May 27, 2006

Since I'm in some sort of weird "posting content" mood

Over at Pratt Hobbies blog, Doug has put up a picture of himself and his son Brian. Brian was my co-timer during the Team America finals.

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.

Posted by Ted at 09:14 AM | Comments (0) | TrackBack

March 18, 2006

Pratt Hobbies Blog (aka "All the cool kids are doing it")

My friend Doug Pratt has started a rocketry blog, and named it, appropriately enough, Pratt Hobbies Blog. It will soon be on the sidebar.

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.

Posted by Ted at 05:58 PM | Comments (5) | TrackBack

January 01, 2006

Rocketry Links

This post will link to some of my favorite online rocketry resources. It is a work in progress, so it will be added to and evolve over time.

Vendors - I've personally done business with these folks, and I'll do so again.

Pratt Hobbies

Vertical Force Rocketry

Performance Hobbies

Edmonds Aerospace

Public Missiles Ltd (PML)

Magnum Rockets

Apogee Rockets

AeroTech Consumer Aerospace

SkyRipper Systems

RATT Works

Red Arrow Hobbies

Hobby Reference

Essence Model Rocketry Reviews


Rocketry Online

Organizations and Clubs

National Association of Rocketry (NAR)

Tripoli Rocketry Association (TRA)

Northern Virginia Association of Rocketry (NOVAAR)

Posted by Ted at 08:31 AM | Comments (2) | TrackBack

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 05:23 AM | Comments (0)

April 03, 2005

Building a simple but unusual rocket

I’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)

Click on the pictures for Saturn V size

tools and materials

X-acto knife or razor blade
Circle template – I used an empty spice jar

Badminton Birdie (aka shuttlecock)
Thin cardboard (from a cereal box or soda 12-pack is perfect)
Cardboard tube (Estes BT-5, or make your own)
Soda straw
Yellow or white glue
Hot melt glue


Motor Mount Tube
The only real complicated step is right up front, and that's only if you have to make your own motor mount tube. I'll explain how, and then suggest a couple of super easy alternatives.

Cut a strip of thin cardboard (manilla file folder is ideal) 2.75" wide and about 4 or 5 inches long. Pre-curl it by running it over the edge of a table. Wrap it around one of the mini-motors, it should wrap two or three times. On the last wrap, squirt a little glue under the layer and use the rubber bands to hold things together while the glue dries. Be careful not to glue the motor inside the tube permanently, it has to be able to slide out. What you'll wind up with is a cardboard tube 2.75" long. Let it dry.

That's the hard way (and it's not all that hard to do). There are some easier ways to do it though. First off, you can buy that size tube from hobby shops and cut it to length, but a package contains 3 18" lengths, which is seriously oversupply for what you need (unless you're making a lot of these). If you go this route, look for Estes BT-5.

You can also buy a rocket kit and salvage the parts from it. Current Estes kits that use BT-5's are the Mosquito, Quark, and Swift. There are probably others, look for a rocket kit that uses the mini-motors (A10-3T, A3-4T, etc. - look for the "T" at the end of the motor designation).

Centering Ring

Using a template (I used a small empty jar), mark a circle on the cardboard. See the picture farther down to judge about how big a circle you need. Take your motor mount tube and use it to mark another circle centered inside the first.


Carefully cut out the inner circle with the X-acto knife, and then cut out the outer circle using scissors or the knife. Be careful, that knife is sharp! Take your time and make multiple light passes instead of trying to cut through the cardboard in one stroke.

Save that inner circle. We're going to use it in a moment.

Assemble the motor mount
Glue that inner circle into the very top of the motor mount. This makes a bulkhead that protects the birdie from the ejection charge of the motor.

When that has dried a bit, fit the centering ring into the bottom of the birdie and then slide the motor tube into place until the top end (with the bulkhead) touches the front of the birdie. Glue the motor tube to the centering ring with a bead of glue where they meet. Remove the centering ring from the birdie and do both sides of the centering ring/motor tube joint. Let it dry.


