3/4 Scale Nike-Designing a cluster with max possible motors to fit

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DannyB

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Hi all, I just bought a 3/4 Scale Nike Smoke from Performance hobbies. It’s 11.5” external diameter. Let’s say that leaves 11” inside to load motors. I added 3ft to the airframe bc it was otherwise too short to carry main chute with a 48” motor packed inside a 60” airframe, but that’s off topic (17ft total after extending). My question is - how much is too much, how much spacing in between motors is too small. Id like to do central 98, 4x75, and 4x54. The 54s would be offset to the side of the 75 at the outer perimeter, leaving just enough room on one side for the fin to pass through and connect to all three motors surrounding/butting up against it. There’s just enough room for the motors to fit and the thrust rings not to touch. In general, is this enough? Is there a rule of thumb? The centering rings are not fiberglass either, they are 1/2” birch. Does that make a big difference in this situation? I will likely reinforce them with fiberglass laminate. I’m working on uploading pics now but having trouble. I have to get ken my cluster design this week so he can cut them, so any help/pointers are greatly appreciated. I did poke around but can’t find anything on the subject, it’s tough to search it bc the words are used in most threads. Thanks in advance for any help!
 
Assuming you're putting tubes in all those holes ( not just naked rings ) I would think the motor mounts themselves would add quite a bit of stiffness to the array.

I am no expert, nor do I play one on TV.
 
Interesting point D. I didn’t think of that because I assumed lateral strength benefit only from them, but with epoxy I believe they would add measurable longitudinal strength as well. Just don’t know if it’s enough...
Thanks D!
 
So I’m having a tough time picturing this, maybe I’ve been out of hard science jobs for too long. How do I calculate the force on the centering rings to see if they’re strong enough? I figure if the motor is snug so there’s zero relative momentum then it should just be the static weight of the rocket, no? Actually crap, I suppose I would certainly have to take drag into account as well. With that said, if I added up weight and drag and applied that force to the central motor wouldn’t that tell me if it’s strong enough? Am I missing something? I feel like I am and that’s why I’m on here now... Thanks for any help!
 
Doesn't play as well with 4 fins I'm guessing?

Sent from my LGL44VL using Rocketry Forum mobile app

Oh it will work , you just gotta think outside of the box ( airframe ) . Slide a 11.5 inch coupler into the airframe , trim the tabs down , then reinforce the hell outta the fins on the outside of the tube . The fin tabs on the big PR kits are not worth a damb anyways .

Eric
 
Watch your center of gravity, big clusters tend to have a lot of weight aft.
When cutting rings have holes for a conduit for igniter wires.

M
 
Hi all, I just bought a 3/4 Scale Nike Smoke from Performance hobbies. It’s 11.5” external diameter. Let’s say that leaves 11” inside to load motors. I added 3ft to the airframe bc it was otherwise too short to carry main chute with a 48” motor packed inside a 60” airframe, but that’s off topic (17ft total after extending). My question is - how much is too much, how much spacing in between motors is too small. Id like to do central 98, 4x75, and 4x54. The 54s would be offset to the side of the 75 at the outer perimeter, leaving just enough room on one side for the fin to pass through and connect to all three motors surrounding/butting up against it. There’s just enough room for the motors to fit and the thrust rings not to touch. In general, is this enough? Is there a rule of thumb? The centering rings are not fiberglass either, they are 1/2” birch. Does that make a big difference in this situation? I will likely reinforce them with fiberglass laminate. I’m working on uploading pics now but having trouble. I have to get ken my cluster design this week so he can cut them, so any help/pointers are greatly appreciated. I did poke around but can’t find anything on the subject, it’s tough to search it bc the words are used in most threads. Thanks in advance for any help!

Hi Dan,

I had a 3/4 smoke with that exact set up. Central 98, four 75s and four 54s. Wood rings, no problems. But I had the one piece fin can version where the fins were essentially surface mounted (no fin tabs).

Skippy has it now.

Don't known if I have many build photos, but I'll check.

Fun rocket!!!
 
Just go MD with it

John you be sniffing Duco cement again! Everybody's knows once he be's packing that 11 in airframe fulls of research propellent he most be likely to have a burn through!

You'se needs to gets modrn and buys that two parts epoxy that's everybody's be use 'in. Kurt:surprised:
 
My 11.5" rocket has a 98 and 8 54s. This is the only flight with all the tubes filled:
Motoreater a.JPG
(The outboards were 38/720s in adapters.)
 
Oh it will work , you just gotta think outside of the box ( airframe ) . Slide a 11.5 inch coupler into the airframe , trim the tabs down , then reinforce the hell outta the fins on the outside of the tube . The fin tabs on the big PR kits are not worth a damb anyways .

Eric

Wow what an awesome bundle of responses to wake up to, thanks guys! Why are the fin tabs bad on PR kits? I was counting on them to pass all the way through to the 98 so I could fillet there, in between the 75/54, and on the inside of the airframe...trying to avoid external filets. If this thing is weak I may have to rethink that, as I don’t want an internal break of the fins.
 
