| 3FNC | Three Fins and a Nose Cone, the description of a generic rocket. |
| Air Start | The starting of a rocket motor while the rocket is in flight. |
| AGL | Above Ground Level. Typically in terms of height above the surface of the ground. |
| AP | Ammonium Perchlorate, the oxidizer used in composite rocket motors. Other components are Aluminum powder (fuel) and polybutediene rubber (the binder holding it all together). This is the propellant mixture that the Shuttle Solid Rocket Booster's use. |
| The highest point in a rockets flight. | |
| Baffle | See Ejection baffle |
| Ballistic Coefficient (Cb) | A measure of a projectile's ability to coast. It is defined as Cb = M/CdA where M is the projectile's mass and CdA is the Drag Form Factor. At any given velocity and air density, the deceleration of a rocket from drag is inversely proportional to this value. Intuitively, it is the principle behind why a tightly crumpled piece of paper can be thrown farther than a loosely crumpled one. |
| Base Drag | A component of aerodynamic drag caused by a partial vacuum in the rocket's tail area. The vacuum is the hole created by your rocket's passage through the air. Base drag changes during flight. While the motor is firing, the drag is minimal since the tremendous volume of gas generated by the motor fills this void. The drag takes a sharp jump at burnout when this gas disappears (note: tracking smoke has very little effect on base drag due to its low density). Base drag can be reduced by the use of a boat tail to transition the main body diameter down to the motor diameter which helps direct air into the evacuated area. When properly designed, a boat tail can reduce base drag below zero (i.e. actually generate a small amount of forward thrust) by making use of the "pumpkin seed" effect. |
| Bernoulli Effect | A phenomenon first described by the 18th century Swiss
scientist Daniel Bernoulli who studied the pressures in moving fluid streams. The effect
states that moving air will have a lower pressure than the still air around it. This is
the principle behind how airplane wings generate lift and why beach balls stay
"balanced" on top of fans in those hardware store displays :-) The effect is significant in rocketry when using altimeters or any other kind of payload that senses the ambient pressure around the rocket. The air moving by the payload section could cause the payload to indicate a lower pressure than the ambient still air, thus giving a false altitude reading. The effect drops to zero at apogee when your rocket stops moving, but the altitude vs. time curve will be wrong. |
| Bernoulli Lock | A phenomenon similar to the Krushnic Effect where the rocket seems to be "glued" to the pad at liftoff. This afflicts larger, flat-bottomed rockets launched too close to pads with flat blast deflectors. The exhaust gasses escape at great speed through the small annular space between the rocket and the pad creating a venturi which generates a low pressure region at the base. This pressure deficit can be significant, and if it is greater than the thrust being generated by the motor, the rocket won't go anywhere! This is quite possible as a 2" dia. rocket has, potentially, over 45 lbs (200 N) of "suction" available to hold it back, while a 3" rocket has over 100 lbs (460 N)! The old Centuri "Point" was an infamous Bernoulli locker when launched from an Estes Porta-Pad with its perfectly matching round blast deflector. |
| Black Powder | Basically, gunpowder. The 'traditional' model rocket motor fuel. Used by Estes and most other model rocket companies through F range. Rocketflite has black powder motors through the H range. See also AP and Composite Motor . |
| Black Powder Motor | A rocket motor containing Black powder (gun powder) as the propellant. |
| Blast Deflector | A Heat Resistant plate on the Launch pad bellow the rocket motor to deflect the blast of hot gasses / flames that appear form the bottom of the rocket motor to stop any heat damage to the ground beneath the rocket at take off. |
| Boat tail | A transition section at the tail of the rocket which gradually narrows the body tube down to the motor diameter. Used to reduce base drag. |
| Body Tube | The body tube of a model rocket is similar to a fuselage in an aircraft. Its purpose is to house the motor/ motor mount and the recovery device. It most cases, the body tube is also the structure upon which the fins are attached. |
| Booster | On a multi stage rocket this refers to the sections (stages) which drop off in mid-flight. On single stage payload rockets, the term is used for the lower powered portion to distinguish it from the payload section. See also Air Start. |
| Bulk Head | A wall within the body tube of the rocket to divide the rocket internal space into compartments. |
| Burn Out | The time when the motor runs out of fuel. |
| Burn Out Velocity | The velocity the rocket is traveling when the motor runs out of fuel. Usually the highest speed achieved by the rocket. See also HyperTerminal Velocity |