Aligning the launch lug
Next you need to place a hole in the centering ring that the launch rod will go through when it's on the pad. Line it up by using the rod and either punch the hole with a hole punch or drill it with the x-acto blade. If it looks like the rod won't pass through the cardboard and birdie smoothly (important!), take a short length of soda straw and glue it into place as a conduit for the rod to pass through.


Gluing it together
Run a bead of hot melt glue around the perimeter of the cardboard ring where it meets the birdie to join the two pieces together. That's it!

Launch Instructions
Put a motor into the motor tube and insert the igniter normally. Slide the rocket onto the pad by passing the launch rod through the straw or holes you made for that. Hook up the igniter to the controller wires, count down and launch.

When the ejection charge goes off, it will eject the motor out the back of the tube, which lightens the birdie enough to recover safely via drag or "featherweight" recovery.

To fly it again, just insert another motor and you're good to go.

Why it works
A badminton birdie stays stable because the rubber nose is heavily weighted compared to the rest of the body and the many holes (feathers) create enormous amounts of drag. These two factors combines keep the birdie flying nose first, but it also decelerates quickly when the thrust ends (either by striking with the racquet or by our rocket engine).

On recovery, the extreme amount of surface area compared to the light weight combine to keep the speeds low. It's the same principle as a whiffle ball, no matter how hard you throw it, the area/mass ratio means it'll slow almost instantly.

Posted by Ted at 10:31 AM | Comments (2)

May 18, 2004

Finding rockets with GPS

There's been a misunderstanding about how I intend to use GPS when tracking and recovering my rockets. I'll talk a little bit about what's available now, the excellent suggestions given, and then explain the technique that I'll actually use.

(in the extended entry)

There are non-GPS low-power transmitters that can be put into a rocket, including a system developed by Walston. The club that we occasionally fly with in Whitakers, North Carolina has the Walston system. Each rocketeer buys a transmitter on a different frequency, and they split the cost of the receiving unit. The nice thing about the Walston unit (as I understand it) is that you don't need a ham radio license from the FCC, because the unit is extremely low-powered. You have to use a directional YAGI antenna, and there's an art to the technique of tracking down your rocket once it touches down. This explanation on using the Walston Tracking System is the best I've ever seen. The author, Sue McMurray, is a wonderful lady who was head of motor testing and certifications for the national high-power rocket organization for a time. She also offered her assistance when a local girl scout troop leader decided that "rockets aren't something that girls do". The lady can flat-out write, but more importantly she builds and flies some impressive rockets.

Back to tracking. It's also possible to use a higher-powered transmitter, but in that case you'll need to obtain your ham radio license. From what I've heard, it's not difficult to become a ham, especially since you no longer need to know Morse code as a prerequisite.

These systems are costly, and the rockets have to be designed to contain the transmitter antenna. They really do work, both out west where they tend to much higher altitudes (it helps to fly on the desert), but their recovery area is exponentially greater, and here in the east where we are more limited on altitude but the recovery areas tend to be cropland and woods. Trust me, wading through high cotton, tobacco or corn is no way to spend a summer day searching for a rocket.

Neither of those options are GPS though, they're just simple beacon transmitters, and you triangulate on the signal to locate the rocket. It's possible to lose the signal behind obstructions, which is where the art of the search applies. Picking up a blocked signal is made more likely by understanding the way everything works and how to take advantage of it.

Putting a GPS into a hobby rocket introduces new problems. You'll still need the transmitter, but instead of a simple beacon signal it needs to transmit its coordinates. It also has to be able to maintain the GPS signal on the way down and on the ground, regardless of how and where it lands. There is also more government regulation on transmitting signal strength and frequency.

My intended method is simpler and doesn't rely on having a signal transmitted from the rocket at all. In fact, it's just using the GPS to refine the search area. It does require seeing the rocket come down, otherwise I'm back to covering the general area in a search pattern until I get lucky or give up.

