# Cluster Question

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#### astronboy

##### Well-Known Member
I am building an upscale clone of the ESTES SPEV, and am looking at several clustering options. Right now, I am leaning towards a central E9-8, surrounded by triple D12-0s or D11-Ps for a good kick off the pad.

My idea is to use booster or plugged motors in order to simplfy ejection ducting. Only the center motor will have a stuffer up to the chute.

Has anyone ever tried this before? Any comments?

#### mtmind

##### Well-Known Member
if the motor tubes are closed, use plugged motors. When boosters burn through the front of the propellant grain it produces a bunch of little pieces of propellant which burn faster due to the increased surface area. this increases the chamber pressure dramatically, and pushes the motor out the back. I had this happen once, and couldn't believe I hadn't thought of it. I'm now looking for an easy way to plug 18mm booster motors since no one seems to make them.

Peace,

Mike

#### Darian Rachal

##### Well-Known Member
Mike, Just fill the ends with epoxy & let it dry. That should work to plug the booster, although I've not tried it.

#### powderburner

##### Well-Known Member
This was covered a few months back on another TRF thread, sorry but I don't have that thread ident handy.

Someone had already done some testing and found plugs could be added to boosters and single/upper stage motors as long as there was room to pour in epoxy to a depth equal to the casing inside diameter. It seemed to work effectively. I recall we talked about the epoxy used for this was the cheapo 5-minute-brand stuff that you get at W-world. The main hazard seems to be that plugs added to single/upper stage motors will cause the nozzle to blow out when the ejection charge ignites. I did a few tests on the ground, and they worked fine.

Since Estes (Quest also?) does not offer much in the way of plugged motors, I wonder if this modification makes them illegal to the NAR?

#### rbeckey

##### Well-Known Member
I had experimented with plugged motors for saucers. Plugging single stage motors is not a good idea. When the ejection charge goes off, there is a high pressure thrust out the nozzle end. High enough to remove the motor mount from a saucer built with polyurethane glue. There was also one CATO, with a length-wise case split. The nozzle is blown out, and that could become an issue if it is pointed at a person when that happens.

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
Rbeckey is right:
if you guys must alter your motors. and that's what plugging is. only plug -0 delay booster motors.
Astronboy: I've been clustering model for a very long time, I just don't see your need to limit your chances of ejection by only using the ejection charge form the core motor. Over the years I've found it is just as easy to add a coupling long ejection charge mixing chamber attached to the forward cluster centering ring, with the stuffer tube on the forward end of the coupling. its quick, its very easy and only adds a tube coupling and one more centering ring to the weight. If your worried about longeivity I have a number of cluster models with dozens of flights without a failure or burnthrough. If you feel the need you can also coat the inside of the chamber with epoxy. With this system you'll have the option of using booster, or same delay or even longer delay motors for the boosters. I don't have a really good photo of the chamber but maybe this one of a 4 E cluster tubes with chamber on my 3X OT will give you and Idea of what this beast looks like

#### astronboy

##### Well-Known Member
Thanks Micro... I was thinking of channeling all of the charges together as well, but I was worried about losing ejection pressure due to the size of the chamber.... or even wondering if it needed to be a funneled chamber into the stuffer. Any rules of thumb for the size of the chamber.... length x diameter?

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
Astron:
I only work with bp so if your looking at Apcp these finding may be somewhat different.
I've always gone with a standard "estes" coupling size for all 13mm, 18mm and 24mm motor clusters. The standard JT-60 and JT -80 's I've been using are 1.5" long and seem to do a great job as an ejection mixing chamber. I've used cardstock, foamcore, and 1/8" aircraft plywood for the cluster and stuffer tube centering rings. I always epoxy coat the underside of the forward ejection chamber/stuffer tube ring whatever the material.
During last years TAC most of the teams we had contact with tried using a conical cluster/sustainer funnel to direct the sparks into the upper stage motor nozzle. This worked well for stageing but I don't really see a need to do this on a straight cluster unless you increase the funnel length enough to allow use of 1/64th aircraft plywood as the funnel and epoxy the heck out of it
As far as leaking gas is concerned, I have a 5 D12 cluster crayon I fly on 3 D12s from time to time I epoxy plugged a couple spent D12 casing to use in the open motor tubes. This is a straight 4" dia crayon with no stuffer tube at all. 3 D12's eject the 36" nylon chute with no trouble at all. My next crayon may have a stuffer tube to shorten the chute compartment not reduce the ejection volumn
Hope this helps