| CA | Cyanoacrylate ('super glue'). |
| Casing | The motor casing is designed to house the other motor components in a safe, convenient package. It functions both to allow pressure to be built up within the motor and to shield your model from damaging pressure and heat. On motors using black powder propellant, the casing is made from tightly wound paper. On single use motors using composite propellant, the casing is usually made of a special plastic. Both types are manufactured to exacting standards and carefully controlled dimensions. In model rocketry, where the majority of the motors are made for single use, the casing should never be re-packed for reuse. The internal stresses exerted upon the casing during the thrust, coast, and ejection phases will have weakened it severely. Reloadable casings are made from aluminium and have screw in forward and rear closures. |
| CATO | A motor failure, generally explosive, where all the propellant is burned in a much shorter time than planned. This can be a nozzle blow-out (loud, but basically harmless), an end-cap blow-out (where all of the pyrotechnic force blows FORWARD which usually does a pretty good job of removing any internal structure including the recovery system) or a casing rupture which has unpredictable, but usually devastating, effects. Another form of CATO is an ejection failure caused by either the delay element failing to burn or the ejection charge not firing, but the result is the same: the model prangs. A CATO does not necessarily burn all of the fuel in a rocket motor (especially true for composite fuels, which do not burn well when not under pressure). For this reason you should be especially careful when approaching a CATO. |
| CG | Center of Gravity |
| CP | Center of Pressure |
| Centering Ring | One or more centering rings are used to correctly position
the motor tube within the body tube. Without this correct position, the thrust of
the motor would be angled incorrectly and cause the rocket to follow an incorrect flight
path. In the worst case, this would cause t rocket to hit the ground while the propellant
is still burning; an extreme safety and fire hazard. The centering ring is one component
of the motor mount. Centering rings are also used to hold other items in the middle of the body tube. |
| Center of Gravity | The point on which all gravitational forces act, (the point of balance) |
| Center of Pressure | The point at which all aerodynamic forces act, |
| CHAD | Acronym for CHeap And Dirty. Used to refer to a quick and inexpensive (but usually inelegant) way to solve a particular problem or produce some end result. |
| CHAD Staging | A simple technique used to make a multi-stage rocket out of a single stage vehicle. A booster motor is taped to the end of the standard, single stage motor in the rocket. The booster is totally external to the rocket. The booster is then ignited in the usual manner. This technique only works with black powder motors. It will only work with models that are VERY over-stable to begin with. When CHAD staging does work, however, it is the most efficient staging method because it minimizes increased drag and mass associated with an added stage. (See Optimum Mass) |
| Chuff | A form of unstable combustion marked by brief bursts of thrust separated by periods of no thrust. Typically, the bursts come faster and become longer as burning proceeds, until stable burning results. The sound of chuffing is similar to that of a steam locomotive starting up. It generally occurs in a composite motor that is ignited too low in the grain. |
| Clip Whip | A number of micro clips on short wires (usually three) all connected at their free end. Used to aid in the ignition of clusters (q.v) where each motor uses a separate igniter. |
| Cluster | The use of more than one motor at the same time to propel the rocket |
| Coast | The time the rocket continues in an upward direction after the motor has burnt out, |
| Composite Material | Hi-Tech materials, other than paper, wood or metal, used in the construction of rockets (see also Phenolic). |
| Composite Motor | A Motor containing a fuel and an oxidiser (Ammonium Perchlorate - (NH4CIO4) The oxidizer used in most composite rocket motors). |
| Composite Propellant | In Hobby Rocketry, any propellant other than black powder. In military parlance (where the term originated) the term is used to denote propellants that are mixtures of oxidizers and fuels and to distinguish them from Single, Double, and Triple base propellants (which are either monopropellants or mixtures of monopropellants). Note that by the military definition, black powder is itself a composite propellant because it consists of separate oxidizers (KNO3 and sulfur) and fuel (charcoal). Further note that by the hobby definition, single/double/triple base propellants are composites because they are not black powder. No ambiguity arises, however, since the military doesn't use black powder (in rockets, anyway), and no hobby rocket motors use single, double or triple base propellants. See also Single Base Propellants, Double Base Propellants and Triple Base Propellants |
