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grandcross

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As part of my Saturn IB scratch build, I'm planning a cluster of 4x38mm motors around a 75mm main motor. The rocket can fly on the 75, but the 38s are what will make it awesome. Not having tried something like this before, I have some questions.

1. Battery(ies). I need to start 4x38mm motors. What sort of battery should I be using and how many? Will one do, or should I (for example) use several 9V (or LiPO or whatever) batteries wired in parallel as my power source? I can loft a bit of weight and I have a fair amount of mounting space available, so short of a car battery I'm pretty flexible here.
2. Electronics. I have a single perfectflite mini timer controller that I plan to set for immediately after take off. Will one be sufficient? Should I have one per motor or per 2 or per all? Is there a way to wire these for redundancy?
3. Timing. My plan is to light the cluster as soon as liftoff is detected. Should there be a delay? If so, how much and why?

And go...
 

MaxQ

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As part of my Saturn IB scratch build, I'm planning a cluster of 4x38mm motors around a 75mm main motor. The rocket can fly on the 75, but the 38s are what will make it awesome. Not having tried something like this before, I have some questions.

1. Battery(ies). I need to start 4x38mm motors. What sort of battery should I be using and how many? Will one do, or should I (for example) use several 9V (or LiPO or whatever) batteries wired in parallel as my power source? I can loft a bit of weight and I have a fair amount of mounting space available, so short of a car battery I'm pretty flexible here.
2. Electronics. I have a single perfectflite mini timer controller that I plan to set for immediately after take off. Will one be sufficient? Should I have one per motor or per 2 or per all? Is there a way to wire these for redundancy?
3. Timing. My plan is to light the cluster as soon as liftoff is detected. Should there be a delay? If so, how much and why?

And go...
Good questions...I'll share what modest experience I've had to start the discussion related to your three questions....1.) onboard power source 2.) onboard electronics 3.) timing/delay

As you've pointed out - I also like to have a reliable central core motor that will fly the rocket by itself, so I've always had outboards for "effect" only.
That gives some latitude as to when they need to light, since the flight doesn't rely on the outboards for anything other than "effect".
(Just as an aside - one of my earliest clusters had a good central motor with two canted outboards, once it left the pad, only one outboard lit..but in true Deuces Wild fashion, it flew straight, despite the unbalanced thrust of the outboards -minus one that did not light).

I've seen airstarts using on board relays with a lot of power - to light large bore motors where the flyer also wanted a noticeable delay after lift off, and they were dramatic with the time delay.
If I've got an adequate on board power source to light the airstarts, and a reliable central motor that will safely fly the rocket on it's own, and the outboards are not high thrust types - the airstart ignition delay is really a matter of preference.
I prefer to see the outboards light quickly off the pad so you can really see them before it goes out of sight.
Of course if you use a very visible motor type in the outboards - you'll see them anyway, even when it is up there.
There used to be a lot of interesting discussion on these forums (and the old Rocketry Online Forum) on low current vs. high current igniters, and voltage drop/brown outs on the fight computer if that was doing all the airstart chores as well as dual deployment.
I think we can retrieve some of that old info about testing resistance in different commercially made igniters.
I assume you won't be having that "brown out" problem - because you are using a separate timer to do the airstarts here.

My experience with airstarts has been limited to using GWiz altimeters. (I'll be moving to a Xavien FIST soon which has more capabilities).
I added a second 9V Lithium to the pyro channel as recommended with the GWiz specs.
The GWiz reliably detected lift off (if the G's on the central motor were enough to trigger the G switch) and fired immediately - so other than using a reliable hand dipped igniter with some home brewed supplemental pyrogen - I never had to worry about setting a timer for a delay and worrying about the outboards coming up to pressure at a certain time.
I can light a few composites ok - but recently reached the limit of the number of Ematch/BP outboard clustered motors with the dual 9V for whatever reason (more than four won't light - which is odd - as there was no brown out in the altimeter as it gave post flight readout).

