Clustering reloadable motors

When I take to people about clustering re-loadable composite rocket motors it is like I was insulting them. Why is that? Is it inadvisable to cluster reloads?

Andrew

Lack of confidence in motor retention?

Fear of losing cases?

Lack of confidence in composite cluster ignition?

I've always viewed model rocketry as an opportunity to learn through failure. However, that requires a willingness to take some (reasonable) risks.

If you aren't put off by the three above issues there's no reason not to cluster reloads. If you are, work up to it by tackling each problem using single use motors. When you're comfortable with your techniques, go for it.

accooper said:

When I take to people about clustering re-loadable composite rocket motors it is like I was insulting them. Why is that? Is it inadvisable to cluster reloads?

Andrew

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That's what I say. I have a couple of designs that I believe would be great as clusters.

Andrew

accooper said:

When I take to people about clustering re-loadable composite rocket motors it is like I was insulting them. Why is that? Is it inadvisable to cluster reloads?

Andrew

Click to expand...

Clustering composite motors require paying attention to detail, and frankly a lot of folks don't have the attention span to do it right.

Boris Katan, Delta22 on TRF, is an expert in clustering. He wrote a great article on Clustering for Sport Rocketry a few years ago, and has successfully lit 30 of 30 motors on his Toginator several times. Several keys to his success is his ability to concentrate on the 100% testing of the igniters that he makes, followed by installation of the igniters into each motor and retesting the continuity afterwards, hardwire connecting the igniters in series strings according to a written wiring diagram and test plan, and retesting for continuity after each step, and then hardwiring the series strings in parallel and again checking continuity.

Boris makes his own igniters, and he performed many test firing in the beginning to make sure his manufacturing methods were consistent. It takes a couple of hours to prep the Toginator, and he uses a written checklist to make sure he doesn't forget to do something.

Smaller clusters don't require quite that amount of time, but do require the same attention to detail. The most important items are to check individual igniter continuity after installation into the motors, and to hard wire the igniters together rather than using clip whips. Do these simple things and follow a check list and you'll have a successful launch.

Bob

i wouldn't light the motors in series, that makes for some big problems. True it does save you if one of the igniters doesn't have continuity, but...if one has a slihgtly lower resistance and it goes first, all the other connections break, which is what happened when eric gates wired that way in the girl swing episode of mythbusters

parallel requires a continuity check of each igniter, but all can go off even when one goes slightly early

I was going to suggest you contact Delta22 but Bob beat me to the punch. I have contacted him about similar questions in the past and he has always been most thoughtful and thorough.

Look him up in the directory and check out his avatar.

I've been clustering a pair of 29mm in a rocket I recently built. I only have two flights on it so I am no where near an expert, or even very experienced with it, but based on BP clusters and what I've seen of the composites, I can say this:

1. You have to have a dependable, repeatable, and consistant igniter that will definately ignite the motor.
2. You want to make sure the igniter leads are long enough that if one motor lights first, the leads can follow the rocket upwards some distance to allow the (second+) motor to ignite.

My first cluster was with a pair of G33J hobby line motors. One had been in my pile for several years and the other I bought from a vendor. That one had so many stains on the liner and grain cases it looked like it had sat for 20 years. I sanded the c-slots and left the dust in place. I then used my own homemade igniters instead of the copperheads that came with the motors. A movie of the launch showed the one motor lifting the rocket about 18" before the second motor kicked in. I think that both motors were lit from the start and one came to pressure before the other, but allowing the igniter wires to lift some if the rocket lifts off could help make sure that other motor ignites.

[YOUTUBE]uSVlwipaDzc[/YOUTUBE]

I am just getting in to clustering and I will second the advice on igniter testing. I make all of my own igniters, I get quick ignition, and almost no ignition failures. Having read a lot about clustering, I know the importance of rapid ignition, so I decided to do some testing of lighting multiple igniters alone under the inspection of a 30 fps video camera. The results were somewhat shocking. Getting all of the igniters to fire at the same time is non-trivial, and required multiple changes to my launch controller and ignition setup.

If you are not already using a "hot" igniter, I strongly suggest upgrading to one and testing it (them, rather) under inspection of video camera with the launch controller setup you will be using it with.

