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.

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.

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 05:23 AM | category: Rocketry Resources
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From today's Aerospace Industry Association press release:

Teams Will Meet in Fly-Off May 21

Arlington, Va. – The stage is set for a fiery showdown of the top teams in the Team America Rocketry Challenge after AIA announced the 100 finalists Friday.

The teams will meet for a final fly-off on May 21 at Great Meadow in The Plains, Va. for the title. It is the third year AIA and its partners are putting on the worldÂ’s largest model rocket contest. A list of the finalists is available at www.rocketcontest.org.

A total of 712 teams from 49 states and the District of Columbia – and even an American middle school in Germany – took part in the preliminary round of the competition. That represented close to 10,000 middle and high school students.

AIA President and CEO John W. Douglass said interest in the contest shows it is succeeding in reaching out to middle and high school students.

“We are excited to see the enthusiasm surrounding TARC and look forward to another great final day of competition,’’ Douglass said.

This yearÂ’s competition tasks students with launching a one- or two-stage rocket and having it fly for exactly 60 seconds. The payload of one or two raw eggs must return safely to the earth, and each flight receives a score according to performance and weighted for the number of stages and eggs. Teams had until last week to send in preliminary scores to see if they made the finals, which features schools from 28 states.

AIA created the contest two years ago as a one-time event to mark the 100th anniversary of flight, but overwhelming interest turned it into an annual event. The goal is to promote aerospace to students to attract more young people to careers in the industry. The contest is also sponsored by the National Association of Rocketry in partnership with NASA, the American Association of Physics Teachers and 34 AIA member companies. The winning teams share a prize pool of $60,000 in savings bonds and cash. NASA also provides top-performing teams with grants for students to build more advanced rockets and for teachers to attend workshops and meet space program engineers.

For more information about AIAÂ’s Team America Rocketry Challenge, including details on how to sponsor a high school team and to apply for press credentials to attend the finals, visit www.rocketcontest.org.

Mookie and I will be attending again as volunteers. We've worked all three (they hold the finals at our home field), and it's fun and exciting to see the various ways that the student teams solve the challenging task they're given.

Spectators welcome.

Posted by: Ted at 06:50 PM | category: Rocketry
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From this American Bosch Arma advertisement (1959):

This nuclear-fueled reconnaissance craft is preparing to land on Mars' outermost satellite, Deimos— 12,500 miles away from the "red planet" (center) and 35 million miles away from the Earth.

Under the ad is the (I assume) title of the illustration: Mars Snooper.

The reason it caught my eye - other than it's a rocket - is that Estes used to produce a rocket kit known as the Mars Snooper, and it's remarkably similar and obviously based on the illustration. Cool.

Posted by: Ted at 11:55 AM | category: Rocketry
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Where: Great Meadow Equestrian Center, The Plains, Virginia
When: 9am – 6pm
Who: Northern Virginia Association of Rocketry (NOVAAR)
Weather: 60Â’s, variable gusty winds, sunny.

This was a two-day launch packed full of activities. In addition to flying several contest events, NOVAAR was handling TARC students making flights on the last weekend before qualification closed. Also, there were students from a high school physics class making flights, a scout group, a Junior ROTC group, plus a team from Oakton high school making final test flights for their NASA Student Launch Initiative flight (I supervised construction of their Aerotech I300 motor). All this going on while still managing plenty of just-for-fun launches, both model and high power.

The evening before I called Ken at Performance Hobbies and ordered some high power motors for delivery at the field. He arrived not long after I did and once he got set up I picked up my motors and got ready to fly.

1. Groove Tube Upscale - H165R-M - Beautiful boost (as usual) from this rocket. She coasted to an estimated 1500 feet before ejecting the 36” parachute right at apogee. I’d gone with an undersized chute – normal is 45”, I need a 40” – to minimize drift and she landed softly on the thick grass without damage. (takeoff photo here).

