ROCKETRY 101

I have to apologize. This blog was written with the expectation of it being read by some other rocketeers. I didn't imagine anybody else would even be interested in it, it seems such a nerdy thing to me. Well one day my sister says she reads parts of it, but doesn't really get some of the stuff I write about. So in fairness to those not familiar with model rocketry (I guess 99.9% of the literate population, we are a small community), here is my short primer on rocket science. I'll just clue you in to some of the concepts and terms - and there will be no test at the end. I'll also attempt to explain why this is fun for me.

Why rockets? Well, it is enjoyed mostly by guys, I don't know why but I suppose the smoke, fire, and high speeds and high altitudes have something to do with it. For some, it's the modeling aspects, the hands-on crafting. These are the guys with the shiny, pretty rockets. For others it's the flying, these are the guys with the ugly, beat-up rockets. I personally like to make them pretty comming out of the shop, but let the real world beat them up so they look tough. I will even apply the touch-up paint poorly so you can see the scars from the rocket's experience. Some enjoy the competition aspects of it. There are many ways to compete, going for a certain altitude, flight duration, or landing in a particular spot are the most popular.

EDUCATIONAL

Rockets are a lot safer than most pursuits, including cycling, running, football, skateboarding or most other activities kids are encouraged to do. Statistics prove this unquestionably. Rockets are also exciting to most kids and actually are educational. I've seen first-hand how interested kids are in this hobby. They show a genuine desire to learn how to do this. I've also seen how they can't help learning things they don't realize are science facts and practical skills. If more middle and high-school teachers could see how kids respond to this activity, both boys and girls, they wouldn't have to complain about apathetic students. While I don't have kids myself, I've seen it countless times at the monthly rocket club launches, and lived it myself as a kid! Now I ended up as an engineer with a good paycheck and out of jail!

ROCKET SAFETY

Rockets are safe because the rocket motors - the theoretically unsafe part - are pre-manufactured, loaded and sealed. They have been tested by the millions over the past 50 years, and are certified by the NFPA (National Fire Protection Assn.). They are started electrically using an ignitor from a distance of 15-30 feet away, with a fireproof plate underneath called a blast deflector. They are designed so you can't light them with a match if you tried. There is a written safety code (the "Model Rocket Safety Code") on model construction and use, voluntairly printed and shipped with almost any rocket product sold, and all are encouraged to follow. This includes when and where to fly, aiming rockets, staying out of trees and power lines, using light and soft materials, etc. All easy and common-sense stuff.

MODEL ROCKET MATERIALS
Rockets are made out of lightweight materials, both because light flies better and is safer if it were to hit your car or you. Some of my rockets are 20 to 30 inches or more tall, yet weigh about as much as a McDonald's burger (that's not a lot). Hollow cardboard tubes are the main ingredient along with balsa wood - a wood so soft and light you could crush it in your hands. Paper, string and plastic sheets, toothpicks, masking tape or cloth for parachutes are also popular materials. Only the tiny-est parts are made of metal such as a 1/8" screw-eye or a little springy motor clip. Many nose cones and fins and other detail parts are made of plastic in commercial kits. All cheap, light, and safe.

ROCKET MOTORS AND POWER CLASSIFICATIONS
The motor is started from a remote electrical controller. Generally they are made so you must insert a safety key to allow the launch, so accidentally bumping the button will not launch the rocket. The motor burns, produces a thrust, and the rocket lifts off, guided by a launch rod or launch rail for the first few feet until the rocket is going fast enough thru the air to be stable by using the fins on the back of the rocket. The thrust of the motor determines how fast the rocket accelerates. For a given total power level, a motor designated "A10" will have a higher average thrust than a motor designated "A3". Once the rocket is off the launch rod or rail, it can keep accelerating to infinity if given enough time, there is nothing but air resistance and gravity keeping them back. As long as the thrust of the motor is more than the weight of the rocket and a tiny bit to push the air out of the way, it will go up. In addition to thrust, rockets require fins in the back to guide that thrust by aerodynamically steering the rocket in the right direction, just like an arrow. Some special variations can use sticks or a widening cone, but the end result is to create some air pressure from the relative wind to keep the rocket on track.

Motors are classified by total power (or impulse), an "A" motor is twice as strong as a "1/2A" motor. A "B" motor is twice as strong as an "A". You can probably guess that a "C" is twice as strong as a "B", and so on through the alphabet. Smaller rockets stop at D or E. Total energy of a rocket motor is measured in Newton-Seconds. (Newtons are an international standard measurement - analogus to pounds in the U.S.) This defines the amount of force (newtons) and the amount of time (seconds). A motor could produce 20 newtons for one second, while another produces 10 newtons for two seconds. Either way, the total energy is the same: 20x1 or 10x2. Some motors push strong and quick, others push more weakly but for a longer time. A rocketeer will choose the type of motor depending on the flight conditions.