My 11.5" rocket has a 98 and 8 54s. This is the only flight with all the tubes filled:
View attachment 332987
(The outboards were 38/720s in adapters.)

Those are some mighty but launch lugs!! What did you do, just thread the giant lugs over the top of a 1010 rail?!

[emoji12]


Sent from my iPhone using Tapatalk Pro
 
Hi Dan,

I had a 3/4 smoke with that exact set up. Central 98, four 75s and four 54s. Wood rings, no problems. But I had the one piece fin can version where the fins were essentially surface mounted (no fin tabs).

Skippy has it now.

Don't known if I have many build photos, but I'll check.

Fun rocket!!!

Are you kidding me Eric?! That’s awesome! What are the odds, there can’t be that many of these out there. I think that conclusively answers the question for this particular setup. I suppose anything will work so long as there is spacing for thrust rings. The more I thought about it the more I realized that D is totally right. The tubes will transfer so much load that it doesn’t matter. What I may do is add another centering ring or two, it only comes with three and with a four foot 98 mm surrounded by 8 more motors a couple extra should probably be used. Eric, any chance you could save me some time by sharing a rocksim file with me? Otherwise I would take a 5.5” model and have to scale it up and redo the cluster, not a big deal but any time saved is always good. I’m at [email protected] if you have one. Don’t suppose anyone else out there has a 3/4 Nike Smoke model with molded fins in rocksim if Eric doesn’t? Although the mmt is really what I’m after. Did you like the rocket Eric? Was it a PR and how were the fins? Thanks bud!
 
Watch your center of gravity, big clusters tend to have a lot of weight aft.
When cutting rings have holes for a conduit for igniter wires.

M

Music to my ears M! That’s exactly what I was hoping for since I’m going to have a high margin extending it by 3ft. Speaking of that, Eric, did you find that there wasn’t enough space for a main in the airframe? Did you go main in payload? It’s a 60” section, take away 48” for motors and 9” for coupler/av bay and you have a whopping 3” for a giant chute!
 
Why not just say screw the 54's and do a cluster of 1 98 and 6 75's? You can pack more ns in with the 75's , 9200ish ns with the 4 54's vs almost 20000 with the extra 2 75's ?

Would love to do that but I wanted fins to contact the 98mm. After hearing that the fin tabs aren’t great I may reconsider... I also wanted the 54s more for effect not power, and then there’s the issue of coming up with 6 75mm hardware sets all with the same size nozzle, while I have plenty of hardware I wouldn’t want to cut 6 nozzle throats just for one rocket. It’s still an idea I’m pondering though, I figure I could always use an adapter to make some 54s if I wanted. Thanks for the input!
 
Are you kidding me Eric?! That’s awesome! What are the odds, there can’t be that many of these out there. I think that conclusively answers the question for this particular setup. I suppose anything will work so long as there is spacing for thrust rings. The more I thought about it the more I realized that D is totally right. The tubes will transfer so much load that it doesn’t matter. What I may do is add another centering ring or two, it only comes with three and with a four foot 98 mm surrounded by 8 more motors a couple extra should probably be used. Eric, any chance you could save me some time by sharing a rocksim file with me? Otherwise I would take a 5.5” model and have to scale it up and redo the cluster, not a big deal but any time saved is always good. I’m at [email protected] if you have one. Don’t suppose anyone else out there has a 3/4 Nike Smoke model with molded fins in rocksim if Eric doesn’t? Although the mmt is really what I’m after. Did you like the rocket Eric? Was it a PR and how were the fins? Thanks bud!

Hi Dan,

Yes, we had the Performance Rocketry 3/4 scale Smoke. The fin can was molded in 4 pieces, joined into one at Perf. Rocketry. No fin tabs. The can needed reinforcement internally and externally as it was a bit....uhhhh....fragile. The later version went to foam filled fins added to a core tube which appeared to be a far superior setup.

Did I like it?
It was a love/hate relationship.
The good: The thing looked sharp in the field and in the air when it worked. It was an attention getter and I'm glad to have built and flown it.

The bad: THAT FINCAN! Was flimsy to begin with so the fins got filled with 2 part urethane foam from US Composites. When filled with foam, they split at the seams, so the seams needed to be glassed on the OD, faired and filled, etc. And there was no support at the fin base; just a 90 degree change in direction for the cloth which tore easily. And it didn't fit the airframe. And so on....spent a lot of time on that can! Again, the later revision with individual fins that arrive foam filled appears much superior at the expense of a bit of room on the internal side (conflicts with motor mounts).

