Cluster Thrust VS Single Large Motor

[techrat]

Okay guys, a math question here: I am currently building a Quad Runner, HOWEVER, I am converting from a 4, 18mm cluster to a single 24mm motor mount, simply because I consider clusters to be problematic, especially where I launch regularly, seen too many failures there.

Here's the Math issue: 4 "C6-5" motors in terms of total impulse would equal an "E" motor, but I do not think that the thrust is simply 6x4=24, meaning an E12 would be fine. In my mind, thrust is how the motor burns through the propellant, so a C6 is "6", no matter how many of them there are burning simultaneously. So an E12 is double the thrust regardless of how the cluster would have played out.

Am I totally wrong here? I'm going to epoxy the fins on regardless, and there's plenty of fin-root area, so I'm in no danger of ripping off fins even I put in a 24mm F motor. I just want to make sure that I'm thinking the way I should be thinking, or is the math guesswork in my head completely off???

Thanks in advance for helping this confused rocketeer.

 

[Steve Shannon]

You are totally wrong. Thrust is instantaneous. Four motors which each have six Newtons of thrust provide a total of 24 Newtons if they are all burning at the same time.
Total impulse also sums.

 

[rfjustin]

 

[techrat]

OK then.... so an E12 would be slower off the pad than a quadruple cluster of C6-5s, since the cluster would equal an E24. Very interesting. So composite motors would be OK in my configuration. Wow, that totally changes things in my head... Thanks!

 

[Steve Shannon]

OK then.... so an E12 would be slower off the pad than a quadruple cluster of C6-5s, since the cluster would equal an E24. Very interesting. So composite motors would be OK in my configuration. Wow, that totally changes things in my head... Thanks!

Generally speaking, yes, but to see the actual thrust at a particular time, you should look at the thrust curve of the motors. If that E12 has a spike at the beginning that exceeds the sum of the 4xC6 motors, that must be considered.
Look at ThrustCurve.

 

[smstachwick]

You can get some interesting results with clustered composites, too. A quad mount of C18 Q-Jets is equivalent to a nearly-full impulse E72.

But as Steve says, check the data from ThrustCurve or OpenRocket.

 

[BDB]

I think the reason to cluster is that it's fun to try something different and challenging. Generally, you can get better and more reliable performance from a single motor. But it's a hobby, so build it however you want.

 

[BABAR]

RocketReviews.Com also has a calculator that allows your to creat a thrust curve fo individual and clustered motors.

https://www.rocketreviews.com/combine-motors.html
I dont cluster much, except when I add an auxilliary parachute pod to a long gap stage booster, in which case I usually use a very small motor cuz all I need is a motor with a short delay and an ejection charge to pop the chute 2-3 seconds after separation (I learned the hard way that you DO want the booster to slow down a bit before you deploy its chute, it does NOT instantaneously STOP at separation).

i use the COMPARE two motors a LOT especially in decisions like whether to go for a larger motor mount, or comparing the Quest composites to the Estes black powder motors.

an interesting one is the Estes 24mm D vs the Quest 18 mm D


https://www.rocketreviews.com/compare-motors-508946.html
@Antares JS , I have an Estes Outlander in my build pile, I think a while back you said based on experience the Quest D was NOT a good motor choice? I had planned a while back when I bought it to mod it for an Estes 24 mm D, as I heard the Estes C recommended motors were woefully underwhelming.

 

[waltr]

Do use Thrust Curves to determine speed off the rod/rail and overall performance.
Thrustcurve.org is a good resource as well as OpenRocket simulator. I would build the rocket in OpenRocket using the cluster then do Saveas to a different name and change this one to a single motor. Then compare performance on the two configurations.

Be careful about motor numbers, it has been noted (a thread here) about Estes motors numbers. A C6 does not have 6 Newtons average thrust but only 4.7N average and with a large initial thrust of about 11.5N. There is history about why these are not average thrust in Newtons whereas on almost all modern composite motors the number is the average thrust in Newtons.