Are there specs with the your Perfectflite Timer that say anything about recommended power source for cluster ignition?
Everything I've read says if you've got the power...and the right igniter, - wiring in parallel only.
What kind of igniters do you plan on using for the outboards? IIRC you were planning on using Cessaronis mellow yellow on the outboards, which should light quickly, maybe w/o a need to add pyrogen, blue thunder shavings, etc.
Keeping the igniters firmly in place in the outboards may be all you need to worry about.

With all the new Lipos and other stuff out there, I too would also like to hear what others are doing.
Thanks for posting this.

General discussion on the topic:
http://s605282183.onlinehome.us/wp-content/uploads/2016/01/Staging-Air-Starting.pdf
Various info/comments from the brain trust here:
http://www.rocketryforum.com/showth...-the-experts-needed/page2&highlight=airstarts


-The timing depends on what the goal is. If it is to achieve the max altitude I suggest building a sim of the rocket, and running some sims with varying ignition times.
If you are airstarting motors I would stick with something quick to light- maybe Blue Thunder. Again, depends on the goals. Some folks airstart smokers for effect.
-Faster burning propellant light more reliably for me
-Initiate the air starts at main motor burnout, it takes a few for the air starts to light.
-Make sure none of the igniters can move or fall out under thrust.
-IMO, the best time to light them is shortly after burnout of the main motor.
-Use a high thrust main motor to maximize the velocity when the outboards are lit.
-Best motors for the outboards are propellants that produce a lot of smoke (White Lightning, Imax, Smoky Sam, BlackJack, Skidmark, Dark Matter).
-I usually build the igniters into the outboards to prevent them from pulling out early.
-Do NOT use extremely high thrust outboards. If they don't light together, the rocket will be thrown off course.
-I'm wanting to do the same thing with a LOC Magnum - a K711 and 2 H115DM. I'm going to still coat the igniters with some blue thunder shavings and secure to the smallest wooden dowels and tape those to the OD of the nozzle.
-I used a Minitimer 4 and set it to .9 seconds after burn-out. I think an additional second would have looked cool. I was not sure how long the delay was going to be from Minitimer trigger-start until actual up-to-pressure so I kept it short.
-Apparently the 3cell Lipo lit things pretty aggressively as my guess is that they were up-to-pressure in under .5 second (or less).
 
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grandcross

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Great info! Thanks!

Perfectflite doesn't have any specific recommendations, other than ground testing to make sure it's sufficient. The main problem I see with this is I've never seen Cesaroni igniters sold separately, so they're not available for testing. I know they use 3rd party e-matches but they don't say which ones (used to be Oxral). I may contact them if no one else knows which they are. I'd definitely like to ground test first.

They recommend testing with 2x the number of igniters. For me, that would be 8. For the Oxral ematches, that would be 4A for 0.05 sec, well within the limits of the Perfectflite timer (up to 45A for 1 sec, 5-16V) as long as the battery would support it. But that may be different for their current ignitors.

You're right about my motors. I'm planning on 3G mellow yellow motors with low thrust primarily for the 7 second burn. It should look awesome :) It's not for altitude or speed - purely for wow factor!
 

sharkbait

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As part of my Saturn IB scratch build, I'm planning a cluster of 4x38mm motors around a 75mm main motor. The rocket can fly on the 75, but the 38s are what will make it awesome. Not having tried something like this before, I have some questions.

1. Battery(ies). I need to start 4x38mm motors. What sort of battery should I be using and how many? Will one do, or should I (for example) use several 9V (or LiPO or whatever) batteries wired in parallel as my power source? I can loft a bit of weight and I have a fair amount of mounting space available, so short of a car battery I'm pretty flexible here.
2. Electronics. I have a single perfectflite mini timer controller that I plan to set for immediately after take off. Will one be sufficient? Should I have one per motor or per 2 or per all? Is there a way to wire these for redundancy?
3. Timing. My plan is to light the cluster as soon as liftoff is detected. Should there be a delay? If so, how much and why?

