Model rocket

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A typical model rocket during launch

A model rocket is a small rocket capable of being launched by anybody, to generally low altitudes (usually to around 100-500 m (300-1500 ft) for a 30 g (1 oz.) model) and recovered by a variety of means.

According to the National Association of Rocketry, (NAR) Safety Code[1], model rockets are constructed of paper, wood, plastic and other lightweight materials. The code also provides guidelines for motor use, launch site selection, launch methods, launcher placement, recovery system design and deployment and more. Since the early 1960s, a copy of the Model Rocket Safety Code has been provided with most model rocket kits and motors. Despite its inherent association with extremely flammable substances and objects with a pointed tip traveling at high speeds, model rocketry historically has proven to be a very safe hobby and has been credited as the most significant source of inspiration for children who eventually become scientists and engineers.

The launch of a scale model rocket

Contents

[edit] History of model rocketry

While there were many small rockets produced over the years for research and experimentation, the first modern model rocket, and more importantly, the model rocket engine, was designed in 1954 by Orville Carlisle, a licensed pyrotechnics expert, and his brother Robert, a model airplane enthusiast. They originally designed the engine and rocket for Robert to use in lectures on the principles of rocket powered flight. But then Orville read articles written in Popular Mechanics by G. Harry Stine about the safety problems associated with young people trying to make their own rocket engines. With the launch of Sputnik, many young people were trying to build their own rocket engines, often with tragic results. Some of these attempts were dramatized in the fact-based movie October Sky. The Carlisles realized their engine design could be marketed and provide a safe outlet for a new hobby. They sent samples to Mr. Stine in January, 1957. Stine, a range safety officer at White Sands Missile Range, built and flew the models, and then devised a safety handbook for the activity based on his experience at the range.

The National Association of Rocketry (NAR) was founded in 1957 to help promote not only the hobby, but to promote the safety of the activities related to model rocketry.

[edit] Companies

The first American model rocket company was Model Missiles Incorporated (MMI), in Denver, Colorado, opened by Stine and others. Stine had model rocket engines made by a local fireworks company recommended by Carlisle, but reliability and delivery problems forced Stine to approach others. Eventually Stine approached Vernon Estes, the son of a local fireworks maker. Estes founded Estes Industries in 1958 in Denver, Colorado, and developed a high speed automated machine for manufacturing solid model rocket motors for MMI. The machine, nicknamed "Mabel", made low cost motors with great reliability, and did so in quantities much greater than Stine needed. Stine's business faltered and this enabled Estes to market the motors separately. Subsequently, he began marketing model rocket kits in 1960, and eventually, Estes dominated the market. Estes moved his company to Penrose, Colorado in 1961. Estes Industries was acquired by Damon Industries in 1970. It continues to operate in Penrose today.

Competitors like Centuri and Cox came and went in America during the 1960s, 70s and 80s, but Estes continued to control the American market, offering discounts to schools and clubs like Boy Scouts of America to help grow the hobby. In recent years, companies like Quest Aerospace have taken a small portion of the market, but Estes continues to be the main source of rockets, motors, and launch equipment for the low-medium powered rocketry hobby today.

Since the advent of High Power Rocketry, which began in the mid-80s with the availability of G through J class motors, a number of companies have shared the market. Low Power Air Rocketry began in the late 1900s. (Each time a motor's classification goes up by one letter, the total energy in it doubles; thus a G motor is twice an F motor and so on). By the early 1990s, Aerotech Consumer Aerospace, LOC/Precision, and Public Missiles had taken up leadership positions, while Aerotech and a host of engine manufacturers provided ever larger engines, at much higher costs. Companies like Aerotech, Vulcan, and Kosdon were widely popular at launches during this time as high powered rockets routinely broke Mach 1 and reached heights over 3,000 m (10,000 ft). In a span of about five years, the largest regularly made production motors available reached N, which had the equivalent power of over 1,000 D engines combined, and could lift rockets weighing 100 kg (200 lb.) with ease. Custom motor builders continue to operate on the periphery of the market, often creating propellants which produce colored flame (red, blue, and green being common), black smoke and sparking combinations, as well as occasionally building enormous motors up to O-R class for special projects such as extreme altitude attempts over 17,000 m (50,000 ft).

Engine reliability became a significant issue in the 80s and early 90s, with CATOs (Catastrophe At TakeOff) occurring relatively frequently (est. 1 in 20) in motors of L class or higher. At costs exceeding $300 per motor, the need to find an alternative was apparent. Reloadable motor designs (metal sleeves with screwed-on end caps and filled with cast propellant slugs) were introduced by Aerotech and became a popular way to reduce the price of launches, since the motor housing was cleaned and reused each time. These engines dominate the market today. At this time (2009) single-use motors above G class are quite rare, and many are collectibles. Aerotech, Dr. Rocket, Animal Motor Works, Rouse-Tech, Cessaroni, Ellis Mountain, and Loki Motorworks provide the majority of reloadable systems today.

