cluster comparison

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radiO

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so i was wondering if anybody has or can give me a comparison of multiple motors to single. now i am not talking exact here or anything, just a rough idea. and i'm just talking clusters up to 3 E's. for example, i heard 3 C6's is close to a E18. is three D's close to an F? maybe a list of different configurations from B to E? my dad is working on a 2-C with a D in the middle??? for all i know this might be a stupid question, but it comes up in the field when me and my dad are talking about clusters and what they might compare to.....
as always, thanks for your time!
 

utahrc

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Three C6s will provide the same average impulse as an E18, though the total burn time may not be the same, depending on what the total impulse of the C and E are. An estes C6 has a total impulse of around 10 N-sec, meaning it burns for about 10/6 = 1.67 seconds.

It's not really possible to make a listing of conversions just based on the letter size of a motor, as each letter denotes a range of total impulse. The range of a D is twice the range of a C, but a D at the low end of the D range will only me slightly more powerufl than a C at the top end of the C range. A big D, however, may be more than two times as powerful as a small C. I believe an Aerotech D is about the equivalent of 3 Estes Cs.

You can see a list of the ranges for each letter here:
https://www.ninfinger.org/~sven/rockets/rmrfaq.1.html#q6
 

powderburner

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utahrc gave you some good pointers on assessing the true differences in impulse classes

here are some more thoughts on clustered BP v.s. single composite motor-

with a clustered design you have an added level of complexity in the ignition system, and maybe a bigger battery required

in a clustered design you run the risk of delayed or failed ignition of part of the motors----this would effectively reduce your peak thrust and may make stability (immediately above the launcher) a bit dicey

with a clustered design you tend to have more weight in the back end of the rocket, which changes (for the worse) the size or amount of fins required for proper stability

with a single mid- or high-powered motor you have the (relative) problem of powerplant availability-----I know this is no big deal for some guys, but in many areas there are no stores to walk into and pick up an Aerotech (or equivalent) and you are forced to plan ahead, mail-order, and wait--- with BP motors you can usually find them in a hobby store, in stock

with a composite motor you begin to get seduced into using the reloadable motor casing thing, which means your rocket is now carrying an additional $20-30-40 investment ---kind of painful when it lands in the top of a tree or blows away in the wind

launching a composite motor generally makes more noise, which can tend to attract the (unwanted) attention of local residents who call the police who tell you to stop launching--- it has happened to me!
 

OKTurbo

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Here's a great tool for finding the total impulse of your cluster.

https://www.info-central.org/index.cgi?propulsion

Some good points have been brought up here...but be careful when you pick the delays on your cluster if you're using different sizes. You don't want the chute coming out too soon. Either plug or vent some of the motor mount tubes. For example...don't use 2 C6-5's with an E9-6...the C6-5's will pop the chute while the E9 is still burning.

John
 

radiO

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wow that is EXACTLY what i was looking for! that total cluster calculator is awesome! and thanks for the tip on the delays, that makes total sense!
 

powderburner

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use two (or even three) of the motors to send ejection gas to the recovery bay; vent the others overboard or let the empty casings eject themselves

you can try plugging, and a lot of people have had success at doing this themselves
be aware, however, that this amounts to 'modifying' a motor and you will not be able to launch at a NAR-sanctioned event (for whatever that's worth to you)

the reason you want to plumb the insides of your model to use more than one motor's ejection gas is simple: if you use only one motor then you are gambling on getting that one ignited on every launch
if you have a flight where all the other motors ignite EXCEPT your 'recovery' motor, then you are in trouble

you can greatly increase the odds of a successful flight if you use at least two motors to provide ejection for your recovery system
you do not have to worry about excessive pressurization due to simultaneous ejection pulses because there is a very very (did I say very?) low statistical probability of this----there are too many variables at play for this to happen
 
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