looks potentially dangerous, to a significantly greater (if not very large) degree than most of the rockets any of us launch.

You're certain that tension in the strings keep them rigid enough to transfer force to the body, but I confess I'm skeptical (not that I'm anywhere near sure they won't). You're not so sure it will work that way after burnout, and I have very little doubt on that part; it won't. (You aptly point out that at that point it's a hazard only to the rocket, assuming a 'chute or streamer deploys.)

Still, because of the substantially larger than normal chance of totally unpredictable behavior immediately upon leaving the launch rod, please don't launch this anywhere near other people.

Do you plan to prop the drive section up on the rod so that the ring is dangling, i.e. the strings start out straight?
 

DrewW

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I feel rather hypocritical urging @Senior Space Cadet not to launch his dirt dive recovery rocket, even on a deserted soccer field miles from anyone, while saying this looks like a cool experiment and eagerly waiting for the results. I assume you've got a 'chute or streamer in there, but that's not the point. It looks potentially dangerous, to a significantly greater (if not very large) degree than most of the rockets any of us launch.

You're certain that tension in the strings keep them rigid enough to transfer force to the body, but I confess I'm skeptical (not that I'm anywhere near sure they won't). You're not so sure it will work that way after burnout, and I have very little doubt on that part; it won't. (You aptly point out that at that point it's a hazard only to the rocket, assuming a 'chute or streamer deploys.)

Still, because of the substantially larger than normal chance of totally unpredictable behavior immediately upon leaving the launch rod, please don't launch this anywhere near other people.

Do you plan to prop the drive section up on the rod so that the ring is dangling, i.e. the strings start out straight?
So I read your post and thought about it, read it again, thought some more, checked open rocket, and did some math before responding.

Thankfully I live in a relatively low density suburb, in the deserts of Arizona so it isn't hard to find places away from activity. I think the comparison to senior space cadet's project is valid in some ways but not all ways. Given my rocket isn't stable in flight it will likely spin out of control dissipating energy, whereas, in his project he has a spin stabilization built in meaning it will be a ballistic projectile intending to strike the ground. Under normal range conditions this can be safe as long as he has a ~10º rod angle and a spotter who is keeping the downrange corridor under control, I don't have a problem with his experiment though I feel like I know what the answer is going to be without running the experiment at all.

OR estimates a peak altitude of 120ft on a A8 cluster and 190ft on a B6 cluster, which makes me feel confident a 1000ft box should be way more than sufficient (I'll go with the MPR safety box) in case things go sideways...literally I guess. Taking weight and max velocity into account this rocket packs as much punch as well...a punch from an slightly better than average person.

After ejection charge, and separation (shoot or no) the dynamics will bring terminal velocity well short of anything dangerous. It's safely getting to the separation event I have my eye on.
 

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