#### astronboy

##### Well-Known Member
Thanks for the details Micromister,

Due to the need for proper scaled up tube sizes, I am using heavier walled tube: 4" to 3.1" to 1.7" heavy walled tubing.

Based on your designs and techniques, I will be having all 4 engines vent into a 3.1" dia by approx 3" long epoxy lined chamber, that will in turn spill into a BT-60 stuffer tube up throught the balsa transition (4" to 3.1") into the 3.1" diameter recovery section that houses the parachutes.

I will be going with BP as a cluster. The center motor surrounded by three allows me to use empty D motor casings plugged with a balsa plug in the outer tubes if I want to use a single central AP F motor.

This just gives me some motor options.

Am I on the right track?

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
Couldn't be doing a better job! Not to throw a fly in the ointment but isn't there enough room to fit another three 24mm motor tubes around the 29mm core?
OK that's two 3 packs of D12's for the flight but wouldn't that be sweet!
When pluging your casings you might want to use just a piece of paper or alum foil under about a quarter inch of 5 minute epoxy to keep the extra weight down. plugging an empty 29mm casing also works

#### powderburner

##### Well-Known Member
If you want to know a good way to contain/direct/deliver the ejection gas while maintaining its effectiveness, read on. Otherwise, ignore.

Pressurized gas will flow best over short distances if you give the gas path a slightly expanding duct. By slightly I mean that the cross-section should probably increase by about 5 to 10% per foot. If you start with a BT-50 (with an inside diameter of 0.95 inches) you have a flow cross-section of 0.71 square inches. So a foot downstream, your flow area should be 0.75 to 0.78 square inches, or 0.97 to 1.00 inch diameter. But building a tapered duct is a pain in the ***. What difference does the taper really make? Probably very little.

In a theoretical world, the flowing ejection gas will form a boundary layer against the inside of the duct walls just like the external airflow does on the airframe external surfaces. Over a long enough distance, the boundary layer can build up and begin to block the flow. I dont have numbers for you, but I dont think that will pose a problem for anyones model rocket (or mid-pwr, or high-pwr either). Simple is also good, and a constant-diameter duct will carry the ejection gas just fine in the real world. Ejection charges seem to have plenty of power to spare to reach through a passage and still kick out a recovery system.

Complications begin when you allow the ejection gas to expand into an internal space and then try to re-direct the flow into a smaller duct. You would have this situation if you build your multi-motor mount to exhaust the ejection charges into the main BT and then use a centering ring to close down or converge the flow to fit into a stuffer tube. The gas will cool (talking theoretically again here, and cool only with respect to gas conditions inside the front of the motor case; the expanded gas will still be plenty hot) as it expands into the BT volume and the pressure will drop slightly. Facing the obstacle of the CR/stuffer tube, however, the gas will lose energy as it gets pushed into the smaller passage.

Over in the real world, we are beginning to run the risk of ejection problems IF we combine such a mount/stuffer design with a large/long rocket, a long/small-diam stuffer tube, and a large recovery compartment. You are not always assured of having all ejection charges go off at once (actually, you are pretty much guaranteed that the ejection charges will be staggered). For practical purposes, we really are talking about using the FIRST charge to deploy recovery; subsequent charges may serve as back-up. But for any and all motors in the cluster that are intended to provide ejection, you are relying on each individual charge to be capable of pressurizing the entire convoluted gas path and popping out the recovery system.

