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Engine Specifications - Definition of Terms
What do all those technical terms mean?

  • Propellants: The Oxidizer and Fuel (listed in that order) used by the rocket engine. For solid fuel motors only the word 'Solid' is shown. For nuclear or electric motors, the fluid used by the motor to form the exhaust is shown. For air-breathing motors, 'Air' is indicated as the oxidizer.

  • Thrust(vac - kgf): The thrust the motor generates in a vacuum, expressed in the terms of 'kilograms-thrust (kgf)'. This is not a term officially recognized by the scientifically proper establishment but was used by the Russians in their rocket engineering (the Americans used pounds thrust, thank you).

  • Thrust(sl - kgf): The thrust the motor would develop at sea level in terms of 'kilograms-thrust'. This may be zero for motors designed for upper-stage operation in a vacuum (see Expansion Ratio, below).

  • Thrust(vac - kN): For purists, the thrust of the motor in vacuum in officially-correct kiloNewtons (= (Thrust-kgf) * g / 1000) where g = acceleration of gravity on Earth at sea level = 9.80665 m/sec^2.

  • Thrust(sl - kN): For purists, the thrust of the motor at sea level in kiloNewtons.

  • Isp-sec: The specific impulse of the motor in vacuum. The higher the number, the more efficient the motor. The units here are seconds - because specific impulse represents the kilograms-thrust the motor generates per kilogram of fuel per second of operation (kgf/(kg/sec)) = sec). Important relationships are : effective vacuum exhaust velocity of the motor = Isp * g (this is also the specific impulse in kN thrust terms); fuel consumption of the motor = (Thrust in vacuum) / Isp.

  • Isp (sea level)-sec: The specific impulse of the motor at sea level. This is the same fraction of the vacuum specific impulse as the sea level thrust is to vacuum thrust. It can be zero (see Expansion Ratio, below)

  • Burn time: The total operating time of the motor. For solid motors, this is the more-or-less unstoppable period of thrust until all of the propellant is consumed. For liquid motors, this is the rated thrust duration of the motor for a single operation. This is greater than the thrust time of the motor in actually use on a particular stage. The total tested or rated operating time of a liquid motor between overhauls (if it is reusable) is typically many times the total rated operating time per each use.

  • Mass Engine: For liquid motors, the mass of the engine dry (without propellant in the lines).

  • Diameter: For liquid motors, the maximum envelope diameter of the motor. This is usually the same as the diameter of the nozzle exit, but in some cases may be more than that due to pumps, pipes, or structural attach points. For solid motors, the diameter of the main motor body.

  • Length: For liquid motors, the length of the motor and the pumps, pipes, etc. usually mounted above the thrust chamber. For solid motors, the total length of the motor.

  • Pc-bar: For liquid motors, the pressure of the combustion chamber in bar or atmospheres sea level pressure. The proper definition for the chamber pressure is the throat stagnation pressure (total pressure in the critical cross section). A small loss exists between the injector end of the chamber and the throat. American engineers usually give the throat stagnation pressure, Russians usually gives the injector end pressure. This is usually not stated explicitly and thus may be unknown. For solid motors, the pressure in the motor casing during operation.

  • Expansion Ratio: The ratio between the area of the combustion chamber exit and the area of the nozzle exit. A large area ratio improves the performance of a motor in a vacuum since the exhaust is expanded further, thus converting potential energy into kinetic energy. However, at sea level a high area ratio can result in flow separation, which can drastically reduce or eliminate the net thrust of the motor.

  • Oxidizer to Fuel Ratio: The ratio between the mass of oxidizer burned per mass of fuel burned (liquid motors only).

  • Thrust to Weight Ratio: The ratio between the vacuum thrust of the motor and the mass of the motor (liquid motors only).

  • Coefficient of Thrust vacuum: A measure of the flow expansion of the motor in vacuum.

  • Coefficient of Thrust sea level: A measure of the flow expansion of the motor at sea level.





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