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Welcome to Rocketry

This page is designed to introduce people to our hobby (or sport): Rocketry! (welcome mat) I hope this answers some of your questions and to see you join this fascinating hobby. This page is organized similarly to a FAQ (Frequently Asked Questions) page, but is written in a conversational style so you can also read it straight through. If you want to jump to a particular section, the topics covered are:

  SciLinks Selected by the SciLinks program, a service of National Science Teachers Association. Copyright 2001.
   

Categories of Hobby Rocketry

(lockers) The term "non-professional rocketry" is so broad that it is typically broken down further in general use. There are four common categories used to describe the sub-groups:

"Hobby rocketry" is also used to describe model, mid-power and high-power rocketry. Specifically rocketry where the motors are pre-manufactured (as opposed to amateur rocketry).

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Anatomy of a Rocket

The picture below is from the Handbook of Model Rocketry by G. Harry Stine, which is one of the original and best introductions to hobby rocketry. This is definitely the book to start with and is currently in its sixth edition.

rocket anatomy

One interesting thing is that as large and as complex as rockets get, they're still just variations on this basic design. There are fundamental physical principles which restrict what a stable and efficient rocket will be like. Of course, details of design and selection of materials differ widely, but most rockets really are "rocket shaped" and made up of these basic pieces.

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Terms You'll Encounter Frequently

(books) Every activity has its own jargon and rocketry is no exception. Here are some terms in common use which will make what you read comprehensible.

There are many other terms in use, more than we could possibly cover here (and doubtless more than I know), but understanding these basic will allow you to participate in rocketry conversations. Another great source of information is the rec.models.rockets FAQ.

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Motor Information

(catapult) Rocket motors need to provide all the elements for combustion themselves (as compared to a jet engine, which uses atmospheric oxygen). So there are four key elements to a rocket motor:

Rocket motors fall into three broad categories: solid-, liquid- and hybrid-fuel motors. Solid fuel motors combine all materials into a mixture which is poured or packed into the motor liner and which hardens into a solid. These motors are easy to use and safe to transport because the components don't require special containers and the motors are very unlikely to ignite accidentally. Liquid fuel motors keep the two materials in separate tanks and combine them only during actual motor firing. Hybrid motors use as gas and a solid, which are combined during firing.

(explosive A) Solid fuel motors are the simplest to use and the safest to transport and store and are by far the most commonly used in non-professional rocketry. Model rocket motors use black powder as both the oxidizer and fuel. Mid- and high-power rocket motors generally use ammonium perchlorate as the oxidizer and various binders as the fuel. Amateur solid-fuel motors generally use A.P. as well, but also use ammonium nitrate and other oxidizers. Metals and other additives are included in the motor to affect burn rate, alter the thrust curve and even make colorful flames. (Most substances which are good oxidizers are classified as explosives and regulated to some extent.)

Liquid fuel motors are more complex to set up and control, but provide a better thrust-to-weight ratio than solid fuel. Because liquid fuel motors generally use liquid oxygen as the oxidizer, the weight and complexity of these motors require very large rockets (they don't scale down well). Various liquids are used as the fuel such as kerosene, alcohol or diesel fuel. Liquid fuel motors are rarely used because of the difficultly in building them and the difficulty and danger of dealing with liquid oxygen.

Hybrid motors attempt to blend the best of solid and liquid fuel rockets. The oxidizer is a gas (usually nitrous oxide) and the fuel can be almost anything which burns well (PVC is common). Hybrids are more complex than solids because the gaseous oxidizer needs to be pumped into the motor on the pad, requiring a complex launching system. However, the reloads are much cheaper than equivalent solid fuel reloads ($25 for a hybrid J grain as compared to $60 for a J solid fuel reload). Perhaps best of all, none of the components of hybrids are regulated by the B.A.T.F. Hybrid motors have even been introduced into high-power rocketry with systems from HyperTek and AeroTech which are certified for use at high-power club launches.

In hobby rocketry, motors are given a letter designation which specifies the total impulse (overall productive energy) of the motor with a range. Each letter indicates a doubling of the energy in the prior range so a "B" motor is twice as powerful as an "A" and a "C" is four times as powerful as an "A" (and twice as powerful as a "B"). That said, not every "C" motor has the same total impulse. Any motor whose total impulse is greater than 5 Newton-seconds (Ns) and less than or equal to 10Ns is a C motor, even though a 5.01Ns motor is only half as powerful as a 10Ns motor. By the way, commercial motors are made up to "O" size (the limit of high-power rocketry) and a full O is 40,960Ns, equivalent to 2,048 "mighty D" motors. For motor information and simulator data, see ThrustCurve.org.