For most model-rocket-sized designs, deployment is probably not a problem; each ejection charge will probably be big enough. The more serious problem is over-pressurization of the compartment ahead of the motor mounts and facing the convergent CR (how many Ds do you have stacked together?). The ejection hot gas will go into the main BT compartment and stall momentarily while it tries to flow out the forward stuffer tube, effectively pressurizing this structural compartment. (One aspect of these pressure loads that I have never heard addressed is that they are applied very quickly, almost explosively; the severe onset rates of these loads can also cause problems if construction is not up to the job.) Structurally speaking, it does not matter whether this pressure lasts for three days or three micro-seconds, it will still act against the sides, front, and rear of this internal space. If you dont build this compartment to withstand the pressures, temperatures, and abrasive flow, you may have a surprise deployment of your motor mount. Or a surprise collapse of your internal structure and stuffer tube.

That is why micromisters comments are extremely important to study and understand. It may be necessary to use a bit more beef in constructing the compartment. Note his experimentation with foam-core bulkheads and centering rings; that stuff is pretty darn strong and still very light.

Another option would be to build extensions of the individual motor mount tubes projecting forward through what would otherwise be the stuffer tube zone. You would still need a good bulkhead at the front of the extensions, to seal off the internal volume of the center airframe and minimize the compartment volume that must be pressurized for recovery deployment. This will deliver ejection gas most efficiently to the front of the rocket, but will add complexity to the structural guts of your design. This step is probably NOT necessary if you can build a good, solid compartment around the front of the motor mount.

Yet another option would be to make a short internal cone-shaped duct to funnel ejection gasses down to the stuffer tube. The rear of the duct would have to cover the front of any/all motor tubes that are to provide ejection. All connections would require solid attachment and good reinforcing fillets. The internal face of this duct should be coated with epoxy or CA to improve its fire resistance. The outside of this duct might even be wrapped with reinforcing materials.

Keep in mind, most of the above discussion is really only important for situations where you think the ejection/deployment might be marginal. For model rockets with a gas path of up to three or four feet, and probably for airframes up to BT-60 size, you probably dont have any problem at all.

And yes, many old men like me know quite a bit about passing gas.

#### astronboy

##### Well-Known Member
Micromister:

I am looking to build a rocket that can easily be flown on both a BP cluster or a single AP motor, so although I have the room to make this a 5 or 6 cluste rocket, I believe that I will stay with a center and 3 outer motors.

My Rocsim gives my aprox 650 feet with all 4 as D engines, 850 with three D and a centered E, and 700 feet with an F21. THis is fine for the flying field that my local club has access to.

As for the plugged casings, I was thinking of using balsa plugs with a coating of epoxy on each side.

Powderburner:

Thanks for your comments and insight. I am aware, at least generally, of all that you have written, and that is why I posed this question to the forum. The 'ejection canister' will be 3.1": thick walled tube, with plywood bulkhead/centering rings on either end, and will be epoxied together for strength.

My only concern is that I will be stuffing a single engine's ejection into a 3.1" x 3" ejection canister, then constricting it into the 2.6" stuffer that will be about 7" long, then it will enter the 3.1" diameter recovery section which will be 8" long. I believe that this should do the trick however.

#### powderburner

##### Well-Known Member
I have to wonder, why bother at all with a 2.6 inch stuffer in a 3.1 inch airframe?
You would do just fine with a 1.6 (or a 1.0) inch stuffer, and it would be lighter and cheaper too.
And I suspect that you would do well to include a funnel between the motor mounts and the stuffer, especially if you are worried about successful deployment.
The plywood certainly ought to do the job, did you select it because of eventual use with composite motors? For me, plywood is a LOT of work, I will choose foam core every time even if I have to double it up and add webs between the motor tube and the airframe.

#### astronboy

##### Well-Known Member
The stuffer is needed as the lower airframe is a 4" tube with a balsa transition to a 3.1" tube. I need a stuffer that can pass through the balsa transition without my having to remove so much wood that I will compromise the strength of the balsa piece.