| Continuity Check | A group of electrical techniques for checking the firing circuit through the igniter to ensure that the circuit is functional. This usually involves some type of light or audio tone activated by a push-button. The techniques range from a simple current limiting light bulb or buzzer placed in series with nichrome igniters, to sophisticated bridge circuits for sensitive, low current flashbulbs and electric matches. |
| Copperhead | An igniter supplied with Aerotech motors made from two pieces of copper foil sandwiched together separated by an insulator, this then appears as a single lead ( they are prone to the foil cracking if bent in a tight radius, and have hence been given the name of crapperheads) |
| Core Sample | Synonyms describing a failure mode where the model comes down fast and hard (nose first) and ends up tail-high in the ground (this is where large, colorful fins come in handy :-). Often the nose cone has separated (taking the recovery device with it) and the body tube ends up containing a nice 'core sample' of mud/dirt when pulled out of the ground. Also known as: Auger In |
| Crapperhead | see copperhead |
| Cruise Missile | A rocket which has failed in such a way that it ends up flying horizontally while still under power. A common example would be a multi-stage rocket which stages "dirty" (due to stability or structural problems) causing the upper stage to bend to near horizontal at ignition. Severe launch rod tip off or high winds have also been know to cause a cruise missile attitude. |
| Delay | The time between the motor burning out and the ejection charge being fired. |
| Delay Element | The part of the rocket motor that burns slowly producing little or no thrust, this is the element that produces the tracking smoke. |
| Double Base Propellant | A solid propellant consisting of two monopropellants (usually nitroglycerin and nitrocellulose) and various additives. Double base propellants are used as smokeless powders in ammunition. They are also used in smaller military rockets but have been largely replaced by composites in larger vehicles. Double base propellants are not used in hobby rocketry. See also Composite Propellant |
| Drag Coefficient (Cd) | A dimensionless number used in aerodynamics to describe the drag of a shape. This number is independent of the size of the object and is usually determined in a wind tunnel. It is part of the basic drag equation F=.5*rho*V^2*Cd*A where F is the drag force, rho is the air density, V is the air velocity and A is the cross sectional area. All of these, except Cd, are directly measurable in a wind tunnel so Cd can be thought of the "fudge factor" that accounts for all of the aerodynamic peculiarities of a shape. The Cd for most sport type hobby rockets is in the range of .4 to .5. See also Reynolds Number. |
| Drag Form Factor (CdA) | The Drag Coefficient (q.v.) of an object multiplied by its cross sectional area. This is used to scale the drag value for a particular object from the dimensionless Cd. Theoretically, every object of a similar shape will have the same Cd regardless of its size, meaning that both a grain of rice and a Zeppelin would be the same. Multiplying by the area allows comparisons of the true drag between dissimilar objects. For example, the original Honda Civic had a horrible Cd, and makers of large luxury cars, with a little edge rounding, were easily able to beat it and proclaim "Lower drag than a Honda Civic!" in their ads. This is patently absurd as the Honda had such a tiny cross section, thus much lower *actual* drag. See also Optimum Mass |
| Drop Staging | See CHAD Staging |
| Effective Exhaust Velocity | See Impulse (Relative) |
| Ejection Baffle | A device used in some rockets to eliminate the need to use wadding to protect the recovery system. Usually composed of some type of metal wool or mesh to absorb the heat of the ejection gases before they reach the recovery compartment. |
| Ejection Charge | a small black powder charge used to deploy the recovery device |
| Electric Match | A type of igniter originally designed to set off fuse-type blasting caps (i.e. a match that can be set off from a great distance electrically). It requires a very low electrical current (~10 mA range) to activate. |
| Epoxy | A two part glue, When the two parts are mixed a catalytic reaction takes place to set the glue, can come in varying setting times from about 5 mins. to 24hrs. |
| Fin | A flat surface protruding from the side of the rocket to produce a stabilising effect by using air pressure acting upon it. |
| Fillet | A reinforcement of the joint between the fin and the body tube of the rocket to improve the rocket's aerodynamics and to strengthen the fin mount. |
