Cluster Quesitons

[Zonker]

Hi everyone. I did some searching around and came across this forum.....looks like a great place for information. I'm not sure I'm posting this in the right area, but I have a question about clustering. I'm in the process of designing my first cluster rocket (I'd like to use 4 Ds). My question is about the ejection charges. I've heard of people using 4 D12-5s for example.....would the combined force of 4 charges going off simultaneously be a problem?? I know that plugged versions are available, but they're sometime harder to come by. Any advice/suggestions?

Thanks

 

[Silverleaf]

Howdy Zonker,

1st off welcome to the forum. This is a fantastic place to talk about rocketry, and the people here are truly wonderful. Looks around, uh-oh, what have I said ?..now their melons won't fit through the door. 8)

Clusters - ok..heres a couple ideas.

1) You could use a baffle, but in some instances 4 D's can blow right through the baffle and everything else. Its is a good option though if your baffle design is strong.

2) How about vent holes on 3 of the engines, and the 4th has a containment tube leading upward to the wadding/chute ?

In essence your venting away 3 of the charges out of the design.

If you go this route, just choose one of the engine mounts and extend a tube upwards and make a Bulkhead with a hole drilled offset to that tube only. Then add 3 holes around the perimeter of the rocket, below the bulkhead assembly. (Heck you could even angle this tube towards the center of the bulkhead as well.)

Then use black paint and create a 4th hole to match.

3) Plugged engines on 3 of the engines would be the simplest route, and yeah they are a bit expensive, but this is rocketry after all. lol

Seriously though, if you want the easiest route, 3 plugged, 1 normal would be good.

4) You could use boosters motors, but you'd then have to worry about the particles being shot forward - less you vent those as well, similiar to 1 & 2 above.

Hope that helps, and once again welcome to TRF.

Cheers,

 

[sandman]

Well, here is my thought on this.

Why not find the optimum delay like say a D12-5 motor with a 5 sec delay.

Use two of them. Two ejection charges in a body tube the size of a BT-80 would be no big proble.

Now for the other two engines, how about 2 D12-7's. this way the nose cone is already out...now big deal just an extra puff of smoke on the way down.

BUT! if even with two D motor ejection charges the nose cone and chute don't deploy, you get another chance 2 seconds later! Sort of a redundant back-up system.

Make sure you stufff some wadding into the tail end of the motors before you insert them. If you get an engine that doesn't light the ejection charge from the others "could" light it from the back end!

sandman

 

[Rocketmaniac]

Originally posted by sandman
Now for the other two engines, how about 2 D12-7's. this way the nose cone is already out...now big deal just an extra puff of smoke on the way down.

Sandman,

Would this lead to a problem of having the shock cord burned by the 12-7's..... I mean the 12-5's would have already ejected the nosecone, chute and the recovery wadding....... What would protect the shock cord at this point?

 

[sandman]

I usually use a nice heavy Kevlar leader so it wouldn't be a problem.

sandman

 

[Mike]

Originally posted by Zonker
I know that plugged versions are available, but they're sometime harder to come by.

Just be aware that the Estes plugged motors are D11-Ps, they have a different thrust curve to a D12-5. I'm not too sure how much of an issue it would be but it's worth being aware, perhaps someone else can shed some light.

Mircromister, a member here, has a very good article about cluster ignition which is well worth reading.

 

[powderburner]

Welcome to TRF!!

I think it was the great Flis, in an old post about clusters, who pointed out the following considerations:

--Because of the foibles and vagaries of cluster ignition, it is highly doubtful that you will accomplish simultaneous ignition of all portions of your cluster within the same nanosecond

--Because of the further variables and inconsistencies of cheaply man-made items like rocket motors, it is also doubtful that the timelines for the burn/delay/ejection will match all that closely within your cluster

--Therefore, chances are just about nil that you will ever have to worry about even two of the ejection charges going off closely enough (timewise) to cause any serious overpressure to the BT. And once the first one pops, the laundry will be (a) moving or (b) gone, and subsequent ejection charges will just make noise.

Personally, I like Sandman's suggestion with the -5 and -7 delays. It would be kind of cool to hear the second wave popping off.

