Basics Of Model Rocketry

This article is written by David Parkin to explain the various disciplines in model rocketry. So that both the beginner and individuals wishing to under stand the hobby can. Since most people I come in contact with have little or no understanding of what's really involved.

Disciplines of Sports Rocketry

Model rocketry is a safe, fun and rewarding hobby. Like all hobbies there is always a good way to start. Starter sets are the best way in model rocketry. Both Estes and Quest make model rocketry starter sets. These are the best way to get started and with a verity of different models provide lots of choice for the individual. Some of the verities made by Estes are the Shell-shocked/ that is capable of lofting an egg to 300 feet. Astrocam RTF that can take aerial photos of your launch site. The Free fall launch's your favorite action figure and the Recon with its mini satellites. Quest all so makes an impressive selection of different starter sets. From the tiny micro max Saturn V and space shuttle to the Shockwave starter set.

A typical starter set consists of a launch pad; launch controller and a rocket. All you need to purchase are some batteries and suitable motors.

The fun does not stop with the starter sets there are many types of models to add to your fleet. The model rockets available vary from simple sports models like the Alpha, which are simply colorful and fun, to multi staged rockets that ignite and separate in mid flight. Including scale models of the real thing and rockets that glide back to earth. Once you have built a couple of models and have a basic understanding of the principals involved you might want to try kit bashing or design your own.

Multi Staging

Multi Staging is where two or more engines are used one after the other. Usually they are pyrotechnically lit either engine butting or electric match. Why stage? Staging is done for effect or to gain a greater altitude on smaller motors. In the early days of model rocketry motors greater than D engines were not legal in Australia and still have a number of restrictions on them. So to achieve more altitude it is necessary to stage. Staged rockets also has the advantage of as each motor is used the empty casing and apiece of the airframe are jettisoned, leaving less weight to carry to the same altitude as a bigger motor or clusted rocket. The disadvantage of staging is that for each engine you add the further back the centre of gravity moves this practically limits the number of stages to three, even if you use electronic timing to move the engines further a part. If this is done right moves the center of gravity forward.

Clustering

Clustering is where more than one engine is used. (Usually two side by side.) But can consist of three, four or even five. The largest number of engines I've seen clustered was seven. However this is not normally done. Three engines can be clustered together or side by side. Four engines are normally stacked two by two. The advantage of clustering is that you can lift more weight or launch a rocket at nearly twice the speed of a rocket with only one of the same motor. The biggest disadvantage it is complex and can only achieved by someone with an in-depth understanding of model rockets.

Payload lofting

Payload lofting can be considered a discipline within its self although some people consider it part of competition flying. Flying a payload can be electronic, an experiment or for competition. Competition payloads either consist of a 1oz weight or an egg. The idea being to loft it as high as possible, Experimental payloads are usually to find the answer to a question. And electronic are either related to an experiment, tracking or to record the flight.

Boost gliders

Boost gliders and rocket gliders are not for the faint of heart. A boost glider is where a pod is attached to a glider. The pod contains a separate recovery system. The BG is launched like a conventional rocket, when the ejection charge fires the pod separates and the glider comes down separately. A rocket glider is an all in one package. The rocket motor stays with the glider through out the whole flight. The ejection charge is used to burn through a string to make the elevator spring up or slide the wing forward.

Scale and semi scale models look spectacular when launched and challenge the craftsman skills of any modeler.

Medium and high power rockets are a completely different thing all together, Medium power ranges between E to G powered motors the construction technique is completely different to low power up to D engines.

Before you learn the disciplines you need to understand a few basic things.

A model rocket essentially consists of a few basic components and no matter how complex or large the rocket is. Their functions never disappear, these parts are

1. Motor mount

The job of the motor mount is to secure the rocket motor to the rest of the rocket. A motor mount can be as simple as a thrust ring glued to the body tube with masking tape wrapped around the rocket motor casing, to increase friction enough to prevent the motor from being kicked out by the ejection charge. Generally the motor mount consists of motor tube, two centering rings and engine hook joined to the body tube. (See Dia 1)

2. Body tube

The body tube is usually cylindrical in shape. It's main purposes being, is to join things like the motor mount to the fins and to the parachute and shock cord. Sometimes this will include a payload section, a second chute and a nosecone.