| Hang Fire/Misfire | Terms which refer to abnormal ignition. With hang fire, the motor usually ignites after a considerable delay. Misfires never ignite. Hang fires often appears as a misfire until the motor ignites some time later. This is the main reason the safety code advises not to approach a misfired rocket for one minute. |
| High Power Rocket(ry) (HPR) | Hobby rockets that exceed the total weight, total propellant or single motor total impulse restrictions of model rockets as defined in NFPA 1122 (q.v.) but otherwise conform to the same guidelines for construction materials and pre-manufactured, solid propellant motors. High power rockets have no total weight limits, but do have a single motor limit of no more than O power (40,960 NS total impulse) and have a limitation of 81,920 NS total impulse. |
| HPR Lite | A term used to describe rockets using motors in the 'E', 'F', and 'G' power classes. Formerly called "Medium Power Rocket" (a term nobody used), it describes rockets which fall between the old NFPA 1122 weight limit of 1 lb (454 grams) and the current model rocket weight limit of 1500 grams. Rockets in the 'E' through 'G' class aren't normally considered high power rockets but, to be successful, must be built using many of the same construction techniques as the larger rockets. Also, any rocket over 1 lb requires an FAA waiver to fly legally. |
| Hybrid | A Motor that has a solid fuel and gaseous or liquid oxidiser |
| HyperTerminal Velocity | A situation where a rocket is traveling faster than terminal velocity (q.v.) for a given motor. This is possible, for example, with a staged model with grossly mismatched motor combinations such as an F-100 staged to a B6. At staging, the upper stage will already be beyond its terminal velocity for the "B" motor. In this case, the upper stage will actually *decelerate* during thrusting and approaches terminal velocity from above. |
| Igniter | A device used to ignite motors, usually a nichrome element coated in a pyrogen, the nichrome wire glows hot when current is supplied hence igniting the pyrogen. |
| Impulse (Relative) | A measure of the efficiency of a rocket engine. Similar to Specific Impulse, it is defined as the Total Impulse (q.v.) divided by the mass of the propellants. A little dimensional juggling shows that this gives the same units as velocity (ft/sec or m/sec) hence is sometimes called Effective Exhaust Velocity. How quickly the reaction mass leaves the nozzle is a good measure of efficiency. |
| Impulse (Specific) | A measure of the efficiency of a motor/propellant system. It is determined by taking the Total Impulse (q.v.) and dividing by the weight of propellants. This carries the potentially confusing units of "seconds" (as if it had something to do with the burn duration) but is due to weight and thrust both being force parameters hence canceling out (e.g. lb-sec/lb or N-sec/N). This is actually very handy since it makes the term independent of the units system (metric or English) since they both use "seconds" for time. |
| Impulse (Total) | A measure of the total momentum imparted to the rocket by the motor. It is defined (for those who know calculus) as the integrated area under the thrust-time curve. For the rest of us, it can be thought of as the motor's average thrust times the duration of the burn. Measured in N-sec or Lb-sec |
| Kato | See Cato |
| Kicked | A term used to describe a motor which is ejected from the rocket while in flight. This often results in the failure of the recovery system. It is usually caused by not fitting the motor into the motor mount properly. See also Prang |
| Krushnic Effect | A very dramatic phenomenon where your rocket makes a tremendous amount of noise and smoke but doesn't go anywhere! This happens when the motor is recessed into the body tube by more than one tube diameter. If so recessed, the cylindrical volume below the motor forms a secondary expansion chamber which allows the exhaust gasses to expand below atmospheric pressure before leaving the rocket. Surrounding air aspirated into the exhaust stream causes turbulence which negates much of the thrust, along with creating the characteristic roar. A multi-stage model that ejects its booster motor, but not the airframe, is a perfect example. Very damaging; it almost always destroys the lower body tube beyond use. Named for Richard Krushnic, the rocketeer who characterized the effect in the late '60s. Not to be confused with Suction Lock. |
| Land Shark | A rocket which has failed in such a way that it ends up on the ground while still under power. Upper stages of unstable multi-stage rockets often end up like this, as do some (too) heavy HPR rockets with long-burning, low thrust motors. |
| Launch Controller | Electronic device used to activate the igniter |
| Launch Lug | A device (normally a tube) that is attached to the side of the rocket and holds the rocket to the launch rod allowing the rocket to travel the length of the rod freely. |
| Launch Pad | A Structure that hold the Launch Rail or launch Rod in a stable Vertical or near vertical position |
| Launch Rod | A rod that guides the rocket until sufficient speed has been obtained for the rocket to become aerodynamically stable |