Since I planned to return on Sunday, that was the extent of my high power flying for the day. Everything else was from the Sport Range.

2. Pacifyer - D12-5 - This battleaxe shaped rocket always gets noticed. Beautiful boost and good altitude, sheÂ’s picked up a bit of spin since repairing a broken fin. Recovered nicely very close by.

3. YJ-218 - C6-7 (x2) - Arrow-straight boost for this dual-engined cluster and a perfect recovery.

4. Vampyre - A10-3T - This mini-engined ring-fin always surprises people by how fast it is. 3-2-1 and gone. IÂ’m used to it and followed it all the way. Recovered undamaged on a streamer.

5. Zen Doggie - C6-7 (x3) - It’s been quite a while since I flew this rocket. Remembering that her last flight was a little squirrely (the fins are a tad undersized), I added some clay to the nose to increase stability and asked it to be announced as “heads up”. Only two of the three motors ignited. The delay was about two seconds too long due to missing one-third of the thrust during boost phase, but the climb was stable and she recovered nearby without damage. I’ve regained confidence in the design and I’ll start flying her again.

6. Sparrow Upscale - B6-6 - It doesnÂ’t take much to boost this plain-jane rocket way up there, and the B6 did a nice job. She came down on one fin and slightly cracked it. Already repaired.

At 4:30 I took over as Range Safety Officer (countdown and button-pushing guy) and had a great time announcing and launching some fun flights, including some TARC qualifiers.

On Saturday night it became apparent that I wouldnÂ’t be out flying on Sunday because IÂ’d gotten so wrapped up in events that I never applied sunscreen and wound up with a severe and painful sunburn on my neck and ears. Since I do this every year on the first or second launch, youÂ’d think IÂ’d have learned better by now. Sheesh.

Posted by: Ted at 12:26 PM | category: Rocketry
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Something I've found among rocketeers is the willingness to share equipment and knowledge. You need something? Ask the guy next to you, and chances are he'll gladly lend it to you, and if he doesn't have it, he'll point out someone who does.

That, more than anything, has impressed me since day one about this hobby.

Saturday was a perfect example. I had some parts that I no longer needed, but I knew that Bart, a good friend could make use of them. I also remembered talking to him about rail buttons, and mentioned that I had a bunch and would give him some to try. So I added those to the bag and found him talking to another club member, Mitch. Bart wanted to pay me for it, but I refused. Instead I asked for some advice.

My Level 2 rocket has been designed and I've collected all the parts except for the fins. I was going to use plywood, but more and more people were suggesting that I go with fiberglass sheet. I asked my friends what thickness they'd recommend considering the motor I was going to use. Mitch immediately said "use the thickness of the ones I'm going to give you for free right now," and he handed me three large sheets of G10 fiberglass. The stuff they make circuit boards out of. They're about 15" square, in pristine condition, and Mitch salvaged several hundred of them from his workplace when they started to throw them away. All in all, he gifted me with about $50.00 worth of fin material, along with instructions on how best to cut it into shape. He told me to consider it payment for giving Bart the rail buttons.

This kind of stuff happens all the time.

PS. Yep, I'm at home today. The sunburn is still bad enough that I can't wear a collared shirt, let alone a tie. I'll tough it out tomorrow, but for today I'm still slathered in aloe burn gel.

Posted by: Ted at 10:10 AM | category: Rocketry
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Man, I was an idiot yesterday. I had such a wonderful time at the rocket launch. Everything I flew worked to perfection, there were boy scouts and a high school physics class and junior ROTC and TARC rocket contest teams all over the place, in short, big fun the entire time. So much fun, in fact, that I never got around to putting on any sunscreen.

I am one hurtin' unit right now. I wore a ballcap to protect the ol' solar panel, but my ears and back of my neck are toasted to a screaming hot pink. I've got a nice little 'V' where my shirt was open too. Fortunately I'm prepared for this, because I'm stupid like this once a year or so, and I've got this soothing blue aloe gel that I've been coating myself in. Just going out into the sun in painful today, so no launch for me. Dammit.

