Rocket Fundamentals

• A rocket engine converts chemical energy in the fuel into thrust. The fuel and an oxidizer combine in a combustion chamber to produce hot exhaust gases. The velocity of the gas as it leaves the rocket through the nozzle along with the pressure at the base of the rocket determines the amount of thrust produced. The ambient air pressure outside the rocket depends on the external air temperature and altitude above sea level. This pressure difference affects the thrust and efficiency of the rocket.
  
  

Nozzle Basics

• A rocket nozzle includes three main elements: a converging section, a throat, and a diverging section. The combustion exhaust gas first enters the converging section. The gas moves at subsonic speeds through this area, accelerating as the cross sectional area decreases. In order to reach supersonic speeds, the gas must first pass through an area of minimum cross sectional area called the throat. From here, the supersonic gas expands through the converging section and then out of the nozzle. Supersonic flow accelerates as it expands.
  
  

Performance

• The performance of a rocket nozzle depends on the ratio of the cross sectional exit area to the cross sectional area of the throat. This is called the expansion area ratio. This value normally remains constant since nozzle geometry normally does not change. Combustion chamber pressure and mass flow rates also remain fairly constant during engine operation. The rocket designer chooses the nozzle characteristics and configurations based on its expected operating conditions.
  
  

Types of Nozzles

• Conical and contoured nozzles are the most common types. Conical designs nozzles have straight sides all the way from the throat to the exhaust. The constant angle does not force the exhaust gas to turn, eliminating energy loss through the resulting shock waves. The constant angle also makes this type of nozzle easy to design and build. Contoured or bell-shaped nozzles turn the flow, which reduces efficiency and thrust due to the shock waves resulting from the turn. More advanced designs, such as annular or expansion-deflection nozzles, maintain their efficiency over larger pressure ranges. These are more complex and expensive to produce.
  
  

Design Considerations

• The rocket designer must trade the weight, size, and complexity of the engine, including the nozzle, against its performance requirements. The rocket's operating conditions, such as outer space or sea level, and its required thrust dictate the expansion area ratio. A smaller, lighter nozzle requires the flow to turn more in order to reach the required exit area. This leads to a less efficient flow and reduced thrust. An engine requiring maximum efficiency across a large external pressure range will be more complicated and expensive than rockets operating over narrow sets of conditions.