Medium power is generally around E, F or G. At this level it starts to get serious, speeds and energy are higher and the sound is scary to some small children (and me). Higher-power models will use H, I, J ...where the largest and most expensive motors top out at around N, O, P. (FYI, the Saturn V Apollo first stage would be far, far beyond Z.) So when you read "I'll try an A", that means a small motor, C is a bit more powerful, E, F, now you are getting serious. These larger rocket kits and motors are obviously meant for and only used by experienced adults or kids using them under the guidance of an experienced adult.

FLIGHT COSTS
These smallest motors are a buck or two to buy in quantity. Larger ones are more expensive, up to about $10 each. Medium power (E-F-G..) may cost up to $20-30 each. The rocket kits themselves are very cheap, ranging from a few bucks for the simplest ones to 25-30 retail and twice that for the more exotic or higher power rockets. Prices are much less, by about 30-50% less via web/mail order. At least for me, the cost of the paint is about as much as the kit.

DELAY AND EJECTION

Most small motors only burn for a fraction of a second or up to two seconds. After that, the rocket has to coast and slow down. During this time the motor is just delaying while it produces a tracking smoke to make it easier to see. After a set delay time, a small charge (ejection charge) fires in the opposite direction - up through the rocket. This is made to pop off the nosecone and release the parachute or some other recovery method. That delay time is critical. If too short, the rocket pops open while still traveling 50 - 100 or more miles-per-hour and will likely destroy itself. Too long a delay and it may turn over and drop like a lawn dart with a bad ending. After a while, you may come to read "...I'll try a -3 on this...", which means a three second delay (+/- 1 second, it's hard to exactly predict a burning motor.)

RECOVERY

The parachute and rocket generally need to come back together. This is where the shock cord is used. It connects the rocket parts and parachute. Because the parts of the rocket are literally "blown apart", the force generally needs to be absorbed. The shock cord is often made of elastic or rubber, hence the name.
That small ejection charge that pushes out the parachute is hot - if not protected in some way, the recovery device may just melt. This is prevented by using wadding - small paper treated with borax to make it flame proof. Other folks use "dog barf", a slang for a flame-proof treated shreaded paper used as insulation for building construction. These are light, cheap and biodegradeable so if lost outside they will disintegrate with the next rainstorm. Others use a baffle or ejection baffle - which is like a muffler to allow the gasses to cool before hitting the parachute. Small sheets of fireproof Nomex cloth is also used a lot. That's the stuff firefighters and auto racers wear. Baffles and Nomex stays with the rocket, whereas wadding and dog barf is lost and needs to be replaced after each flight.

I've been mentioning parachutes, but there are other ways to get the rockets back safe and slow. Extremely light rockets can come back with featherweight recovery, they are so small and light they can't fall fast enough to be damaged. Streamers are long -well- streamers that flap in the wind to slow the rocket down. Other rocket designs use rotating wings like a helicopter, or glide back like an unpowered airplane or the Space Shuttle. There are more ways to bring a rocket back safely than there are states in the US. Many use a combination of several recovery methods.

AGAIN WITH THE SAFETY!
Again this is all extremely safe. I challenge anybody reading this to find a documented fatality from a model rocket using a commercially made rocket motor in the past 50 years. There aren't any! You'll have to go back to the '50s and the Sputnik era where kids were attempting to make their own rocket motors. That's why Estes and others developed the modern rocket motors back then - to prevent curious little "Leave-it-to-Beaver" type kids from injuring themselves like they were. Popular and routinely encoraged sports like football, swimming, skating, I suppose all of them, have dismal safety records in comparison, with not only serious injuries but also occasional deaths (I know that has happened with football, not certian about the other sports). Yes, that is the sad truth. The popular myth about model rockets being unsafe is not at all true. Most people just don't know the facts, and think of them as fireworks (which sometimes use rockets that explode!)

WHERE'S THE CHALLENGE (FUN)?

While all this rocket stuff sounds pretty simple and safe, there is an art to it. Not just building them but successfully flying them. What fascinates me about the hobby is how difficult it is to be able to consistently fly the perfect flight. Something always seems to go wrong - parachute is crumpled, burned, tangled, didn't open - wrapped too tightly, rocket veered away and got lost or stuck in a tree, got stuck on the rod and never flew, a fin cracked, landed on the stones, tube bent ...the list goes on and on. It's not too difficult to have a perfect flight, but it is near impossible to consistently have only good flights. That's kind of like bowling a perfect 300 where every toss is a strike. That is what keeps me flying - trying to get it perfectly right all day long. It's kind of like wanting to master that video game, you just keep playing over and over thinking "This time I'll get it!"

