Nozzle Shapes - The Bell Nozzle
These nozzles are the epitome of what people think of when you say "rocket nozzle." Their shape resembles that of a bell, hence the name. This nozzle has the unfortunate problem of being unable to alter its geometry, which means that as local back-pressure changes the performance of the nozzle becomes sub-optimal. The exit diameter is fixed, which corresponds to maximum efficiency at a particular back-pressure.
As a rocket ascends to the upper atmosphere, the local pressure decreases greatly. Whether designed for operation at ground level, or high in the atmosphere, a bell shaped nozzle will only experience optimum operation for a small percentage of its flight.
As a rocket ascends to the upper atmosphere, the local pressure decreases greatly. Whether designed for operation at ground level, or high in the atmosphere, a bell shaped nozzle will only experience optimum operation for a small percentage of its flight.
As shown in this picture, the bell nozzle operates at maximum efficiency only when local conditions match the design pressure. When the back-pressure is greater than that level, the exit flow collapses inward resulting in a shock wave forming within the exit jet.
When the back-pressure is lower than the design pressure, the exhaust flow expands suddenly after the exit plane. Both flow collapse and flow expansion represent sub-optimal nozzle efficiency.
Despite this major drawback, bell nozzles are still used are the vast majority of rockets. From missiles used in combat, to the massive launch vehicles used in delivering satellites into orbit, bell nozzles are used to provide propulsion.
When the back-pressure is lower than the design pressure, the exhaust flow expands suddenly after the exit plane. Both flow collapse and flow expansion represent sub-optimal nozzle efficiency.
Despite this major drawback, bell nozzles are still used are the vast majority of rockets. From missiles used in combat, to the massive launch vehicles used in delivering satellites into orbit, bell nozzles are used to provide propulsion.
JR