[edit] Precaution

Model rocketry is a safe and widespread hobby. Individuals such as G. Harry Stine and Vernon Estes helped ensure this by developing and publishing the NAR Model Rocket Safety Codes [2][3][4] and by commercially producing safe, professionally-designed and manufactured model rocket motors.

One of the main motivations for the development of the hobby in the 1950s and 1960s was to provide young people the opportunity to construct flying rocket models without having to engage in dangerous construction of motor units and direct handling of explosive propellants.

[edit] Model rocket motors

Anatomy of a basic model rocket engine. A typical engine is about 8cm long. 1. Nozzle; 2. Case; 3. Propellant; 4. Delay charge; 5. Ejection charge; 6. End cap

Most small model rocket motors are single-use engines, with cardboard bodies and lightweight molded ceramic nozzles, ranging in impulse class from fractional A to E. Model rockets generally use commercially-manufactured black powder motors. These motors are tested and certified by the National Association of Rocketry, the Tripoli Rocketry Association or the Canadian Association of Rocketry. Blackpowder motors come in impulse ranges from 1/8A to E, although a few F blackpowders motors have been made.

The physically largest blackpowder model rocket motors are typically E-class, for black powder is very brittle. If a large black powder motor is dropped, or is exposed to many heating/cooling cycles (for example, in a closed vehicle exposed to high heat), the propellant charge may develop hairline fractures. These fractures increase the surface area of the propellant, so that when the motor is ignited, the propellant burns much more quickly than it should, producing greater than normal internal chamber pressure inside the engine. This pressure may exceed the strength of the paper case, causing the motor to burst. A bursting motor can cause damage to the model rocket ranging from a simple ruptured motor tube or body tube to the violent ejection (and occasionally ignition) of the recovery system.

Rocket motors with power ratings higher than D to E, therefore, customarily use composite propellants made of ammonium perchlorate, potassium nitrate, aluminium powder, and a rubbery binder substance contained in a hard plastic case. This type of propellant is similar to that used in the solid rocket boosters of the space shuttle and is not as fragile as black powder, increasing motor reliability and resistance to fractures in the propellant. These motors range in impulse from size D to O. Composite motors produce more impulse per unit weight (specific impulse) than do black powder motors.

Reloadable composite-propellant motors are also available. These are commercially-produced motors requiring the user to assemble propellant grains, o-rings and washers (to contain the expanding gases), delay grains and ejection charges into special non-shattering aluminum motor casings with screw-on or snap-in ends (closures). The advantage of a reloadable motor is the cost: firstly, because the main casing is reusable, reloads cost significantly less than single-use motors of the same impulse. Secondly, assembly of larger composite engines is labor-intensive and difficult to automate; off-loading this task on the consumer results in a cost savings. Reloadable motors are available from D through O class.

Motors are electrically ignited with an electric match consisting of a short length of pyrogen-coated nichrome, copper, or aluminum bridgewire pushed into the nozzle and held in place with flameproof wadding, a rubber band, a plastic plug or masking tape. On top of the propellant is a tracking delay charge which produces smoke but essentially no thrust as the rocket slows down and arcs over. When the delay charge has burned through, it ignites an ejection charge, which is used to deploy the recovery system.

[edit] Motor nomenclature

Rocket motors. From left, 13mm A10-0T, 18mm C6-7, 24mm D12-5, 24mm E9-4, 29mm G40-10.

Model rocket motors produced by companies like Estes Industries and Quest Aerospace are stamped with a code (such as A10-3T or B6-4) that indicates several things about the motor.

The Quest Micro Maxx engines are the smallest at a diameter of 6mm. The company Apogee Components made 10.5mm micro motors, but those were discontinued in 2001. Estes manufactures size "T" (Tiny) motors that are 13 mm in diameter by 45 mm long, while standard A, B and C motors are 18 mm in diameter by 70 mm long. Larger C, D, and E class black powder motors are also available; they are 24 mm in diameter and either 70 (C and D motors) or 95 mm long (E motors). Some motors, such as F and G single-use motors, are 29mm in diameter. High-power motors (usually reloadable) are available in 38mm, 54mm, 75mm, and 98mm diameters.

[edit] First letter

The letter at the beginning of the code indicates the motor's total impulse range (commonly measured in newton-seconds). Each letter in successive alphabetical order has up to twice the impulse of the letter preceding it. This does not mean that a given "C" motor has twice the total impulse of a given "B" motor, only that C motors are in the 5.01-10.0 N-s range while "B" motors are in the 2.51-5.0 N-S range. The designations "1/4 A" and "1/2 A" are also used. For a more complete discussion of the letter codes, see Model rocket motor classification.