Before the actual launch of our rockets, if I've got extra eyes (Mookie) to help track I'll send her out a ways in the general direction of expected drift. If I'm at the club launch alone I'll enlist a friend to track from the pad and I'll head out a ways to get a different angle on the flight. Once it comes down, I make note of a landmark on the line of sight and estimate how far away the rocket landed. Often it's by judging whether it came down in front of or behind treelines, which means it can be very imprecise (the field behind that treeline might be half a mile wide). Then I head along that sight line, while Mookie (if she's available) does the exact same thing from where she was watching. In a perfect world, where we come together is where the rocket landed. In reality, one of us didn't track the rocket all the way down, or we have to scramble around and over obstacles which throws off the line of sight, or many other gotcha's that keeps you from walking a straight line in nature.

And this is where the GPS comes in. Some models allow you to shoot an azimuth with a compass, orient your unit to it, then enter 'waypoints'. By doing this, the GPS tells you how far off your line of sight you've wandered as you head towards the rocket. Entering a second set of waypoints - from where Mookie is standing - increases the accuracy.

It's not the most efficient use for GPS, but it's definitely an improvement over the guess-and-by-golly method we use now.

Posted by Ted at 11:24 AM | Comments (1)

January 03, 2004

How high did that rocket go?

Here's a simple and inexpensive way to get a fair estimate. There are three diagrams, so expand the extended entry to read the surprisingly easy method we use.

You actually only need a couple things to figure out the altitude of your flights. A theodolite, a tangent table, and a pencil. For reasonably accurate readings, you can make the simple theodolite shown in figure 1. It's basically a 1"x2" piece of wood, 2 foot long, with a plastic protractor screwed to the side. Add a couple finishing nails to sight along, a string with a fishing weight at the end to indicate the angle, and you're set.

figure 1

The tangent table can be found in any trigonometry textbook. Use the one shown in figure 2, or find one to your liking, they're all the same.

figure 2

Still with me? Good, believe me, this is simple. In fact, this explanation takes longer than the process.

Figure 3 shows the basic concept of determining altitude:

figure 3

The 'tracker' takes the theodolite and stands a known distance from the launch pad. In the diagram, it's where the black and blue lines meet. This distance is the baseline, and the farther the better (as long as you can see the rockets from there). Our usual launch area is a football field, so our tracker is usually 300 feet (100 yards) away from the pad. The tracker on one goal line, the launch pad on the opposite goal line.
When the rocket launches, the tracker follows the rocket with the theodolite, sighting it like a rifle, until the rocket reaches apogee (it's highest point). The angle is read (where the string marks it on the protractor), and this angle is written down.
Time for some simple math. The formula is on the diagram. Look up the tangent for the angle on the table, multiply that number by the baseline, and that is the altitude in feet. Simple!!!

An example: baseline is 300 feet. measured angle is 40 degrees. The tangent for 40 degrees is .839, so 300 * .839 = 251.7 feet.

This is only one method, there are many others. But this one is cheap, simple, and accurate enough for our purposes. You can find more information about altitude tracking in the Handbook of Model Rocketry, by G. Harry Stine.

Accuracy can be improved by using two trackers placed at 90 degree angles to each other to compensate for rockets that don't fly perfectly vertical. This is the usual method used at altitude contests. We don't bother when we're flying for fun. Truth be told, we seldom worry about altitude anyway, we just guesstimate using the good ol' Mark I eyeball.

Posted by Ted at 09:45 AM | Comments (6)

October 20, 2003

Model Rocket Q & A for Beginners

Several years ago I put together a web site devoted strictly to model rocketry. One of the most popular pages was an introduction set up in question and answer format. Looking back on it, I can see that we've come a long way since those early days. I've copied that page into the extended entry, and added links where I could.