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Starting Out in the Hobby

(armchair) Building a model rocket isn't difficult or expensive and it gives you a sense of what goes into a rockets and how they work. The best advice for someone brand new to the hobby is to build a model rocket and fly it at a local club launch. You can buy model rockets from Estes and Quest from most hobby shops and large toy stores. Most people enjoy the building itself and building model rockets is fun and satisfying and you can always put more effort into it to improve on your next model.

Beyond model rockery, you will need to get involved with a club. (see the Clubs section below). Because the logistics of flying high-power and amateur rockets are too difficult for most individuals to tackle, club launches are mandatory for more advanced rocketry. Not to mention that meeting and talking to more experienced rocketeers is the best way to find out more information. Through a club, you'll find out what kinds of rockets can be flown from their field and what things they have used and like. This is a great way to advance because you can get direct help and feedback.

The on-line community, primarily Rocketry Online and the rec.models.rockets newsgroup, provide a good forum to find out what other people are doing, get more information and ask questions. Take a look at the rec.models.rockets FAQ and Rocketry Online's INFOcentral for the two most organized repositories of information.

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Designing Your Own Rockets

Once you've built a few model rockets, you'll want to design your own rocket and you should definitely go ahead and do it. One of the best things about model rocketry is that you can experiment with rocket designs cheaply and easily.

(wacky rocket) There are three major things to consider when designing your own rocket:

Motors affect the design of the rocket in two ways: their physical size dictates some elements of construction and their average thrust dictates the weight limit they can lift. Pre-manufactured rocket motors come in several standard diameters: 13mm (¼A & ½A), 18mm (A-C), 24mm (D-F), 29mm (E-G), 38mm (H-J), 54mm (I-K), 76mm (K-M) and 98mm (L-O). Motor lengths vary quite a bit, depending on how much propellant they contain. A through D engines are typically 3" long, and larger ones can range up to 8 feet.

Motors are categorized in many ways, but the two most important ones are total impulse and average thrust. Total impulse refers to total amount of productive energy the rocket develops during its burn. Average thrust refers to the average amount of "push" the motor has. Total impulse defines the motors letter category (see Motor Information above) and is most useful for determining the altitude the rocket will reach. Average impulse determines how much rocket the motor can lift. The rule of thumb is that the motor can left a rocket which weighs less than 5 times its average thrust.

(experiment) A rocket is stable if it will fly straight up when launched. Stability is affected by many things: rocket design, motor selection, the launcher, and even the weather! If you follow the rule of thumb described above when selecting your motor, fly when there is little or no wind and use an appropriate launcher, the only remaining major factor is rocket design.

Most model rockets look similar because that basic shape is the most efficient. Long and sleek with medium-sized fins at the aft end is the shape allows the rocket to be stable without special arrangements. (Commercial and military rockets use "active guidance" systems which control the flight in real time through on-board sensors, computers and adjustable fins or motor nozzles.) You can actually make fairly unusual shapes fly, as long as you understand the forces which act on a rocket in flight.

For a basic understanding of stability, you need to understand the centers of gravity and pressure. The center of gravity (C.G.) is the point along the length of the rocket at which it balances. The finished rocket, with the chosen motor installed, is balanced like a see-saw. The balance point is the C.G. The significance of the C.G. is that this is the point that the rocket will tend to rotate around. A tube with no fins would spin around this point instead of flying in a straight line.

The center of pressure (C.P.) is the point, also along the length, at which the aerodynamic forces are equal. Imagine a cross-section of your rocket along the length; most of the body is a thin tube and at the aft end, the fins stick out. The C.P. is the point along this length at which the area of the cross section is the same forward and aft. In fact, this method (called the "cardboard cutout method") is a simple way to determine the C.P. The cross section cutout of the rocket is balanced, with the weight of the cutout representing the surface area, and the balance point determines the C.P. The more common way to calculate the C.P. is through software. The best all-around program for rocket design is Apogee's RockSim (also see the ThrustCurve.org simulators page for more references).

