Quote

“In every real man a child is hidden that wants to play.” -- Friedrich Nietzsche

Backpack Info

Courtesy of REI, a nice little infographic on selecting, packing and adjusting a backpack.

HYOH

HYOH stands for "Hike Your Own Hike," and is one of the major bits of etiquette on the hiking trails. What it means is that everyone does things their own way, and unless they ask for your advice or opinion, or they're obviously doing something unsafe, then you should respect their way of doing things.

If you've backpacked much, you'll have run across the guy or gal who's eager to tell you exactly what you're doing wrong. Usually in great detail.

I'm reminded of the story of the long-distance hiker (we'll call him LDH) who was halfway through his hike of the Continental Divide Trail when he came across a couple of well-equipped hikers (let's call them the Bean Brothers, you'll see why in a moment) out for a long weekend. These two looked like they stepped straight from the pages of the L.L. Bean catalog, while LDH (who had already put over 1200 miles in), looked rather ragged (understandably so, I reckon). They had all stopped for lunch, so the Bean Brothers, with all the good intentions in the world, proceeded to explain to LDH everything that he was doing wrong, why his gear sucked, and what they would do differently (basically everything). After a good hour break, LDH thanked them and got ready to head on up the trail. The Bean Brothers asked if he was almost to his car, because they were genuinely concerned about his outdoorsmanship. LDH chuckled and explained that he was still about 2 months from his end point and more than 2 months into his hike, but he appreciated their input. He left them speechless.

The lesson here is to not be a know-it-all. If someone asks you for your opinion, or why you do things a certain way or carry a specific bit of gear, talk about it. I tend to listen more than I talk, and I will ask questions about things that intrigue me. I learn a lot that way. Sometimes a new way to do things, sometimes a way that I don't want to try.

Hike Your Own Hike.

Quote

We have won a moment in an unfolding universe. Isn't that worthy of our notice?

-- Hannah Hinchman

Building A Model Rocket - part 5

Parachutes.

Technically, what comes with your typical model rocket are parasheets, and real parachutes aren't measured in diameter, but in square inches (or feet) of canopy. Model rockets have been doing things their own way since the beginning, and it works just fine. The Fat Boy has a purple and white chute, which is 18" across. For reference, Estes 12" chutes are orange and white, and 24" chutes are red and white.

Lately, Estes has been including pre-assembled parachutes in its kits. If you have one, all you really need to do is make sure that the knots are tight. If you want to make the parachute better, follow along as I explain the steps to construct one of the Estes chutes, and re-do a couple of simple things.

Lay out the plastic sheet and, using an x-acto knife and metal straightedge, cut it out on the outside lines. It's a hexagonal shape, and the parachute shrouds will be tied into each corner.

At this point I always stick a binder reinforcement onto each corner. These little self-adhesive paper rings are available in the stationery section of most stores, and keep the strings from tearing through the plastic. Alternatively, you can use a small square of duct tape (about 1/4" square). Whatever you use (if you use anything) make sure it lays flat so the strings can't catch on it.

Stretch the string out and fold it back on itself twice. You're going to cut it into three equal lengths. While we're at it, we'll start calling them "shroud lines" too instead of the "strings".

Using a sharp pencil point or thick needle, punch a hole in each corner of the chute, inside the reinforcement ring or tape square. Thread an end of a shroud line through, then tie a double knot and pull it tight. Tie the other end of the shroud line to the corner immediately to either side. Do all three shroud lines in the same way, so that each corner has one line attached and you have three loops of line coming off of the chute.

While it's flat, decide whether you'd like to cut a spill hole. This is a hole in the apex of the canopy that lets the air out from underneath. The reason for it is that without it, a chute will tend to oscillate in the air as the air spills out from the edges of the canopy. If you remove enough, it's also a good way to increase the speed that the rocket comes down beacuse you're removing a part of the canopy. On real chutes, adding a spill hole can actually increase the efficiency of a canopy, which can decrease its descent rate (make it come down slower). Parachutes are subject to the same laws of aerodynamics as rockets, airplanes and birds.