 

[powderburner]

I forgot to mention something else from an earlier thread by Jim Flis:

When prepping and loading motors for a cluster, insert a plug of ejection wadding inside the front of each motor casing.

Seems that Jim has had occasions where a motor that did not ignite with the others, at launch, coasted aloft and was 'backwards' ignited by the other ejection charges. Said uncooperative motor then proceeded to fire out the front of the case, into the rocket interior, thoroughly roasting everything. Not good.

 

[Weekends]

I have flown my LOC Viper III at least 3 times (maybe 4 hehe) and not had a problem with strong ejection charges. Although this is a 3 cluster of 24mm and not a 4 cluster.

Anyhoo,
Hope it helps.

Weekends

 

[Zonker]

For starters, I just want to say thanks......I didn't exepect so many replies

You guys have some great ideas I never thought of. I had another idea....what about a kevlar heat shield? Would it provide enough protection? I have no experience with a heat shield...maybe it's overkill. Just don't want any flaming wrecks

Zonker

 

[Micromeister]

Zonkers:
First Welcome to the forum you have indeed come to a great place for info.

Like Sandman I've been flying clusted D13 and D12's for over 30 years. 2 to 8 motors cluster.

First and foremost, you will never have all the motor ejection chargers going off at the same time due to the 20% manufacturing tolerances, I can't remember ever hearing less than the number of motors at and just after ejection 4 motors you'll Hear POP,pop, pop, pop, in fairly rapid succssion but seperate times, 3 motor 3 pops 8 motors 8 pops and so on.

Pluging motors is a NO NO! You are altering the manufactuers "intended performance" of the motor. This is not a legal thing at NAR Competitions or any sanctioned launch. This activity I believe voids your insurance if your with the NAR. Not to mention is doesn't work very well!.. more often then not causing a case splitting or just blowing the Plug out. Estes just introduced a factory plugged A10-0P for their rocket cars. I'm waiting to see how long it is before they pull these motors form the market.
I have a couple dozen 3, 4, and 5 motor clusters I've been flying for many many years. Some with as many as 50 or more clustered flights. I normally match delays or use booster motors in external pods. Someone mentioned venting which is ok and works very well. Baffles are also good if you remember to make them so they can be cleaned out. For large body tube (BT-60 & larger) models I'd stay with the multi ejection charge as a redundant deployment back-up. Why would you choose to rely on a single motor in a cluster that has the potential for not igniting as your only means of recovery deployment? Stick with the multi motor ejection charge, they don't hurt the models at all. OBTW I only use Estes type .013 and .021 wall tubing, without extra glassing or other unnecessary "Heavy up" junk I see so prevalent on this forum. If you are flying D12's all that stuff is totally dead weight. The 3X upscale " Hobby Goblin" picutred below is a standard BT-101 without a stuffer tube or other reinforcing, She has a least 15, 5 D12-5 motor flights without any kind of burn through or crimping. Look up the thread on "clustering the classics" in the low power section for loads of pic of clustered upscale and standard models.
One more cluster maniac note. the only reliable way of cluster ignition is a Relay and HI amp/hr battery. Like I mentioned earlier 30 plus years of multi motor clusters and I can still count the number of non lite motors on 1 hand.
Forgive me if this message seems a little strong, Clustering is my #2 passion, with so much experience I hurts me to see new cluster folks unintentionally lead down less then the best paths.
My other screen name is mrcluster
Hope this helps a little.

 

[Silverleaf]

Dang..that means that mega cluster with 319 motors would have sounded like a string of Chinese fireworks all going off harri-karri style. What a wonderful visual that would be, a virtual cacophany of sound for us all. 8)

Kevlar would help, and I believe Flying Silverado's Thrustline Aerospace site has Kevlar - I better check that before I say for sure...

Yeah, just checked. Try this link:

https://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3153095237&category=2567

Here is the link to his Ebay site headquarters:

https://stores.ebay.com/id=29124660&ssPageName=L2

Hope that helps,

 

[jflis]

wow, little to add here, that's for sure...

