Urgent Cluster Question: How many Ejection charges needed?

I've completed the "Scrambler" cluster rocket from SemRoc and will prepare to launch it and an egg, action figure, plastic egg, hard-boiled egg or similar payload once or twice Saturday at our final club launch.

As I am assembling my rockets and motors and mentally counting how many I need of each value, it occurs to me that I've overlooked something simple and basic when planning the use of THREE motors on each launch.

I understand that the three should be of the same value.... THREE A8-3, or THREE B6-6 or THREE C6-5
or whatever the recommended motors are. But as I look over my packages, I find I have a set of B6-0 boosters that i was planning on using up slowly on a two stage rocket later.

Why could I not use them in the Cluster rocket? That is, why not TWO B6-0 and ONE B6-6 in each firing. Or TWO A8-0 and ONE A8-3 or TWO C6-0 and ONE C6-5

After all, how many ejection charges are really needed to blow the top off a nosecone? Aren't the use of THREE such motors a bit of over-kill? Or does it really matter?

Is it more important to use all three motors from the same package so that they are identical in every way, or should I save resources and only use one such motor with an ejection delay, while the other two could be straight boosters?

Thanks for a speedy response. I will only have a limited amount of time for final packing late tonight, and tomorrow's launches will be a frigid affair (lucky if it's above freezing at any time) and probably running quickly to get them done and out of the cold in SE Ohio.

I'm no expert, but I would first calculate how much "work" the ejection charges need to do (how big is the parachute compartment (volume). I personally would then use a known-rocket as a baseline (for example, Big Bertha- BT-60 with about 10" of length for the compartment), which I know fires well with one 18mm estes charge. If the volumes are equivalent, you should be good to use only one charge. If they're double, or triple, then there you go.

Do keep in mind, however, that a -0 designation does not mean "plugged", so while you wont get an ejection, you will get a nice roasty blow-through that will surely ruin your day (and your build, probably).

Forgot about the rest of your question, sorry- I personally wouldn't worry about using three motors from a pack, particularly if your cluster is arranged properly (not with a great deal of distance between the motors, so if one doesn't light, it's not going to affect trajectory).

If you fly at un-organized launches then pour enough epoxy in to plug it, really doesn't take much. The Powers That Be have stated that this is a modification and not allowed. On my own I've done it for years and at a Tripoli launch they tend not to care, low power is a sideshow to them. Just remember that the motors in a package can vary 20% +/-, that's a big variable. In reality the first one does the work, the other two just go "POP". Problems can come up if they all "pop" at the same time, like blowing the cone a mile

Roll up a square or two of wadding into the -0. I have flown -0s with -X's in clusters to use what I have on hand, and to keep pressure down in the event of simultaneous charges going off. As long as they are all the same impulse it works fine.

As hinted at above, the danger is that if you go with more than you need, you could break your shock cord and send your nose cone off into the ether somewhere. Go with soopirV and do the best calculations you can. Never hurts to confer with a more experienced cluster guy at a launch. Or, PM Micromeister, also known as "Mr. Cluster".

Use 3 of the same from the same pack, with same ejections on all. If you mix boosters with ejections in the same bt, when the boosters burn through they will cause early separation.

Verna
www.vernarockets.com

In a three motor cluster if I needed a C6-5 for the delay I would fly (1) C6-5 and (2) C6-7.

The -5 should pop the nose, but if it doesn't you get two more chances to avoid a lawn dart without risking too powerful an ejection event at the desired time.

I don't fly clusters but I've seen hundreds go up.
I've always thought that if you were planning on using three C6-5 motors, wouldn't it be a good idea to use two C6-5s and one C6-7 as a failsafe?

{Cl(VII) posted almost the same advice as I did just after I started typing}

What these guys said. Use staggered delays for redundancy and to take it easy on the shock cord. All 3 the same will be fine (I've done it several times) but you will wear the shock cord down much faster.

-0's may act like ejection charges when they burn through if they aren't plugged, which has already been covered here. Ever seen those staging shots where you can clearly see sparks and burning bits of BP flying away at the instant of staging? Those things burn through with quite a bit of oomph. I would also think "unmodified" -0 motors would also expose the inside of the BT to more flame than an ejection charge would.

Thanks for the advice. It sounds like a better plan is to "stager" the ejection charges to some degree so that they won't all "gang up"... and still give a second chance at deploying the chute.