Estes chutes have the optional spill hole already marked. Just use your xacto knife to cut out the dotted lines around the center logo. I do recommend doing this for the Fat Boy, because that 18" chute is awfully big for the weight of the rocket.

If you ever want to make your own model rocket parachute, it's easy to do. Any plastic bag material will work, or you can use the heavier plastic from those rolls of picnic tablecovers. Whatever you use, add some color if needed with permanent markers or hilighter marker because a clear plastic chute will be invisible at altitude.

For shroud lines, you can use heavy carpet thread, braided nylon, dacron or kevlar, or a brand of dental floss called Glide. The Glide is made of teflon and is fire resistant, which is a good thing for our purposes.

To attach the parachute, gather all of the shroud lines and thread them through the plastic loop on the nosecone. Pull the lines through and open them enough to slip the canopy through. Keep tightening the lines by lightly pulling on the canopy until the shroud lines snug up against the nosecone loop.

Alternately, you can attach the chute to a fishing swivel using the same steps. This way, you can move the chute from one rocket to another just by opening the swivel and reattaching it to another nosecone loop. You might need to use needlenose pliers for this. There's a picture of fishing swivels in part 2 of this series. The shroud lines go through the small loop at one end, and the big end opens like a safety pin so you can attach it to the nose cone.

Now a little bit about aerodynamics and what makes these rockets safe to fly. For the Fat Boy kit, it should be perfectly stable as built, assuming you didn't add a bunch of weight at the aft end. Not all kits are naturally stable, so if it comes with a chunk of clay in the kit, you'll need to put it inside the nosecone as the kit instructions direct. In any event, you should at least do a quick check on a completed kit. The following tells how and why.

On standard rockets - fins at one end, nose cone at the other, nothing really odd going on in between - there are two places on the rocket that are critical to stability. First is the Center of Gravity (CG) and it's the point where the rocket weighs the same in either direction, like a fulcrum of a teeter-totter, or perfectly balanced scales. In the exact same way as a teeter-totter, you determine the CG by balancing the rocket on a pencil or some such (I use my finger - it's close enough). The point where it balances is the CG. Put a little pencil mark there.

I talked a little bit about the CG here without naming it (the bit about the hand out the window). The CG is the point that the rocket will rotate around as the fins correct the flight path.

The second place is called the Center of Pressure (CP). This one is a little harder to explain, but just like the Center of Gravity is where all the weight of a rocket balances, the CP is where all the various aerodynamic forces balance. These forces include thrust, drag and gravity, as well as the roll, pitch and yaw of the flying rocket.

To determine the CP, the easiest way is to make a cardboard cutout of the rocket outline, then balance it on something like you did for the CG. The difference here being that the cardboard is only two dimensional. It also represents the rocket flying through the air sideways (90 degree angle of attack), since it's presenting the largest possible cross-section to view. What this does is give the most conservative CP of the airframe. This CP will be farther forward - toward the nose - than any other angle of attack.

Your rocket will be stable if the Center of Gravity (CG) is in front of the Center of Pressure (CP) by at least one diameter of the main body (caliber). So if the CG is twice as far in front of the CP as the body diameter, then the rocket has two calibers of stability.

All this is great for regular rockets, but the Fat Boy is rather short and squatty, so the margin for stability is shortened a bit, and you'll find you probably have around 3/4 of a caliber stability, which is fine for that kit.

To move the CG forward, you can add weight to the front of the rocket, or add length. To move the CP backwards, you can either add length to the rocket, or increase the size of the fins, or the number of fins, or sweep them backwards.

Having the CG too far ahead of the CP is called 'overstable', and can cause the rocket to be overly sensitive to wind gusts. It can behave like a weathervane and cock sharply into a breeze, just like a... uh, weathervane.

One last thing, you should measure the CG when the rocket is prepared to fly - motor, chute and the works, because that's how the rocket will actually fly. Sounds dumb, but it's not. The motor can shift the CG significantly backwards.

