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?
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:
Various info/comments from the brain trust here:
-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).