I'm thinking of the initial test flight with an Iron Man action figure with an A8-3 trio of motors, but only have two left in my tackle box at the moment. If all goes well, and I hope it does, I was going to switch out the Iron Man plastic figure for a raw egg inside a "raincoat" (thank you Micromaster for the term) packed in sponge rubber foam, and then go for a trio of B6-4 or B6-6 depending on what I have available. Now, maybe I'll mix and match those B6-4 and B6-6 as per your advice!

On a completely side note: Does anyone know why suddenly I have images of a half naked old man leaning on a motor cycle popping up in the middle of my posts here on TRF? Is it happening to everyone, or are they targeting just me? ( I'd hate to think they think I'm pursuaded by a half naked man....)

Couple of things not mentioned:

Use the longest ignition wire leads available to allow the clips to ride up during those first few milliseconds of thrust. A reluctant motor may light on the way up.

Add a small amount of wadding to the aft end of each motor before loading. Cheap insurance against a back burn.

Good luck with the Scrambler. Launch report with pictures perhaps ?

Kirk G said:

I've completed the "Scrambler" cluster rocket from SemRoc and will prepare to launch it and an egg, action figure, plastic egg, hard-boiled egg or similar payload once or twice Saturday at our final club launch.

As I am assembling my rockets and motors and mentally counting how many I need of each value, it occurs to me that I've overlooked something simple and basic when planning the use of THREE motors on each launch.

I understand that the three should be of the same value.... THREE A8-3, or THREE B6-6 or THREE C6-5
or whatever the recommended motors are. But as I look over my packages, I find I have a set of B6-0 boosters that i was planning on using up slowly on a two stage rocket later.

Why could I not use them in the Cluster rocket? That is, why not TWO B6-0 and ONE B6-6 in each firing. Or TWO A8-0 and ONE A8-3 or TWO C6-0 and ONE C6-5

After all, how many ejection charges are really needed to blow the top off a nosecone? Aren't the use of THREE such motors a bit of over-kill? Or does it really matter?

Is it more important to use all three motors from the same package so that they are identical in every way, or should I save resources and only use one such motor with an ejection delay, while the other two could be straight boosters?

Thanks for a speedy response. I will only have a limited amount of time for final packing late tonight, and tomorrow's launches will be a frigid affair (lucky if it's above freezing at any time) and probably running quickly to get them done and out of the cold in SE Ohio.

Click to expand...

NO TWO motor will eject at exactly the same time! Just be sure Your model internal volume is correct to eject the laundry with a SINGLE ejection charge.
You do not have to worry about the number of ejection charges. As a matter of fact we cluster buffs count the Pop,Pop Pop's at ejection as an early read on how many motors actually lit.
There is a 10% +/- factor built into BP motor production, Add to that the differences in time it takes for igniters to heat, differences in lead length and resistance, time it takes the BP in each motor to reach ignition temperature, etc. just match up you motors as best you can (BY Weight) before each flight.

samb said:

Couple of things not mentioned:

Use the longest ignition wire leads available to allow the clips to ride up during those first few milliseconds of thrust. A reluctant motor may light on the way up.

Add a small amount of wadding to the aft end of each motor before loading. Cheap insurance against a back burn.

Good luck with the Scrambler. Launch report with pictures perhaps ?

Click to expand...

What's a "back burn"?

I'm familar with boating terminology, but "Fore" and "Aft" would seem to imply that you want me to "add a small amount of wadding to the REAR end of each motor before loading." WHY? Won't it ignite under concentrated burn of a motor? I'm confused here.

Kirk G said:

What's a "back burn"?

I'm familar with boating terminology, but "Fore" and "Aft" would seem to imply that you want me to "add a small amount of wadding to the REAR end of each motor before loading." WHY? Won't it ignite under concentrated burn of a motor? I'm confused here.

Click to expand...

a back burn is a really bad thing that is always possible when the forward end of your motors are exposed to the ejection burst. The burning bits can and do burn though the clay caps setting the motor off backwards usually destroying the inside of the model.

Cheap and very easy prevention is to use a 1/4 sheet of FP wadding in the forward end of the clusted motors held in place with a bit of masking tape. (SEE Photo)


If you just sport flying using a C6-5 an two C6-7 is an excellent remedy for your concern HOWEVER if your using this model in Egg-Loft competition the extra weight will cost you. When flying my 3 motor clustered SR-71's I use a C6-5 in the core and C6-7's in the outboards that are ducted into the main body.