A simple test for stability is called the 'swing test'. Find the rocket's CG (remember, ready to fly configuration), and tie a long piece of string around it at that point - use a spot of tape to hold it in place. Then take the string and swing the rocket around your head like you were using a rope lasso. The rocket should settle into place and look like it's flying horizontally around you. Sometimes it will settle in tail first, that's ok. And for certain weird cases, a rocket will tumble as unstable, even though in actual flight it'll be fine. But for 99% of the time, this is a good test, and even scale models of real rockets have been checked this way by engineers in informal testing.

Or you can trust the kit. :) Knowing where the CP and CG are become critical when you design and build your own rockets.

The math to determine the CP isn't that difficult, and was worked out in general form by Jim Barrowman in 1966. Known as the 'Barrowman Equations' (duh - and the link is a .pdf document), they simplify the process by making several assumptions about the rocket and aerodynamic environment. They're still a useful approximation and are still frequently used.

So what kinds of practical use is all this CG and CP hocus-pocus?

Well, for our rockets, we want them to be stable so that they fly straight and safe, especially since model rockets are unguided, and rely on fins to keep it going straight up.

In general, an airplane (real or model), wants the CG and CP to be closer together, so that they're neutrally stable. That way, the plane is easy to steer because the airframe isn't fighting to keep itself pointing in the same direction. A military fighter is going to be closer to unstable, and thus more nimble, than a passenger jet.

Military missiles, especially air-to-air versions like the Sidewinder, are purposely designed to be unstable. They can turn-on-a-dime, figuratively speaking, and the only thing that allows them to fly straight at all is the onboard guidance computer, and controls like fins that rotate, tiny steering rockets along the sides, or thrust deflection. Larger missiles without fins steer by changing the direction that the engine bell is pointing, using the rocket thrust itself to steer.

Next up: Paint!

Cloud Appreciation

If you take the time to really look at them, clouds can be incredibly cool. You might see shapes in the fluffy ones. They can evoke emotion, such as dread in a darkened sky just before a big storm breaks. With experience you can read them to get clues about the coming weather, and scientists create man-made clouds with sounding rockets to study the upper-atmospheric winds.

Here's a website devoted to all the various types of clouds: The Cloud Appreciation Society. Click that link, look at their photo gallery and be prepared to be amazed.

Quote

Flying is learning how to throw yourself at the ground and miss.

-- Douglas Adams

Building A Model Rocket - part 4

Cardboard tubes, balsa, paper and plywood all fall into the category of wood products, and the best adhesive for these materials is wood glue. A well-fit joint will be stronger with wood glue than even epoxy or industrial grade cyano-acrylate (CA) - aka 'superglue'. The technique to make these indestructable glue joins is called the 'double-glue method'.

That 'indestructable' claim isn't exageration either, because the materials being joined will break before the glue bond fails. It's not unheard of to have a fin break just beyond the glue.

So what's this secret way to glue rockets together? It's simple.

1. Lay a light bead of glue along the root of the fin.
2. Put the fin into place so the glue gets onto both parts to be joined. In this case, the fin root and the body tube.
3. Pull the pieces apart. There should be a light coat of glue on both pieces. Let it dry almost completely.
4. Put another bead of glue on the fin root, then press the two pieces together for good.

The reason this works is because the first coat of glue penetrates the materials to be joined, and the second coat chemically bonds with the first, locking eveything together. This makes for an incredibly strong joint.

For the Fat Boy, I suggest just sliding the motor mount assembly into position without glue (the metal motor hook will be sticking out the bottom), then gluing the fins into position. Use plenty of glue on the fin tabs which go into the slots of the body, and less on the parts of the fin root that don't fit into the slots.

It's fairly important to get the fins straight, but vertical alignment is more important that being perfectly spaced or perpendicular to the body tube. Since the Fat Boy has slots that the fins fit into, this is taken care of for you. When gluing the fins into place, take the nose cone off and set the rocket body upside down on your work surface so that you can look down at it and better gauge alignment. Looking from above, all three fins should point to an imaginary spot directly in the middle of the motor mount tube. Wipe excess glue away with your finger, smoothing it into the joint where the fin meets the body tube.



