Tuesday, July 12, 2016

Rocket Camp - Kit Selection

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One of the first and most important decisions you'll make when teaching a rocketry camp or unit, be it at a middle school, high school, scout troop or public library will be: What rockets will we build? There are a lot of choices available, and the answer you come up with will depend on the parameters of the class. You'll want to consider the age group of the kids you're teaching, the likely experience level they'll have, and the length of time you'll have with them.

The obvious choice for leading a group of kids in a rocketry class is an Educator's Bulk Pack. Most major model rocket manufacturers carry these. Estes and Quest Aerospace, the two biggest, have a pretty good selection of bulk packs, but there are others to consider. Apogee Components has bulk packs of different skill levels, as does FlisKits, and even Balsa Machining Services, which mainly specializes in rocket parts, has a  "School Rocket."

The School Rocket from Balsa Machining Services.

You'll need to decide on the skill level of kits, the type, and whether you want a variety pack with several different designs, or a bulk pack of the same kit. You should also consider the size of the field you will launch from, as well as the surface - whether it be grass, gravel, asphalt, etc.

What "Skill Level" means varies from company to company, and even rocket to rocket. There are some Skill Level 2 rockets which are very simple, and some Skill Level 1 rockets which have more parts or require cutting plastic. There's not much rhyme or reason to it, other than that once you get to Skill Level 3 and beyond, it's going to be too difficult and frustrating for first-time builders.

The Estes Goblin - a Skill Level 2 rocket which is simple to build.It does fly very high on
D motors, though, and does not come in a bulk pack. I just mention it as an example.
If you're working with younger kids or have a limited amount of time - say, a single afternoon - or if you are looking for something simple, for example, to demonstrate some principles of basic physics, then Ready-To-Fly (what Estes calls "RTF," and requires no assembly), or Easy-To-Assemble ("E2X," which requires some very simple assembly) kits will probably be best.

But if you have more time, older kids, kids with more experience, or if you are trying to teach the kids the craft of rocketry, so that they will feel comfortable continuing on their own when they leave your camp or class, you'll probably want to choose something that takes a little longer and a bit more attention to build.

When I mention students' experience, I'm not necessarily talking about experience with model rockets. You may be working with a scout troop who have built pinewood derby cars. Maybe you're doing something with 4-H, with a group who have experience with various crafts. Perhaps you'll be going into a shop class or even incorporating rocket building into an art class, with students who are used to using paints and glues. These are all valuable, transferable skills to rocket building. Especially working with glue - your first-time builders are going to need some guidance with glue, believe me!

As for type, you'll consider whether to use simple 3-fins-and-a-nose-cone rockets, or something with a payload section, or an "odd-roc" - a rocket which doesn't look like a typical rocket. A saucer is a common kind of odd-roc.

Estes Blenders - a type of saucer "odd-roc." These are interesting and don't fly too high.
The Blender is a more advanced build. Image from eRockets.biz.

 Before you choose, try to find the instructions. For most Estes kits, you can download the instructions and read them ahead of time. Same is true for other companies as well. That way, you can see if you'll need any special tools. You might find a so-called "Skill Level 2" rocket which you like, and which in fact is pretty simple to build. You might also find Skill Level 1 rockets you'd rather avoid.

Whether you get a variety pack or a bulk pack of all the same rocket will also depend on the factors I've mentioned above. Younger kids and inexperienced builders will need some guidance. If you simply hand them the instructions and let them have at it, they will very likely build too quickly. Some kids don't have the patience to let glue dry on certain parts, and you might end up with a class full of rockets with iffy construction. When in doubt, I'd recommend everyone have the same kit, so that you can build with them, step by step, and you can guide when it's time to set the parts down to let the glue dry while you do something else with them.

Finally, consider the size and type of field you will launch from. If you're flying on a small field, you'll either want a rocket that doesn't go too high (which is largely due to the motors you use, but will also depend on the rocket - a lightweight, thin rocket will fly much higher than a fatter or heavier rocket), or you may want to select a rocket that uses streamer recovery instead of a parachute. If you're likely to land on a hard surface, such as rocky ground or asphalt, you'll probably want to go with parachute recovery*.