Q: Why do you think rockets are such a great thing to do with your kids?
If I sit down to play video games with the kids, or we watch sports together, or read in the same room, we might be spending time together, but it's not necessarily 'together' time. Model Rocketry is more interactive for us, there is a give and take, and an exchange of ideas. It's not just spending time together, it's spending time with each other.
My kids have picked up some very good habits from rocketry; setting goals, planning, following directions, working together, teamwork, and keeping records.
They've also felt satisfaction. Imagine the look on 9 year old Rachaels' face as her U.S.A., designed, built, and launched all on her own, roared off the pad for a perfect flight. As it drifted down on its pink streamers, everyone was cheering and congratulating her. I don't know who was more proud at that moment, her or me.
And they've learned how to deal with the minor tragedies of life. The lost rockets, and the ones dinged when the parachute didn't deploy (because Dad forgot the baby powder).
Flying rockets teaches about science too. You'll see practical demonstrations of aerodynamics, physics, chemistry, and more. The kids become engineers, meteorologists, photographers, and journalists, without any pain, and possibly without even realizing it.
One thing we've discovered about rocketry is that the only way to get bored with it is to quit dreaming. We've yet to launch anything bigger than a 'C' motor, but that's ok. We've still got clustered rockets to try, and staged rockets, and 'gap' staging. We haven't done near enough glider or helicopter recovery. How about night launches, how can we make these smaller rockets visible in the dark?
My kids have a million ideas, to go along with my one or two.
I probably should also mention that model rockets are fun.

Q: Isn't model rocketry like launching fireworks?
There are some basic differences between rocketry and fireworks.
To start with, model rockets are never launched by lighting a fuse. The ignition is electrical, with the power supplied by batteries. This lets you stand back a ways from the rocket when it is launched. Much safer.
A second difference is that model rockets are designed to be recovered. This means that you can reuse a rocket over and over. There are various ways of recovering a rocket, such as parachutes, streamers, gliding, and more (there's more about recovery later).
Another difference is the use of a launch rod. This is simply a guide for the rocket to follow for it's first few feet of flight, keeping it straight up until it's going fast enough to be stable on its own. Once again, it's a safety thing.

Q: Is this really safe enough for kids?
Model rocketry is an amazingly safe hobby, provided you follow the Safety Code. When you read it over, you'll find most of it is just common sense. Over the years, there have been literally millions of rocket engines fired safely.
As for kids doing rockets, if you insist on following the safety code, and have adult supervision, it's almost impossible to get hurt. Explain that each and every one of them is responsible for safety when launching rockets.
I have normal kids, they get into their share of mischief. But when we launch, they know what is expected of them, because it's been that way since day one. A brief example that really happened:

My youngest, Rachael, was doing the countdown. When she got to '3', her brother TJ yelled 'STOP' from where he was standing (about 100 feet away). Rachael immediately pulled the safety key and put the launch controller down. Then we saw a mom chasing a toddler, who was running full steam towards the rocket.
After mom corraled her child (he never even got within 20' of the rocket), we made sure the area was clear again, and started the countdown over. It was a perfect launch.

Some rules we use:

The countdown is LOUD.
ANYONE can stop a countdown at any time, for any reason.
When someone yells 'stop', that's it. No exceptions.
The only time the safety key is in the launch controller is during a countdown.
We don't resume a countdown from where it stopped. We start over.
Before a countdown starts, everyone has to give an 'OK', meaning they're in position, ready, and the area is clear.

We have never had anyone hurt, or been even remotely close to having an accident. It's not luck, it's designed to be that way. And by the way, that mom and child stayed and watched us for about an hour that day, and still stop by occasionally when we are launching a few.

Q: What's the easiest way to get started?
I'd suggest an Estes Starter Set. They start around $15.00 [~$20.00 in 2003], and you can get them at stores like Wal-Mart, K-Mart, Toys-R-Us, hobby shops, and even some craft stores like Michaels or MJDesigns. The starter set includes almost everything you need, except batteries and glue. There are even some 'Ready To Fly' starter sets out now, where the rocket is pre-built for you. Other sets have a variety of rockets (1 or 2) that you have to build yourself. Rockets like the Alpha 3 and Sabre goes together quick and easy. Other sets have 1 simple rocket, plus another that takes a bit more skill to assemble. Another company, Quest, also makes starter sets, but I've never seen one. I hear they're pretty much the same.

Q: Launch controller, recovery wadding... What's all this stuff really do?
I'm going to assume that you are looking at a starter set, and I'll just run down the assembled parts.