Tip: If you've already bought motors, put one in the motor mount because it's easier to 'aim' the fins at the small nozzle than it is at the imaginary point in the empty tube.

Leave that be and let's assemble the shock cord mount. Hopefully you've taken my suggestion and picked up a package of 1/8" sewing elastic, because the length supplied with the kit is just too short.

In the instructions is a diagram for the standard Estes shock cord mount, sometimes called a 'paper sandwich'. If you're building a different kit then follow whatever directions you've got, or use the following diagram to make one like ours (click it and it gets bigger). At the end, you should have a truncated pyramid shape folded twice with one end of the elastic embedded inside. Here, you should be generous with the glue, yet squeeze it out so that it's as flat as possible.



Let everything dry. Be patient, give it a couple of hours.

For each side of each fin, run a small bead of glue along the edge where the fin meets the body tube. Then take your finger and smooth the glue into the crease. Don't wipe too much glue away, just try to leave a smooth rounded fillet. If you're using brown carpenters glue, the gel formula will keep the glue from running and you can do all the fin fillets at one time. Otherwise, just do one or two at a time and let it dry before moving on to the next. These glue fillets add lots of strength to the fin joint and you should always do them.

We didn't glue in the motor mount before attaching the fins, so lets do that now as well. Apply the glue fillet to the seam where the centering rings meet the body tube, just like you did with the fins. Smooth it with your finger, and since we're using wood glue I recommend putting a second coat on after the first is dry. Do this for both ends of the motor mount. The top fillet is deep inside the body tube, so what you can do is take a long scrap of the balsa that the fins came from, and use that to apply the glue. Don't worry about being perfectly neat, the important thing is getting the joint glued.



The instructions tell you to mark a line between the fins to help you align the launch lug. Instead of that, I usually install the lug in the corner where the fin meets the body tube. This way the lug is automatically lined up vertically (the pre-cut fin slots help), plus it's stronger for the extra surface to glue against.

Finally, it's time to glue the shock cord mount into place. You should have a "paper sandwich" which has the elastic coming out of one end. Use a good bit of glue, and attach the mount to the inside of the top of the body tube, with the elastic pointing up towards the nose cone. Make sure you get it far enough down inside the body so that it doesn't interfere with the shoulder of the nose cone.



You can trim the elastic to a length of about 24" or so before or after gluing the mount into place.

The reason for making the shock cord longer is a phenomenon known as the 'estes dent'. What happens is that during the flight, the nose cone is propelled forward by the ejection charge. If the shock cord is too short, then the nose cone stretches the elastic until it zings right back at the rocket, crunching the top of the body tube. Using a longer shock cord prevents this from happening. A good rule of thumb is to make the shock cord 2-3 times the length of the body tube.

Once the shock cord mount is dry, use more glue to make sure it's firmly glued into place. This part is going to keep your rocket attached to the parachute and nose cone, so use some care here. You also want to make sure it's as flat as possible, so that there's nothing to snag the parachute on it's way out.

When everything is dry, tie the end of the elastic shock cord to the plastic loop of the nose cone. Use a double knot and make sure it's tight.

At this point, the rocket is ready to fly except for the parachute. Next Friday we'll put up the next part talking about the parachute and discuss flight stability a little more. We'll also get ready to paint the rocket.

Every Day Carry (EDC)

Each and every day, I have the following things with me in my pockets:

Wallet
Comb
Handkerchief
Swiss Army knife
Cell phone
Chapstick
Fisher Space Pen
Paper for quick notes
Keys

What about you?

Quote

The power of imagination makes us infinite.

-- John Muir

Building A Model Rocket - part 3

It's time to spread some glue! I know, finally, right?

This is a series of posts where we're building a basic model rocket online. Each post shows part of the process step by step, including pictures and passing along tips and tricks I've learned along the way. To learn more about what model rocketry is about, see this Q&A.