Whatever kits you decide on, I highly recommend building one yourself a few days before class begins. A lot of experienced builders will modify their rockets slightly, changing out the shock cord for another material, for example. But I would suggest you build according to the instructions, as the kids will build. This will alert you if there's anything you need to be on the lookout for. Are there parts which don't fit together just right? Does it come with a two-piece nose cone which requires plastic cement? Do you have to make your own parachute, and if so, how tricky might it be for little fingers? These are the things you'll want to figure out before you get into the classroom. It's also nice to have a well-built demonstration model to show the kids on the first day. It can be hard to visualize what a pack of parts can really look like when it's assembled, and most bulk packs do not come with a face card with a nice photo of the rocket.

I went with Skill Level 1 kits, preferring balsa fins over plastic or card stock.

*In my case, I had both a small launch area and asphalt, as we launched from a parking lot. The lot would be coned off, but there were still cars parked in the vicinity, and there was the community college roof to be concerned about, not to mention a busy road not too far away. My biggest concern was that road, and though we could launch pretty far back from it, I decided I'd need to keep our altitudes to about 300-350 feet with parachute recovery.

Launch site in the upper left corner lot. We could be nearly 1000 feet from the busy road at bottom, but the width of the site was only about 450 feet. Sometimes there were cars parked next to the building, so we had to pay attention to the wind.

 * * *

I had suspected last year that I'd need all the kids to have the same kit. My suspicions were confirmed after the Estes Alphas I had requested did not arrive, and the first week kids each ended up with different rockets. It was great that they got to build whatever they wanted, but it meant I couldn't guide them through it. There was the issue of kids using too much glue, or trying to stuff a motor mount into the back of a rocket before the glue was dry, or the one kid who glued on his launch lug directly in line with one of his fins, so that the rocket wouldn't be able to go onto the launch rod. (We pulled that one off before the glue had totally dried and got it in the right place, but I might have missed it).

For the second week, we got a pack of the Estes Viking, because it was available at a local hobby shop.

This is a nice little rocket with card stock fins, with a wide variety of fin configurations. Kids could build with three, four, or five fins, and they could be attached in a variety of directions, so that each kid could make a slightly different rocket. (Balsa fins must be attached with the wood grain parallel to the fin leading edge, so there is only one right way to attach a balsa fin).

The Viking is what's called "minimum diameter." It's very narrow - only as wide in diameter as it needs to be to accommodate the rocket motor. Narrow rockets have less aerodynamic drag than larger-diameter rockets. That means they can fly very high - which kids love, but which cost me some money (I'll explain when we get to altitude tracking)! Fortunately, they weren't likely to drift too far. The Viking uses streamer recovery, which isn't ideal for asphalt, but with such a lightweight rocket, hopefully we'd get them all back with minimal damage to the fins and body tubes.

One drawback to the Viking is that the rocket has no motor hook on the back. The motor hook is really convenient for kids, because it snaps into place and keeps the motor from falling out the back of the rocket. The Viking requires a "friction fit," which means you must wrap masking tape around the motor until it's nice and tight - just tight enough that it won't fall out when the ejection charge fires, but not so tight you can't get the motor out and put a new one in when you're done. It's a very fine line, and one I still have a bit of trouble judging. Kids will sometimes have a motor fall out at apogee, or never be able to get the used motor out without damaging the rocket.

Also, because the rocket is minimum diameter, it has no motor mount - the body tube is the motor mount. I really wanted to show the kids as typical a model rocket as possible, with all the parts they're likely to encounter on most builds.

My solution was to put together a quick scratch build, a rocket I called Sounder II.

It had all the basic parts, but I didn't glue the motor mount into place until after the first day. When I was showing the parts of a rocket on day one, before launching, I pulled the motor mount out of Sounder II, showing the centering rings, thrust ring/engine block, motor hook, etc. I then glued it in, showing how this was done.

Sounder II also turned out to be very useful later in the week when talking about stability and rocket design. And it flew very well. It's always good to show kids a few scratch builds - scratch building was pretty standard in the early days of model rocketry, and it's a good confidence builder. A kid who understands stability and how a rocket goes together should eventually be able to learn to design and build his or her own.

Sounder II, with markings for the center of gravity (CG) with an A8-3 motor and a C6-5 motor.
Also marked is the center of pressure (CP). Though the difference is small, the
rocket is stable with the A, and marginal with the C. This will come back later.

On week 3, I went in a different direction - the Quest Astra.