And go...
The most I've down is 6, used two mini timers with regular 9V batteries and fired 3 then fired the other 3.
Also have done 4 off a single mini timer and used a 2s LiPo battery

Best E-match for air-starting is the orange Chinese E-match's available on Amazon or E-bay, the actually burn and don't just pop. If you are using CTI motors you'll be fine with a stock E-match, I also augment them by dipping them in Duco cement and then rolling them in 777 pyrodex. You must make sure the match stays on or in very close proximity to the ignition pellet at the top of the top grain in the CTI motors. You can accomplish this several ways. I use very small diameter wooden dowels taped to the match and/or small coffee sturring straws taped to the match.

My suggestion would be to set your timer to lift off with a 1 to 1.5 second delay to accomplish the flight profile you were describing.
 

MaxQ

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I'm hoping someone better at electronics than me can explain some basics about how altimeters and flight computers fire pyro channels, and how long they draw current to do so, during the flight.

Over the years - before there was a lot of information available on Forums - I did the "old school" method using thermalite (not to be confused with thermite) fuses to outboards, tried and failed with nichrome igniters in BP outboards, and various ematches that did (and in some cases did not) light. Curious about the length of time an altimeter will send the pyro charge to the airstart motors....I suspect they are all different, and the additional 9V battery I used on the GWiz, as advised for the pyro channel, should make more igniters fire, but it didn't seem to do that for me.

BTW: If anyone here is curious about ground starting (igniting) large clusters as opposed to airstarting clusters - just do a search for discussions with the cluster king himself....Boris Katan.
http://bpasa.com/
 
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NateLowrie

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The most I've down is 6, used two mini timers with regular 9V batteries and fired 3 then fired the other 3.
Also have done 4 off a single mini timer and used a 2s LiPo battery

Best E-match for air-starting is the orange Chinese E-match's available on Amazon or E-bay, the actually burn and don't just pop. If you are using CTI motors you'll be fine with a stock E-match, I also augment them by dipping them in Duco cement and then rolling them in 777 pyrodex. You must make sure the match stays on or in very close proximity to the ignition pellet at the top of the top grain in the CTI motors. You can accomplish this several ways. I use very small diameter wooden dowels taped to the match and/or small coffee sturring straws taped to the match.

My suggestion would be to set your timer to lift off with a 1 to 1.5 second delay to accomplish the flight profile you were describing.
John's suggestions are excellent. I second the orange lead ematches from Amazon. I use them for ejection charges and haven't had one fail yet. Just measure your resistance on them and make sure they come out right.

One point to make: CTI "igniters" are really just ematches. CTI can use a small ematch because they use a Pyrodex ignition pellet at the head of the motor to augment the match. The match lights the pellet and the pellet brings the motor up to pressure. If you don't position the match next to the pellet the motor won't ignite.

That said, the ematch/pyrodex system is more reliable for airstarts because of the power requirements. An ematch typically has an all fire current of 1A. Three of them wired in parallel will be fine for most altimeter channels which typically have FET limits of 5A. Because of the lower power requirements a cluster of ematches tends to ignite closer to one another than a cluster of higher current igniters.

Regarding batteries: A 9V will work for ematches. I would use a lipo. A small 300mah 2S lipo should be lighter than a 9V, has 3 times the capacity, and several times the current capacity.
 

new2hpr

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If you're using the CTI motors with the BP pellet in them and their included ematches, you should have no problem whatsoever lighting 4 with the PerfectFlite Minitimer. I've done 3x 38's airstarted with the Minitimer 3 and a Duracell 9V battery (yes, just one). I used a couple second delay, but you can set it where you want for best effect. Just make sure the main motor will give you enough G's for the timer's g-switch to detect liftoff.[video]http://vid424.photobucket.com/albums/pp321/new2hpr/Rocketry/Excess%20In%20Moderation/ExcessInModeration-airstartflight.mp4[/video]
-Ken
 

Rocketjunkie

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NOTE! The Perfectflite timer requires time after liftoff. Minimum time to firing is 1/2 second after liftoff detect!
 
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