# Model rocket engine-clusters: Should they be connected in series or in parallel?

#### Stanly

Joined Mar 29, 2014
35
Hi everyone,

Recently, I have become interested in model rockets. (One of the national organizations involved in this field is the National Association of Rocketry.)

Typically, for low-power model rockets, a solid propellant engine is used to power flight. One of the major producers of such engines is the Estes Corporation, and other companies exist as well. A small device called a starter provides an electrical current to an igniter which is inserted into the engine, and this current ignites the propellant causing the rocket to lift off. (I think that I am describing the process more or less accurately, but I may well be wrong in details.) A quick search on model rocket engines would lead to vast amounts of information on the topic. If you would like to go to one Estes site on the subject, you may click on https://www.estesrockets.com/rockets/engines.

Usually, one engine powers the model rocket. However, some model rockets are configured to hold several engines, and such a configuration is called a cluster. A cluster could contain any number of engines, but the usual number is three or four engines.

Now, here is my question please: Having researched the issue, I find out that the cluster of engines can be hooked up either in series or in parallel. So, what are the advantages and disadvantages of each method? Should I connect the engines in my engine cluster in series or in parallel?

Thank you.

Stanley

#### Ylli

Joined Nov 13, 2015
1,060
Using the standard Estes igniters, you want to connect them in parallel. These are cheap devices and there is no guarantee they will maintain continuity when they fire. With a series connection, if one motor ignites, burning and opening its igniter, then the others may not go as there is no longer current flow in the loop.

If you want to spend $1-$2 each on high quality ematches, then you can connect them in series as these are designed to maintain continuity after they fire.

#### Stanly

Joined Mar 29, 2014
35
Hi Ylli,

I intend to be flying a three-engine cluster. So I would be willing to pay a few dollars extra to more likely insure that all the engines fire.

Could you please provide me a link showing me where I could purchase high-quality ematches? I had not heard of those.

Stanley

#### Ylli

Joined Nov 13, 2015
1,060
Most of the 'better' igniters are meant to be used with larger composite (APCP) motors. But here's one for BP motors that you might consider: https://electricmatch.com/rocketry/see/23/6/bp-rocket-starter

But the basic Estes Solar motor starter is likely to be reliable enough for a cluster of 3. Just wire them in parallel, and use 6 - 12 volt battery that can deliver 5 amps or more. Don't try a cluster with the little estes launch controller.

Last edited:

#### Stanly

Joined Mar 29, 2014
35
Hi Ylli,

But now I am confused. I searched Estes Solar motor starters, and the image that I am getting looks like the regular starters that I get when I purchase Estes black-powder engines. So first, are you saying that I don't need to buy the item that I saw in the link that you sent me?

And second, what device do I use to deliver 5 or more amps? I am not good at rigging things up by myself. I need to purchase something.

Thank you.

Stanley

#### Ylli

Joined Nov 13, 2015
1,060
The Estes solar igniter (motor starter) is the one that comes with the motors. As long as you handle it carefully and insert it all the way into the motor, it should be 95% reliable. Most people are satisfied with that.

If you are trying to fly a cluster then you need more current that the little Estes controller can provide. A car or motorcycle battery, or a 12 volt 'gel-cel' will provide enough juice. Some also use li-ion packs such as you might get with a power tool.

Ideally, you want to have a safety interlock switch, and a 'Launch' button. Attached a picture. You don't really need the LED/resistor, and you will have the three igniters connected in parallel.

#### Stanly

Joined Mar 29, 2014
35
Hi Ylli,

Well, thank you very much for your assistance. I now realize, however, that launching a clustered rocket is beyond my skills. I had also had my eyes on a two-engine clustered rocket, but apparently any kind of clustered arrangement requires more knowledge and skill than I have.

For the time being, at least, I am going to have to give up the idea of launching clustered rockets.

Thanks again so much for your help.

Stanley

#### Wendy

Joined Mar 24, 2008
23,065
When I was int the hobby I had a lot of problems with clustered engines.Theonly time I got it to work was to use flashbulbs. whicharen't being made any more

#### Stanly

Joined Mar 29, 2014
35
Hi Wendy,

Thank you for the message.

Yes, I can see that I had better concentrate on single-engine rockets.