If I go with a BT-60 stuffer, I should not have to use a funnel as I would with a smaller tube.

Yes, I chose 1/8" ply to ensure that I can use AP motors as well as BP. It is not that hard to make them... I just fill out the spreadsheet from BMS!!

#### mtmind

##### Well-Known Member
It's amazing how a thread can grow in a day. To reply to something quite a ways back, and to set minds at ease, I agree with the "don't plug upper stage motors" statement. I mean only to plug booster motors, which effectively makes them just like standard plugged motors.

As far as ejection, at least round or taper any square corners. Personally, I think I would have a cone at the base of the transition and above it to keep the square corners from robbing the ejection charge of energy. However, if just the middle motor is going to provide ejection, I would recommend extending the stuffer from the top of the transition all the way down to the motor tube, or bulkhead. That way you don't have the first constriction slowing the ejection.

Peace

Mike

#### astronboy

##### Well-Known Member
I agree Mike, I would not recommend plugging a 'regular' non booster motor... that just a way to blow out the nozzle.

I have plugged A10-0Ts in the past with about 1/8" of epoxy. They worked fine as outboard boosters.

Although a funnel is most necessary for staging, I am not so sure that it would be needed for ejection, as the gasses are expanding in all directions. I believe that reinforcement (epoxy) is more neccessary.

Well, I believe that I will be sending my BMS order tonight, and I will add pics to this thread once things get rolling. Thanks everyone for their advice and assistance.

#### Stymye

##### Well-Known Member
Powders thesis got me thinking as well
Astron
I did some calculating and I came up with one D engine would be" just enough"
to generate about 15 psi in the recovery section,,(If I did it right)

best I can measure you need near 1 grm charge weight
I didn't factor in chute and wadding.

tip-a good dose of celulose wadding will reduce the volume and help.I asumed.the pressure should more or less blow thru the wading rather than have to factor in the weight of wadding into the calculation(I got lazy on that part)plus I dont know what you plan to use

24 mm motors have about .85 grms charge weight

this is also assuming a combustion gas temp around 3000/R(combustion gas constant) *yes it's that high

by the way...this all hinges on deployment at or near apogee. not at a high rate of speed

powder ---there should be no problem pressurising an ..as you put it... "convoluted " path.If everything holds up it will happen very quickly plus you have to consider the pressure at ground level is already there to build up from ,, regardless good info!

#### astronboy

##### Well-Known Member
OK, so if I kick the 'ejection chamber' idea for this model, and just go for plugged booster motors outboard, I can run a 24mm x 14" long heavywall stuffer tube right through the transition and up into the recovery chamber. (3.0" ID, x 8" long) That should do the trick, no? It would also be much lighter than an epoxied ejection chamber. (for stability and altitude performance)

I am looking at 2 parachutes right now: a 36" for the bottom end, and a 24" for the 'capsule', so I want to be sure I can kick them both out at ejection.

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
So the first and last time your clusters center motor fails to ignite
you'll be kicking yourself for not using a ejection mixing chamber. The MINOR, Tiny amount of additional weight from your chamber construction is more than offset by the comfort factor, knowing you will have an ejection. Paperwork Theory doesn't always lend itself exactly to real world workings. Both powderburner and stymye bring up great material, and I'm sure powderburner will agree with my reasoning here that redundancy is always a good thing when is comes to remote operation of just about anything..Recovery system deployment for sure

#### astronboy

##### Well-Known Member
Hello Micromister.... I agree with youon the concern about center motor ignition. It is one of the reasons I created this thread. However, I am also very concerned about not blowing the 'chute out if I use a BT-60 stuffer. I feel like I am left with 2 equally undesirable choices:

1) Use one center motor for assured recovery, IF it ignites.

or

2) Use clustered ejection with a chamber, but then worry that the chutes will deploy......

#### Stymye

##### Well-Known Member
I like the chamber strategy.your not relying on one shot that way

#### Micromeister

##### Micro Craftman/ClusterNut
TRF Supporter
I'm Sorry Astron you lost me somewhere.

Why would you be concerned at all about the chutes ejecting using the chamber method. Did I miss something in my explanation?