While I'm being negative: another item that you're addressing, and Tim Dixon was set to address also (see his thread) was the limited recovery space. Sticking to scale and using the 20 or 22" couplers, plus that goofy fincan, you didn't have much room for anything recovery related! I set up mine for a couple of options...there was NOT enough room in the lower section for a traditional drogue setup, so it was initially setup for tethered main out the top. It also flew single deploy with the nose and airframe recovering on their own chutes. Further, there was a full length of 6" phenolic built into the nosecone. If I were to fly it again, one or more altimeters would've gone in the nose and the main would've come out of that 6" tube. I *think* you're intent of adding 3' will take care of the tight recovery quarters!!!

One more negative: the rocket was cursed! First flight: launch system malfunction led to a disaster. Second flight: launch system issue led to a great flight, just a bit more exciting than anticipated. Third flight went well; the curse was apparently lifted so we ditched the rocket!

Photos of the last flight at Midwest Power 10: (1) 98-17,500 Sconnie White airstarting (4) 54-2550 Sconnie Sparkies for effect!

24807263038_1efb379af2_z.jpg
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I don't have a Rsim file...not much of a fan. I have Cad drawings and more photos somewhere, but that goes back a few computers, cameras, and years! I'll see what I can dig up!

Cheers!
 
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Four ninety eights and a prayer. 7.7" then. You won't need any extra effects. But you could always fill the remainder with a circular cluster of 38's for a bunch of cheap sparkles. 10.6" out of 11.5". The smoke cloud may obscure launches for a half hour and the loud roar should set off all the car alarms. A lot cheaper to fly than a Q and you could source reloads on site. 3/4th Scale Nike gonna Nike. (Foxholes and bunkers not included).
 
What is the dimensions of the fin tabs on this? I was curious so I laid out the centering ring in cad. it looks very plausible for the 1-98, 4-75, 4-54 setup.

would foaming the MMT/Fin tabs help bond it all together?
 
So I’m having a tough time picturing this, maybe I’ve been out of hard science jobs for too long. How do I calculate the force on the centering rings to see if they’re strong enough? I figure if the motor is snug so there’s zero relative momentum then it should just be the static weight of the rocket, no? Actually crap, I suppose I would certainly have to take drag into account as well. With that said, if I added up weight and drag and applied that force to the central motor wouldn’t that tell me if it’s strong enough? Am I missing something? I feel like I am and that’s why I’m on here now... Thanks for any help!

Unless you’re using a thrust plate, the force on the centering rings in-flight will be equal to the combined thrust of the motors.


Steve Shannon
 
Easy - an 8-inch "Q" is all you need...........
Nah. One Q is overrated. What this rocket needs is four 1515 rails externally mounted with end caps and a cluster of Q's set to drag separate. Then Government may approve orbital satellite chunkin' contests with a whole bunch of forms from Wallops or something nuts. Lowball the bid to win.
 
OK Danny, I'll get serious now since you wouldn't bite on the MD

Here's what I do on my big multi-diameter clusters.

1. Put your 2 Swiss cheese CR's in at the bottom and top of the fin tangs
2. You won't put any more than 24" of 54mm motor in those tubes so put another CR in at 25" up without the 54mm holes
3. You won't put more than 40" of 75mm motor in those tubes so put one more CR in at 42" with only a 98mm hole in it

Adjust as needed based on what you think your max 54 & 75 motor lengths will be. This is a strong mount and will also block off those MMT's that you may not use sometime and keep your ejection charge gases from going down the tube as they say.
Also if you are planning to air-start some of these, leave room to run conduits down through for igniters
 
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John you be sniffing Duco cement again! Everybody's knows once he be's packing that 11 in airframe fulls of research propellent he most be likely to have a burn through!

You'se needs to gets modrn and buys that two parts epoxy that's everybody's be use 'in. Kurt:surprised:

If only Scott would make a 280mm liner :)
 
OK Danny, I'll get serious now since you wouldn't bite on the MD

Here's what I do on my big multi-diameter clusters.

1. Put your 2 Swiss cheese CR's in at the bottom and top of the fin tangs
2. You won't put any more than 24" of 54mm motor in those tubes so put another CR in at 25" up without the 54mm holes
3. You won't put more than 40" of 75mm motor in those tubes so put one more CR in at 42" with only a 98mm hole in it

Adjust as needed based on what you think your max 54 & 75 motor lengths will be. This is a strong mount and will also block off those MMT's that you may not use sometime and keep your ejection charge gases from going down the tube as they say.
Also if you are planning to air-start some of these, leave room to run conduits down through for igniters

Hey John, thank you for pointing out the obvious that I definitely didn’t think of. Man I was about to put 9 holes through all 5 plates. You’re totally right, I need two for 54s, 3 for 75s and 5 for the 98. This solves the other problem where I was going to put a solid bulkplate above the motors to prevent ejection pressure from escaping when it’s not packed with all 9. Genius man, I can’t believe I didn’t realize that. With this and knowing how much load is transferred through the tubes to the forward rings, I’m plenty comfortable. And I’ll definitely be running a conduit pipe for air starts. I appreciate the tip John!
 
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