Stanley

#### be80be

Joined Jul 5, 2008
2,070
Clustered rocket is not easy to do I made front page in Kansas city Kansas 45 years ago.
I fired a 3 stage 5 engine 3 fire on take off and then fired the stages I took the 12 battery
out of my Moms car to fire it I misfired with the controller I had that used 4 D batteries.

Was a lot of fun seeing it go up but kind of hurt I lost the rocket tho.
Next thing I did was work on a tracker that what got me started really with electrictronics.

#### Wendy

Joined Mar 24, 2008
23,065
The problem as I see it is timing When one engine ignites the connections or the igniters are yanked out.The flashbulbs were taped to the engine nozzles and a quick fuse under the tape made by centauri rockets carried the ignition to the engine.I believe centauri rockets are long defunct.the fact the arrangement was firmly connected mad the difference. The article I got the idea from they successfully lit off 36 engines in a test bed rocket I think heavy amperage would make a difference.

#### WBahn

Joined Mar 31, 2012
27,868
Every time I every launched a cluster (always a cluster of three C or D engines) I used the standard Estes solar igniters. I used a homemade jury-rigged box with two 6-V lantern batteries in parallel. May not have been the best choice -- I was in sixth grade at the time and didn't know didley about battery internal resistance, I just knew that the tests I did using C cells didn't work too good but the lantern batteries worked every time.

#### Wendy

Joined Mar 24, 2008
23,065
It may be you need to heat igniters as quick as possible.

#### Ylli

Joined Nov 13, 2015
1,060
Stanley, sorry if I confused you too much. Perhaps you might want to see if there are any NAR clubs near you and contact them. Or go to one of the organized launches. Look here: http://www.nar.org/find-a-local-club/nar-map-locator/ You'll find a local club friendly and eager to help a newcomer.

#### Stanly

Joined Mar 29, 2014
35
Hi Ylli, Wendy, be80be, WBahn, and everyone else,

First, I want to thank you for your helpful and instructive comments regarding my questions on clustered-engine rockets. I learned a tremendous amount. (Ylli, you by no means confused me. You explained a lot to me.)

Second, the issue of engine clustering continues to fascinate me, and therefore I could not drop the subject so easily. I looked again at one of the places that I check for model rockets, which is Apogee Components. I noticed that one of the clustered-engine rockets that they sell has the option of being flown with either three or with only one engine. The name of the rocket is Micro Sentra SRB. If you want, here is a link to that rocket as shown on the Apogee Components website:

https://www.apogeerockets.com/Rocke...o-Sentra_SRB?zenid=hqi2hhtl8hgud1jorjp4g9k1r6

The one engine in the middle holds a 29 mm engine by default, but an engine adapter is included in the kit for a 24 mm engine. Thus, a low-powered Estes engine could be placed in the central location.

Using the very educational rocket-flight simulator that I purchased from Apogee Components, which is called RockSim version 7, I have tested flying this rocket with only the one central engine. In particular, I have now had acceptable success using an Estes D12-5, and only slightly less success with an Estes D12-7.

I am seriously considering buying this rocket, although building it will be highly challenging. If I succeed in constructing it, however, then I will be able to fly it as a one-engine low-powered rocket — much more suitable to my still elementary rocket skills. And then, at some future time, I will have the possibility of attempting to fly a clustered-engine rocket.

For everyone’s information, a cluster containing an even number of engines could never safely be flown on fewer than all its engines. For example, if the rocket had four engine mounts in the shape of a square, and if one of the engines failed, then the rocket would be seriously unstable and might well fly dangerously off course. But all of you skilled and knowledgeable people probably realized this already.

So thanks to all of you again, as I now proceed with my rocket experiments.

Stanley

#### Ylli

Joined Nov 13, 2015
1,060
That is probably a little bit advanced for a first timer, but doable. Have you tried a D12-3 in the sim? And do look into finding a local club. Again, you will find other rocketeer friendly and always ready to help.

#### WBahn

Joined Mar 31, 2012
27,868
For everyone’s information, a cluster containing an even number of engines could never safely be flown on fewer than all its engines. For example, if the rocket had four engine mounts in the shape of a square, and if one of the engines failed, then the rocket would be seriously unstable and might well fly dangerously off course.
What if two opposed engines failed to light? That would be fewer than all its engines yet would be a stable thrust configuration.