For instance, a B6-4 motor from Estes-Cox Corporation has a total impulse rating of 5.0 N-s. A C6-3 motor from Quest Aerospace has a total impulse of 8.5 N-s. [5]

[edit] First number

The number that comes after the letter indicates the motor's average thrust, measured in newtons. A higher thrust will result in higher liftoff acceleration, and can be used to launch a heavier model. Within the same letter class, a higher average thrust also implies a shorter burn time (e.g., a B4 motor will burn longer than a B6).

[edit] Last number

The last number is the delay in seconds between the end of the thrust phase and ignition of the ejection charge. Black Powder Motors that end in a zero have no delay or ejection charge. Such motors are typically used as first-stage motors in multistage rockets as the lack of delay element and cap permit burning material to burst forward and ignite an upper-stage motor.

A "P" indicates that the motor is "plugged". In this case, there is no ejection charge, but a cap is in place. A plugged motor is used in rockets which do not need to deploy a standard recovery system such as small rockets which tumble or R/C glider rockets. Plugged motors are also used in larger rockets, where electronic altimeters or timers are used to trigger the deployment of the recovery system.

[edit] Reloadable motors

Reloadable motor cases. From left: 24/40, 29/40-120, 29/60, 29/100, 29/180, 29/240

Reloadable motors are specified in the same manner as model rocket single-use motors as described above. However, they have an additional designation which specifies both the diameter and maximum total impulse of the motor casing in the form of diameter/impulse. A reload designed for a 29mm diameter case with a maximum total impulse of 60 newton-seconds carries the designation 29/60 in addition to its impulse specification.

[edit] Model rocket recovery methods

Model and high-power rockets are designed to be safely recovered and flown repeatedly. The most common recovery methods are parachute and streamer. The parachute is usually blown out when the engine's recoil creates pressure and pops off the nose cone. The parachute is attached to the nose cone, making it pull the parachute out, and make a soft landing.

[edit] Featherweight recovery

The simplest approach, which is only appropriate for the tiniest of rockets, is to let the rocket flutter back to earth after ejecting the motor. This is slightly different from tumble recovery, which relies on some system to destabilize the rocket to prevent it from entering a ballistic trajectory on its way back to earth.

[edit] Tumble recovery

Another simple approach appropriate for small rockets—or rockets with a large cross-sectional area—is to have the rocket tumble back to earth. Any rocket which will enter a stable, ballistic trajectory as it falls is not safe to use with tumble recovery. To prevent this, some such rockets use the ejection charge to slide the engine to the rear of the rocket, moving the center of mass behind the center of pressure and thus making the rocket unstable.

[edit] Nose-blow recovery

Another very simple recovery technique, used in very early models in the 1950s and occasionally in modern examples, is nose-blow recovery. This is where the ejection charge of the motor ejects the nose cone of the rocket (usually attached by a shock cord made of rubber, Kevlar string or another type of cord) from the body tube, destroying the rocket's aerodynamic profile, causing highly-increased drag, and reducing the rocket's airspeed to a safe rate for landing. Nose-blow recovery is generally only suitable for very light rockets.

[edit] Parachute/Streamer

A typical hassle with parachute recovery.

The approach used most often in small model rockets, but can be used with larger rocket models given the size of the parachute greatly increases with the size of the rocket. It uses the ejection charge of the motor to deploy, or push out, the parachute or streamer. Typically, a ball or mass of fireproof paper or material is inserted into the body before the parachute or streamer. This allows the ejection charge to propel the fire-proof material, parachute, and nose cone without damaging the recovery equipment. Air resistance slows the rocket's fall, ending in a smooth, controlled and gentle landing.

[edit] Glide recovery

In glide recovery, the ejection charge either deploys an airfoil (wing) or separates a glider from the motor. If properly trimmed, the rocket/glider will enter a spiral glide and return safely. In some cases, radio-controlled rocket gliders are flown back to the earth by a pilot in much the way as R/C model airplanes are flown.

Some rockets (typically long thin rockets) are the proper proportions to safely glide to Earth tail-first. These are termed 'backsliders'.

[edit] Helicopter recovery

The ejection charge, through one of several methods, deploys helicopter-style blades and the rocket autorotates back to earth. The helicopter recovery usually happens when the engine's recoil creates pressure, making the nose cone pop out. There are rubber bands connected to the nosecone and three or more blades. The rubber bands pull the blades out, and let them 'copter down.

[edit] Other model rocketry

[edit] Aerial photography

Cameras and video cameras can be launched on model rockets to take photographs in-flight. Model rockets equipped with the Astrocam, Snapshot film camera or the Oracle or newer Astrovision digital cameras (all produced by Estes), or with homebuilt equivalents, can be used to take aerial photographs.

