Liquid Rocket Engines
The propellants we’ve talked about are used in liquid rocket engines. These powerful devices burn the fuel and oxidizer under extreme pressure and heat to produce the thrust that propels rockets into orbit, and to the planets.
But it takes more than just letting the chemicals explode in a chamber. If that’s all that happened, little thrust would be created. The gases generated by the controlled explosion must be sped up to supersonic velocity. This is accomplished utilizing what is known as the Venturi effect, named for Italian physicist Giovanni Venturi who discovered the principle in the 18th century.
This effect is such that a fluid (or gas in the case of a rocket motor) flowing from a large cylinder to a smaller cylinder increases its velocity as it flows through the smaller cylinder. Those of you who live in a city I’m sure have experienced this effect as you’ve walked between buildings and been subjected to a sudden increase in wind velocity.
The gases from the large cylinder flow through the smaller cylinder until they reach the local speed of sound. If they are then allowed to expand into another larger cylinder, they will go supersonic.
That is the basis of a rocket motor. The propellants are burned in a combustion chamber (the large cylinder), flow through a small cylinder, called the throat, and into another large cylinder called the nozzle. The nozzle is critical to the rocket engine. The gases must be allowed to expand to reach the ambient pressure of the medium they will exhaust into. Nor do rocket exhaust just expand in a linear manner. They expand in a slightly bowed shape. Thus the familiar bell shape of today’s rocket engines.
In the early days of rocketry, this was not known, and the V2 engine, for example, had a simple straight nozzle. Even the Redstone had a straight nozzle.
Even with the bell-shaped nozzle, it is critical that the gases be expanded so their pressure matches that of the surrounding pressure while the gases remain in contact with the surface of the nozzle. If the gases reach ambient pressure before they exit the nozzle - overexpanded - they will separate from the surface. If they have not reached ambient pressure when they exit the nozzle, they will balloon out as they do - underexpanded. In either case thrust will be significantly reduced. That means a motor meant to operate mainly in the atmosphere must be shorter than one that is to operate mainly in space. The illustrations show how over and under expansion can affect the exhaust.
Over expansion causes shock diamonds to form in the exhaust because the gases are dropping below supersonic and making 'sonic booms' within the exhaust stream. It’s a fine sight, but reduces thrust.
Under expansion reduces thrust as well. Engineers had a particularly thorny problem with the Shuttle's main engines, because they had to operate from sea level to the zero ambient pressure of space. This meant the nozzle expansion had to be a compromise to get the greatest efficiency throughout the boost phase. If you've seen Shuttle launches from the long-range cameras, you have seen that the main engine exhaust begins to balloon out slightly as the spacecraft approaches orbit. It is underexpanded at that point.