 

[Antares JS]

@Antares JS , I have an Estes Outlander in my build pile, I think a while back you said based on experience the Quest D was NOT a good motor choice? I had planned a while back when I bought it to mod it for an Estes 24 mm D, as I heard the Estes C recommended motors were woefully underwhelming.

Other way around, you MUST use either the Q-jet D16 or the 18/20 D13 reloadable with the shortest delays. I haven't tried the D20 but I suspect it's too much thrust. It's the C5-3 that I said does not work.

 

[dr wogz]

there is also a weight trade off. 4x C6-5 motors outweigh a single E12. (At least, I believe so / would assume so) that that will have an effect on your flight profile.

 

[heada]

Not specific to this configuration but could be a consideration in others: a cluster of motors, especially in 24mm, can have a total propellant mass above the FAA class 1 limit and so force the need for an FAA waiver. FAR 101.22 limits class 1 to 4.4oz of propellant total. 4x E12 is 5.2oz If you were to try and cluster 12x C6 motors, you'd go above that limit too.

 

[BABAR]

Just a conceptual thought comparing model rockets to NASA and commercial payload rockets.

the NASA and commercial rockets have gimbaled motors and rely on computers and motor tilt adjustments for stability and minimal if any fins, (thus can launch from a standstill from a tower and don‘t need a rod or rail.) MODEL rockets have fixed motors and no directional control on boost and stability(the notable exception@ are @JimJarvis50 and @georgegassaway designs which I believe have some active fin guidance, Jim’s I think are tilt correcting and George’s I believe were ”sunseekers.” I guess @burkefj remote control boost gliders have SOME stearability on boost also.)

anyway, back out of the rabbit hole.

whereas non-model rockets use clustering and staging for performance (altitude, payload weight, or both) where available single motors are either unavailable, insufficient, or impractical, MODEL ROCKETS for the most part accomplish the same goals EASIER by a SINGLE larger motor than with clusters or stagers.

for CLUSTERS, you can usually achieve more thrust or longer thrust with a single larger motor than with clusters with less weight penalty, and often with a smaller diameter rocket (which depending on payload my not help that much, although would reduce base drag.)

for STAGING, because model rockets use fins for stability, and you need a whole lot more fin area (or nose weight or both) to keep stagers stable, again for altitude you are usually better off going with a larger single motor (at least for my world in L-0 land, aka ”low power.” High power guys and gals may know different.)

point is (yes these is one), there really only is one good reason to do clusters or stagers: for the FUN of it (or challenge, kind of same thing.)

it’s also why for stagers I recommend going with the lowest power available motor practical for the sustainer, at least for first flight, and as far as I am concerned for all flights. If I want altitude, I can get it simpler and more reliability with a larger single motor Minimum diameter bird. For the booster, I worry about getting the stack off the rod or rail at a good speed to maximize stability and minimize weathercocking, so I use a big enough motor to confidently get the bird convincingly into the air. A booster motor with a big initial spike and a relatively short burn time works great, I get it off the pad, staging occurs low where I can see it, and the small sustainer motor gives me a satisfying second “whoosh” and burns out so I have a short walk, particularly if staging is off vertical, in which case a powerful or long burn sustainer results in a long (and sometimes disappointingly unsuccessful) trek to find the @#$&&$$ thing!

 

[techrat]

I agree with the above. While clusters are fun, and I'll get to that eventually, I'm enjoying the hobby with a single larger motor. That's why this Quad-Runner I'm building now is going to be a Mono-Runner. I'll get to the clustering, although if I've got access to 24mm E30's as well as some F motors, I can't see what exactly I'd get out of a cluster of C6's that I can't get out of a single larger motor except less reliability and the opportunity for my rocket to fly sideways if some motors do not ignite.

 

[BDB]

Now I'm just bragging...but my daughter loved to launch clusters. But more motors also means more chance for a CATO on any given flight.

And you can really up the complexity by clustering and staging at the same time.