Follow along and when we're done you'll have built and flown your first model rocket. Questions asked from previous posts are answered too, so if you have questions, please leave them in the comments or email me.

This time we're going to put together the motor mount. It's a simple process. If you're building the Fat Boy, then the motor mount consists of the motor tube, two cardboard centering rings, a metal engine hook, and the black engine holder ring. Every model rocket has this setup, with minor variations. I'll talk about that after assembly.

Test fit the centering rings on the motor tube first. I had to widen the inner holes a little bit by reaming it out with a pair of scissors. The rings should slide on easily, don't force it.

Mark the motor tube (it's not quite 3" long) according to the instructions. Carefully push the tip of your x-acto knife into the tube at the proper mark to make a small slit. The slit only has to be wide enough to accept the width of the motor hook.

Push the "L" shaped end of the motor hook into that slit, so that the motor hook lays flat along the length of the tube. Then slide the black engine holder ring onto the tube and over the motor hook. Don't glue anything yet.

Now slide the rings onto the tube. If one ring has a notch in the inner cutout, then that notch fits over the squiggly end of the motor hook. The idea here is to allow you to lift the overhanging end of the motor hook out of the way to insert and remove the rocket motors.

I recommend putting a couple of wraps of masking tape around the motor tube and hook right where the hook goes into that slit you cut. It's not strictly necessary, but it's simple insurance to prevent a potential problem later.

Now it all looks like this. Nothing is glued yet, but we're ready to go.



Put a bead of glue all the way around the place where the motor tube goes through the centering ring. If you're using the gelled stuff that won't run, do both sides of both rings all at once, otherwise just set the motor mount on end and do the 'top' surfaces. When dry, flip it over and do the other sides.

You don't need a ton of glue here, but use enough to completely circle the tube. Use your finger to lightly smooth it into the corner of the joint and then straighten out the centering ring again if needed.

Set it aside to dry.

While's it's drying, I'll explain how this whole assembly works. The rocket motor goes into the motor tube and rests against the hook (the one through the slit). When the motor ignites it pushes against that hook, which is secured to the motor tube, which is glued to the centering rings, which will be glued to the airframe. Simply put, the motor takes off, and everything else goes along for the ride. That's the reason for the wraps of masking tape I recommended earlier - to keep the hook end in place. If the hook slips out of the slot, then the motor will just thrust straight up through the rocket and blast off by itself, knocking the nosecone out of the way on it's way through. Entertaining, but not in any way a successful flight.

Some kits use a 'thrust ring' to prevent this instead of, or in addition to the motor hook. It's just a cardboard ring that is glued inside the motor tube where the hook enters, to give the motor something substantial to push against.

The other end of the motor hook (the squiggly bit), has an important function as well. Besides letting you move the hook out of the way to extract an expended motor, it also keeps the motor in place when the ejection charge goes off, which deploys the parachute.

Isaac Newton's third law of motion states that for each action there is an equal and opposite reaction. The ejection charge of a model rocket motor fires forward (towards the nose, which means that the body of the motor is forced backwards. Without the motor hook in the way, the motor would eject out the back of the rocket and the nosecone would stay in place (meaning no chute). Lawn dart.

If your rocket doesn't have a motor hook, then you can do a couple of things. First off is what they call friction fit. This is simple and wonderfully effective. Use pieces of masking tape (I use enough for about a half-wrap) around the end of the motor case closest to the nozzle end, until the motor is a very snug fit in the motor mount. The idea is to make it easier for the nosecone to come off than it is to expel the motor, 'path of least resistance' style. Another method that I've used is to put the motor into place, and then use a couple wraps of masking tape around the motor and motor mount tube. You can also do both, but that's usually overkill.

Next up will be the shock cord mount, and putting the motor mount into the body tube. Maybe a little bit about the chute too.

Sky Maps

Each month, Skymaps.com publishes a free, printable map of the night sky to help you identify stars, constellations, planets and more. Seriously cool.

Quote

Reputation is what men and women think of us; character is what God and angels know of us.

-- Thomas Paine