Cluster vs Large Motor...

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Joekeyo

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Upon further review, I don't see any direct conflict with the safety code.

Take a two stage two engine cluster (4 engines total). Worse case scenario: One engine lights on launch; the rocket cocks because of off- center thrust and this problem compounds when the 2nd stage fires. Potentially, the rocket could be pointed toward the ground when the 2nd stage fires. I expect this probability of this occurring would increase with an increase in the number of motor used. Maybe not a safety code violation, but a potentially dangerous situation that would be hard to mitigate (?)

Last time I launched a 2 "C" engine cluster, I overheard, "A 'D', the hard way." LOL
 

smstachwick

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Take a two stage two engine cluster (4 engines total). Worse case scenario: One engine lights on launch; the rocket cocks because of off- center thrust and this problem compounds when the 2nd stage fires. Potentially, the rocket could be pointed toward the ground when the 2nd stage fires. I expect this probability of this occurring would increase with an increase in the number of motor used. Maybe not a safety code violation, but a potentially dangerous situation that would be hard to mitigate (?)
One provision of the Model Rocket Safety Code is that simultaneous launches of 10 or more rockets be conducted at a distance from spectators at least 1.5x the expected maximum altitude of any of the rockets. I’m sure a rocket like this could be given this treatment, just out of an abundance of caution.

The builder may also elect to have redundant electronic airstart, a “no-start” condition/command, an eject-parachute failsafe abort, or some other safety provision that could be granted through autonomous flight computers or telemetry or a combination of both.

But I feel like something like that would be most workable within the HPR realm anyway. You might be pushing the weight limit with electronic payloads like that and you’re also introducing additional points of failure. Doing a staged cluster like this might just be best done at a generous distance.
Last time I launched a 2 "C" engine cluster, I overheard, "A 'D', the hard way." LOL
I’ve made this joke in relation to a very large cluster not too long ago, but there’s no denying that rocketry tends to attract people who like a challenge, even one that’s not strictly necessary.

EDIT:

My own contribution here: one thing I haven’t seen anybody point out is that clustering can make a rocket very versatile. I personally know one rocketeer who clustered a 54mm with a pair of 29mms, arranging them 1-2-3 in a line inside a….6-inch body tube, I think?

He used it to do an L1 re-certification flight with two Gs in the 29mm mounts, took the L2 test, and re-certified for L2 the same day with the same rocket.
 

Mike Haberer

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Upon further review, I don't see any direct conflict with the safety code.

Take a two stage two engine cluster (4 engines total). Worse case scenario: One engine lights on launch; the rocket cocks because of off- center thrust and this problem compounds when the 2nd stage fires. Potentially, the rocket could be pointed toward the ground when the 2nd stage fires. I expect this probability of this occurring would increase with an increase in the number of motor used. Maybe not a safety code violation, but a potentially dangerous situation that would be hard to mitigate (?)

Last time I launched a 2 "C" engine cluster, I overheard, "A 'D', the hard way." LOL
Not really. The more motors you have, the less impact one not igniting has on the off center thrust profile. Hence, more is better...
 

Joekeyo

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Good stuff all, Gentleman. My opinions deal only with LPR rockets and multistage with "one layer of cellophane tape." I have no idea how to do an airstart. I like the idea of increasing distance out of an "abundance of caution". This is a good common sense approach. Keeping a safe distance is a popular technique in the safety code. The chances of a powered flight into valued property would decrease with distance. Powered flight into terrain is ugly under most circumstances.

That being said, I love clusters and I love staging. TFC, 1 stage, one engine, comes down on chute. You can only do it so many times. Then it is time to move on to something a little more exciting.
 

beeblebrox

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Note: For a model rocket, each individual motor has to have an average thrust <= 80nt. there is nothing in the code about clusters. Therefore if total rocket propellant weight is less than 4oz, no problem. I.E. a cluster of 3 F32's is ok. Total avg thrust = 96nt, but each motor is less than 80 and total propellant is less than 4 oz. Also, you can buy large quantities of small motors without hazmat fees... The rocket in my avatar was all loaded with model rocket motors, total power was a full "J"
 

Mike Haberer

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That being said, I love clusters and I love staging. TFC, 1 stage, one engine, comes down on chute. You can only do it so many times. Then it is time to move on to something a little more exciting.
Can't agree more.