* Launch pad - Usually has 3 or 4 legs, with a blast deflector and launch rod sticking up from it.
The launch pad holds the launch rod and blast deflector. The wide legs keep it from tipping over in a breeze, and you can adjust the pad to tip the rod a few degrees for launching into the wind. The launch rod is what guides the rocket until it's moving fast enough for the fins to keep it stable. In the starter sets, the launch rod is usually sectional, always use both pieces. The blast deflector keeps the engine exhaust from hitting the pad and ground. Safety again. There is also a rod cap included. Put it over the tip of the upright launch rod, and it helps prevent injuries where someone leans over the top of the rod while preparing a rocket for launch. Make sure you remove the rod cap just before the countdown, and replace it immediately after.

* Launch controller - This is where the batteries go, usually 4 AA size. It has a continuity light or buzzer that tells you when the rocket is set up properly for launch and the safety key is inserted. The safety key must be inserted before pushing the 'fire' button has any effect. In other words, keep the safety key with you when you work around the rocket, and no one can accidently launch it when someone could get hurt. Coming out of the launch controller is a long wire (about 15 feet) that ends in two small microclips. These clips connect to the ignitor, explained below. When you launch, the length of the wire makes it easy to stand back at a safe distance.

* Rocket - A simple rocket is 3 or 4 fins and a nose cone. These are connected to each other by the body tube. On the side of the body tube is the launch lug, a small tube or loop which is slipped over the launch rod prior to igniting the engine. Connecting the nosecone to the body tube is the shock cord. This keeps the pieces of the rocket together as it comes down. Inside the rocket is the recovery system, often a parachute (there is a whole section on recovery later on). The recovery wadding protects the parachute from the ejection charge, which is what deploys the recovery system. Finally, at the bottom of the rocket is the motor mount. This is the place where the engine goes, and it transfers the thrust of the engine to the rocket itself.

* Engine - The 'whoosh generator', also called a motor. This small cardboard cylinder is actually quite complex in design and function. That doesn't mean it's complicated to use. First turn the engine upright so the small hole is facing up. That's the nozzle, the business end of the engine. The ignitor is a small U or V shaped piece of wire. Drop the point of the ignitor into the nozzle, and gently make sure it goes in as deep as possible. There will be two wires sticking out of the nozzle quite a bit. Next take an ignitor plug (color coded, check the directions in the set), and gently push it into the nozzle. This holds the ignitor where it needs to be to fire the engine. Insert the engine into the rocket motor mount and you're almost ready to go!
When ready to launch, connect the controller clips to the ignitor. After everyone is away from the rocket, insert the safety key, and the light should light (or buzzer buzz, depending on your controller). This means that the rocket will be launched when you push the button.

Q: What do the motor numbers and letters mean?
This is an easy code to provide complex information. Here's the bare minimum needed to start with.

A sample engine code might be: B6-4

The 'power' range of an engine is indicated by the letter, in this case a 'B'. The codes start with 'A' and keep right on going up the alphabet. So B is twice as powerful as A, C is twice as powerful as B (and 4 times more powerful than A), and so on. This is overly simplified, but you'll absorb the details as you gain experience.
Bigger engines (higher letters) achieve higher altitudes, or lift heavier rockets.

The '6' is the average thrust of the engine. It's measured in 'newtons', but don't worry about it for now. Just keep in mind that a '6' has a higher average thrust than a '4'.

The '-4' is the delay, measured in seconds. This means that 4 seconds (more or less) after the propellant burnout, the ejection charge fires. That deploys your recovery system.

There are '-0' engines. These are booster engines designed for multi-staged rockets. As soon as burnout occurs, the ejection charge fires to ignite the next engine. Don't use these on a single stage rocket. '-P' engines are plugged, and have no ejection charge. They're made for gliders.
Some Estes engines have a 'T' listed after the delay time. This means it's a mini-motor, and has a smaller diameter casing.