I'm enjoying these posts and have been researching past ones on cluster ign. I received alot of great tips a few weeks back here on TRF regarding my LOC ViperIV. Last week I made my L1 Cert Flight, so now i'm "legal?" to attempt my first cluster launch at next months SRA club launch.(We have 10k'FAA waiv) I'll use clubs 12v system, but was going to use my own homemade whips, or maybe wire one leg of ign to next ect. Still not sure which,(ideas?) I purchased Quest 4" igniters I heard their pretty reliable. Motors: E9-4, friction fit w/taped nozzle thrust retain, screw/washer motor retn. I've been waiting to fly this BEAST for too long...only 3 more weeks! First attempt will be E's but I really want to see the sight/sound of 4 F21-8's.....Lots of good advise is available right here on TRF, so theres no excuse to go to a launch too unprepared for anything new...but experience only starts after 1st attempt!...PS, I recieved great info from Jdcluster,Cluster King, and Handeman,as well as many others on motor clusters..

edwinshap1 said:

i wouldn't light the motors in series, that makes for some big problems. True it does save you if one of the igniters doesn't have continuity, but...if one has a slihgtly lower resistance and it goes first, all the other connections break, which is what happened when eric gates wired that way in the girl swing episode of mythbusters

parallel requires a continuity check of each igniter, but all can go off even when one goes slightly early

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Get a copy of Delta22's article. Series ignition is succcessful because the igniters don't burn out as it's the pyrogen that lights and ignites the motors.

Bob

bobkrech said:

Get a copy of Delta22's article. Series ignition is succcessful because the igniters don't burn out as it's the pyrogen that lights and ignites the motors.

Bob

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Series ignition is not recommended with standard igniters. With E-matches, it can be a good idea, but standard igniters have enough variability that one can indeed burn out the bridgewire before another one in series has a chance to fully light. In addition, series strings of igniters, even if consistent, significantly reduces the voltage to each igniter, necessitating either a higher voltage source or a lower voltage igniter than is commonly used for high power rockets.

cjl said:

Series ignition is not recommended with standard igniters. With E-matches, it can be a good idea, but standard igniters have enough variability that one can indeed burn out the bridgewire before another one in series has a chance to fully light. In addition, series strings of igniters, even if consistent, significantly reduces the voltage to each igniter, necessitating either a higher voltage source or a lower voltage igniter than is commonly used for high power rockets.

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Again. Get a copy of Delta22's article in Sport Rocketry. He makes his own igniters from a kit and has tested and flown hundreds of them.

You are correct that some high current igniters like copperheads are not suitable for series connections, but lower current Estes type igniters are designed for 6 volts, not 12 volts, and 4 Estes igniters consisting of 2 dual strings in series paralleled at 12 volts draws what a single Estes igniter does at 12 volts. If you check continuity at each step of the process, the series/parallel approach works fine.

Series ignition is succcessful because the igniters don't burn out as it's the pyrogen that lights and ignites the motors. If you use the wrong type of igniters, it may not work, but Boris did a large number of ground tests before he launched his clusters successfully, and has been doing it successfully for several years, and I have witnessed most of them.

Bob

bobkrech said:

Series ignition is succcessful because the igniters don't burn out as it's the pyrogen that lights and ignites the motors.

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It is *extremely* informative to experiment with different bridge wire material types, thicknesses, length, and supply voltage.

Short, thin bridge wires supplied with sufficient current will vaporize almost immediately. Longer, thicker wires will take a few seconds to start glowing cherry red. Depending on how long, how thick, and supply current, the bridge wire may continue to glow red, or progress to white and pop.

Once you have an understanding of how firing a single igniter works without any pyrogen, then you can experiment with watching how multiple igniters fire without pyrogen. Then you are ready to watch multiple igniters with pyrogen.

Even how you wire them up makes a difference. If you have an igniter that is slower to come up to pyrogen ignition temperature, a slight difference in resistance, due to supply wire length or resistance at the connections, *will* cause igniters to fire at different times.

After running these tests, I now understand why so many people have problems with reliable cluster ignition!

rockets4kids said:

Once you have an understanding of how firing a single igniter works without any pyrogen, then you can experiment with watching how multiple igniters fire without pyrogen. Then you are ready to watch multiple igniters with pyrogen.