These aerial photographs can be taken in many ways. Mechanized timers can be used or passive methods may be employed, such as strings that are pulled by flaps that respond to wind resistance. Microprocessor controllers can also be used. However, the rocket's speed and motion can lead to blurry photographs, and quickly changing lighting conditions as the rocket points from ground to sky can have an impact on video quality. Video frames can also be stitched together to create panoramas. As parachute systems can be prone to failure or malfunction, model rocket cameras need to be protected from impact with the ground.

There are also rockets that shoot short digital videos. There are two widely used ones used on the market, both produced by Estes: the Astrovision and the Oracle. The Astrocam shoots 4 (advertised as 16, and shown when playing the video, but in real life 4)seconds of video, and can also take three consecutive digital still images in flight, with a higher resolution than the video. It takes from size B-6-3 to C-6-3 Engines. The Oracle is a more costly alternative, but is able to capture all or most of its flight and recovery. It is generally used with "D" motors. The Oracle has been on the market longer than the Astrovision, and has a better general reputation.

There are also experimental homemade rockets that include onboard videocameras, with two methods for shooting the video. One is to radio the signal down to earth, like in the BoosterVision series of cameras. The second method for this is to record it on board and be downloaded after recovery, the method employed by the cameras above (some experimenters use the Aiptek PenCam Mega for this, the lowest power usable with this method is a C or D Motor).

[edit] Instrumentation and experimentation

Model rockets with electronic altimeters can report and or record electronic data such as maximum speed, acceleration, and altitude. Two methods of determining these things are to a) have an accelerometer and a timer and work backwards from the acceleration to the speed and then to the height and b) to have a barometer onboard with a timer and to get the height (from the difference of the pressure on the ground to the pressure in the air) and to work forwards with the time of the measurements to the speed and acceleration.

Rocket modelers often experiment with rocket sizes, shapes, payloads, multistage rockets, and recovery methods. Some rocketeers build scale models of larger rockets, space launchers, or missiles.

Some high altitude rockets deploy a smaller 'second stage rocket' during flight. Once the main rocket engine begins to die out, a second stage is fired from the main. This greatly increases altitude as the speed of the second rocket adds to the speed of the first rocket. For example if a rocket is traveling at 250 km/h (150 mph) then the second stage deploys at an additional 100 km/h (60 mph) from the main, the second stage is now at 350 km/h (210 mph). However, this is not perfect as other variables such as weather may influence the flight.

[edit] High power rocketry

Main article: High Power Rocketry

As with low power model rockets, high power rockets are also constructed from lightweight materials. Unlike model rockets, high power rockets often require stronger materials such as fiberglass, composite materials, and aluminum to withstand the higher stresses during flights which often exceed Mach 1 (340 m/s) and over 3,000 m (10,000 ft.) altitude.

High power rockets are propelled by larger motors ranging from class H to class O. Their motors are almost always reloadable rather than single-use in order to reduce cost. Recovery and/or multi-stage ignition may be initiated by small on-board computers, which use an altimeter or accelerometer for detecting when to ignite engines or deploy parachutes.

High powered model rockets can carry large payloads, including cameras and instrumentation such as GPS units.

The NAR and the Tripoli Rocketry Association successfully sued the US Bureau of Alcohol, Tobacco, Firearms and Explosives over the classification of Ammonium Perchlorate Composite Propellant (APCP), the most commonly used propellant in high power rocket motors, as an explosive. The March 13, 2009 decision by DC District court judge Reggie Walton removed APCP from the list of regulated explosives, essentially eliminating BATFE regulation of hobby rocketry.

[edit] Controversy in the United States

Both amateur and model rocketry have come under controversy in the United States following the terrorist attacks of September 11, 2001, as federal and state authorities allege that model rockets can be modified to act as weapons.

Authorities argue that all members of the hobby should have to be licensed and their purchases recorded and reported to federal agencies. Critics of such policies, particularly those involved in the hobby itself, argue that while building model rockets capable of high speeds and fairly impressive altitudes is a relatively simple feat, guidance systems are exceedingly difficult to design and expensive to implement. G. Harry Stine has stated in his Handbook of Model Rocketry that

"A model rocket literally disintegrates when it hits something because its airframe absorbs the energy of impact by destroying itself. This is the same principle used in modern automobiles where “crush zones” absorb the energy of a crash by deforming and collapsing. Model rockets have been deliberately launched directly into sheets of window glass; these experiments completely destroyed the models but didn’t even scratch the glass."

[edit] References

  1. ^ Model Rocket Safety Code
  2. ^ http://nar.org/NARmrsc.html Model Rocket Safety Code
  3. ^ http://nar.org/NARrcrbgsc.html Radio Control Rocket Glider Safety Code
  4. ^ http://nar.org/NARhpsc.html High Power Rocket Safety Code
  5. ^ National Association of Rocketry web site: http://nar.org/SandT/NARenglist.shtml

[edit] See also

[edit] External links

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