 

[heada]

Consider a modular mmt configuration so that you could swap motor configurations. LOC has it in their 2.6" line where you can swap between 4x 18mm, 3x 24mm, 1x 24mm, 1x 29mm or 1x 38mm. If you could build in something like that you give yourself more options later.

 

[techrat]

Consider a modular mmt configuration so that you could swap motor configurations. LOC has it in their 2.6" line where you can swap between 4x 18mm, 3x 24mm, 1x 24mm, 1x 29mm or 1x 38mm. If you could build in something like that you give yourself more options later.

I believe the current incarnation of the Quad Runner has this capability, but I'm building an older model that only came with the 18mm quad, so I ordered a Semroc 24mm mount with the plywood centering rings... I also did not like their rotating "star" to hold the motors in place. I'll design/build something better in the near future. I may be able to mod the current rocket as nothing is glued in yet, so, I'll see what I can do. Perhaps I can build a switch-able Motor mount into the current build.

 

[Back_at_it]

While your math is not accurate I am completely with you on clusters be problematic and not worth the hassle. I have removed cluster mounts from a number of rockets including some early Estes pro-series in favor on a single mount. My own Quad-Runner was built with a single 24mm mount well before they started offering this as an option.

Absolutely build it as a single motor rocket and go enjoy it.

 

[smstachwick]

Just a conceptual thought comparing model rockets to NASA and commercial payload rockets.

the NASA and commercial rockets have gimbaled motors and rely on computers and motor tilt adjustments for stability and minimal if any fins, (thus can launch from a standstill from a tower and don‘t need a rod or rail.) MODEL rockets have fixed motors and no directional control on boost and stability(the notable exception@ are @JimJarvis50 and @georgegassaway designs which I believe have some active fin guidance, Jim’s I think are tilt correcting and George’s I believe were ”sunseekers.” I guess @burkefj remote control boost gliders have SOME stearability on boost also.)

anyway, back out of the rabbit hole.

whereas non-model rockets use clustering and staging for performance (altitude, payload weight, or both) where available single motors are either unavailable, insufficient, or impractical, MODEL ROCKETS for the most part accomplish the same goals EASIER by a SINGLE larger motor than with clusters or stagers.

for CLUSTERS, you can usually achieve more thrust or longer thrust with a single larger motor than with clusters with less weight penalty, and often with a smaller diameter rocket (which depending on payload my not help that much, although would reduce base drag.)

for STAGING, because model rockets use fins for stability, and you need a whole lot more fin area (or nose weight or both) to keep stagers stable, again for altitude you are usually better off going with a larger single motor (at least for my world in L-0 land, aka ”low power.” High power guys and gals may know different.)

point is (yes these is one), there really only is one good reason to do clusters or stagers: for the FUN of it (or challenge, kind of same thing.)

it’s also why for stagers I recommend going with the lowest power available motor practical for the sustainer, at least for first flight, and as far as I am concerned for all flights. If I want altitude, I can get it simpler and more reliability with a larger single motor Minimum diameter bird. For the booster, I worry about getting the stack off the rod or rail at a good speed to maximize stability and minimize weathercocking, so I use a big enough motor to confidently get the bird convincingly into the air. A booster motor with a big initial spike and a relatively short burn time works great, I get it off the pad, staging occurs low where I can see it, and the small sustainer motor gives me a satisfying second “whoosh” and burns out so I have a short walk, particularly if staging is off vertical, in which case a powerful or long burn sustainer results in a long (and sometimes disappointingly unsuccessful) trek to find the @#$&&$$ thing!

Agreed. I love staging enough that I currently reference it in my signature and see it as a worthy challenge for any rocketeer. Specifically, when I see that purple label that says “upper stage”, I take that as a challenge to use the motor that way instead of in a very light, narrow, altitude-hungry single-stage rocket.

But the utility of staging usually ends there, as a challenge, not as a shortcut, solution, or magic bullet. It’s normally something to be done because it’s there, not because it’s the key to new possibilities in and of itself.