Zonker, you got your answer Plus some

These comments are right-on. I wouldn't worry about the overall ejection power as they won't all go off at the same time anyway. Although I will also say that Sandman's suggestion of 2 D12-5 with 2 D12-7 is the most elegant.

As for your concern for the shock cord, do not worry. The main cause of damage to shock cords during ejection is because the shock cord is wadded up in a confined space with the hot ejection particles. once deployment has occured, the hot particles will shoot out the top of the tube and dissipate/cool before they could *ever* harm the shock cord.

point of note: clusters add a great deal of weight to the tail of your rocket. Verify stability before flying and ALWAYS use a good strong (12VDC car battery is recommended) power source! And REMEMBER to use a bit of recovery wadding in the top of each motor (very important! )

good luck and keep us posted!

 

[Vicious]

I just ordered one of Fliskits' 5 motor cluster mounts (1 x 24mm & 4 x 13mm) I was thinking of using it with A10 plugged motors outboard and a D or E in the middle, is this wrong? My thinking is that I would want to use the ejection charge of the D or E motor and not the ejection charges of the smaller motors. If I didn't use plugged A's wouldn't the smaller motors eject while the D or E is still under thrust? Maybe I'm confused, this is a new hobby to me but the cluster deal has me intriged.

Thanks, Butch

 

[jetra2]

I would think that you would be OK with whatcha got. As long as the BT-70 tube isn't too long, you should be okeedokee. If it's a little too long, say, greater than 20 inches from the motor to the end of the tube, you can go ahead and replace the central BT-50 24mm motor mount tube with a single length of BT-50, which should be 18" long. You'll need one more ring at the end of the stuffer tube to support the end of the BT-50 tube.

HTH,
Jason

 

[nomopbo]

jetra2,
You mentioned something I was wondering while I was reading this thread... The length of the body tube. I am working on my first cluster as well. I have two BT-80's coupled. (about 28 inches) Ejection separation will be at the nose cone. I used a three "D" engine cluster. I now know I don't need to worry about the pressure since all engines won't eject simultaenously, but is the body tube too long? Is it a trial and error thing?
Thanks

 

[eugenefl]

Originally posted by nomopbo
jetra2,
You mentioned something I was wondering while I was reading this thread... The length of the body tube. I am working on my first cluster as well. I have two BT-80's coupled. (about 28 inches) Ejection separation will be at the nose cone. I used a three "D" engine cluster. I now know I don't need to worry about the pressure since all engines won't eject simultaenously, but is the body tube too long? Is it a trial and error thing?
Thanks

Too long? Pssshh. No way man. You'll be OK. The FlisKits <a href="https://www.fliskits.com/products/rocketkits/kit_detail/richter_recker.htm">Richter Recker</a> is a whopping 92" in length! It used 3 D's in a BT70 (2.2" dia) tube. You'll be fine.

 

[Zonker]

Wow! I'm glad I came here.....lots of great info. It probably would have cost me quite a few $$ to learn some of these tips the hard way As jflis pointed out, there is a weight distribution (CG/CF) issue to consider. What are some recommended ways to add weight to the nose?

Thanks

Zonker

 

[powderburner]

You have to start with the minor matter of whether you are dealing with a plastic (hollow) NC or a balsa (solid) one.

For balsa, drill a hole as deeply as you dare from the base toward the nose or tip. Don't go too close to the tip or your added ballast could break through sometime in the future (like if a parachute snaps open at high speed, or your rocket lands hard, or you get a lawn-dart). Anyway, if you need only a little weight then use modeling clay or anything you have handy. If you need a lot of trim weight you may have to step up to copper shot (buy a pkg of BB ammo at your hardware store) or lead shot (buy a box of shotgun shells, pry open the front of a cartridge, pour out the shot, and dispose of the rest SAFELY). Pour some epoxy into the hole you drilled, pour in the shot, cover with some more epoxy.

Side note: Making a hole in a NC (as described above) also allows you to add a more secure point for attaching a screw eye. Drill your hole with a bit that matches whatever dowel rod size you can find at your local hardware store, like 1/4 inch, or 5/16, or 1/2. Fill the front of the hole with ballast, and fill the back of the hole with a plug cut from the dowel rod. This gives you a hard piece of wood into which you can securely insert your eye screw.

