May 27, 2006
Meanwhile, for the true tech-geek out there, check out this mashup of Google Maps that lets you track the orbital positions of satellites as well as letting you know when and where they'll appear in your sky over the next 48 hours. Tres cool! Kudos to Dick's Rocket Dungeon for the info and pointer.
March 18, 2006
He's off to a great start, including a post about how the BATFE is reacting to their recent smackdown by the Federal courts regarding rocket motors. Short answer: they are not taking it well, and it seems that the retaliation against the hobby has started. Read more over at Doug's blog, and, like most of us, he wanders off onto other topics as the fancy takes him. Check it out.
January 01, 2006
April 23, 2005
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.
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.
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.
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.
April 03, 2005
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...
May 18, 2004
(in the extended entry) more...
January 03, 2004
October 20, 2003
October 13, 2003
The back row, from left to right:
Quest MicroMaxx, about 1"x.25" diameter.
Estes mini-engine, 13mm diameter.
Estes standard engine, 18mm diameter.
Estes "D" engine, 24mm diameter. These first four can be purchased in a lot of Wal-Mart type stores, as well as some craft stores. They all use a kind of black powder for propellant.
AeroTech "E" engine, 24mm diameter.
AeroTech "F" engine, 29mm diameter.
AeroTech "G" engine, 29mm diameter. These three all use Ammonium Perchlorate based propellant. In general, each 'letter' is twice as powerful as the one before.
Two Dr. Rocket Reloadable Motor Casings for "H" motors. For these, you buy reload kits that provide solid slugs of Ammonium Perchlorate propellant and all of the necessary parts to assemble the motor. The casing on the left holds one more slug than the one on the right, so it's the more powerful motor. The casing on the right is a fully assembled motor. There's no danger here, because the motors need to be electrically ignited to fire. These are both 29mm in diameter.
This is the motor for the Air Munuviana. It's a RATT-works "H", again in 29mm diameter. The reason for the length is that this is a hybrid motor, and a tank for nitrous oxide is incorporated into the design. The fuel is a slug of PVC plastic. I've designed the Air Munuviana to handle up to "J" motors and the motor mount will accept motors up to 38mm in diameter.
A little about the diameters. Standard diameters for rocket motors are 10.5mm, 13mm, 18mm, 24mm, 29mm, 38mm, 54mm, 75mm, 98mm, 3 inch, 4 inch and 6 inch. As you can see, I still fly at the smaller end of the range, but I'm slowly working my way up. [insert Tim Allen grunting noises here] more...
October 26, 2002
When scaling a model (or anything else), the first step is determining the scaling factor. We'll upscale an Estes Mosquito to demonstrate. The original BT-5 tube measures .544" in diameter, and the desired BT-60 tube measures 1.637" in diameter. Dividing 1.637 by .544 results in a scaling factor of 3.00 (rounded, use more decimal places and/or forget the rounding for more precision). In other words, a Mosquito built using a BT-60 body is three times larger than the original, or 300% bigger. Sometimes youll see this mentioned as a 3x upscale.
This scaling factor is what you will multiply every measurement by for the new model. So the original 3" length of BT-5 would become a 9" length of BT-60, and you would multiply each of the fin dimensions the same way for the upscaled version. There are two possible exceptions to this. One, you dont always want to upscale the thickness of the fins, or that upscaled Mosquito will have fins 3/16" thick. Its up to you. The second exception is nose cones. Unless you have a truly scaled version of the original nosecone, the length is probably wrong to some degree, or the shape is slightly (or not so slightly) different. What I do in these cases is to measure the length of the true upscaled nosecone, compare it to the length of the available nosecone, and then adjust the length of the body tube to make up the difference. For instance, if the upscaled nosecone should be 4" long, but the nosecone you have is only 3" long, my solution is to make the body tube 1" longer to compensate. Close enough is usually good enough.
To reverse the above, and downscale an Estes Big Bertha, well take 1.637" (diameter of the original BT-60) and divide it by .544" (diameter of the desired BT-5), which gives a scaling factor of .33. So a BT-5 Big Bertha would be a 1/3 sized downscale, or 33% as large as the original. Just like above, all dimensions are multiplied by the scaling factor, which makes the original 18" long BT-60 a 6" long BT-5. Adjusting for fin stock thickness and nose cone/body tube lengths are done the same way as well.
Doing the measurements and calculations in metric (millimeters), makes things much easier.
Thats the theory and the math. Below is a table I keep handy by my workbench with measurements and scaling factors for many common sizes of tubing. To use the table, find the original size body tube down the left hand column, then find the desired size tubing along the top row. Cross index the column and row to read the scaling factor to use. The two columns farthest left on the table have the metric and standard diameter measurements for the body tubes. Adding other sizes to the table is easy to do by using the techniques above. Obvious additions are Apogee 10.5mm tubes and tubes for the Micro Maxx sized rockets.
One neat thing about the table is using it to help scale fin templates using a photocopier. The copier I have access to will make reductions/enlargements from 64% to 155% of the original size. Suppose I want to upscale an Estes Alpha to use BT-80 sized tubing. Looking at the table, this means the scaling factor is 2.72, or the fin template needs to be enlarged 272%. Looking at the table (and knowing the capabilities of my copier), I see I can enlarge the original fin template by 154%, making the template the correct size for a 1Â½" tube. Next, I take that new (enlarged) template and use it as the original, enlarging it again by 148%, for a BT-70 tube. Finally, Ill enlarge this new template by 117%, giving me a fin pattern perfectly sized for the BT-80 tubing Im going to use. Its easier to do than to explain, so just follow it through using the table to see the steps.
Upscales and downscales are fun and interesting. The Mosquito is a classic thats done often, and makes a good first project. After that, the possibilities are endless, just look through past issues of Sport Rocketry and High Power Rocketry for examples, and old catalogs for ideas.
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