Propulsion

| Word                            | Definition                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      | Assosciated Links and Resources                                                          |
| ------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------------- |
| O/ F Ratio or Mixture Ratio     | Mixture Ratio is the ratio of the liquid oxidizer flow rate divided by the liquid<br>fuel flow rate, with both flow rates being measured as mass flow rates. The best performance (highest specific impulse) is obtained at a<br>specific optimum mixture ratio.                                                                                                                                                                                                                                                |                                                                                          |
| Chamber Pressure                | The Chamber Pressure is the pressure in the combustion chamber of an<br>operating rocket propulsion system.                                                                                                                                                                                                                                                                                                                                                                                                     |                                                                                          |
| Propellant                      | Propellant is the stored matter that is energized and ejected. It is commonly in the form of a Liquid<br>Propellant (stored in vehicle or missile tanks) or a Solid Propellant (stored<br>inside its combustion chamber). Rocket propellants undergo a chemical reaction<br>or a Combustion Process (usually at high chamber pressure and high<br>temperature) that transforms them into hot gaseous reaction products or Exhaust<br>Gases, which are then accelerated and ejected through a Supersonic Nozzle. |                                                                                          |
| Specific Impulse                | Specific Impulse is a parameter indicating propulsion system performance. It can<br>be defined as the thrust of an equivalent rocket propulsion system (same chamber<br>pressure, same propellant, same nozzle throat to exit area ratio) that has a<br>propellant mass flow of unity. Higher values indicate a better system.                                                                                                                                                                                  |                                                                                          |
| Total Impulse                   | Total Impulse is the integral of thrust over the propulsion operating time. It is a<br>measure of the total kinetic energy of the nozzle exhaust gas as released by the<br>combustion of all the available propellant in the propulsion system. For constant<br>thrust operation it is the average thrust multiplied by the effective propulsive<br>operating duration or it is also the mass of the total expelled propellant multiplied<br>by the average specific impulse.                                   |                                                                                          |
| RPA                             | Rocket Propulsion Analysis: Software tool that helps model out conceptual rockets                                                                                                                                                                                                                                                                                                                                                                                                                               | [http://www.rocket-propulsion.com/index.htm](http://www.rocket-propulsion.com/index.htm) |
| Cryogenic Propellants           | Cryogenic propellants are subcooled liquids at low temperature (such as liquid<br>oxygen or liquid hydrogen); they are gases at ambient temperatures.                                                                                                                                                                                                                                                                                                                                                           |                                                                                          |
| Nozzle Area ratio               | The Nozzle Area Ratio is the nozzle exit area divided by the nozzle throat area.<br>For optimum gas expansion in a nozzle the gas pressure at the nozzle exit is equal<br>to the local ambient atmosphere pressure. Typical values of this nozzle area ratio<br>are between 4 and 20 for expansion to sea-level pressure and between 40 and 200<br>for operation at very high altitude (space vacuum).                                                                                                          |                                                                                          |
| Ablative Chamber or Nozzle Well | An Ablative Chamber or Nozzle Well absorbs heat (from the hot gases) by<br>having some of the heated ablative well material (e.g., reinforced organic fibers in<br>an organic matrix or phenolic resins) vaporized, endothermically decomposed into<br>gaseous species, or charred. The evaporated gas products flow out of the ablative<br>material and form a protective cooler gas layer at the wall’s surface.                                                                                              |                                                                                          |
| Liquid Engine (LE)              | A Liquid Propellant Rocket Engine commonly has these principal components: one or two<br>propellant tanks, one or more thrust chambers, a feed mechanism (pumps —<br>driven by a turbine or displacing the propellant in the tank(s) by high pressure<br>inert gas), piping and control valves, and sometimes servo- valves (for starting,<br>stopping, throttling, or mixture ratio control).                                                                                                                  |                                                                                          |
| Rocket Propulsion Systems       | A Rocket Engine (it uses liquid propellants) and a Rocket Motor (solid propellants) are the two most common types of Rocket Propulsion System.                                                                                                                                                                                                                                                                                                                                                                  |                                                                                          |
| Components of a rocket engine   | A Rocket Engine usually consists of one or more Thrust Chambers, one or<br>more Tanks for storing propellants, a Feed Mechanism to force the liquids into<br>the thrust chamber, a Power Source to provide energy to the feed mechanism,<br>suitable Piping and Valves to transfer the liquid propellants, a structure to<br>transmit the thrust force to the vehicle, and Controls to initiate and regulate the<br>propellant flow rates.                                                                      |                                                                                          |
| Components of a thrust chamber  | The Thrust Chamber, also often called Thruster (usually for low thrust and<br>repeated starts), has an Injector (where propellants are introduced and metered), a<br>Combustion Chamber (where liquid propellants react and burn) and a<br>converging-diverging Nozzle, where the hot combustion gas is accelerated and<br>ejected at supersonic velocities.                                                                                                                                                    |                                                                                          |
| Solid rocket propellant         | Solid Rocket Propellant typically consists of an oxidizer (usually a crystalline<br>solid like ammonium perchlorate), an organic Fuel (such as a rubbery polymer<br>like polybutadiene, which also acts as the glue to hold the grain together), and<br>various additives to improve performance, storage, thrust-time profile,<br>manufacture, aging, etc. Additives include liquid Plasticizers, Explosives,<br>Burning Rate Catalysts, etc.                                                                  |                                                                                          |
| burn rate                       | The Burning Rate is the rate of regression of the burning grain surfaces as<br>propellant is consumed or burnt (inches per second) in a direction normal to the<br>surface. Surfaces that are bonded to the case walls or to insulators, will not burn.                                                                                                                                                                                                                                                         |                                                                                          |
| inhibitor                       | Inhibitors are layers of non-burning materials that are glued to exposed grain<br>surfaces so that they will not burn. The propellant flow and, therefore, also the<br>thrust are proportional to this burning rate and the exposed burning surface. The<br>burning rate varies with chamber pressure and the initial ambient temperature of<br>the grain.                                                                                                                                                      |                                                                                          |
| internal insulator              | Internal Insulators are layers on the inside of the case wall made of material<br>with low thermal conductivity; they protect the case from the hot combustion<br>gases and prevent it from reaching the temperature where the case material loses<br>its strength                                                                                                                                                                                                                                              |                                                                                          |
| external insulator              | External Insulators are applied to the outside of liquid propellant tanks or solid<br>propellant motor cases to protect against excessive heat transfer from hot air,<br>when flying through the atmosphere at high speed.                                                                                                                                                                                                                                                                                      |                                                                                          |
| Attitude control                | Attitude Control (vehicle rotation) and a limited amount of Flight Velocity<br>Change can also be achieved by a series of small thrusters, which are located in<br>various locations on the vehicle. By Pulsing individual thrusters or pairs of<br>thrusters (repeated short duration operations) the vehicle can be rotated and<br>maneuvered.                                                                                                                                                                |                                                                                          |
| Erosion                         | In uncooled nozzles the gradual Erosion of the nozzle throat causes a small increase in nozzle cross-sectional area and thus a small decrease in chamber pressure and thrust.<br><br>                                                                                                                                                                                                                                                                                                                           |