Time Delay between Cluster Motor Ignitions

Out of curiosity, and perhaps usefulness for future projects, I video-recorded 2 rockets launching simultaneously, using a Rocketarium Two Motor Cluster Cable, 12V battery, and the Aerotech Interlock Controller. Motors used were Estes A8-3.

On reviewing the video, taken with an iPhone 6 at 30 frames/second, I made the following table, noting frame and time where significant events take place.


I'm hoping to do more tests in the future with different motors, but if anyone knows where similar data is already available, I'd be interested. For my purposes, both rockets would ideally weigh the same.


(Frame 13 from the same video.)

What is your mission?

Would like to see this tested with a standard BP motor and a QJet composite.

blackjack2564 said:

What is your mission?

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I want to know what motors are reliable for clustering, and understand why. I know it works for BP motors, but I haven't yet found much information on the APCP variants. Comparing them all seems interesting, BP being the reference.

I guess the basic question is: What APCP motors can be clustered with my current setup, and how can the setup be modified to take care of the rest.

Connect a flash bulb as well to get a t=0 reference.

CTI motors consistently light as soon as you push the button. CTI motors have a BP pellet at the forward end of the forward grain. Great for clusters.

AT takes anywhere from a few factions of a second to several seconds after pushing the button - very inconsistent results even with the same motor over time. Not so great for clusters.

For the times I need to use multiple igniters to get a motor going, invariably it's with AT.

This is my experience using stock, unadulterated igniters.

Euclid said:

I guess the basic question is: What APCP motors can be clustered with my current setup, and how can the setup be modified to take care of the rest.

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So from above..your current setup is just for [2] 18mm motors? [hopefully at least 24mm]
Planning on larger AP motors in future?
Much to learn.

The WHY with AP motors;
Getting the motor up to full pressure quickly is the key. The many types/colors of AP all take different times to pressurize fully. SO to make it work with clusters either use a fast propellent like AT Blue Thunder or somehow augment the process with igniters on steroids or pellet in grains or painting the grain surface with pyrogen. ALL just to get AP motor to pressurize instantly so cluster motors all light at same time.

Depending on what access you have to motor choice. AT Blue Thunder has always been the easiest to light of their propellents. As mentioned CTI uses starter pellets in most of their motors. This can be replicated with AT motors also [add pellet to core] for 29mm high power. Hobbyline size motors are mostly C-slot making the igniter do all the work, as placing a pellet into a slot is nigh impossible.

Quick burst has a new Boron dip for making igniters/re-dipping Estes/AT/E-matches that is instant on I have been using BKNO3 for 5yrs now, lighting many large motors instantly .
My Go to cluster rocket has been a 54mm central with [3] 38mm outboards
I have lit/air-started them, with fuse in teflon tube, from central motor plume...copper-thermite and BKNO3 [Boron]
I now prefer the Boron for ease of use and safety.

Igniter choice or dip used makes huge difference .
Before getting into all that, what is your choice of motors now for your learning curve... size & type?

Simple Answer:
CTI 24mm motors of any propellent would be the simplest choice that would work with your set-up right now....no modification needed.[assuming you can do 24mm]
Because: they use a low current e-match to start a BP pellet pressed into top grain of motor=instant on.

"the rest" requires some research/learning on your parts as to augmenting igniters and motor grains to make light instantly.

Basically you need a cluster mentor. It would jump start you,in your quest.

Thanks for the info. Next thing I would do is try clustering 18mm AT motors incl. Blue Thunder. If I can do this with stock motors, I’m good to go for a while. I just got “back” into rocketry last year (had 1 kit in the 80’s) so it’s still quite a small collection. Building up slowly. No rush.

I still don’t get what it is about White Lightning that takes them longer to ignite. Great to know about CTI motors.

Euclid said:

I still don’t get what it is about White Lightning that takes them longer to ignite. .

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Simply put: the chemical make up of the propellent.
which is easier to light.....wood ...coal...oil, yet they all burn.

After pressing the button for about 2-3 seconds, I heard a soft pop, the rocket popped up an inch, and fell right back. "What now..." I waited a minute and dumped what I then handled as a time bomb in a nearby pond (I know it wasn't, and I went back a week later to retrieve it and properly dispose of it).

That was my first experience with an APCP motor: an AT 18-mm white lightning. Disconcerting to say the least. So if I'm going to cluster anything now, I need to do whatever it takes to prevent that kind of semi-ignition.

When clustering APCP motors, you need to know the typical start-up characteristics of each motor and not just propellant type but propellant type in that grain configuration in that case size using your igniter of choice. This can only be found through experience, either personally obtained or from a mentor. An AT 18mm with a c-slot White Lighting propellant using most igniters is going to be difficult to ignite. AT White is harder to ignite, c-slots more so. 18mm means very small nozzle opening so a small igniter. You'd need to augment it somehow just to get it going in normal use. Using that motor in a cluster is not something I'd advise.