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It's not a useful indicator to watch the bare bridgewire melt, as this won't happen to a properly pyrogen coated bridgewire.

Pyrogen ignites between 300C to 400 C but the melting point of NiChrome is about 1400C, so the pyrogen, properly coated, will ignite long before the bridgewire melts. Bridgewires have a heat capacity and thermal conductivity, and pyrpgen has a heat capacity and thermal coducutivity, so it actually takes some time for the pyrogen to ignite and the wire to melt.

Heat capacity is measured in Joules per gram per degree K. An 0.008" thick (32 gauge) nichrome bridgewire that is 1 cm long weighs 0.0025 grams. The head capacity of nichrome is ~0.5 j/g-C so 1 joule of energy raises the bridgewire ~200 C. If there are no heat losses, 2 joules would be required to heat the bridgewire to 400C, and 7 joules would be required to heat the bridgewire to melting.

1 watt is 1 Joule per second. In previous posts, I mentioned that you want 50 watts or more power to be developed by the ingitor bridgewire. Assuming no losses, it takes 2J/50j/s = 0.040 s to heat a 32 gauge bridgewire to 400C, and 7J/50 J/s = 0.14 s to melt the bridgewire. At 100 watts heating rate, the times are reduced to 0.020 s and 0.070 seconds respectively. In reality, most of the heat loss goes into the pyrogen to raise the temperature from ~25C to 400 C and then it lights. Provided pyrogen is coated on the bridgewire, it is not possible to melt the bridgewire before the pyrogen ignites, and indeed if there is too much pyrogen coated on the bridgewire, it could take a long time for it to ignite, and the bridgewire still would not have melted. That's why uniform dipping is important.

E-matches have much thinner bridgewires and less pyrogen, and will activate 10 or more times faster, typically in 0.002 seconds or less.

Bob

Many thanks. This explains the results I have been seeing after adding pyrogen. I'm down to getting all igniters (coated with pyrogen) to fire within 3 frames of 30 fps video. I think I can do better. Still working on tweaking things. My goal is to get at least 4 igniters to fire regularly on one frame of 30 fps video. Only once I get there (or spend sufficient time trying) will I actually attempt a cluster launch.

Wiretron https://www.wiretron.com/ has the most useful downloadable wire resistance/properties calculator I have found for a Windows based computer. https://www.wiretron.com/wtsetup.exe

There are 2 ways to reduce the pyrogen ignition time. By reducing the wire gauge, you reduce the thermal mass of the bridgewire so it will get hotter quicker. Reduce the diameter by 2 and you reduce the mass by 4, so the bridgewire will heat up 4 times faster assuming there are no thermal losses. (Not exactly true because the wire looses heat to the pyrogen and by conduction out the ends so you might not quite hit 4 times faster.)

Increase the power you put into the igniter. P = IV = I2R = V2/R Double the current through the bridgewire by doubling the battery voltage and you increase the power by a factor of 4 assuming no thermal losses and a battery with a low internal resistance that can actually source twice the current. (Not exactly true because the wire looses heat to the pyrogen and by conduction out the ends so you might not quite hit 4 times faster.)

Bob

accooper said:

When I take to people about clustering re-loadable composite rocket motors it is like I was insulting them. Why is that? Is it inadvisable to cluster reloads?

Andrew

Click to expand...

I launched my first AP cluster last July. It was CTI 54 mm J and 2 CTI 29 mm G in a LOC Magnum. It was a perfect flight!

this year I'm flying an AP/BP combo cluster, 1 AP, 6 BP.

To insanity and beyond!

Tsolo Dann

Cluster King said:

I launched my first AP cluster last July. It was CTI 54 mm J and 2 CTI 29 mm G in a LOC Magnum. It was a perfect flight!

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That's cheating though. With CTI motors, all the effort of clustering is gone :wink:

cjl said:

That's cheating though. With CTI motors, all the effort of clustering is gone :wink:

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then i am really going to be cheating at LDRS :lol:

A OOP Vaughn Brothers WILDTHING

7 CTI 29mm, and a central 38mm CTI :y:

all lit on the ground

I flew a 54mm K + 2 J285 from CTI. That were air-started about 1 sec later.


JD

Lit a M1315 and 4 K700's on Saturday; about the fourth time we have lit big motor clusters. Always have had them lite fast and consistant.