I have one design that is a two stage, 5x24mm booster to 3x24mm sustainer. Uniform airframe size. Gap staging, rear eject engine pod and parachute on the booster. G impulse on D's, H impulse on E's.

Then I have a three stager, 5x24mm to 1x24mm & 2x18mm to 1x24mm, all BP engines. Decreasing airframe size from stage to stage. Gap staging. Rear-eject engine pods and parachutes on the boosters. It's designed to be flown as a 1,2 or 3 stager. 3-stage will be G impulse on C's, H impluse on D's and E's.

So much to do, so little time...
 

Mike Haberer

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Note: For a model rocket, each individual motor has to have an average thrust <= 80nt. there is nothing in the code about clusters. Therefore if total rocket propellant weight is less than 4oz, no problem. I.E. a cluster of 3 F32's is ok. Total avg thrust = 96nt, but each motor is less than 80 and total propellant is less than 4 oz. Also, you can buy large quantities of small motors without hazmat fees... The rocket in my avatar was all loaded with model rocket motors, total power was a full "J"
Not sure what your final point is. My understanding is that it's total impulse that is the governing parameter here. If total impulse = a J, you need to be L2 to fly it...
 

Daddyisabar

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Canting the cluster midway up the rocket, or at the front, employing widely seperated cluster pods or staging clustered motors can lead to flying rockets some may deem as "unnatural." Beware of the Dark Side young Padawan! Single, powerful motors at the back of the rocket in a traditional 3-4FNC configuration will keep you in the good company of the Jedi. ;)
 

smstachwick

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Not sure what your final point is. My understanding is that it's total impulse that is the governing parameter here. If total impulse = a J, you need to be L2 to fly it...
Yup. Model rockets are limited to 320 Newton-seconds for the entire setup. This tends to be a non-factor though, since it’s difficult to get that kind of total impulse out of 125g of propellant. No configuration I’ve looked at with that limit in mind breaks 300 Newton-seconds. Nevertheless, a pair of Aerotech G69Ns or a quartet of Aerotech F52Cs will still yield some impressive performance.

Ah, the challenges of flying “Level 0”. More than enough to fly a lifetime and still have not tried everything.
 

Mike Haberer

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Canting the cluster midway up the rocket, or at the front, employing widely seperated cluster pods or staging clustered motors can lead to flying rockets some may deem as "unnatural." Beware of the Dark Side young Padawan! Single, powerful motors at the back of the rocket in a traditional 3-4FNC configuration will keep you in the good company of the Jedi. ;)
Boring....
 

smstachwick

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Boring....
Yes, only a Sith deals in absolutes….absolute ridiculousness, that is.

I might have one that would have several things on your list, moving it down into the realm of extremely poor rocket design. Pushing all the hallmarks of extremely poor rocket design to the max I am going to have to come up with a really good story to get this one by the top men at the next launch.
View attachment 475454
Excessively long metallic launch lug, fins up front, action figures tied on front of rocket. Cluster tractor motors mounted in styrofoam ball with silicone adhesive, no positive motor retention, total weight 200 grams below max lift off for four F15 4. Four seperate parachutes ejecting from seperate tubes. No fins or motor at the back of the rocket where they should be. Needless Asymmetric action figure drag on top of rocket. Ridiculously high CP. Will CG at Malibu Barbie's head be enough? Slightly canted motors burning above plastic coated with spray adhesive. No rail buttons. Slow lift off using thick launch rod, rails rule, rods drule. Excessively morbid theme not to taste. Action figure abuse like Sid in Toy Story.

OMG. How will such an RSO frightening machine fly?
 

kuririn

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I've flown single-motor models most all of my life, including up to G76 AeroTech RMS motor systems, but never a cluster.
Try it, who knows, maybe you'll like it.
At the very least you'll pick up some construction and ignition skills you didn't have before.
Loc Viper IV (Loc'd 😬 and loaded)
1024210839[1].jpg


And a screen grab of my Apogee Flying Machine (a favorite)
1635101039425.png
 

jqavins

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I'm L2 certified and fly that config at launches with a waiver. It's no different than when I launch my LOC Ultimate with 6x F42 and 1x H180. Combined they're in the J range so you have to be L2 certified but nothing about either is outside of safety rules.
Actually, L2 certification is needed for any single motor over the I range, but L1 allows for multiple engines totaling up to J.