The same could be said for most odd-engine arrangements. A typical triangular 3-engine cluster will be nearly as badly imbalanced if anything other than all three ignite. A three engine cluster that is all inline will only be balanced if the center engine fails.

#### Stanly

Joined Mar 29, 2014
35
Ylli: Yes, I realize now that that was too advanced. I am now looking into a Hydra VII by Semroc Astronautics. At least that is a low-power rocket, although the construction is still advanced. I guess that I am looking for a low-power, clustered rocket that has one central engine.

I have gone to one rocketry event. The problem is that no local club is very close to me. But I am examining the situation.

WBahn: Your point is well taken. I am just saying that if I have one central engine, then I could use only that to get a balanced lift off, as long as that one engine provided enough thrust. You are correct, of course, in noting that a triangular configuration would never allow that.

To anyone, I have this question:

Back to the original issue of connecting a cluster in series or in parallel. Assuming that all the engines did fire, is one circuit preferable to the other? Assuming that all the engines were of the same type, am I correct in believing that either a parallel or a series circuit provides the same voltage and current?

And along the same lines, is it more important that there be sufficient current or sufficient voltage in igniting the rocket engines?

I thank everyone for their assistance.

Stanley

#### Ylli

Joined Nov 13, 2015
1,060
One of my cluster egg loft contest models had a center D12 motor and three D12's surrounding it. Named the rocket "Fat Chance".

I'll reiterate that basic igniters such as the Estes solar igniters need to be wired in parallel. Nothing occurs simultaneously, and these igniters tend to burn open. If in series, first motor lights and then there is no current to the igniters in the other motors.

Estes igniters need at least 1.5 amps each. They need *amps*, but the amps are produced by providing a voltage to the resistance, so the answer to your last question is "yes". You need enough voltage to produce the required current, and a voltage source that is capable of supplying the required current.

#### WBahn

Joined Mar 31, 2012
27,868
Back to the original issue of connecting a cluster in series or in parallel. Assuming that all the engines did fire, is one circuit preferable to the other?
If we can assume that all the engines fire, does it matter?

One circuit is preferably to another primarily in the which has the greater likelihood that all of the engines will fire.

Once we achieve a sufficient likelihood on that, then we can talk about other factors unrelated to achieving reliable ignition of engine clusters. That might include such things as battery selection for each option.

Assuming that all the engines were of the same type, am I correct in believing that either a parallel or a series circuit provides the same voltage and current?
Not really. You use a different circuit for the different connections. The circuit, however, is designed so that each individual igniter gets the same voltage and current that it would if used with a single engine cluster.

If you have an N-engine cluster and you put the igniters in series, then you need a circuit that can deliver N-times the voltage but the same current.

If you have an N-engine cluster and you put the igniters in parallel, then you need a circuit that can deliver the same voltage but N-times the current.

For a small value of N, then either option is reasonable. But for larger values of N (say 5 or more), then you need to consider your options more carefully.

Another option is to do a mix. If you have N = X*Y engines, then put X engines in series and have Y of these sets in parallel.

For higher reliability, you can also put two igniters in each engine (if the engine can accommodate two igniters, which all the Estes engines I've ever used could) with two independent ignition circuits activated by the same button.

And along the same lines, is it more important that there be sufficient current or sufficient voltage in igniting the rocket engines?
This issue isn't which will provide sufficient voltage or sufficient current -- the circuit can easily be designed so that, at least initially, the igniters don't know the difference. Besides, you can only get sufficient current if you have sufficient voltage and vice versa.

Where the big difference comes in are the failure modes. In igniter is broken, then in the series connection none of the igniters will work and so you will have no launch and can fix the problem and try again. In the simple parallel case, you would get ignition on only the good igniters, which is not what you want. So that's an advantage of the parallel case. But, if you are using certain types of igniters, include the usual Estes solar igniters, then they tend to fail open as a normal part of the ignition sequence. So if they are wires in series, the first motor to ignite might break the circuit before one or more of the other motors ignite. But if they are wired in parallel then this doesn't matter.