Welcome to satellite

86/04/17

Chemical rocket propulsion is essentially a controlled explosion of some of the most volatile elements on the periodic chart. The epicenter of the event is the rocket engine thrust chamber.

A satellite is lifted into orbit atop a Delta launch vehicle powered by a Pratt & Whitney Rocketdyne RS-27.
In late afternoon on December 18, 1992, a three- stage Delta II lifted a 4,150-pound NAVSTAR II global positioning system (GPS) satellite into orbit, the 17th entry of an eventual 24-satellite constellation that will enable ground-, air- or sea-based users to accurately determine their true location within 50 feet, as well as the precise time within a millionth of a second under all weather conditions. The primary engine for the three-stage Delta II launch vehicle was Pratt & Whitney Rocketdyne's RS-27A engine, which has a sea-level thrust of 201,000 pounds, and was augmented by nine solid rocket motors that are strapped to the exterior of the vehicle.

Earlier that same year-on June 9-an Atlas IIA carried a 6,000-pound INTELSAT K spacecraft into low Earth orbit that provided new telecommunications links between Europe and North and South America. Primary propulsion for the two-stage vehicle was the Atlas MA-5A engine system, developing 484,000 pounds of thrust.

And in January of 1993, the Space Shuttle roared aloft on mission STS-54 with a payload of more than 46,000 pounds, including NASA's Tracking and Relay Satellite, which will enable high-capacity communication with the shuttle and other launch vehicles. Powering that flight were three Space Shuttle Main Engines (SSMEs), each generating 489,000 pounds of thrust.

The three missions demonstrate a wide variety of payload capacities-in weight and applications. In each situation, the engine and vehicle were closely matched to the declared need, missions that comparatively indicate the wide size spectrum of spacecraft currently being placed in Earth orbit.

Our discussion here, however, will be concerned with more basic aspects of those engines-in particular, the rocket engine thrust chamber. Understanding that portion of a liquid rocket engine is a key to an appreciation of the larger technology.

Common to all three flights was a similar fiery beginning as the main engines ignited with a resounding explosion that continued as the vehicles lifted slowly off the pads and moved upward, gaining speed with each passing moment. What actually occurred was a sustained explosion under carefully controlled conditions, a prime example of the venerable every-action-has-an-equal-and-opposite-reaction law. In each case, the law was put to work in spectacular fashion.

The precise site and the means for controlling that sustained explosion is the engine thrust chamber, the very heart of the propulsion system-and where form truly conforms to function. In the RS-27A, the Atlas MA-5A and the SSME, that form is very much predicated on mission requirements, which traditionally include thrust needs, flight duration and the number of starts the engine will have to endure. And further, the realities of thrust chamber design exert a heavy influence on the rest of the propulsion system.

In a liquid propellant rocket engine, the function of the thrust chamber assembly is to generate thrust by efficiently converting the propellant chemical energy into hot gas kinetic energy. That conversion is accomplished by the combustion of the propellants in the combustion chamber, followed by acceleration of the hot gas through a convergent/divergent nozzle to achieve high gas velocities and thrust.

Basic elements of the thrust chamber assembly are an injector assembly, which contains a propellant ignition device, a combustion chamber and a hot gas expansion nozzle. Depending upon the nozzle expansion ratio and size, which impacts both nozzle fabrication and handling, the expansion nozzle may be either an integral part of the combustion chamber or a separate component part that attaches to the exit end of the combustion chamber assembly.