Decreasing flow area results in subsonic acceleration of the gas. The area decreases until the minimum or throat area is reached. Simply stated, the nozzle uses the pressure generated in the combustion chamber to increase thrust by accelerating the combustion gas to a high supersonic velocity. The velocity that can be achieved is governed by the nozzle area ratio which in turn is determined by the design ambient pressure-the atmosphere into which the nozzle discharges. Low ambient pressure leads to a high nozzle exit area, higher gas exit velocity, and hence, more thrust. High thrust efficiency is achieved as a result of careful design of the nozzle shape or contour.

For an ideal one-dimensional flow where the gas properties are uniform at any point in the flow direction, thrust is simply the gas momentum at the nozzle exit, minus the influence of pressure. In a vacuum or space application, the maximum theoretical performance would be realized by an ideal nozzle with infinite exit area that would expand the combustion gases to zero pressure thereby attaining the maximum gas velocity which can be expressed.

Since the engine flow rate is fixed by chamber pressure, propellant mixture ratio and the nozzle throat area, the nozzle increases thrust by achieving as much as possible, given practical limitations of size, weight and operational altitude. Low altitude operation, of course limits the amount of nozzle expansion available because of higher ambient pressure. If ambient plunger pressure is excessive, the engine exhaust flow will be separated from the nozzle wall and large nozzle side loads may develop due to the random uneven separation of the exhaust jet. These loads must be included in the nozzle structural design process.

With a perfect gas, g remains constant throughout the expansion process. However, when the 8N7005 engine flow is composed of hot combustion products, real gas effects dominate. So as the gas expands, g shifts as a result of changes in temperature and the chemical composition.