| Lawn Dart | Term used when a recovery device fails and the rocket hits the ground nose first |
| Lovelace Effect | A phenomenon where the nose cone is apparently "sucked" out of the body right at motor burnout. It is more prevalent on parabola, ogive and other low drag nose shapes. The theory (as yet unproven) is that since the nose cone has much less drag than the body, its momentum tends to carry it forward faster (or, more correctly, the body's drag decelerates *it* more quickly) putting tension on the nose-body joint. The condition is exacerbated by any nose weights added for stability (which also raise the momentum of the nose) and/or a loose fit of the nose in the body. |
| MagneliteTM | An igniter made by Rocketflite used mainly to start composite motors. A medium power device (2-3 amps at 12 volts), it requires significantly more than an electric match (q.v.), but not as much as a Copperhead (q.v.). It consists of a nichrome bridgewire dipped in a magnesium based pyrogen which burns *very* hot (~6000F), aiding in the ignition of stubborn composites, such as a "Blue Thunder". They come both single and double dipped, depending on how much "oomph" you need. The head is quite large so they work best in 29mm and larger motors. |
| Medium Power Rocket | See HPR Lite |
| MIF | Missing In Flight. A rocket that disappears with no sign of the recovery system deployment, and no other obvious failure mode (e.g. Prang or CATO). Sometimes called "into orbit." |
| Minimum Diameter | A rocket built with the smallest possible diameter body tube for the size of motor casing. Usually done to reduce drag in sport or competition models even though it can increase the difficulty of attaching fins and recovery systems. See also Boosted Dart |
| Model Rocket | An aero-vehicle that ascends into the air by means of a
reaction motor, but without the use of aerodynamic lifting surfaces. The restrictions, as
defined in NFPA 1122, are as follows: The gross launch weight, including motor(s), will not exceed 1500 grams. Motor(s) will not exceed 160 NS of impulse (total) and/or contain more than 62.5 grams of propellant each, and no more than a total of 125 grams of propellant in multiple motor applications (clusters and/or multi-stages). All components of said vehicle will be of wood, paper, rubber, breakable plastic or similar material and without substantial metal parts. |
| Monopropellant | See Single Base Propellant |
| Motor | Something that imparts or produces motion, such as a machine or engine. A device that converts any form of energy into mechanical energy |
| Motor Mount | The motor tube, centering rings and fixings that hold the motor in the end of the rocket. |
| Motor Retainer | The Device used to ensure the motor is not ejected from the rocket when the ejection charge is set off. Also stops motors from falling out when the rocket is descending under a Parachute or streamer |
| Motor Tube | The tube in the rocket that the motor fits inside. |
| Multi Stage | A rocket which uses more than one rocket motor, with subsequent motors igniting when the previous ones have burned out |
| NAR | National Association of Rocketry. A national hobby organization promoting model and high power rocketry in the United States. The NAR promotes rocketry related sport flying, competitions, and education. |
| Newton & Newton-Second | Metric units used to measure thrust and total impulse (q.v.) respectively. One pound = 4.448 newtons. |
| Nose Cone | A traditionally conical structure on the front of a rocket to help reduce the Drag Coefficient, Now available in a number of differing shapes including Ogive, Parabolic and Conical |
| Ogive | A shape defined by the intersection of two circles. It is not the same as a parabola (q.v.). Both ogives and parabolas produce low drag sub-sonic nose shapes. They can be told apart since a parabola always has a rounded nose while an ogive comes to a point. |
| Olympic Torch | A rocket that power prangs (q.v.) with the motor still burning. Coined by Bob Kaplow after almost impaling NAR President Mark Bundick with an RMS (q.v.) powered model that suffered a nozzle failure which dropped the thrust to zero even though the motor kept burning. See also Roman Candle |
| Optimum Mass | For any given motor and Drag Form Factor (q.v.) the liftoff
mass for which a rocket will reach maximum altitude in dense atmosphere. At first this
might seem to be just the lowest possible mass, but there is a two edged nature to mass
covering both powered flight and coasting. Lower mass will give higher burnout velocity,
but will dissipate its momentum to drag faster (think of a feather). Conversely, a heavier
rocket will have more momentum at burnout to coast farther, but too much mass will hold
down both burnout altitude and velocity. Hence, there is a "knee" on the liftoff