He used it to do an L1 re-certification flight with two Gs in the 29mm mounts, took the L2 test, and re-certified for L2 the same day with the same rocket.
I like the versatility point. However, the example seems like an error by the certification team. Cert flights may not use complex configurations, i.e. clusters or staging.
 

tomsteve

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large motors are great.
buuuuuut...

i also loved my thoy nighthawk on 3- G125s and 4- G25s. G125s popped it up and the G25s kept it going.
 

boomtube-mk2

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1635178046792.png

Ah! The good old days when you could still get the Quest Q2-G2 igniters.
They were too good to be true, so they were regulated into oblivion.
 

smstachwick

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I like the versatility point. However, the example seems like an error by the certification team. Cert flights may not use complex configurations, i.e. clusters or staging.
Interesting.

I could be incorrect regarding the use of the clusters for re-cert, I wasn’t there. I imagine flying a rocket like that on just an H or an I would be somewhat akin to flying a Big Bertha on an A, however.
 

jqavins

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Really? How heavy was it, if you know? The 54 mm mount could take a CTI 1 grain I with good thrust, such as their 518I165 or 475I445, and for AT there's the I599 (405 Ns) among other choices. Now, with all three motors going, once the L2 cert is done, one could use a really long burn 54, like the J99 (945 Ns and burns for 10.2 seconds) plus a pair of I205 (345 Ns each and a 1.7 s burn) to get off the pad. (Double the green line plus the red line.) Versatility indeed!
1635182308537.png
 

smstachwick

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Really? How heavy was it, if you know?
No clue. I don’t recall the exact specs but I’d ballpark it as a 6-inch body tube and probably around 5-and-a-half feet tall, with what looked like some pretty heavy-duty interior construction.

To be honest I’m talking out of my exhaust nozzle a bit when it comes to something of that size. I have yet to certify for HPR myself and I haven’t flown on anything bigger than a single F in over a decade. My brain has a way of not truly comprehending that kind of power in a hobby rocket.

I’m planning to get some experience working with staging and reloadables (not in the same airframe) before going for my own L1. I mainly want to use the certification for flying HPR Fs and Gs, since I find those more intriguing than the traditional HPR configurations. I figure spending extra time building and flying low- and mid-power would be wise.
 

Rocketjunkie

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I have been contemplating this question for quite a while, and after just seeing a post regarding a member's first flight of an Estes Patriot using 4 D-motors... well, I just have to ask... WHY?

Why would you build a rocket using clusters vs a single larger motor of equivalent (or better) total impulse power? Why risk one or more of the motors in a cluster not igniting before the remainder shove it off of the pad? Why deal with the complexity of a cluster vs reliability of a single larger motor? Why deal with the COST of a cluster vs a single larger motor?

Not trying to be critical of ANYONE who chooses to fly cluster models; I'm just trying to learn about an area of model rocketry that I have never ventured into. I've flown single-motor models most all of my life, including up to G76 AeroTech RMS motor systems, but never a cluster.

Thanks!
View attachment 487210
Ah! The good old days when you could still get the Quest Q2-G2 igniters.
They were too good to be true, so they were regulated into oblivion.
If you want instant on use ematches. They don't have to touch the propellant to work. I do have a stash of Q2G2s that I use in extreme circumstances. Pictures show ematch in 18 mm motor. The ematch head will fit into the nozzle of 24 and 29 mm Estes motors. Do check and make sure you can see black propellant in all the motors. If you can only see nozzle clay do not use the motor.
Ematch in Estes 2.jpg Ematch in Estes 3.jpg
 

John Kemker

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Here's my philosophy on cluster-vs-large-motor:

Did it fly? Did you enjoy it?

If you can answer "Yes" to both questions, then it's all good.
 
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