Now that you know the C.G. and C.P. of your rocket, you can determine whether it will be stable. The rule of thumb is: the rocket is stable if the C.G. is one to two calibers forward of the C.P. Why is this? Remember when we talked about the rocket having a tendency to rotate around the C.G.? Well, for the fins to be effective, the C.P. must be significantly behind the C.G or the affect of wind pressure will not be enough to overcome the tendency to rotate. (Fins work by using the air rushing past them to correct the rocket's tendency to rotate.) The C.P. can be changed by changing the size of the fins and the C.G. can be changed by adding weight forward or aft

Materials and building techniques are also an important consideration. Model rocket materials (paper tubes, plastic or balsa fins and glue) are not appropriate for more powerful rockets. By the same token, high-power materials are not appropriate for model rockets because they weigh too much for model rocket engines to lift. Steel is not used for airframes or fins because it's too heavy for its strength. Large high-power and amateur rockets generally use composite materials like fiberglass. Follow the same techniques you find in kits of rockets using the same motors you plan to use in your designs.

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Laws & Regulations

This section is describes the situation in the United States. Other countries either outlaw non-professional rocketry entirely (rare) or regulate is much less tightly (common).

There are three U.S. government organizations which affect our sport directly. Also some states (notably California) have additional licensing requirements.

(jail) The most confusing and difficult organization to deal with (at least historically) has been the B.A.T.F. In 2009, the hobby organizations won a legal battle that exempts most hobby rocket motors from the previous requirement of Low Explosives Users Permit (L.E.U.P); see the Joint Statement on BATFE litigation. Rocket igniters which are called "electric matches" still require an explosives permit.

The D.O.T. affects rocketeers the least, because their requirements must generally be met by manufacturers and vendors who ship you motors purchased by mail order. However, you must comply with D.O.T. regulations when transporting motors to the launch site. You can personally transport up to 99lbs. of legally manufactured commercial motors (in all but the very largest sizes) as long as they are still in the manufacturers packaging. Note that it is not legal to ship rocket motors through ordinary means. The Post Office and U.P.S. can ship them as "hazardous materials" for an extra charge.

(stop rabbit) Because rockets could damage airplanes, the F.A.A. requires that people launching rockets weighing more than one pound, made out of hard materials, or flying into controlled airspace obtain a waiver in advance. Waivers will generally not be granted in urban areas or near airports. Typical waivers for mid-power launches are 2500-5000 feet and high-power launches usually have waivers from 5000-25,000 feet. Some launches have waivers up to 100,000 feet, but these are rare and only given in very remote areas. It is rare for individuals to obtain waivers on their own, so generally high-power rockets are only launched at club launches.

Note that most of these rules only come into play with larger rockets and more powerful motors. If you stick to model rocketry (weight < 1 pound, made of breakable materials and flying underneath the controlled airspace on pre-manufactured motors) you can launch in a local park (unless there are local ordinances). Just be careful of the infamous rocket-eating trees!

Related to these laws are the certifications maintained by the U.S. national hobby organizations. Both N.A.R. and Tripoli administer tests and track certifications for high-power rocketry at various levels (N.A.R. has two levels and Tripoli has three). These certifications have no legal force, but are a convenient and common way to control access to pre-manufactured motors. At N.A.R. and Tripoli sanctioned club launches, flyers must have the requisite certification level to fly high-power motors. Level 1 certification allows flying H and I motors; level 2 allows flying J, K and L motors, and level 3 (Tripoli only) allows flying M, N and O motors. Certifications generally don't apply at amateur launches which are not sponsored by the national organizations.

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Clubs to Join and Where to Fly

From the Laws & Regulations section above, you can see that it takes a lot of work to set up a launch. This task is generally undertaken by a rocket club for periodic organized launches. Clubs are basically mandatory for all but model rockets, but even model rocketeers benefit from club membership. You should make an effort to find a local club in your area. See the list of NAR Sections and Tripoli Prefectures for clubs in the U.S. and the list of C.A.R. Clubs for clubs in Canada.

(desert) A club will have a field somewhere near the majority of their membership base where they will have periodic flights. Most model rocket clubs have flights monthly or weekly and most high-power clubs have flights less frequently (some as little as twice a year). Because the launches are periodic, most of the club will attend and this creates a social atmosphere and the launches become "events" with flyers, friends and spectators.