I can't stress enough to WEIGH each of you motors and pick the closest matchs in weight for your clusters.
you will not get exactly the same weight for all three motor from the same pack. Every once in awhile they will be close but rarely have I gotten more then 2 motors from the same pack that have the same initial mass. Remember that 10% varience I mentioned earlier.

I'll mention again if you are serious about flying clustered BP Motor models my Tech-Tip 006 "Clustering BP Motors"
will answer most of the questions you've ask and give you a much deeper insight into what has to happen when you push the button for (Reliable) 2-12 motor clusters. www.narhams.org in the library section under Tech-Tips.

Another thing I'll point out is if you are not using Quest Q2g2 Igniters.. a Relay Ignition system is not A way to fly Clusters, "IT is the ONLY reliably way to ignite clustered motors".

Tomorrow I'll be flying 3, 4 and 7 motor clustered models just for fun.











View attachment Cluster Wiring Diagrams-2c_Dwg(Revised)_09-17-12.pdf



View attachment RangeBox Relay-a2c_Rev Drawing & Wiring_01-31-96 Rev.06-23-10.pdf

I must be missing something. Not only was there no photo, but I followed the link and didn't find any archieves and nothing Tech-Tip jumped out at me. But I'll go look again. EDIT: I found the Tech-Tip #006 in the library. Thanks for the info!
I think I understand the principle of why I should use the half sheets to cover the motors individually to prevent a "back burn".

I'm just a sport flyer here. I only built this cluster cause the club president is trying to lure me down the path to bigger, more expensive rockets... mpr and hpr rockets.... (You know, the darkside...) and I enjoy the lowpower rocket builds, as they generally are less expensive, and with two kids in college and a mortgage and car payments, I've got to watch how much I blow on this hobby.

Kirk G said:

What's a "back burn"?

I'm familar with boating terminology, but "Fore" and "Aft" would seem to imply that you want me to "add a small amount of wadding to the REAR end of each motor before loading." WHY? Won't it ignite under concentrated burn of a motor? I'm confused here.

Click to expand...

Sorry for confusing the terms fore and aft. Putting wadding in the rear of each motor is exactly what I'm suggesting, for the reason Micromeister outlines below.

Micromeister said:

a back burn is a really bad thing that is always possible when the forward end of your motors are exposed to the ejection burst. The burning bits can and do burn though the clay caps setting the motor off backwards usually destroying the inside of the model.

Cheap and very easy prevention is to use a 1/4 sheet of FP wadding in the forward end of the clusted motors held in place with a bit of masking tape. (SEE Photo)

...

Click to expand...

Thanks for clarifying my response Micro. Here's the result of a back burn when my Maxi Alpha clone launched on 2 of 3 E9's. The ejection charges lit the 3rd E9 from the rear.
Chaos ensued.




This doesn't happen often. You may fly 10 cluster flights with no issues. But the 11th flight.... like I said, cheap insurance.

Clusters are still the coolest.

Sorry for some reason it took forever to up-load the photos. they are there now.

When you go to the Narhams web site look on the left side menu. Click on library then the drop down menu will show Tech-Tips
you want #006 "Clustering BP motors".
Let me know if you have any more trouble accessing the Narhams site.

I've almost always enjoyed flying Clustered MODEL rockets, As i'm sure you know by now I do not and Will NOT fly APCP. It' against my religion! Personally think they are the downfall of model rocketry. but that's another story. Anyway I've said many times I'd rather cluster 13 to 24mm BP motors then fly a single APCP motor anytime and I mean it!
Been flying Clustered BP motors since the early 1970's. Seriously flying BP clustered models is as much Art as it is science. Once you get a few under your belt you'll be a lot more confident about what your doing but I do Stress the need for a decent RELAY Igntion system and the highest amp/hr gel-Cell battery your budget will allow. while a 7amp/hr HobbyCo will do for a few clustered flights a day, if you plan on flying other models along with the clusters when you really should look for a higher amp/hr rating say 17 to 26amp/Hr. either or both of these Gel-Cell batteries will deliver the necessary Amps to get 2-7 motor clusters with no problem. and can be recharged by converting a 12V 1000ma 120/12v wall pack overnight or on of the very inexpensive trickle chargers from Harbor Freight. I use these HFT chargers all the time.
Let me know if I can be of any other help