# Over-Under Expanded Nozzle:
Expansion is the process that converts the thermal energy of combustion into kinetic energy to move an object forward. In other words, the hot gases created by burning fuel inside a jet or rocket engine are exhausted through a nozzle to produce thrust. It is the shape of this nozzle that is key to the expansion process. As that high temperature flow is exhausted, it expands against the walls of the nozzle to create a force that pushes the vehicle forward.
Flow passing though a rocket nozzle
The behavior of this expansion process is largely dictated by pressure--both the pressure of the exhaust itself as well as the pressure of the external environment into which it exhausts. Of greatest concern is to design the shape and length of the nozzle so that it converts as much of that thermal energy into thrust as possible. In an ideal nozzle that optimizes performance, the exit pressure (Pexit) will be equal to the ambient pressure of the external atmosphere (P¥). The flow in this case is perfectly expanded inside the nozzle and maximizes thrust.
Unfortunately, this situation can only occur at one specific atmospheric pressure on a fixed-geometry nozzle. As we have seen previously, pressure decreases as altitude increases. Nozzle designers typically must select a shape that is optimum at only one altitude but minimizes the losses that occur at lower or higher altitudes. These losses result from the fact that the atmospheric pressure will either be higher than the exit pressure of the exhaust gases, i.e. at low altitudes, or lower than the exit pressure, i.e. at high altitudes.
The behavior of this expansion process is largely dictated by pressure--both the pressure of the exhaust itself as well as the pressure of the external environment into which it exhausts. Of greatest concern is to design the shape and length of the nozzle so that it converts as much of that thermal energy into thrust as possible. In an ideal nozzle that optimizes performance, the exit pressure (Pexit) will be equal to the ambient pressure of the external atmosphere (P¥). The flow in this case is perfectly expanded inside the nozzle and maximizes thrust.
Unfortunately, this situation can only occur at one specific atmospheric pressure on a fixed-geometry nozzle. As we have seen previously, pressure decreases as altitude increases. Nozzle designers typically must select a shape that is optimum at only one altitude but minimizes the losses that occur at lower or higher altitudes. These losses result from the fact that the atmospheric pressure will either be higher than the exit pressure of the exhaust gases, i.e. at low altitudes, or lower than the exit pressure, i.e. at high altitudes.
(a) over-expansion, (b) ideal-expansion, and (c) under-expansion
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Table of Contents:
- What Are Over-Under Expanded Nozzle?
- Expansion Process
- Ideal Expanded Nozzle
- Over Expanded Nozzle
- Under Expanded Nozzle