It's a great rocket, but different than a standard Estes Skill Level 1 kit. It has through-the-wall balsa fins, so the kids won't get the fins in the wrong spot. Instead of a rubber shock cord with a paper trifold "tea bag" mount, as is used in Estes rockets, it uses Kevlar thread, tied around the motor mount and passing under the forward centering ring. Some of the kids had a little difficulty with this method. On my own Astra, I was so busy helping the kids with their rockets, I never put the launch lug on mine. I painted it without one. (This wasn't a mistake - I decided it was more important for me to show the kids how to paint than to have one more rocket I could launch with them. You can launch a rocket without a launch lug, but you need a tower or piston launcher. These are advanced launch pads more used for competition rocketry.)

* * *

This year, I went in a different direction. Hoping to avoid any purchasing mistakes, and wanting to make sure I selected a rocket which any of the more experienced kids were unlikely to have built before, I turned to Apogee Components. We built the Apogee Avion.

Apogee has a number of great bulk packs. They're not the cheapest you'll find, but they have a variety of great kits, both of their own and from Quest Aerospace. They carry simple rockets, like the Avion, which has balsa fins, and the Apprentice, with a single-piece plastic "fin can." They also have payload-carrying rockets and even a two-stage bulk pack for the truly ambitious.

Another nice thing about buying bulk packs from Apogee is that you can download a free RockSim file for each rocket they sell. This will allow you to show the design file on rocket simulation/design software, such as RockSim, which is sold by Apogee Components, or OpenRocket, which is free. With simulation software, you can get a rough estimate of how high the rocket will go with different motors, and you can also use it to demonstrate principles of model rocket stability, aerodynamics, and design.

I had thought the Avion looked like a cool little rocket for some time, so I ordered those. As I built the demo model, I discovered a few things. The nose cone came in two parts, so we'd need plastic cement. The shock cord is Kevlar, and is supposed to be anchored to the motor mount. The Kevlar shock cord is also pretty short.

The full length of the Avion shock cord,
when built according to the kit insructions
 A short Kevlar shock cord can be a problem. Because it's not elastic, Kevlar can actually damage the rocket. If the parachute ejects when the rocket is moving too fast - either due to a motor delay which is too long or too short - the force of the parachute opening can pull the shock cord back against the opening at the top of the body tube. Because it is so stiff, this can cause the shock cord to rip through the body tube, causing a long, jagged tear known as a zipper.

A zipper - a jagged tear down the body tube of a rocket, caused by the shock cord.
Image from an Apogee Components YouTube video.
Because I had already known about the Kevlar cord, and I knew some of my students really had trouble with the Quest Astra shock cord last year, I decided we would use some sewing elastic and make an Estes-style paper trifold "tea bag" mount.

An Estes paper trifold shock cord mount, sometimes called a "tea bag mount"
Some experience rocket builders don't like the trifold mount, because they sometimes "fail." In fact, it isn't usually the mount itself that fails - it's that the shock cord breaks. A properly-glued paper mount should be quite secure, because wood glue and white glue are said to form a bond that is stronger than the paper tube itself. In reality, I'm sure a well-glued shock cord mount may occasionally come out, but more often than not it's a failure of the shock cord itself.

But first rockets are usually lost or damaged long before that happens. They end up stuck in a tree, breaking because of a poorly-packed parachute, or simply flying so high on a C6-5 motor that they simply "vanish," that an elastic trifold mounted shock cord is probably sufficient. Some elastic cords last decades.

Unfortunately, when I stopped in to check everything the Friday before class began, I discovered that the sewing elastic I'd requested had been forgotten. We'd still use a paper shock cord mount, but just with the Kevlar. Mounting the shock cord near the top of the tube would at least give us a little more length on the shock cord, and if there were a zipper, it wouldn't go more than an inch or two down the tube, stopping at the paper mount.

These Apogee Avions are nice! They are really great fliers - straight up every time. I don't know what it is about them, but I really enjoyed seeing these things launch. Since they're larger than a rocket like the Viking, they don't go as high, and you can keep your eye on them the entire flight (unless you fly on a C motor - then they're capable of reaching 1300 feet - if you've got a large enough field, go for it! Even on A motors, these are exciting rockets to watch. After building a demo version and one each week with the kids, I now have four of them!

In an upcoming post, we'll talk about the building process.

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  1. Great work Daniel. You never cease to amaze me with your dedication to this terrific hobby.

    1. Thanks, Ted! Ya gotta spend your free time on something, right?

  2. Awesome article. I will have to check out the Apogee Avian kit.

  3. Great summary Daniel. Thank you for taking the time to write up your experiences with the kids. Really enjoyable to read and learn.

  4. The new classroom packs of fin alignment guides from Qualman (http://www.qualmanrocketry.com/Class_Packs.html) seem like they'd be really useful.