April 23, 2005

Rocketry - Clusters

Matt asked in my comments section for help on clustering model rocket motors. An excellent topic! This is a beginner's guide at best. It's enough to help you be successful, I don't claim it's definitive.

What is Clustering?
Clustering is when a rocket has more than one motor that ingnites simultaneously. A perfect real-life example is the Saturn V rocket that took men to the moon. The first stage had five engines that lit all at once at lift off, and the second stage had five more smaller motors that fired all at once when the first stage dropped away (that's a good example of a staged rocket too). A variation on the theme is when the main motor(s) lift the rocket and then additional motors ignite in the same stage. These are called "airstarts" and are more complicated and difficult because on-board electronics must be used for the ignition system and the timing has to be correct. Good examples of that concept are today's Delta family of rockets and the ESA's Ariane. In fact, most current heavy lifters use combinations of airstarted boosters to increase their lift capacity and to tailor the thrust profile over the boost phase.

Why do Clusters?
In the early days of model rocketry, motor classes were very limited and the only way to get more power was to cluster available motors. Nowadays the selection of motors is excellent so it's less of a neccessity. That's not to say there aren't still good reasons for designing cluster rockets today. Many TARC rocket contest teams have gone with clustered motors because the smaller Estes motors are cheaper, more reliably ignited and more readily available. Personally, I love clusters because they're cool.

Design Considerations
On the model rocket level, the main consideration must be "what if all the engines don't light?" I've made test flights of my cluster rockets where I intentionally didn't ignite all the motors, to check the performance even when underpowered. You should be trying for a rocket that can still fly safely on half power. It might not be a great flight, but safety is always first.

Another consequence of not lighting all motors is unbalanced thrust. If two motors are firing and the third isn't, then the rocket has to work harder to stay stable because the thrust is trying to tip the rocket over into an arc.

There are a couple things you can do to minimize this. First, you can put your motors close to the main axis of the rocket. If all the engines are tucked in right next to each other then the imbalance is minimized. Conversely, if your motors are in outboard mounts on the fin tips, well, a motor that doesn't ignite is a much bigger problem. I don't recommend fin-tip motors. Ever.

Another way to keep stability is to aim the motors at the rocket's center of gravity. Tilt each motor mount in slightly (or not so slightly - this is an extreme example that works wonderfully), and once again all the motors can easily compensate for the one(s) that didn't ignite. Check out that Delta link above and notice that the booster engine bells are slanted out to achieve the same effect. Obviously, you'll need to have a good idea ahead of time about the design and how it'll balance out. I use an older version of Apogee Components Rocsim to design complex clusters.

One other way is to induce spin in your rocket. Spin increases stability (but increases drag), and if the rocket spins on the way up then the unbalanced thrust is evenly distributed all the way around. What happens is that you wind up with a wacky corkscrew or the rocket looks like it's wagging it's tail end on the way up. Some rocket designs do this on purpose. It's fun to watch.

Igniting Clusters
The key to reliable ignition of multiple motors is to be meticulous.

The battery of your launch controller must be well charged, don't try to ignite a cluster at the end of the day with your worn down AA's. Invest in a small sealed cell motorcycle or lawn tractor battery. They're cheap and deliver plenty of power when you need it. Rechargable batteries used in cordless power tools or RC vehicles work great if you connect them in series. Better yet, find a local club and use their launch setup, it'll almost certainly be good enough to fire clusters all day long.

For model rocket engines, use the Estes igniters. Quest tigertails are too finicky to deal with. You can make them work, but to me it's not worth the extra hassle. Pick through your igniters and select the ones with a good blob of pyrogen on the end. You want the igniters to go instantly when you press the button.

Also, check inside the nozzles of each engine. You should see black up inside. If you see light gray, then there's excess clay from the nozzle blocking the propellant, and it won't matter how good your igniter is, it's not going to help. If you need to, you can gently scrape the inside clean with the end of a straightened paper clip.

All right, your battery is charged up, your motors are ready to go and you've got a handful of blobby little igniters.

Wiring Clusters
Here's where the 'meticulous' bit comes in again. Once you've got the cluster hooked up to the ignition system, take a minute to carefully inspect everything. Make sure igniter wires aren't touching except where they're supposed to. Make sure the clips are hooked up securely and not touching the blast deflector, the launch rod, or other exposed metal. You need everything to be absolutely perfect. It's not hard, just fiddly.

Start by putting the igniters into each motor and inserting the ingniter plug. If you want, you can carefully remove the paper tape that Estes puts on their igniters. I just fold the ends out of the way.

cluster wiring

Click on the image for a bigger picture.

For two-motor clusters (assuming that they're right next to each other), all you need to do is twist one leg of each igniter together. You'll end up with two 'tails' consisting of the two igniter leads, which you hook up to the launch controller clips. Just like in the upper left part of the diagram.

For three and four engine clusters (or more complex motor arrangements), you're going to need a set of clip whips. These are easy to make, see below.

Notice in the diagram for three motor clusters that one leg from each of the three igniters are twisted together in the middle. Then I take two of the remaining leads and twist them together. One ignition clip goes on the set of three twisted together and the other clip is attached to a clip whip. The other, dual ends of the clip whip are connected to the twisted pair and the single lead, respectively.

Four motor clusters in a square pattern are simple. Twist the two leads together from each corner so that each igniter is connected to the ones on either side. This time you'll use two clip whips to connect oppsosite corners together, and then the igniter clips from the launch controller attach to the clip whips. It sounds more complicated than it really is, check out the diagram.

bus bar ignition

Another alternative is to use a "bus bar" setup. With this method, you take a length of heavy solid copper wire and wrap a leg from each igniter around it. If needed, a second bar is used for the other side of each igniter. Finally you hook the bus bars up to the launch controller ignition clips.

There's no need for the bus bars to be straight either. I've seen some people use a three-quarter circle of wire to eliminate the need for a clip whip when doing three-motor clusters.

Making a clip whip

A clip whip is just a way to deliver electrical current to more than one place at once. No matter what kind you make, one end will always have a single clip that hooks up to the ignition clip, and the other end will have two or more clips.

Making a pair of three-whips will cover 99% of your needs. You'll need eight mini-clips (available at Radio Shack) or small alligator clips and three or four feet of solid core copper wire - none of that stranded wire for this.

Cut the wire into lengths between 6"-8" long, then strip the ends. Solder clips onto one end of each wire (you can get by without soldering, but it's not nearly as reliable. If you don't know how, find a friend who can, it's worth the trouble.)

Here's the magic part. Take four wires and twist their ends together, then solder to make a solid connection. Ok, so that's not so magical, but that's really all it is! You can use a wire nut if you want, and/or cover the connection with electrical tape. I lay one wire opposite the other three so that it's obvious which connection is which, but it doesn't really matter. I also use different color wires for the three leads, to help me keep my cluster wiring straight.

So there ya go. That's most everything I know about clustering model rocket motors. There are a few things I've left out, but these are the basics, and if you're careful there's no reason you can't have a near 100% success rate with cluster ignition. Using these exact same methods, I've only had two motors not ignite in the last seven or eight years, and even then the flights were safe.

Posted by Ted at April 23, 2005 05:23 AM
Category: Rocketry Rocketry Resources
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