Q: Where can I launch a rocket?
We launch at the local middle school (Jr. high) field. This is a football field, a baseball diamond, and 2 soccer fields, all bent around an L shape. The bigger the field, the better your chances of recovering the rocket. We've had a few rockets drift away on the wind into, or over the trees. Be aware that it can be calm on the ground, and fairly windy a couple hundred feet up! Because of the L-shape of our regular launch field, we limit ourselves to A and B engines on most rockets. We've got a few heavier birds that fly normally on C's, and on one spectacularly calm day, we launched our little rockets on C's. Straight up well over 1000', and recovered on a parachute less than 30 yards away. For more information, read about rocket clubs below.

Q: How do the recovery systems work?
You spend time to get your rocket looking good, and to fly well. You hate to lose them! Recovery is one thing that keeps this hobby from being glorified fireworks (I'm not knocking fireworks hobbyists). There are many ways to recover a rocket. Here's the most common:

Featherweight - for the lightest rockets. The have such a high surface area compared to weight that they 'float' to the ground, like the name says.

Tumble - for very light rockets that are too stable for featherweight recovery. Usually the nose cone is ejected (it's all connected by the shock cord, remember), and the whole thing comes down. If something wasn't done to ruin the stability, it might come down like a dart. At best, hitting the ground like that could damage or destroy the rocket. At worst, it could hit and hurt someone. There are terms for rockets that accidently come down hard, they're called Prangs or Lawn-Darts. No fun, and very hard on the rocket.

Streamer - this is a long, thin piece of plastic or crepe paper. It creates enough drag to bring the rocket down gently. These are good for days when the wind causes too much drift in a parachute.

Parachute - these range in size from 8" up to 24" for model rockets. To minimize drift, you can cut a spill hole in the center of the canopy. This will help the rocket come down faster, but it hits harder when it reaches the ground. If you cut a spill hole, cut it large because too small a hole can actually increase the lift the parachute generates as it descends. Estes parachutes have a spill hole marked with dotted lines, just cut it out if needed. Another technique to minimize time in the air is to 'reef' the shroud lines. Take a piece of masking tape and wrap it around all the parachute lines about halfway between the rocket and the canopy. This prevents the chute from opening fully.

Glider - It goes up like a rocket, and comes down like a glider airplane. Really cool.

Helicopter - Ever see a maple seed fall? Spinning on one wing is one method of helicopter recovery. Another is to have rotors deploy at ejection, causing the whole rocket to rotate.

Q: What about rocket clubs?
The National Association of Rocketry (NAR) is America's model rocket organization. Their site can be reached from my links page, and from there you can find a local chapter near you. Flying with a club is a great way to learn from others' experience. The NAR also offers insurance for rocketry activities. Sometimes the deciding factor on whether you can fly in some areas (a public park, for instance) is whether or not you have this insurance. On top of that, you receive the NAR rocketry magazine, full of useful tips, plans, and articles. NAR also offers it's Technical Services division, called NARTS. This is where you can get anything from rocket designs to wind tunnel plans to baseball caps. Check out their site, it's worth it!
Another organization devoted strictly to high power rocketry (HPR) is the Tripoli Rocket Association (TRA). Since this is Q&A for beginners, I'll mention that they're there, and not go into HPR. You can find a link to TRA from Rocketry Online.
A new organization is just starting out, the Independent Association of Rocketry (IAR). They are very new, and not yet completely organized. They're worth checking into. [years later, I take this back. they've gone nowhere.]

Q: Can you recommend a book or something to learn more?
Some very good books:

The Handbook of Model Rocketry by G. Harry Stine.
Model Rocket Design and Construction by Tim Van Milligan.
The Art of Scale Rocketry by Peter Alway. [out of print]

At least the first two can be found in your local library, NARTS also offers these books and more for sale. See my links page for Saturn Press, they have the entire collection of Peter Always' rocket books. There's also a link to Apogee Components, where you can find Tim Van Milligan's books. Apogee has a complete line of educational rocketry publications, including 69 Science Fair Projects with Model Rockets: Aeronautics.
The Rocketry Online webpage has all kinds of links to good sites on the web related to rocketry. See my links page for a link to them.
The Rec.Models.Rockets (RMR) newsgroup is a vast source of experience. I've always found the folks there to be willing to answer questions without talking down at you. A great group of people.
The RMR FAQ (frequently asked questions) file will answer many questions you may have. I keep a copy of this handy by my workbench, because it's that useful.