mass vs. altitude graph. For very low impulse motors (say "B" and below) this "knee" is right around the mass of the motor itself, so the rule of thumb is "the lighter the better." The higher impulses, though, have more leeway, and careful calculations should be made to determine the optimum mass for altitude attempts. In a multi-stage rocket with no staging delays, only the dead mass in the upper stage participates in coasting. Extra dead mass in lower stages cannot enhance coast distance, and so lower stages should be as light as possible. Strictly speaking, an undelayed staged rocket has no optimum liftoff mass, but the mass of the last stage may be optimized with respect to the (sub-optimal) lower stages. In dense atmosphere, the best single stage configuration is more efficient than the best multi stage configuration, provided all the propellant can be contained in one stage. Indeed, there are many instances when cluster rockets out perform staged rockets. The opposite is true for rockets operating in the thin atmosphere of high altitudes. In that environment, staged rockets are more efficient (propellant-wise) than single-staged rockets, and lighter rockets always perform better. There is no optimum mass in a complete vacuum. |
| Parabola | A shape produced by the formula y=x^2. Used to produce low drag nosecones. See also Ogive |
| Payload | A payload is an item that is carried within the rocket. |
| Piston | A device used on some rockets to push out the recovery device |
| Phenolic | A Heat-resistant Reinforced resin often used for making body
tubes and occasionally fins for HPR It is made by a reaction of phenol and
formaldehyde. When mixed with carbon black, it is used to make casings for composite
propellant rocket motors. Phenolic body tubes are stiffer than ordinary tubes, but are also more brittle so that extra care must be taken to avoid damage during construction, transportation and recovery. |
| Prang | Term describing a failure mode whereby a rocket comes down
aerodynamically stable, in other words, 'streamlines in'. This is almost always caused by
some sort of recovery system failure, usually the result of a too-tight nose cone,
too-tightly packed parachute or a too-loose motor that ejects out the back. Multistage
models with upper stage ignition failures also result in a prang. The results of a prang range from no damage at all (other than a few grass stains) on lightweight sport models to the total destruction of the rocket (usually a payloader with a VERY expensive payload on board :-(. A Prang that occurs while the motor is still burning (e.g. a marginally unstable rocket that performs one large half loop) is called a 'Power Prang'. Origin: If you insist on it being an acronym, the postwar military sounding rocket program had a quasi-official failure mode category "Parachute Recovery Apparatus No Good." However, like CATO (q.v.), this is another "Post Hoc" definition. The term was in widespread use during WW II in aviation circles to describe aircraft crashes, especially experimental or military ones. Prior to use in the U.S., it was popular in Britain since at least the '30s where the expression "Prang his Kite" was equivalent to our "Auger in" or "Buy the Farm." |
| Pyrogen | A substance that burns readily |
| Quick Match | a quick burning fuse used to ignite clusters of black powder rocket motors, it can be lit by an igniter |
| RASP | The Rocket Altitude Simulation Program. Originally written by G. Harry Stine in BASIC in the late '70s (and included as an appendix in the later editions of the Handbook), it performs a simulation of rocket flight using small time interval approximations. The original was relatively primitive assuming constant Cd, vertical flight and other simplifications. There have been several rewrites into "C" and other languages to both broaden its appeal and increase its sophistication |
| Reef | A series of techniques used to gather the shroud lines of a parachute together to prevent it from fully opening. This is usually done on rockets that reach extreme altitudes or launched on windy days which need higher sink rates to help them land near the launcher. There is also a "traveling reef" technique of placing a soda straw or metal washer on the shroud lines and sliding it all the way up to the chute canopy during prep. At deployment, the parachute is prevented from opening until the chute is fully deployed and the rocket stabilized beneath it. The straw/washer then slides down the shrouds allowing the canopy to open gradually. This is used mostly on large rockets which might have very high speed or high altitude recovery deployment since it allows the rocket to slow and drop considerably before chute opening. |
| Reynolds Number | A dimensionless number used by fluid flow engineers to
characterize the way a fluid (gas or liquid) will behave when passing over a solid
surface. The number combines the fluid's density, viscosity and velocity with the length
it's traveled along the surface. No matter what the fluid is or what size the surface, the
flow conditions (laminar, turbulent, detached, etc.) should be the same at the same Rn.