For a plastic NC things are usually a bit easier. Many of these NCs already have a small hole in the base as a result of the molding/manufacturing process. If not, cut or drill a small hole. Pour in some epoxy and shot (as above) and cover with a bit more epoxy. If you want to be double-darn sure that the ballast will not come loose, fill the remaining empty space with some foam-in-place product.

Return of the side note: If you add ballast to the NC be sure to also add a length of elastic shock-absorbing cord, so the weight of the NC does not cause it to snap the tether. Ballasted free-flight objects descending over the launch area on an uncontrolled ballistic path are generally discouraged.

Night of the living side note: If you are worried about an excessively aft center of gravity, you can help yourself a bit by keeping the recovery system as far forward as possible. Especially if you have a very long BT, the parachute and wadding will tend to slide aft under the force of the motor thrust. You can hold these items forward by placing your ejection baffle (or any other retaining device) about six or eight inches inside the front of the BT to keep things forward. Granted, the parachute usually doesn't weigh much, but it is better to keep it in the nose than let it slide toward the tail.

 

[Weekends]

Originally posted by Vicious
I just ordered one of Fliskits' 5 motor cluster mounts (1 x 24mm & 4 x 13mm) I was thinking of using it with A10 plugged motors outboard and a D or E in the middle...

Not to derail this thread, but lemme know what you end up doing with this motor mount Vicious, I've been meaning to order one or two. I don't quite have an idea for what type of bird to put it in.

Anyhoo,
Weekends

 

[Vicious]

I plan on putting the 5 motor cluster in an Estes Phoenix kit, I've got a 3 x 24mm cluster that I'm putting in an Executioner right now.

Butch

 

[Stymye]

post some pics if you can ,sounds interesting .especially the cluster Phoenix!

 

[Vicious]

I haven't got the Phoenix kit yet but I plan to get one to use the 5 cluster mount in. I got the motor mount in the Executioner today though.

Butch

 

[marvSRG]

Check out my cluster Phoenix!

https://www.geocities.com/mcrocketprojects/phoeflite.html

The last launch wasn't so good, but I may have it ready again for the TRF launch in April

 

[eugenefl]

Originally posted by marvSRG
Check out my cluster Phoenix!

https://www.geocities.com/mcrocketprojects/phoeflite.html

The last launch wasn't so good, but I may have it ready again for the TRF launch in April

Hey, now that's not fair! You can't exactly put a D and 4 mini-A's in it! Please do try and get it ready for Whitakers. I would personally be interested to see how easily AP cluster rockets fly.

 

[jetra2]

Originally posted by eugenefl
Hey, now that's not fair! You can't exactly put a D and 4 mini-A's in it! Please do try and get it ready for Whitakers. I would personally be interested to see how easily AP cluster rockets fly.

Yeah - I didn't even know it at the time, but I actually saw that rocket fly! You'd notice, Eugene, that he picked some of the easier to light motors, being the I435T (Blue Thunder) and Pro38 G69's, which are also easy to light.

Am I close to right, marvSRG?

Jason

 

[marvSRG]

Pro38 G69's

They were still designated as G60's back then...they had just changed over and the old designation had stuck around.

But yes, I did pick the G60's (69's) for ease of ignition, and the fact that the I435T was easy to light was just an added bonus...I picked it because I needed to to get that sucker into the air. I had planned a full up flight, with all Pro38's, but the motors were sold out.

The last flight attempt was all Pro38's, but the G-switch didn't work on the timer so I got a single J330 flight, and the nose cone came off and lawn darted.

I'll be certified when we go to Whitakers and I haven't picked out a motor combination to fly in on yet if I do fly it...any ideas? And to avoid having to have an L2 certified person help fly it, lets stick to L1 38mm motors....it has a 5x38mm mount.

 

[wwattles]

If you build it tight, you can fit 4 24mm mounts into a stock Estes Phoenix. Be prepared to add a LOT of nose weight, and reinforce the inside of the fin can with your favorite flame-proofing material. I used a thin layer of 2-ton epoxy, and even that burned through in a couple places.

WW