The RAF Basset (prototype) streaks forward on a 5 motor cluster.

Igniting Clusters of Composite Motors

After several years of launching cluster rockets in various configurations, I believe I can properly classify myself as a minor expert in what can be done wrong. No doubt there are other clustering faux paus that I haven't managed to discover yet, but by drawing from this experience base I've listed a number of tips you may want to consider on your next clustering attempt.

These are the key success factors, as I perceive them, in order of importance:

1. Continuity: Measure the impedance of each igniter with an ohmmeter AFTER installation in the motor to verify it is still intact. Assure the igniter clips are clean. (Some rocketeers I know refuse to even use igniter clips when clustering and solder directly to the igniter lead.) Your launch system will probably not detect continuity for each igniter individually, so this is critical.



2. Launch Controllers: RTV has been fortunate to use Marc Casanova's cluster launch controller for several complex launches. It's an awesome device, capable of firing over 100 igniters at once. I've also use my own relay launch controller successfully to fire clusters -- both use a strong 12 volt batteries. Just make sure have enough power, whatever you use -- an underpowered launch controller is a quick way to waste a good cluster flight.



3. 
   The TBird motor cluster with too-short igniter leads.
   Igniter Lead Length: I view this as a very important factor. No matter how meticulous you are in your preparations, your igniters will not light at precisely the same instant. Install your igniters with this in mind, and prepare against having the motor that lights first pulling out the other igniters with its blast. Long igniter leads allow you to direct their path around the other nozzles and blast patterns. The early igniter can get ejected without interfering with the other igniters. Long leads also allow you to avoid the use of a clip whip (a wire harness that expands the number of igniter clips from 2 to x number in order to hook up all your leads).



4. Propellant Types:
   • Fast (Blue Thunder, White Lightning, Blue Baboon, Green Gorilla, Etc.) -- I know guys who swear that cluster success probability is increased by using fast propellant (some even use slivers of Blue Thunder propellant stuffed into a motor with the igniter). That may be true -- but when RTV held the Evil Bert Drag Race (essentially a 10 motor cluster spread over five rockets) the only motor that didn't pop was Blue Thunder.

     You should also know that I've heard some rocketeers swear that fast propellant is bad for clusters. Why? One motor can ignite and come up to pressure before the other igniters light the other motors in the cluster, and it leaves the pad with ounly one motor lit. A medium propellant like White Lightning won't come up to pressure as fast, giving more time for all motors to light.

     You decide which makes more sense.
   
   
   
   • Slow (Black Jack, Black Max, Skidmark, Etc); I won't use these in a cluster. Too hard to light.
   
   
   
   • 
     36 out of 36 motors lit on Darth Maul's Revenge.
     This rocket combined black powder and
     composite motors using air-start techniques.
     Matched Propellant Types: Don't ever mix slow propellant in a cluster with fast and attempt to ignite them simultaneously. For example, Blue Thunder will ignite long before Black Max. Instead, light the fast motor on the pad and air-start the slow. You should also use air-start techniques if you mix a cluster using composite and black powder propellant.



5. Matched Impedance Igniters: Several rocketeers I respect insist that the resistance of all cluster igniters need to match precisely. I've experimented with this some, and find that matched igniters still pop sequentially, not simultaneously. But it still isn't bad insurance to make sure they're the same.



6. Igniter Type:
   • No copperheads! I've never tried copperheads in a cluster myself, but I've seen it done. Copperheads have a notoriously high misfire rate. Don't put them in your clusters.
   
   
   
   
   The 2001 RTV Drag Race.
   Four G motors in minimum diameter rockets.
   • What about (fill in the blank) igniter manufacturer? I've tried a number of them, and none (other than copperheads) seems more reliable than any other provided you're using the right controller for the job. There are other variables that seem to have more impact on ignition success than brand name. Once I started controlling the factors listed above, I've had equal or better success with igniters I dipped myself using an Igniterman kit from SMS (using highly suspect quality control techniques, I might add.) This causes me to believe that either igniters types have less of an impact than the manufacturers claim, or the impact is less important than the factors above, or I'm just really good at making igniters.
   
   
   
7. Igniter Augmentation: Thermalite, additional pyrogen dips, Blue Thunder slivers, etc. -- all are intended to intensify the efficiency of the igniters. Make sure all igniters are augmented equally.




The 2002 RTV Drag Race.
Ten G motors in five Evil Bert Muppet rockets.
8. Experimentation: A great way to test your launcher or methods is to do igniter tests. You can also test ignition techniques doing a drag race with a single launch controller -- this allows you test fire a bunch of motors at once without risking a complex rocket (if one or more motors doesn't fire, the rocket just stays on the pad instead of flying under-powered and off-balance). And here's a tip: Challenge a few friends to a race, and you they'll unwittingly sponsor your cluster ignition test!

Any clustering tips you would like to share? I would be happy to add your wisdom bourne from experience to the collection I've posted above. Just contact RTV at the link shown at the top of the page.

There's also some additional clustering advice at the bottom of the page describing Marc Casanova's cluster launch controller. Marc turned clustering ignition into an art form, and the meticulous steps used in his preparations explain his success!

Click [HERE!] to go to the Lessons Learned Index