Q: Couldn't I save money by making my own rocket engines?
No. When you factor in the cost of the chemicals, equipment you'd need, and materials, the store bought motors are actually a pretty good deal. Also consider that a home-made motor is more likely to malfunction, which could destroy your rocket or, worse yet, hurt someone. The commercial motors are reliable and consistant performers, and you'd have to make literally hundreds of motors yourself to even come close to that kind of reliability.
Now let's talk about safety. It's dangerous to deal with some of these chemicals unless you know what you are doing. Even among experienced rocketeers, there is a surprising amount of 'lore' and common knowledge that is just plain wrong. It's not safe to try to make your own motors, please don't do it.
If you absolutely have to make homemade motors, check out the RMR FAQ (links page) where there is information about a course in making rocket motors. The Rec.Pyrotechnics newsgroup has folks that can help too.
Simply put, Model Rocketry means using commercially available motors. To save money on these, you can mail order them (or order from companies on the internet), or buy them in bulk packs at your local store.

Q: I remember these cool rockets I saw as a kid. Are the old companies still around?
Estes is still with us. They absorbed Centuri a while back, and once in a while release an old Centuri design. There are many small companies producing quality rocket kits today, check the Rocketry Online website for links.

Q: I can't believe that white glue is strong enough for rockets. Shouldn't I use model glue or epoxy?
For gluing plastic to plastic, model airplane glue is best. There are some times and places where epoxy is handy. But for Estes kits, white or yellow glue is king (yellow is superior). A bond you make between the cardboard body and the balsa or cardstock fin will be so strong that the tube itself will tear before the glue joint breaks. Two secrets to getting even stronger joints; lightly sand the body tube to remove the glasine coating (the glossy stuff), and use the double glue method. The way to double glue is to apply a small amount to the pieces to be joined and press them together. Pull them back apart, and let the glue dry for a few minutes. Apply a little more glue, then join like normal. This technique results in super strong bonds that will easily handle A-D engines. I've heard of rockets built with just yellow glue that fly on G motors. [I've flown H motors this way.]

Q: It goes up, it comes down. What's next?
If you look at rocketry webpages out there, you will find a hundred people experiencing rocketry in a hundred ways. I mentioned in passing cluster rockets, staging, scratchbuilding, high power rocketry, scale modeling, gliders, and more. I didn't mention payloads, or contests, or arial photography, or altitude records, or... The list just goes on and on, and you can decide what suits you best.

Do it safely, and have fun!

Posted by Ted at 05:18 AM | Comments (3)

October 13, 2003

Rocket motor sizes

If you click on the extended entry, you'll find a picture of the common rocket motors that Mookie and I fly, to give you an idea of the range available. The 3.5" diskette in the background gives some scale. These are commercially available motors of three basic types.

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.

Second row:
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.

Front row:
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]


Posted by Ted at 01:48 PM | Comments (1)

October 26, 2002

Upscales and Downscales

Don't confuse these terms with 'scale' modeling, which is building a detailed version of a real rocket. Rather, 'scaling' a rocket design means building a larger or smaller version of the original, like a big Mosquito or miniature Big Bertha.

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 you’ll 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 don’t always want to upscale the thickness of the fins, or that upscaled Mosquito will have fins 3/16" thick. It’s 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, we’ll 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.

That’s 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 1½" 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, I’ll enlarge this new template by 117%, giving me a fin pattern perfectly sized for the BT-80 tubing I’m going to use. It’s 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 that’s 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.

0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18
0.544 13.8
... 1.36 1.80 2.04 2.44 2.48 2.79 3.01 3.72 3.90 4.12 4.64 4.76 5.57 7.43 10.22 11.14 13.93 21.18

Posted by Ted at 01:32 PM | Comments (0)
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