By using a coupling size chamber directly ahead and attached to the forward end of the motor tubes on your BT-80 airframe model with BT-60 stuffer tube up to your 12 or 14" chute compartment you are ensuring that if only one of your 4 motors ignites you will eject the chute(s). In my 3X OT I have a 36" main and an 18" nose cone chute just for looks The stuffer is BT-60 and If memory serves me it is 28 or 30 inches long. 4 D12's fly the model and usually the chutes are out before all 4 ejection chargers are heard going Pop,pop, pop,pop. Ejection charges almost never go off at the same instant which I think is what Stymye was driving at in has post, At the given expansion of the gases..one motor "Could" do the job. My point was if the center motor is the only ejecting capable motor with only plugged boosters you are relying on that center motor to do the job. NO center motor ignition.. NO Ejection at all.
Sometimes we may over think these concerns. I can't guarentee you will never crash your clusterd model(s) if you use the ejection chamber method, I can only state with 100 percent confidence its a better method than single motor ducting with plugged tube boosters. The only system that would be better would be on-board DD with R/C ignition, but I've seen them fail as well
This has been a great thread. just the kind of tech I like. I hope this is helping you a little. and easing your eggbag. I know it's tuff building a new model using new methods. That's why this forum is SOOOO great.

#### astronboy

##### Well-Known Member
Micro: I guess it was Powderburner's/Styrme's posts that had me worried about ejecting the chute.

I completely agree with you as far as not depending on ignistion of the single central motor... I will return to my ejection chamber design....

#### powderburner

##### Well-Known Member
Originally posted by astronboy
Micro: I guess it was Powderburner's/Styrme's posts that had me worried about ejecting the chute.
Astronboy,
The main reason I responded with all that rambling and raving was that you had posted:
"I was thinking of channeling all of the charges together as well, but I was worried about losing ejection pressure due to the size of the chamber.... or even wondering if it needed to be a funneled chamber into the stuffer."
Somewhere in all the prose, I opined that your ejection system would probably work fine, but that you needed to build well to avoid damage due to internal pressure. You then indicated you had that part under control (plywood certainly should do the job). Sounds to me like your design should work just fine, sorry if I caused confusion with any other indication.

#### astronboy

##### Well-Known Member
Thanks Powder...

I am very pleased with this post. We are really exploring all of the nooks and crannies of this situation, and I am getting a better feeling about the project daily.

Thanks again to everyone who has responded!!

We are go for 4 live engines, funnelled into the chamber!!

#### Stymye

##### Well-Known Member
Astron ,
Looking forward to seeing some ,construction pics,,(hint hint)
I'm a clusterholic too ,good luck with the build

#### astronboy

##### Well-Known Member
You know I will post pics!! I am just waiting for my order from BMS... (custom burned centering rings!!)

#### KermieD

##### Well-Known Member
This is why it's great to hang out here, and easy to moderate here. There's no work involved!!! Anywhere else on the web, this misunderstanding could (and probably would) have become a flame war. Everyone here acted like rational people, clarified their positions, and landed close to the pad. Man, I love you guys!

(no, I don't want your Bud Light...if you drink any of that light swill, I take back all the compliments)

#### astronboy

##### Well-Known Member
Light Beer?

Now Them's fightin' words!!

Make mine a pint o' Porter!!

Seriously: This has been a great thread. Despite being a rocketeer for many years, I have learned a great deal from everyone involved in this thread. Thanks again!!

This debate-like discussion sure beats spending all the time and  it will take to build this Beast, only to see it crash due to ejection failure!! A rocket coming down with 4 24mm motors would not create quite a heap!!

#### KermieD

##### Well-Known Member
Hehehe...come to NARCON in March...I'll show ya around the brewpubs in Milwaukee...just 1/2 hour north of there....not to mention the Safe House. It's an incredible spy themed bar. Better know the password to get in!