Model Rocketry Terms

Terms in this set (158)

The class of non-professional rocket beyond HPR. Amateur rockets use structural metal parts and very often the motor casing doubles as the airframe (as with professional rockets). These rockets can be very large and powerful, capable of placing payloads many miles up. Activities in this field (one can scarcely call it a hobby) include formulation and manufacture of propellants and thus can be EXTREMELY hazardous. This is the main reason that amateur rocketry is not to be attempted alone. Another is expense as these vehicles can run many hundreds or thousands of dollars and take months to build. The equipment necessary to safely pursue amateur rocketry (sandbagged bunkers, loading pits, standby fire truck, etc.) are quite beyond the resources of most individuals. Not all amateur rockets are so large. Many of the "beginner" vehicles would qualify as HPR or even model rockets in terms of liftoff weight and total impulse, but fail the NAR/Tripoli codes due to their metal airframes and user-compounded propellants. Note There is a fine, but significant, difference between using a metal cased reloadable motor with pre-manufactured fuel slugs and packing a pipe with zinc/sulfur (a common amateur beginner fuel). Liquid fueled vehicles are becoming more popular among amateur groups. These can produce up to 1,000 lbs of thrust for up to a minute from a LOX/Kerosene engine which can propel the vehicle to altitudes of over 40 miles. Some hobby! Neither Tripoli nor the NAR sanction amateur rocket activities. See also
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.