Discovered by Osborne Reynolds in the 19th Century while studying the flow of water in
pipes and channels, it has proven most useful to aerodynamic engineers and naval architects
in scaling up wind/water tunnel test results to full size. Carl Dowd, a model aviator and NASA engineer, found it helpful to think of Rn as the "coarseness" of the air seen by a body. Move the body faster, and more particles will pass over it in a given unit of time, increasing Rn. Make the body larger, and there will be more particles over the body at any instant, increasing Rn. |
| RSO | Person who enforces the rules, to ensure the safety of all participants and spectators |
| Range Safety Officer | see RSO |
| Recovery Device | Usually a parachute or a streamer used to slow the descent of the rocket, both for safety and to help protect the rocket when it lands |
| Reload | Kit containing propellant, ejection charge etc. to be used with reusable rocket motors |
| RMSTM | Reloadable Motor System. The trademarked name of the AeroTech/ISP reloadable motors. Often used (incorrectly) as a generic name for all reloadable technology. |
| Rocket Motor | A Motor that burns fuel to cause a stream of hot high pressure gasses to escape through a small opening thus creating a thrust in the opposite direction. |
| Roman Candle | A failure of the motor restraint (thrust ring or engine hook) where the rocket stays on the pad while the motor flies out of the body (usually pushing the nose cone and recovery system ahead of it). Sometimes mistaken for a CATO . |
| Safety Code | A set of common-sense rules which should be followed when building and flying rockets |
| Sectional Density | A projectile's mass divided by the square of its diameter. Used as a measure of a round projectile's ability to coast. See also Ballistic Coefficient |
| Shock Cord | Elastic used to relieve some stresses involved when the recovery devices are deployed |
| Shred | A model which has lost one or more fins due to aero loads and/or acceleration. Also used to refer to a model which has completely come apart during takeoff. Can be used as either a verb or noun. See also Strip |
| Streamer | A long ribbon of paper or plastic used as a Recovery Device |
| Solar IgniterTM | Estes Industries brand of Igniter. Made from two wire conductors with a piece of Nichrome wire connecting them at one end. The nichrome wire tip of the igniter is dipped in a pyrogenic compound which flares to ignite the rocket motor |
| Spill Hole | An opening cut in the top of a parachute to increase the sink rate (thus decrease drift distance) and aid recovery on windy days. It also reduces the swing of the rocket while under the parachute caused by air spilling from under the edge of a parachute |
| Swing Test | A simple method for testing the stability of a model. A
string 6ft to 10ft long is tied to the center of gravity of a fully prepped rocket which
is then twirled overhead in a circle. If the nose points in the direction of the spin and
the rocket does not wobble then it is very likely a stable design. The swing test is not very reliable since it introduces another component, namely radial acceleration, that is completely absent in normal flight. When you tie the string to the rocket at the CG, it's not really at the CG but attached to the outer surface of the body tube *above* the CG (which is actually inside along the center of the tube). In order for the rocket not to twirl, the projection of the string has to pass through the CG. This is fine as long as the rocket is moving in a linear fashion. But when you start swinging it, it's no longer moving linearly, but being constrained to a circle. This forces the rocket (if it's stable) to assume an angle of attack in order to keep pointing into the "relative wind". This angle means that the projection of the string no longer passes through the CG, but slightly behind it. You have to move the string slightly forward for the string to point through the CG while you swing it. |
| Strip, Stripped | Terms describing a parachute that has had one or more shroud lines pull free due to opening shock. Usual cause is recovery deployment at too high a speed, but can also be due to age (of the tape disks on a plastic chute) or poor construction. Can be used as a verb or noun. See also Shred and Reef |