However, it’s for this reason that I disagree with the idea that multistagers should be flown on the lowest impulse available. Sure, that’s a prudent thing to do for a test flight, but once successful recovery on an A8-0 to A8-5 stack has been achieved, why not swap that sustainer motor out for a B6-6 and go again? Why not work up to a C6-0/C6-7 stack and see if you can do that without having to replace either stage? That’s what I’ve been doing and I’ve learned much from that.

 

[lakeroadster]

Motor clustering is well worth the risk, it pays big dividends... I'd have thought a guy by the name of @techrat would be all in for some complexity?

Use the correct launch controller, and the correct ignitors... no worries.

 

[Grog6]

Clustered engines are also more weight, both propellant And casings.

 

[lakeroadster]

Clustered engines are also more weight, both propellant And casings.

Yawn...

 

[bjphoenix]

I agree with the above. While clusters are fun

I think black powder clusters are fun but I haven't done one in a long time. Estes has had cluster rockets in their catalog almost since the beginning, partly because back then the biggest motors we had were B. I built an Estes Ranger when I was young and launched it on both types of B motors. It's not in flyable condition right now and I don't plan to launch it again but I have another old BT70 rocket with 2-motor cluster that I would like to fly again. It has flown on a pair of B motors, now I can fly it on a pair of C motors.

This is the old Ranger on core-burner B motors.

 

[Tractionengines]

Not specific to this configuration but could be a consideration in others: a cluster of motors, especially in 24mm, can have a total propellant mass above the FAA class 1 limit and so force the need for an FAA waiver. FAR 101.22 limits class 1 to 4.4oz of propellant total. 4x E12 is 5.2oz If you were to try and cluster 12x C6 motors, you'd go above that limit too.

Hahaha....It must be late...I just sat here scratching my head at this for 10 minutes : going that doesn't seem right... then it dawned on me ounces vs grams. The limit is officially 125grams which is close to 4.4oz.

This is the one limit that I occasionally forget when planing a cluster launch...till someone reminds me. THANKS for the reminders.

 

[cls]

Years ago, TARC rules were for cluster 1st stage and single second stage. Target altitude was 1000 feet maybe? Lots of 3x D12s burned that year. No better way to get the required grunt off the pad. Was fun but glad the rules changed.

 

[jhill9693]

OK then.... so an E12 would be slower off the pad than a quadruple cluster of C6-5s, since the cluster would equal an E24. Very interesting. So composite motors would be OK in my configuration. Wow, that totally changes things in my head... Thanks!

You should also consider how the thrust curve compares, especially how high the peak is (makes a difference on speed off the pad) and how long the thrust lasts (affects average thrust). E12s have longer burns than C6s.

 

[techrat]

E12s have longer burns than C6s.

Which is a good thing in my book. Less thrust but longer burn == more interesting flight. Instead of the rocket disappearing in a "whoosh" (remember the mosquito?), you actually get to see it leave the pad and go up on a pillar of flame. Low and Slow is what I like. I actually built a super-draggy scratch-build I dubbed "drag queen" for an E12 and it flies better than I expected, with a nice slow lift off.

 

[smstachwick]

Which is a good thing in my book. Less thrust but longer burn == more interesting flight. Instead of the rocket disappearing in a "whoosh" (remember the mosquito?), you actually get to see it leave the pad and go up on a pillar of flame. Low and Slow is what I like. I actually built a super-draggy scratch-build I dubbed "drag queen" for an E12 and it flies better than I expected, with a nice slow lift off.

Mosquito? What Mosquito?

 

[Tractionengines]

I think this one.. here is part of Estes Description... NOT A CLUSTER, but

" When powered by the A10-3T mini engine, heights of 800 feet can be reached! That’s more than 200 times his own height! But buyer beware, with him being so small and using the most powerful mini engine we make, he might not return! So consider yourself warned. "

https://estesrockets.com/product/001345-mini-mosquito/
I think we have made 6 flights with 1 recovery. It just disappears.

 

[techrat]

I remember buying 2 of them as a kid and launching them..... They went up, and as far as I know, they never came down. Just... gone.