Another good reason to attend club launches is that the club provides the launch equipment. Launchers and ignition systems are complex and bulky and having the club provide these facilities makes it much easier to come out and fly. Also, many clubs have vendors who will sell motors (and other parts and equipment) right at the launch site. Even if you don't plan to enter the hobby yourself, look for a club in your area and attend a launch to see first-hand what it's all about.

(questions) Probably the best reason to join a club is the chance to learn from more experienced rocketeers. The Internet is great for gathering information, but nothing beats seeing other's rockets and showing them yours for gaining knowledge. Just seeing what other people are doing is the best way to get your own creative juices flowing and thinking about cool new things to do.


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Sources for Kits and Materials

(lab) The best way to get started in rocketry is to buy an Estes or Quest kit at your local hobby shop or larger toy store. These rockets are complete, easy to build, and come with detailed instructions. Model rockets generally have a level of difficulty indicated which ranges from completely pre-built rockets (ready to fly) to "level 4" rockets which require cutting out pieces from raw materials and complex painting. Two great Estes kits to start with are the Alpha III which is very easy to build and the Black Brant II which is more fun for those with some model-building skill. (Estes now has a web site.)

For mid-power rocketry, the largest company is AeroTech and their Initiator and Arreaux rockets make good introductory kits. The AeroTech kits come complete and with detailed building instructions. These are more work than model rockets, but don't require any very advanced skills.

For high-power rocketry, the two largest companies are Loc/Precision and Public Missiles, Ltd. Anything in the Loc/Precision 1" through 4" kit series makes a good starter mid-power kit with high-power construction techniques and the Graduator and Forte are popular high-power kits. The P.M.L. Ariel and D-region Tomahawk are also good starter kits, and for the more adventurous, the Bull Puppy makes a great-looking rocket.

Kit manufacturers will list several recommend motors. Take their recommendations seriously. Too weak a motor will result in an unstable flight and too powerful a motor will destroy the rocket. See Designing Your Own above for background information. The ejection delay is also critical for a successful flight because too short or too long will case the rocket to open up while it is still traveling too fast and the two sections will come apart (a separation).

Amateur rocketry is all about doing everything yourself, so no kits here. Some larger launches attract companies such as Thunderflame Associates which hold motor-making classes, but your best bet is to find other amateurs in your area and work with them. Scratch-building supplies can be purchased mail-orders from many vendors; when you browse my site, you will see kit and supplies vendors mentioned as appropriate.

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Safety Information

(rocket chasing man) Rocketry is not inherently a dangerous hobby, although misuse or ignorance can cause severe personal injury and property damage. We use powerful motors and the rockets commonly fly hundreds of miles an hour. (Some high-power and amateur rockets even break the sound barrier.) A 20lb. rocket traveling at 500 mph is a force to be reckoned with! Even a rocket falling without a recovery system ("coming in ballistic") can do serious damage. I have seen a rocket drop through the roof of a motor home and trash a table inside.

(crash) Now that I've scared you, what are the things to be careful of? If you build a kit from a reputable manufacturer, following the instructions carefully, and use a pre-manufactured motor appropriate to the rocket, you should be safe. A common failure is a custom rocket design which is not stable. Unstable rockets will not fly straight and could go anywhere. Poorly designed or constructed rockets can break up in flight ("shred"). Before you start designing your own rockets, make sure you have experience with kits using the same motors you're planning to use in your own design and make sure you understand the principles of stability (see the section on Designing Your Own above for the basics).

Materials and building techniques are also important. Model rocket materials (paper tubes, plastic or balsa fins and glue) are not appropriate for more powerful rockets. By the same token, high-power materials are not appropriate for model rockets because they weigh too much for model rocket engines to lift. Steel is not used for airframes or fins because it's too heavy for its strength. Large high-power and amateur rockets generally use composite materials like fiberglass. Despite what you saw in October Sky, steel is not a good material for building your rockets because a steel rocket tube falling from the sky makes a very dangerous projectile. Anyone hit by it will most likely be killed.

Please, please, please: carefully read the safety codes published by the National Association of Rocketry: the Model Rocket Safety Code and the High Power Safety Code.

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Links for Further Reading

This page is just a quick introduction to the hobby. There is an immense amount of information on-line. Below are some of the high points from the web.

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