You know how playing a comercial video game at a bar or arcade is a little more exciting than playing the same game at home? It's because you have some quarters on the line. You get a little bit of the gambler's high because you have something to loose. Rockets are a lot like that. You spend hours, days and weeks preparing your rocket for the ultimate flight. When it's actually out there with nature and it's fickle winds and about to be set on fire, are you sure it will fly straight and true or destroy itself? It climbs out of sight, and you wonder if it will ever be seen again. Will the parachute open OK or will it tangle and get crushed on impact? Will it drift into the top of the one single tree to the East, the power line to the West or will it land on the highway to be crushed by a Honda Civic? The anticipation of the unknown, the commitment of putting all your efforts on the line make you a bit brave and crazy at the same time. The breath you finally take when the rocket slowly touches the grass and settles to rest makes it all worth it.

ROCKET DESIGN

Model rocketry design - and to some extent all modelers participate in this with their choices - is an unending series of trade-off decisions. If it comes down too fast then use a larger parachute, right? Well that larger chute may be too heavy or too big to fit in a small rocket tube. Won't go high enough? Use a bigger motor. But that bigger (heavier) motor may imbalance the rocket and then you need larger fins to correct that, but those larger fins with more air resistance may require more power and the weight will require a larger parachute to bring it down slowly, so add more motor, use a bigger, heavier tube, more parachute, and now the string to attach the rocket to the parachute is too light and breaks. So use a heavier string - now the string is strong but now the big, tough parachute rips the string out of the rocket... it goes on and on like the infinite possibilities of a crossword puzzle.

It would be easy to make the perfect, 100% reliable rocket that will not wear out. Problem is that it would end up looking and flying like a brick. Try it sometime!

ROCKETS AND AIRCRAFT
Most model rockets fly to a height of barely 100 feet up to 1000 feet high and more. Some will go higher but the smaller rockets are usually too small to be seen at that distance and will likely be lost forever. Larger models and multi-stage models can double that altitude to 2000 feet or more, about 1/2 a mile up. Most experienced fliers will not go that high though, usually after loosing a few favorite rockets. A lot of folks wonder how this works with airplanes and jets.

Concerning small airplanes, they generally travel at the lowest about 500 to 1000 feet AGL (Above Ground Level). The actual FAA rule is for 500' AGL over sparsely populated areas (farmlands, forrests etc.) and 1000'AGL over densely populated areas (towns, cities, stadiums etc.). So if you see a small plane flying below that level, it's very likely they are breaking the FAA rules, unless near an airport for arrival or departure or a declared emergency. I constantly see small aircraft flying below FAA mandated minumums! In a few areas of the country (out West, like in the deserts or wide-open areas or in Alaska), where the airspace is classified as class G, these rules don't apply, but in any event, aircraft must remain beyond 500 feet from any building or structure. As for the larger aircraft like the commercial transports, they are usually several thousand feet or more up, they just look like they are low because they are so large! Note that rotorcraft (helicopters) are excluded from these rules. Which is why I suppose they are just so noisy and annoying.

So while there is some potential overlap of model rockets and airplanes, there are two factors that prevent disasters. One, the lightweight cardboard and balsa of a rocket stands no chance of inflicting any significant damage to an airplane. Two, model rocketeers are taught to look for low-flying aircraft before starting the countdown to launch. Few of them would want to have their rockets destroyed anyway! For those few evil souls that are bound to exist, it would be near impossible to time a launch and get the angle, direction and speed right for any aircraft that is flying directly overhead at over 100 mph anyways.

The larger rockets can travel 2000 or more feet, and some of the most powerful can easily hit 4, 7 or even 10 thousand feet. Any rocket over a certain weight or amount of propellant (rocket fuel) or expected altitude can't legally be launched without first applying for and getting an FAA waiver for the flight. Clubs generally arrange this for a whole day of flying with the FAA, and the FAA tells them how high they can go.

The FAA also publishes a NOTAM (Notice To Airmen) that warns pilots of the areas to avoid. NOTAMS are normal routine and pilots are legally required to familiarize themselves with NOTAMS pertaining to their flights. NOTAMS have useful information such as closed runways, bird migrations, construction cranes, military training flights, skydivers, balloon festivals...etc. so pilots should be checking them before taking off. Not all of them do.

I once flew my Bandit to about 400 feet one day, and after it was climbing above the trees a multi-million dollar business jet came shooting out of unseen, from the tree line to the north, probably just a hundred feet higher than the rocket. There are certianly no airports within 5 miles of the launch area big enough to handle the jet. That pilot was flying irresponsibly low and could have easily destroyed the Bandit.





If you have what appears to be a common question - remember none of them are "dumb", drop me a comment and I'll try to answer it for everybody.

- from the guy who was Born Again into Rockets.