Some General Terms

I thought it was worth separating these from the glossary as this is all to do with the effects acting upon a rocket as these may be useful to someone not wanting to read the glossary but wanting the scientific information.

Aspect Ratio - The ratio of length to width. In rocketry, a rule of thumb is that the aspect ratio of a model rocket’s length to its body diameter must be at least 10:1.

Average Thrust - The total impulse (in Newton-seconds) divided by the length of burn time. Example; a K185 motor has a total impulse of approximately 1400 Newton-seconds, and burns for 7.5 seconds. 1387.5 Newton-seconds divided by 7.5 equals 185 newtons average thrust.

Ballistic Coefficient - A rocket’s weight divided by its drag form factor. The drag form factor is equal to a rocket’s drag coefficient multiplied by its frontal area. The greater a rocket’s ballistic coefficient, the higher it can coast after finishing the thrust portion of its flight.

Barrowman Method - A mathematical technique for calculating the center of pressure of a subsonic rocket at low angles of attack. Named for James S. Barrowman, who developed the method in 1966.

Base Drag - Drag produced by the airflow moving from the sides to the rear of a model. As the airflow reaches the rear of a moving rocket’s airframe, it separates from the rocket at the base. This creates a low-pressure region at the base of the rocket, and impedes its forward motion. During the powered portion of a rocket’s flight, base drag is reduced by the production of gases from the rocket motor.

Bernoulli’s Principle - A physics principle that states the pressure exerted by air perpendicular to its direction of travel decreases with an increase in velocity.

This is the theoretical principle upon which airplane wings work. An airplane wing’s airfoil is unsymmetrical, its upper surface having a greater curve than its lower surface. This causes air flowing across the upper portion of the wing to speed up, since it must cross a greater surface area in the same length of time. The increase in air velocity on the upper surface causes the air pressure in the region just above the wing to be reduced. The higher pressure below the wing presses upward on it, creating lift.

Coefficient of Drag (Cd) - The aerodynamic drag of a rocket as a function of its speed and cross-section. An indication of a rocket’s aerodynamic efficiency. See Drag Force.

Drag Coefficient - See Coefficient of Drag.

Drag Force - The resistive force (Fd) on a body moving through a fluid, such as air. It can be calculated using the formula: Fd = ½ * Cd * ? * S * v2, where:

Cd = the drag coefficient for the body

? = the density of the fluid

S = the cross-sectional area in the direction of motion

v = the velocity of the body relative to the fluid.

Isp - Symbol for Specific Impulse

Krushnic Effect - The effect caused when rocket motor exhaust is confined in an open tube, such as a booster stage airframe. The exhaust gas over-expands and loses its velocity, reducing the motor thrust. Also called over-expansion. Usually occurs when a booster motor separates but the booster stage does not.

Laminar Flow - A smooth flow of air, noted for lack of turbulence. In current theory , it is modeled as consisting of layers of air having similar speed and density.

Neutral Stability - The condition in which a rocket’s CP and CG are at the same spot. Dynamically, this creates a situation in which a rocket may fly steadily in dead air but can’t recover if the angle of attack is greater than zero.

Newton - The amount of force required to accelerate one kg, one meter per second per second. 4.45 newtons equals one pound of force.

Newton-Second - The typical unit of measurement for rocket thrust. One newton-second is one newton of thrust maintained for one second.

One-Caliber Stability - A widely-accepted rule of thumb for rocket stability: The center of gravity should be approximately one body width (one "caliber") ahead of the center of pressure.

Optimum Weight - The mass required to lift a rocket to the highest possible altitude on a given motor.

Over-Expansion - See Krushnic Effect.

Reynolds Number - A nondimensional co-efficient that measures the compression of air due to scaling. This allows scale models to be tested in wind tunnels.

Specific Impulse (Isp) - The total impulse of a motor divided by unit weight of propellant.

Stability - The tendency of a rocket to move in a straight line in the direction it is pointed at launch.

A hobby rocket employs "passive" stability, in that it depends on its inherent stability (the relationship of its centers of pressure and gravity) and the counterbalancing pressure of the airflow on its fins to keep it moving in the correct attitude. Missiles and spacecraft usually employ "active" stability schemes, in which attitude jets or movable vanes help correct the rocket’s course in flight.

Stability involves a rocket’s ability to continue to fly straight when the angle of attack is greater than zero. A stable rocket continues to fly in its intended direction, even if moderate wind or other influences try to change its course.

A poorly-designed rocket can be either unstable, marginally stable or excessively stable. An unstable rocket (whose center of gravity is behind its center of pressure) will fly erratically when launched. A marginally stable rocket (whose center of gravity is less than one diameter ahead of its center of pressure) might be able to maintain a straight flight attitude if there is no side wind (angle of attack is zero.) However, if outside forces such as wind intervene (angle of attack greater than zero), the rocket can be thrown off its flight path and behave in an erratic manner. An excessively stable rocket (whose center of gravity is much more than one diameter ahead of its center of pressure) will tend to turn toward those outside forces rather than remain on the intended line of flight. See Weather cocking.

Total Impulse - The total thrust produced by a rocket motor across its full burn time. Usually expressed in Newton-seconds.

This new page is a re-write of the old hpr.org.uk pages with usefull scientific formulas and explanations that are usefull to understand Rockets from the basics of Newtons Laws to more complicated equations used to design motors and predict their performance. The old HPR.ORG.UK web site contained a lot of out of date information that has now been archived, if you are looking for any of the old information and cant find it please contact us.

Information on this page can be used for educational purposes as long as this site is credited, the contents of this site can not be copied without our permission to other websites or for commercial gain.