Dia 1

3. Fins

The fins main purpose is to guide the rocket during flight. A Rocket flies straight because the center of gravity (CG) is ahead of the center of pressure (CP) in old term's center of drag. What this basically means is that the rocket moves forward the fins pull it straight. See Dia 2.

Dia 2

4. Recovery System

A Recovery system normally consists of a parachute and shock cord attached to the body tube and nose cone. In some rockets 2 parachutes are used one for the body tube, motor mount and fins. The other parachute being attached to a payload section and nose cone. Other types of recovery systems are streamer (26gms or less), tumble (this is where at ejection the rocket is destabilized aerodynamically either by moving the cg back or simply separating the nose cone from the body. 10gms or lighter), glide recovery and helicopter recovery.

5. Launch Lugs

Launch lugs guide the rocket at the beginning of the flight until the rocket has achieved enough speed for the aerodynamic surfaces to have effect (the fins). On most low power rockets (less than the total impulse of a D engine) the launch lug is a simple cylinder with an internal diameter of 1/8 of an inch, glued to the side of the rocket. This enables a launch rod (1/8 steel or aluminum) to pass through the center. Some other methods are tower launches, rail launches and tube launches, these are advanced methods used only by experienced modelers (SeeDia 3).

dia 3

6. Nose Cone

The function of a nose cone is for assisting penetration of the atmosphere. Nose cones come in various shapes round, cone, round orgive, sharp orgive (Dia 4) and Von Carmon. The last of which can only be worked out mathematically.

dia 4

THE ROCKET MOTOR

How does it work? The rocket motor is the main component and usually consists of three phases. The first is the thrust that pushes the rocket off the pad. Followed by the delay that produces smoke but no thrust to enable the rocket to coast up. The ejection charge follows; this sends hot gases up the body tube and pushes the recovery system and nose cone from the body tube. (Dia 5). The exceptions to this are hybrids, plugged motors and boosters.

dia 5

In order to select the right motor for the job, each engine is identified by a code printed on the side or top of the motor e.g. A8-3 the A stands for total impulse in Newton seconds

1/2A= 0.625 - 1.25 Newton's

A = 1.26 - 2.50

B = 2.51 - 5.00

C = 5.01 - 10.00

D = 10.01 - 20.00

This means each letter represents up to twice the power of the letter before it. The 8 stands for average thrust in Newton's e.g. The A8-3 have a peak thrust of 13 Newton's or 3 pounds. The 8 stands for average thrust of the whole propellant charge even if the thrust curve drops below this. A Newton is the force required to move one kilogram over one meter for one second. To get thrust time divide the total impulse by the average thrust.

The delay is the time between propellant and the ejection charge, which would mean that an A8-3 has three seconds between propellant burnout and the ejection charge firing or a total running time of 3.32 seconds. On the kit and starter set packs you will find several different types listed. These are the motors best suited to the kit. Now I here you say why list A8-3 if the Rocket will take a C6-7 and fly higher. There is always one motor type that will have First Flight next to it, use this to become familiar with the rockets flight pattern. Rockets don't just fly up and come down in a straight line. A rocket will curve in to the wind when launched and drift downwind when the recovery system is deployed. By learning the decent rate you can change the angle of the launch rod to assist in recovery and also select the best motor for the wind and your launch site dimensions.

Different motor types

Even in solid motors there are different types of motors and propellants. Black powder is common in small motors like Estes, Composite propellant is normally used in motors larger than 20 Newton's. Hybrids are the most recent change in motor type in model rocketry. Hybrids consist of a solid fuel and a gaseous Oxidizer either oxygen or nitrousoxide.

Still more to come