On the plus side, yesterday was excellent! I said that, didn't I? Oh well, a detailed report (like you care) will be coming in the next day or two, but for now, I'd like to show you a picture that was taken yesterday by a student from Oakton High school. His name is Enrique, and I understand that he's the official photographer for the school's SLI team.

What's SLI? That stands for NASA's Student Launch Initiative, and these young ladies and gentlemen are headed to Huntsville, Alabama to take part in it. They're the next generation of engineers that're going to take us to Mars and beyond.

Enjoy the picture, click for full size. The rocket is an upscale of the Centuri Groove Tube, which was a tube-finned kit from the 70's and 80's. My version is about five foot tall, is 2.6" in diameter, and it's taking off on an Aerotech H165 Redline motor. The intensity of the flame washes out in photographs, but it's bright screaming red (yes, sorta like my ears). This is an amazing photo, and Enrique did an excellent job capturing a difficult shot.

Posted by: Ted at 01:00 PM | category: Rocketry
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Today and tomorrow. Yay!

Supposed to be beautiful weather here, hope it's as nice where you live. Have a great weekend.

About the picture: These are some downscaled models I built based on classic Estes kits from the 70's and 80's. They are (from left to right) Cherokee D, Big Bertha, Der Red Max, Alpha and Goblin. They're not all to the same scale. The Red Max (center) is just under 6" tall, and the brown cylinder next to it is a standard Estes rocket motor.

Posted by: Ted at 07:32 AM | category: Rocketry
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IÂ’m going to show you how to turn an ordinary badminton birdie into a real launchable rocket. These are easy to make and bigtime fun to fly, plus they don't go so high that you'll lose it.

Best of all, they fly on Estes "mini" motors. You can find these in the toy department at WalMart, and a pack of four will cost around five bucks. You're going to need one to help you construct the rocket, so pick up a pack before you start. Look for motors labeled A10-3T or A3-4T, they'll be a little less than 3" long and about one half inch in diameter (pinky sized).

If you need more information about rocketry, check out my Rocketry archives, there's lots there, plus links to even more.

I'm going to assume that you have a launch pad and controller. The ones that come with Estes or Quest starter kits work fine. Starter sets are cheap, include everything you need and the value is very good.

And finally, just to prove I'm not a complete loon, here's the original plans for the birdie rocket as it originally appeared as an Estes rocket kit.

(in the extended entry) more...

Posted by: Ted at 10:31 AM | category: Rocketry Resources
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1 Go for it,Ted.The birdy is on my future to do list also.
I have a first coming up soon,too.My first boosted glider for the upcoming contest(B glider).I was going to try it for the first time this weekend but you know how that went.I scratch built it from old CMR Mantra plans I found in Sport Rocketry.The plans call for it to either boost on it's own or to attach to the two staged Omega.
Although I built both I don't have the Omega rigged to fly the glider......yet.
I want to keep working with this glider design to see if I can improve upon it.Competing with myself....blah,blah.
Another first I have in mind is helicopter duration or at least just a heli.I've been a hardcore heli nut since I first saw Apocalypse Now in the theatre when I was a kid.Been hooked on them ever since.Helicopter?Rocket?Rocket helicopter?I'm there dude!I have my eye on an Apogee Heli-Roc soon.

Posted by: Russ at April 03, 2005 08:55 PM (ObxzR)

2 You'll love the Birdie Russ. I've given several away to little ones at the launches. They're perfect for toddlers who want to fly with the big kids.

For the contests, I'll fly what I brought if it'll help the club. I don't fly serious competition. An Alpha makes a fine duration rocket because you can stuff a huge streamer or chute into it, and I almost won a glider contest once with a Mach-10 because the wind was too strong for lightweight gliders.

Posted by: Ted at April 03, 2005 09:32 PM (ZjSa7)

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