| Suction Lock | Severe Base Drag at ignition. See Bernoulli Lock |
| Terminal Velocity | In the powered phase, the speed where the motor thrust equals the combined forces of gravity and aero drag. Theoretically, the rocket would continue ascending at a constant speed (i.e. no acceleration) with these forces in balance. This doesn't actually happen since motor thrust varies with time and aero drag with altitude. A second meaning is, when descending, where aero drag balances the weight of the descending model. If under a 'chute or other high drag recovery aid, this is quite slow. If in core sample mode this speed can be several hundred feet/sec. See also HyperTerminal Velocity |
| Thermalite | A material, originally used to detonate plastic explosive, which burns at a controlled rate and high temperature. Used with rocket motors as an ignition enhancement. It can be ignited by electric (nichrome) means, flash bulbs or the exhaust of a previously started motor. It comes in three burning speeds color coded as pink (slow), green (medium) and white (fast). For a rough order of magnitude, slow is around 1/2 in/sec and fast is 2 1/2 in/sec in free air, but this can be affected by temperature, humidity, pressure and whether or not the fuse is sheathed in a tube. |
| Through the Wall (TTW) | An HPR fin attachment technique which provides much greater strength than the typical surface mount used in model rocketry. To use TTW, slots are cut in the body tube where the fins mount and the fins are built with extended tabs on the root edge which fit through these slots. In one form of TTW, the tabs are short and just provide a surface to build up epoxy fillets on the inside as well as the outside. In a stronger version of TTW, the tabs reach all the way to the motor tube where they are glued forming a very rigid box structure (also known as TTW-GTMT for "Glued To the Motor Tube). |
| Tiger TailTM | An igniter sold by Quest Aerospace consisting of two very thin copper foil leads separated by and even thinner plastic insulator with the pyrogenic compound at the tip. Essentially a mini Copperhead, its name comes from the orange and black striped tape strip provided to allow it to be used with ordinary alligator clip ignition systems |
| Tracking Smoke | The smoke that the Rocket motor releases, after the period of thrust, to aid in the tracking of the rocket until the recovery device is deployed. |
| Triple Base Propellant | A solid propellant based on three monopropellants and additives. In practice, the monopropellants are usually nitroglycerin, nitrocellulose, and nitroguanidine. In military rockets, such propellants have been largely replaced by composites. Triple base propellants are not used in hobby rocketry. See also Composite Propellant |
| Thrust Ring | The thrust ring is a ring of paper material that fits inside, and usually at the front most end of, the motor tube Its purpose is to keep the motor from breaking away from the motor mount and traveling up through, and damaging, the rocket. These are not used on HPR they are only for model rockets. |
| Tumble Recovery | A method of recovery for very lightweight rockets and booster stages, whereby the rocket tumbles safely to the ground |
| Recovery Wadding | Flame-proof substance (usually a boron treated tissue) used to protect Recovery Devices from the Ejection Charges hot gasses |
| UKRA | United Kingdom Rocketry Association see links to other sites |
| Zipper, Zipper Motor Effect | A devastating side effect of mounting the shock cord to the
motor mount (which is often done for strength or to anchor a piston ejection system). If
strong and thin cord is used (e.g. Kevlar) and the recovery system opens at too high a
speed and/or the piston comes all the way out of the body, then the line can
"zip" open the body tube all the way down to the motor mount :-( A sufficiently
strong top mounted shock cord can partially zip a body tube if opened at a high enough
speed. This can also happen at launch if the motor mount is not fixed securely to the inside of the body tube, where the motor thrust pushes the motor or the motor and motor mount up through the body tube. |