Page 76 - Intro to Space Sciences Spacecraft Applications

Propulsion

Overexpansion Underexpansion Perfect Expansion 63
Figurn 3-4. Nozzle exhaust expansion. It is desirable to minimize the
difference between exhaust pressure and ambient pressure.

higher exhaust pressure in this equation far exceeds the gain in thrust due
to the pressure thrust term.

Perfect Expansion. In this case, the exhaust nozzle expands the propel-
lant gases such that pe is exactly equal to pD. and the pressure thrust term
equals zero. By reason of default of the other cases, this is the best situa-
tion when considering the pressure thrust term and nozzle design.
The practicalities of nozzle design mean it is not always possible to
maintain a perfect expansion condition throughout the operating range of
the rocket. For instance, launch vehicles are used to carry payloads from
the surface of the earth, through the atmosphere, into the near vacuum of
space. Although variable exhaust area nozzles have been designed to con-
tinually match exhaust pressures with the ambient conditions during tran-
sit through the atmosphere, these systems are too complex to be practical
and the thrust benefit is not worth the expense. In most cases, a median
operating pressure is chosen and the nozzle designed to expand the pro-
pellants to this pressure, accepting the small changes in thrust produced at
other pressures. For systems that operate only in space, the situation is
simpler and the nozzle is designed to expand the exhaust to as low a pres-
sure as possible. This results in large, long rocket nozzles as exemplified
by the Apollo service module nozzle depicted in Figure 3-5.

ORBIT ESTABLISHMENT AND ORBITAL MANEUVERS

As was mentioned at the end of Chapter 2, a spacecraft’s propulsion
requirements may include delivery to space, maneuvering into position,
and maintenance of the spacecraft position and orientation. While posi-
tion-keeping systems are usually designed as an integral part of the space-